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Petrologic and trace element geochemical constraints on the petrogenesis of igneous units cored during ODP Legs 170 and 205, Middle America Trench

offshore Costa Rica

B. Dreyer June 4, 2003

Abstract

Drilling on Ocean Drilling Program (ODP) Legs 170 and 205 transected the Middle America Trench (MAT, Pacific margin offshore Costa Rica) and encountered sill-like gabbros intruded into post-16.5Ma sediments, rather than the upper volcanic portion of the igneous basement expected from seismic profiling, presumably ~24Ma East Pacific Rise (EPR) lithosphere. Plate reconstruction indicates that the sill intrusion likely occurred nearly atop the E-W trending Cocos-Nazca spreading center (CNS) during an interval of higher crustal production rates due to the proximity of the active Galapagos Hotspot. The intrusions, which may have exploited nearby tectonic boundaries, may be related to coarser igneous units of normal spreading center (EPR or CNS) crustal building or later hotspot-related overprinting. Petrographic differences and a bimodal distribution of incompatible trace element abundances and ratios allow the gabbros to be categorized into two groups.

Geochemical differences between hotspot and spreading center materials were used to characterize the affinity of these gabbros. Mixing relationships reveal the importance of regional mixing between depleted and enriched sources, represented by spreading center (EPR and CNS) and plume (Galapagos Island hotspot) sources, respectively. Modeling of partial melting and subsequent fractional crystallization processes using incompatible trace element ratios suggest that the gabbros are products of ~10% batch partial meting of a moderately (~25%) enriched mantle source dominantly in the spinel field. Inter-group variations appear to be controlled by differences in degrees of enrichment rather than fractional crystallization. The gabbroic intrusions may therefore represent periodic tapping of magma source variably enriched by the entrainment of plume material. Small differences in partial melting may be the cause of intra-group differences, although the resolution of fractional crystallization effects on incompatible trace element ratios awaits further analyses. Regionally, variations in geochemical enrichment of seafloor basalts are often associated with variation in the structure of the incoming Cocos plate; these geochemical provinces may affect the generation and geochemistry of magmas beneath the Central American volcanic arc.

Introduction: At convergent margins, the structure of the subducting lithosphere may affect

magma generation beneath the volcanic arc. Isotopic studies indicate that the

geochemical composition of the oceanic crust also affects arc magma geochemistry

(Ishikawa and Nakamura, 1994; Brenan et al, 1995; Elliot et al, 1997; Regelous et al,

Brian Dreyer 6/5/2003

1997; Moriguti and Nakamura, 1998; Ishikawa and Tera, 1999; Patino et al, 2000; Clift

et al, 2001), although new data indicating elemental and isotopic fractionation from the

subducting crust suggest that this relation may not be straightforward (Straub and

Layne, 2002). The ocean floor presently subducting on the Pacific margin of the MAT

has a complicated history of hotspot and spreading center interaction (Hey, 1977). The

igneous basement is composed of materials formed at the East Pacific Rise (EPR) and

Cocos-Nazca Spreading Center (CNS), and the long-lived hotspot beneath the nearby

Galapagos Islands has thermally and geochemically overprinted the lithosphere of the

incoming Cocos plate; these variations in bathymetry and composition are thought to be

partly responsible for differences in the structure of the overriding Caribbean plate and

possibly the geochemistry of volcanic arc lavas in Central America (Carr, 1990; Werner

et al, 1999). Characterizing the bulk geochemical inventory of the subducting slab is

crucial to studies tracing subduction components in arc lavas, which requires detailed

analysis of the composition and volumetric contributions of both sedimentary and

igneous units. The role of sediments in element recycling in subduction zones has

received the bulk of the attention (e.g., Patino et al, 2000), but comparatively little work

focused on the upper igneous section.

Igneous units were cored during ODP Leg 170 from the incoming Cocos Plate at

the Middle America Trench off the Pacific coast of Costa Rica (Fig. 1). Drilling

encountered gabbroic sill-like intrusions, as confirmed by Leg 205, with geochemical

characteristics consistent with an origin related to Galapagos hotspot activity (Kimura,

Silver, and Blum et al, 1997; Morris and Villinger et al, 2003, in press). The intrusive

complex was unexpected, as the upper volcanic section of basement, inferred through

pre-cruise seismic profiling, was thought to exist at this depth. These intrusions share

some geochemical and petrologic similarities with a deeper, massive igneous complex

cored near the same location on Leg 205. It has also been suggested that the deeper

igneous unit on Leg 205 could represent coarser spreading center material, implying

local interaction between hotspot activity of distal origin and activity related to crustal

building at spreading centers (Morris and Villinger et al, 2003, in press).

This study uses new trace element analyses of Leg 170 gabbros and shipboard

analyses of Leg 205 samples to generate models of mantle mixing and partial melting


mechanisms. These models are constructed to investigate the petrogenesis and

geochemistry of the Leg 170 gabbros in relation to regional seafloor volcanism. This

represents an initial step in evaluating hotspot and spreading center interaction, which

affects the structure and composition of the seafloor, and may ultimately affect magma

generation beneath the Central American volcanic arc. The tectonic structure of the

incoming Cocos is well studied and may provide further constraints on the timing and

location of intrusion of these gabbroic units.

Regional Geologic Background: The Incoming Plate:

The oceanic Cocos and Nazca plates are subducting beneath the western edge

of the Caribbean plate at the Central American convergent margin (Fig. 1). The oceanic

crust of the Cocos plate is characterized by variations in origin, morphology, and

thermal structure along the strike of the Middle America Trench (MAT). These

variations occur on several scales, are largely due to the conjunction of lithosphere

generated at the EPR and CNS and interactions with the Galapagos Hotspot, and affect

the composition and behavior of the subducting oceanic crust.

The initiation of the Galapagos Hotspot was likely signaled by the emplacement

of the Caribbean Large Igneous Province (CLIP) ~95Ma (Montgomery et al, 1994; Hauff

et al, 2000; Hoernle et al, 2000; Werner et al, 1999; Sinton et al, 1998, Hauff et al,

1997), although debate persists over the initial location of emplacement (c.f., Meschede

and Frisch, 1998; Meschede, 1998). The initiation of spreading centered on the CNS is

better constrained; the Farallon plate broke up at 22.7Ma (Barckhausen et al, 2001;

Meschede and Barckhausen, 2000) to form the Cocos plate to the north, and the Nazca

plate to the south (Fig. 1). Convergence of the Cocos plate beneath the MAT is broadly

similar throughout the region, with the vector oriented at ~N30°E at a rate of ~88mm/yr

(DeMets et al, 1990). Subduction becomes shallower beneath central Costa Rica;

subduction dip angles change considerably >100km downdip, from 84° beneath

southern Nicaragua to 60° beneath central Costa Rica just north of the subducting

Cocos Ridge (Protti et al, 1995).


Differences that exist in Cocos plate morphology and bathymetry are largely an

expression of the juxtaposition of smoother lithosphere formed at the faster-spreading

EPR and rougher lithosphere formed at the slower spreading CNS, historically known

as the “rough-smooth boundary” (RSB of Hey, 1977; Fig. 1). Smaller-scale crustal

morphological variability also exists in the region. For example, the more northerly

EPR-generated lithosphere offshore southern Nicaragua is highly faulted with extensive

back-tilted horst and graben structures (Kelly and Driscoll, 1998), while EPR-generated

crust offshore Nicoya Peninsula has a much less faulted structure with smoother

morphology. South of the traditional RSB is the seamount domain, characterized by

~40% coverage of seamounts (von Huene, 1995) and underlain by lithosphere

generated at the CNS. Further south is the submarine Cocos Ridge, a track of the

Galapagos Hotspot. This linear ridge is a product of extensive hotspot volcanism on the

Cocos Plate, and is characterized by anomalously thick and buoyant crust with

geochemistry typical of ocean island basalts (OIB; Hey, 1977, Stavenhagen et al, 1996).

Analyses of basalt glasses dredged on topographic highs reveal geochemical

characteristics that suggest widespread hotspot influence on seamount volcanism as far

north as the Fisher Seamount, about 150km northwest of the Cocos Ridge (Werner et

al, 1999; Fig. 2).

Recent correlations of magnetic anomalies have lead to a reconstruction of the

history of the CNS, refinement in the tectonic segments in the Cocos plate, and a

clearer definition between tectonic and morphological boundaries (Barckhausen et al,

2001; Meschede and Barckhausen, 2000). Spreading history along the CNS is

characterized by rate fluctuations, southward ridge jumps, and rift propagation (Hey,

1977; Barckhausen et al, 2001; Meschede and Barckhausen, 2000). The bathymetric

RSB represents the triple junction between the EPR and CNS after a ridge jump at

~19.5Ma, and is recognized by a gentle fold escarpment, with crust slightly thicker to the

south (Barckhausen et al, 2001; “Ridge jump” in Fig. 2). Just to the north, the ~80km

fracture zone trace (Fig. 2) represents the location of the Farallon breakup and the first

generation of spreading on the CNS (22.7-19.5Ma), and corresponds to the magnetic

RSB (Barckhausen et al, 2001). Thus, the magnetic and bathymetric RSBs represent

the first and second generation of spreading of the CNS, respectively, which are


separated by a southward ridge jump ~ 19.5Ma. The identification of a rift propagator

as an additional major tectonic boundary is significant, as its landward extension trends

to an offset of Wadati-Benioff zone isodepths beneath the volcanic arc in Costa Rica,

the Quesada Sharp Contortion (QSC, Protti et al, 1995; Fig. 2). The oblique orientation

of the FZT and ridge jump relative to the convergence direction of the Cocos plate

results in northward migration of their respective subduction locations. The orientation of

the propagator parallels the convergence vector, allowing the subduction of this feature

at the same position beneath the margin for some time, suggesting a link with the QSC

(Barckhausen et al, 2001).

These ocean floor morphological segments (smooth, seamount, and Cocos

Ridge domains) and tectonic boundaries are reflected in variations in the structure of

the western Caribbean margin (von Huene et al, 1995, Barckhausen et al, 1998, Ranero

et al, 2000, von Huene et al, 2000). For example, indentation of the continental slope

where seamounts are subducting just south of the Nicoya Peninsula suggests

significant subduction related tectonic erosion. Seafloor structural and morphological

variations are expected to not only have an effect on the dynamics of the subducting

materials (e.g., sediments and upper oceanic crust), but perhaps also affect the nature

of magma generation and arc volcanism in the overlying plate (von Huene et al, 2000;

Ranero and von Huene, 2000). For example, high-Mg andesites (“adakites”) in the

Cordillera Talamanca in Costa Rica and Panama are thought to be the product of

remelting of the leading edge of the Cocos ridge as it began subducting beneath

southeastern Costa Rica (Abratis and Werner, 2001).

The influence of the Galapagos Hotspot on the incoming Cocos plate is revealed

through morphological, thermal, and chemical overprinting. The most obvious influence

of the hotspot on the incoming plate is morphological, as evidenced by the Cocos,

Carnegie, Malpelo, and Coibas volcanic ridges (Fig. 3A); these hotspots tracks are

characterized by thickened, buoyant lithosphere with OIB geochemistry (Werner et al,

1999; Hauff et al, 2000; Hoernle et al, 2002 and references therein). Second order

influences of the Galapagos hotspot on the Cocos plate include: 1) abundant seamount

volcanism, 2) numerous southward ridge jumps of the CNS toward the Galapagos

Hotspot that may counteract the northward migration of the CNS, and 3) rift propagation


that may exploit newly formed lithosphere (Hey, 1977; Wilson and Hey, 1995;

Meschede and Barckhausen, 2000). Paleogeographic restorations and K-Ar ages of

hotspot products indicate that the currently subducting plate was overprinted by

Galapagos Hotspot volcanism 12- 14.5Ma, although other sources have been proposed

(Fig. 3B; Meschede and Barckhausen, 2001; Werner et al, 1999). Furthermore, ~30m

thick gabbroic sill, intruded into post-16.5Ma (latest Early Miocene) sediments, cored on

Legs 170 and 205 shows geochemical similarities with the Galapagos Hotspot,

suggesting hotspot influence even further to the north (Fig. 2). Reconstruction of the

Cocos plate to ages that bracket the latest hotspot overprinting and the earliest sill

intrusion (~12-16.5Ma) places the intrusion much closer to the CNS and Galapagos

Hotspot influence (Fig. 4); this may suggest a link between sill intrusion and increased

production rates. In a similar manner, Barckhausen et al (2001) suggest the ~14Ma

Fisher Seamount chain (for location see Fig. 2) is the result of exploitation of previous

zones of weakness as the plate moved over the hotspot.

Heat flow measurements have shown that significant anomalies exist seaward of

the Pacific coast of Costa Rica (Global Heat Flow Database; Langseth and Silver, 1996;

Kimura, Silver, and Blum et al, 1997; Fisher et al, 2001; TICOFLUX I and II and Meteor

cruise 54/2 as reported in Morris and Villinger et al, 2003, in press). In particular, a

sharp thermal boundary exists between lithosphere generated at the EPR and that of

the CNS (Fig. 2). North of this triple junction trace, heat flow values are only 10-30% of

those expected for normal lithospheric cooling of seafloor of this age (Stein and Stein,

1992); values for similar aged seafloor south of this boundary are closer to those

expected. This anomalous pattern continues beneath the prism (Meteor 54/2). The

existence of a basement fluid flow systems could account for the extremely low

conductive heat flow on EPR lithosphere (Silver et al, 2000; Kastner et al, 2000 and

references therein) and may have significant implications for alteration and shallow

thermal structure in the downgoing plate.

Caribbean Plate Geology:

The Central American isthmus lies on the westernmost edge of the Caribbean

Plate, and it can be broadly divided into three crustal provinces (Fig. 1): 1) crustal


blocks underlain by pre-Mesozoic basement rocks in the countries of Guatemala,

Honduras, El Salvador, and northern Nicaragua, which are not discussed further, 2)

Mesozoic to Cenozoic primitive island-arc in southern Nicaragua, Costa Rica, and

Panama, and 3) thickened oceanic crust (Meschede and Frisch, 1998), likely related to

the initiation of the Galapagos plume and the creation of the Caribbean plate in the

Middle Cretaceous. In southern Nicaragua and Costa Rica, young and thin arc crust is

built atop the western edge of this thick plateau basalt sequence; there is little isotopic

evidence of crustal contamination in ascending volcanic front magmas compared to

magmas ascending through pre-Mesozoic basement rocks (Carr, 1984; Patino et al,

2000). Numerous igneous complexes on the Caribbean plate are genetically related to

the Galapagos hotspot plateau basalt (Meschede and Frisch, 1998) including the

Nicoya, Quepos, and Herradura terranes accreted to the western coast of Costa Rica

(Hauff et al, 1997; Werner et al, 1999; Fig. 3A). Hoernle et al (2002) correlated these

igneous complexes to the hotspot tracks, seamounts, and the Galapagos Islands,

providing strong support for the ~95Ma near-continuous eruptive history of this plume.

Segmentation of the Central American volcanic arc has been confirmed by both

geophysical and geochemical discontinuities. A prominent volcanic arc offset is present

between southern Nicaragua and immediately adjacent northern Costa Rica (Fig. 2).

Northward, the margin is characterized by: 1) a 180-190 km arc-trench gap, 2) a 180-

200 km depth to Wadati-Benioff zone beneath the volcanic arc, 3) smaller (~30-50%)

volcanic output compared to the Nicoya region of Costa Rica (Patino et al, 2000; Carr et

al, in preparation), 4) complete sediment subduction to the site of magma generation,

and 5) more frequent earthquakes of magnitude 7 or greater, and 6) a ~20 km updip

limit of seismicity (Newman et al, 2002). The Nicoya region of Costa Rica to the south

shows an arc-trench gap of ~165 km, a ~120-130 km a depth to Wadati-Benioff zone

(Protti et al, 1995), ~10 km updip seismicity limit (Newman et al, 2002), and probable

subduction of only the basal carbonate sediment unit (Patino et al, 2000).

In sum, geophysical segmentation of the Central American volcanic arc is a

reflection of the structural and morphological variations of the incoming Cocos plate.

Geochemical discontinuities have also been related to variations in the downgoing slab,

with a variety of mechanisms invoked, including variations in 1) sediment dynamics, 2)


timing and mechanism of deep fluid extraction (Rupke et al, 2002), and 3) slab-mantle

geochemistry (e.g. Feigensen et al, 1996).

Previous Work on Legs 170 and 205 Igneous Units:

Drilling during Ocean Drilling Program Legs 170 and 205 transected the Middle

America Trench, penetrating both the undeformed section of the incoming Cocos plate

and the toe of the Caribbean plate margin (Fig. 5). Gabbros cored on Leg 170 are part

of a sill complex, as revealed by Leg 205 drilling that penetrated the sill completely and

drilled through ~30m of underlying sediments (Morris and Villinger et al, 2003, in press).

Evidence suggests that the sill drilled on Leg 170 is composed of multiple magma

injections, including increased rates of penetration (ROP) during drilling suggesting the

presence of softer sediment horizons and the presence of chilled margins (Kimura,

Silver, Blum et al, 1997).

Evidence from Leg 205 also supports the notion of multiple magma injections.

Shipboard interpretation, pending detailed chemical analysis, of the mafic igneous units

drilled during Leg 205 suggest that the upper subunit, 4A, is the equivalent of the

gabbro sill encountered, but not completely penetrated, during Leg 170 (Fig. 5). A

second, deeper igneous subunit, 4B, was also drilled during Leg 205; initial

interpretations suggest that this unit may either be another gabbroic sill, a series of thick

and slowly cooled lava flows (similar but not identical to those cored on Leg 206, see

below), or records a transition from sill to extrusive basement at ~513 meters below sea

level (Morris and Villinger et al, 2003, in press). Shipboard analyses of cored samples

indicate repeated magnetic polarity reversals, and wireline logging shows cycles of

variation in borehole diameter, density, resistivity, and P- and S-wave velocity within the

lower igneous unit (Morris and Villinger et al, 2003, in press). These observations

suggest multiple episodes of magma injection but cannot specify whether the intrusions

occurred as part of normal igneous activity at or near the ridge crest, or as later intrusive

igneous overprinting.

Shipboard analysis of 18 thin-sections from Leg 170 show that these rocks can

be generally characterized as microcrystalline to fine-grained augite gabbro with

medium-grained glomerocrysts of plagioclase and, less commonly, augite, in a nearly


holocrystalline matrix (Kimura, Silver, and Blum et al, 1997). Minor primary phenocryst

phases include altered olivine, rare orthopyroxene, ilmenite, and magnetite. Secondary

phases include zeolites, chlorite, vein-filling calcite, smectite, and products of glass

alteration tentatively identified as saponite and celadonite, suggesting only low-

temperature (probably less than 100°C) alteration. Textural evidence, including an

increase in grain-size and the appearance of fine laminations near the base of some

sections, suggest some degree of crystal settling. Accordingly, the gabbros cored on

Leg 170 do not represent primitive or primary liquid compositions.

During Leg 205, several petrologic and physical property changes were identified

within the igneous subunit 4B centered at ~513 meters below sea floor (mbsf).

Variations in many physical properties, geochemical spikes in fluid-mobile elements,

change in secondary mineralogy, increased glass and alteration, and the occurrence of

a cryptocrystalline basaltic horizon has led to a suggestion that Leg 205 may have

penetrated into the upper volcanic section of the basement (Morris and Villinger et al,

2003, in press).

The relationship between the igneous subunits cored during Legs 170 and 205

and their connection to seafloor igneous activity provides the impetus for further

geochemical and petrological examination between these local and regional igneous

“complexes”. Accordingly, comparisons are made between the samples cored on Legs

170 and 205 (collectively, “gabbros”) and regionally representative basaltic volcanism,

namely, the Galapagos Islands, CNS, and EPR between 5-10°N latitude. This section

of EPR lithosphere was chosen to minimize possible complications introduced by the

EPR-CNS triple junction (presently ~1-3°N latitude), which did not exist when the

basement underlying Legs 170 and 205 was formed.

Methods: Instrumental Neutron Activation Analysis:

Trace element data were collected post-cruise on 16 gabbroic samples (12 from

site 1039; and 4 from site 1040; see Fig. 5) by instrumental neutron activation analysis

(INAA) at Washington University in St. Louis following sample irradiation at the

University of Missouri Research Reactor in Columbia, Missouri. Replicate analysis on


samples previously measured by XRF was not practical because of the spectral

interferences caused by contamination from the tungsten carbide barrel used aboard

Leg 170.

Rock chips from paleomagnetic studies were ground on a diamond wheel to

remove surface contamination, soaked in an ultrasonic bath of Nanopure® (~18mΩ

resistivity) H2O for ~5min, and dried overnight at ~110°C. Samples were split into

smaller fragments in a hydraulic press between two plastic sheets. Fragments were

then powdered in an agate ball-mill; contamination was minimized by cleaning with pure

silica sand and self-contamination cycles between powdering samples intended for

analysis. Powdered samples, ranging in weight from ~250-300mg, were encapsulated

in high-purity silica tubing with an outside diameter of ~5mm and an inside diameter of

~3.4mm before 12hr irradiation in a neutron flux of ~5x1013 cm-2s-1. During irradiation,

samples are continuously rotated to dampen effects of flux heterogeneity.

Irradiated samples were subsequently radioassayed by gamma-ray

spectroscopy, following the methods of Korotev (1996) with additional upgrades, using

four high-purity Ge detectors spaced at different lengths according to sample activity.

Samples were continuously rotated to minimize geometry effects during counting. Four

separate radioassays occurred between seven and 30 days after irradiation in order to

maximize detection of disintegrations from both short and longer-lived nuclei. Raw data

was compiled by the Canberra Genie-ESP AlphaStation 255/233, based on a Digital

Equipment Corporation (DEC) 233 MHz AlphaStation 255 (64-bit). Data was reduced

using an updated version of the TEABAGS (Trace Element Analysis By Automated

Gamma-ray Spectroscopy) software developed by Lindstrom and Korotev, 1982.

Standard reference materials JGb-1 (gabbro, Geological Society of Japan), NBS-688

(basalt, National Bureau of Standards), GSM-1 (San Marino gabbro, USGS) were run

as unknowns and received 8 (JGb-1 and GSM-1) or 9 (NBS-688) radioassays.

Estimated one-sigma precision is given in the caption to Table 1. Accuracy was

evaluated by comparing the measured and certified concentrations of JGb-1. Results

show that all elements analyzed are within one-sigma analytical error except Zr, Sm, Hf,

Ta, and Th, which are all within estimated two-sigma analytical error. The measured

concentration of Yb does not fall within two-sigma estimates of analytical error of the


certified value. However, as reported in Korotev, 1996, replicate analysis (n=4) of JGb-

1 produces a mean concentration of 0.90ppm, which is within one-sigma estimates of

analytical error of the measured concentration reported here.

X-Ray Fluorescence:

Major and selected trace elements of the gabbroic sill cored at Sites 1039 and

1040 of Leg 170 were analyzed shipboard by X-ray fluorescence. Of ~30m of gabbro

cored, a total of 16 samples from site 1039 (11 samples) and site 1040 (5 samples)

were analyzed; results were previously reported in Kimura, Silver, and Blum et al, 1997.

Samples were ground in a shatterbox with a tungsten carbide barrel, resulting in

significant contamination of W and lesser amounts of Ta, Co, and Nb contamination;

XRF analyses of these elements are not used in this study. Estimates of external

reproducibility were made though replicate analysis of basaltic standard BIR-1, and

results show that major element data are precise to within 1-2%, except MnO and P2O5

which are precise within 5-10%, and K2O, which varies by ~30% at these low

concentrations (~0.03wt%, Shipboard Scientific Party, 1997). The trace elements Nb

(potentially contaminated), Rb, Ce, and Ba are near or below detection limits, and other

trace elements are generally precise to within 2-3%; however, trace element

concentrations of BIR-1 are generally lower than the gabbroic samples cored on Leg

170 (Shipboard Scientific Party, 1997). Accuracy was evaluated by comparing the

replicate analyses of BIR-1 with the certified values; results indicate that all major

elements are accurate within one-sigma analytical error, except P2O5 and K2O, which

have concentrations in BIR-1 near the detection limit. Trace elements Nb, Zr, Rb, Ni,

Ce, and Ba are accurate within one-sigma analytical error, while Cu, Cr, and Sr are

accurate within two-sigma.

Inductively Coupled Plasma- Atomic Emission Spectroscopy:

Analysis of basaltic and gabbroic samples was performed on Leg 205 by

shipboard ICP-AES (Morris and Villinger et al, 2003, in press). Estimates of precision

are based on the comparison of measured values of five standards, run as unknowns,

and the certified value. In general, reproducibility is estimated to better than 3% for the


major elements and 5% for the trace elements, except when concentrations approached

the detection limit (Shipboard Scientific Party, 2003a). Based on replicate analyses and

certified values of five references materials of basaltic composition, accuracy is

estimated to be within one-sigma analytical error for the all the reported major element

oxides with occasional exceptions for SiO2, MgO, MnO, and Na2O and all trace

elements, except Sr and occasionally Ba.

Replicate analysis at Washington University of samples previously analyzed by

XRF was avoided due to the shipboard powdering technique that introduced trace

element contaminants, which would interfere with analytical peaks of other elements

during INAA (R. Korotev, pers. comm., 2002). However, data quality can be addressed

by comparison of measured concentrations of samples of the same standard reference

materials used with more than one method. The results of this comparison are shown

in Table 2. Major element data between XRF and ICP-AES analyses agree within 5%,

except P2O5 and K2O; available trace element data agree to within 5%, except Zr.

Major element analysis is limited with INAA (gamma-ray spectroscopy), but FeO, CaO,

and Na2O show good agreement with ICP-AES data; available trace element data show

very good agreement between Sc and Cr (<1% difference), agreement to within 10% for

Sr and Ba, and poor agreement for Zr. X-ray fluorescence and ICP-AES analyses of

BIR-1 are indistinguishable at the one-sigma level for the elements Ti, Al, Fe, K, Mg,

Mn, Ca, Na, P, Ni, and Y (two-sigma for Si, Cr, and Sr, and three-sigma for V and Zr).

For INAA and ICP-AES analyses of Jgb-1, the elements Fe, Ca, Na, Sc, Cr, and Ba are

indistinguishable at the one-sigma level, while Sr and Zr are within two-sigma.

Results: Petrography:

Six thin sections were analyzed post-cruise to assess petrogenetic relationships

between the Leg 170 gabbros. Mineral assemblages and visually estimated modes

agree with the shipboard findings, although rare (<1%) highly altered olivine is

tentatively identified in most of the thin sections, pending microprobe analysis.

Mulitstage crystallization is suggested by the presence of three generations of


plagioclase crystals observed in all thin sections: glomerocrysts, phenocrysts, and

microphenocrysts. Plagioclase glomerocrysts are often subhedral to sub-rounded,

sometimes poikolitically enclosing augite, typically ~5mm, and have An60-65 (using

Michel-Levy estimates, pending microprobe analysis). Plagioclase phenocrysts exhibit

a range in sizes but are typically ≤4mm, occur as euhedral to subhedral laths and

sprays with An55-60; the larger phenocrysts are commonly An55-70, euhedral, embayed,

and have altered glass (?) inclusions and sericite weathering concentrated near the

crystal core. Augite phenocrysts are commonly subhedral to rounded, ~1-2mm, but

occasionally occur as ~3mm euhedral phenocrysts.

Relatively fast later stage crystallization may be suggested by sector zoning

observed in some smaller phenocrysts and microphenocrysts of augite, although this

interpretation of augite sector zoning is debated in the literature (see discussion in

Brophy et al, 1999). Rapid crystallization is suggested by skeletal plagioclase

microphenocrysts in the groundmass, which have An50-55. Anorthite content in

plagioclase glomerocrysts and phenocrysts decreases core-to-rim, indicating magma

conditions becoming less calcic as crystallization proceeds; oscillatory zoning is

common, particulalrly in larger, more Ca-rich plagioclase phenocrysts, suggesting

changing magma compositions during crystallization. In general, An content also

decreases with plagioclase grain size.

Two groups are discernible in these thin sections, based on the abundance and

type of glass and secondary alteration phases, rock texture, and the abundance of

higher-Ca plagioclase phenocrysts. The first group has common (~5-10%) amber

colored glass, which is less devitrified and altered (fibrous orange-brown product)

compared to the second group, which has distinctly fibrous to acicular brown-green

altered glass and is clearly more altered overall (up to ~30%). The first group tends to

have seriate crystal size distribution and intergranular texture, within a network of

plagioclase crystals; the second group is dominantly porphyritic to intersertal with large

glomerocrysts and phenocrysts (plagioclase ~An65-75) and a microcrystalline to

cryptocrystalline groundmass. There are small differences in the mineral modes for the

two groups. The second group tends to have a higher proportion of larger phenocrysts

of augite, more opaque phases concentrated in the groundmass, and lack olivine; the


latter may be a primary feature or due to complete replacement by secondary alteration.

Finally, the second group contains a higher proportion of vesicles and possible small-

scale magmatic contacts, and textural evidence suggesting a partially cumulate origin,

including crystal settling and layering. Completely altered glass occurs at the interfaces

of magmatic contacts, but recrystallization of primary minerals is not apparent. The first

group of thin sections appears in the upper section of Hole 1039C; the second group is

found in the lower section of Hole 1039C and the sole sample from Hole 1040C.

Petrographic analysis of these thin sections indicates changing magma chemistry

during crystallization as well as changing crystallization rates, and indicates that these

gabbros do not represent primary melts. The analyses described herein cannot

determine if these are part of the same parental magma, but allow two groups to be

discerned based on differences in rock texture and secondary mineral modes. Changes

in chemistry, form, and size among the occurrences of plagioclase crystals found in

each thin section suggest that fractional crystallization of An-rich (probably bytownite,

An70-90) plagioclase from the parental magma(s) was pervasive. Changes in crystal

habit and abundances in augite and olivine seem to also suggest changing magmatic

conditions. Mapping of mineral chemistry through microprobe analysis of early and late-

crystallizing primary phenocryst phases will further constrain the evolution of the magma

and may potentially reveal parental compositions.

Geochemistry:

Three geochemical data sets are used in this study to address the issues

outlined above, as discussed in the Methods section; however, no sample has been

analyzed with more than one method. Interpretations drawn from comparison of these

geochemical data sets, and discussed in the following sections, is therefore restricted

to: 1) major elements, particularly between XRF and ICP-AES and 2) selected trace

element analyses from INAA. These data are generally precise within 5% and accurate

within (one-sigma) analytical error.

Rocks analyzed from Legs 170 and 205 are characterized by small variations in

major element oxide composition, as is typical for ocean floor igneous environments.

These gabbros are low- to medium-K (generally K20 <0.4wt%) subalkaline mafic rocks,


and show a Fe-enrichment trend characteristic of ocean floor tholeiites (Figs. 6, 7A). All

samples contain between 46-50wt% SiO2 with Mg# [=Mg2+ / (Mg2+ + Fe2+)] ranging from

0.44-0.60, consistent with their basaltic composition. Primitive basalts typically have

Mg# ~0.70 (Roeder and Emslie, 1970), and these gabbroic samples have therefore

experienced a significant degree of fractionation of high-Mg# phases, such as olivine

and pyroxene, similar to the majority of spreading center basalts. Figure 7B shows that

whole rock basalts from the EPR between 5-10°N latitude and basaltic glasses from the

CNS generally exhibit slightly higher Mg# at a given SiO2 as compared to Legs 170 and

205 gabbros and Galapagos Island basalts, possibly suggesting less fractionation of

high-Mg# phases for spreading center basalts en route to surface. Analyses of CNS

whole rock samples show slightly lower Mg# for a given SiO2 and a larger range in

composition than CNS basaltic glasses, reflecting the effects of fractional crystallization.

Normative mineralogy with diopside, olivine, and hypersthene, generally

occurring in decreasing abundance, suggests a slightly silica-undersaturated character

of the gabbros (Yoder and Tilley, 1962; Thompson, 1984; Fig 8). The gabbros from

Legs 170 and 205 cluster around the center of the olivine tholeiite (di-ol-hy) field of the

basalt tetrahedron of Yoder and Tilley (1962), a feature shared with the majority of

spreading center basalts (White et al, 1993; Alt, Kinosha, Stokking et al, 1993). Basalts

from the Galapagos Islands tend to cluster around the olivine-diopside join, suggesting

moderate silica-undersaturation, although they exhibit a greater range in normative

mineralogy than the spreading center glasses.

Leg 170 gabbros are trace element enriched compared to average EPR-MORB,

as illustrated by (La/Sm)N >1 (Table 1). Furthermore, there appears to be a bimodal

distribution in (La/Sm)N values (~1.8 and ~1.4), as well as in abundances and ratios of

other immobile, incompatible trace element (e.g., Hf/Ta and Sm/Lu). Chondrite-

normalized rare earth element diagrams of Leg 170 samples are shown in Figure 9A.

The range in normalized abundances of the Leg 170 igneous rocks lies entirely within

the fields of spreading center (EPR and CNS) and Galapagos Islands basalts (Fig. 9B).

However, the REE patterns of the gabbroic samples are clearly distinct from spreading

center basalts, having lower heavy REE (HREE ~16x chondritic) and higher light REE


(LREE) abundance patterns, and are similar to more enriched [i.e., higher (La/Sm)N]

basalts of the Galapagos Islands.

Gabbros cored from Legs 170 and 205 show moderate alteration including

devitrified glass, secondary clay, and olivine alteration (Shipboard Scientific Party, 1997

and 2003a; see “Previous Work” above). Probable alteration effects on whole rock

chemistry include increased abundance or variation in abundance of fluid mobile

elements (e.g., Ba, K, Sr, etc.) with relatively little change in major and immobile trace

elements. A comparison between XRF and ICP-AES data shows that the abundances

of large ion lithophile elements (LILE) are positively correlated with loss on ignition (LOI,

which approximates volatile loss and infers degrees of alteration). Similar alteration

behavior between Hole 1039C and subunit 4A and between Hole 1040C and subunit 4B

is suggested based on the trends observed in LILE vs. LOI. Ratios of fluid-mobile to

fluid-immobile incompatible elements were used to qualitatively evaluate the extent of

alteration (e.g., Rb/Zr for XRF, K2O/Zr for ICP-AES, and Na2O/Hf for INAA). Spikes in

these ratios are found to be proximal to sediment-gabbro contacts (recovered or

inferred by increased ROP), veins, vesiculated areas, or near possible magmatic

contacts, as identified shipboard. Finally, gabbros that are more altered and glass rich

tend to also have lower concentrations of fluid mobile elements; this may be primary or

could suggest fluid-mobile element leaching during alteration.

Discussion: The geochemical and petrological analyses of Leg 170 gabbros are used to

address their possible origin, emplacement, and relationship to previously described

seafloor domains in the eastern Pacific offshore Central America. The gabbros have

been subdivided into two groups through petrographic differences and incompatible

trace element abundances and ratios. These ratios are used to fingerprint possible

source regions, form plausible mixing relationships with geochemical end-members of

regional influence, and examine the depth of magma generation. Partial melting and

subsequent fractional crystallization models are used to infer mechanisms that could

produce the differences within and between the gabbro groups. Finally, we speculate


on hotspot influence on the geochemical makeup and thermal state of the subducting

plate, which may ultimately affect magma generation beneath volcanic arcs.

Igneous Provenance:

A goal of this study is to determine the provenance of the igneous units cored on

Legs 170 and 205. More explicitly, do these gabbros owe their origin to volcanism

associated with normal spreading center crustal building or to distal hotspot activity?

These questions have important implications concerning volumes and distances of

hotspot- vs. ridge-related emplacement. These processes may alter the geochemical

and thermal character of the subducting oceanic plate and, ultimately, the composition

of volcanic arc magmas. Geochemical distinction is possible because hotspot and

spreading center sources generally have different trace element characteristics.

Petrologic relationships indicate that Leg 170 and 205 igneous rocks do not represent

their original liquids, and the consequences of fractional crystallization must be

addressed when attempting to elicit parental liquid composition.

The small degree of differentiation between the igneous samples cored on Legs

170 and 205 (SiO2 varies by 4wt%, MgO by <2.5wt%) makes interpretation of major

element trends necessarily speculative. However, the lack of coherent major element

trends suggests that the gabbros have complicated histories. The major element

variations are consistent with differing proportions of low-pressure fractional

crystallization of olivine, plagioclase (bytownite, An70-90), and/or augite from a more

primitive magma before injection (Fig. 10A), consistent with petrographic inferences

described above. Source compositional or melt degree differences could also cause

the variations observed, and cannot be excluded. Small ranges in the abundances of

major and minor elements of ocean floor igneous rocks largely reflect phase equilibria

control, rather than differences in source composition. Source compositional

differences are elicited from trace element systematics.

Minor and trace element trends are generally more coherent for Leg 170

gabbros, sometimes occurring as segments with different slopes. For example, V and

TiO2 are well correlated into two groups, suggesting that the presence of ilmenite and/or

titanomagnetite controls the concentration of these elements in the residual liquid (Fig.


10B). Both V and Ti are highly compatible in these refractory phases but incompatible

in the bulk system. The offsets suggest distinct magma sources and/or separate

magma generation events, as comagmatic events are expected to form correlated linear

trends. Somewhat less correlated trends are also observed in Na2O, Ni, Cr, and Sr vs.

Zr and TiO2. The lower-TiO2 trend includes the base of Hole 1039C, all of Hole 1040C,

and all of subunit 4B from Leg 205 Hole 1253A (i.e., group 2). The higher-TiO2 group

includes all of Hole 1039C, except the lowermost two samples, and all of subunit 4A

(Fig. 6B), distinctions that are similar to the petrographic groupings of Leg 170 gabbros.

The complicated history of the gabbros and relatively limited major element data

set does not permit reliable fractionation normalization calculations (i.e., abundance

normalization to 7wt% MgO along a derived liquid line of decent in order to minimize

fractionation effects) as described in Klein and Langmuir, 1987. Therefore, this study

uses immobile, incompatible trace elements and ratios in an effort to circumvent many

of the complexities caused by fractionating phases and post-crystallization element

mobility.

Data from INAA (this study) and previously published results of immobile,

incompatible trace elements are used to determine likely source regions and address

plausible melting and crystallization scenarios. As described above, the division

between the two groups of igneous rocks cored from Legs 170 and 205 is revealed in

incompatible trace element abundances and ratios. Group 1 is characterized by higher

concentrations of incompatible elements (Nb, Hf, Ta, Zr, Th; Table 1) and higher ratios

of highly incompatible to moderately incompatible trace elements such as (La/Sm)N,

Ta/Hf, (Sm/Yb)N. Groups 1 and 2 have a similar average Mg# (0.52 and 0.54,

respectively), indicating only slightly greater fractionation of group 1, assuming similar

parental magma composition. However, small differences in degrees of fractionation

alone are unable to explain the relatively larger differences in incompatible trace

element ratios (and rare-earth element (REE) patterns, see below) between the two

groups. Therefore, group 1 rocks are interpreted as more enriched than group 2.

Two distinct groups within the Leg 170 gabbros is also evident in the REE

diagram (Fig. 9A), corresponding to groups 1 (higher REE abundances) and 2 outlined

above. Because the gabbros have distinctly different slopes, fractional crystallization


alone is not expected to be the cause of the difference in REE pattern between the

groups. Differences in degree of melting and source composition are mechanisms that

could cause the separation and LREE enrichment; however, as discussed in a following

section, a combination of small differences in source composition and melt degree best

explains the available data.

Are the Gabbros part of the Igneous Basement?:

Evidence collected during Leg 205 led to one possible hypothesis that drilling

may have encountered the top of the volcanic portion of igneous basement at ~513msbf

(Morris and Villinger et al, 2003, in press). Below this depth the rocks are more altered

and glass-rich. If this suggestion is correct, then gabbroic units recovered from deeper

levels would be from coarser parts of the igneous basement, such as slower cooled

sheet flows beneath quenched hyaloclastites and/or pillow lavas. Sediments are

generally absent at the ridge axis of fast spreading centers, and the recovery of an

unlocated boulder recording a baked sediment-gabbro contact may suggest that at least

some igneous activity was located off-axis; however, the relative stratigraphic location of

this contact and the observed changes at ~513mbsf is uncertain.

Drilling on ODP Leg 206 (Fig. 1) penetrated the igneous basement, and

comparisons can be made with the lower igneous unit from Leg 205. Plate

reconstructions place EPR-derived basement at the sites of both Leg 205 and 206 near

the same paleolocation, separated by <10Ma (Shipboard Scientific Party, 2003b),

although the spreading rate that generated basement drilled during Leg 206 was ~30%

faster (Wilson, 1996). Leg 206 shipboard studies have revealed an upper volcanic

section that is >500m and composed of multiple horizons of cryptocrystalline basalt

sheet flows with numerous intervals of hyaloclastites, pillow lavas, and massive ponded

lavas (Shipboard Scientific Party, 2003b). Whether the ponded lava flows of Leg 206

are the equivalent of the lower igneous complex cored on Leg 205 beneath ~513mbsf

remains an intriguing question. However, the vast majority of igneous rocks cored on

Leg 205 have holocrystalline matrix, while the interiors of the massive (>35m) ponded

lava units cored on Leg 206 are generally fine-grained basalts (Shipboard Scientific


Party, 2003b). Leg 206 units are described as slightly altered with a normal mid ocean

ridge (N-MORB) geochemistry.

The textural relationships and depleted geochemistry of the basalts cored from

Leg 206 are also found in basement basalts cored from Holes 504B and 896A near the

Costa Rica Rift (Fig. 1), the easternmost segment of the intermediate-rate CNS (Alt,

Kinosha, and Stokking et al, 1993; Brewer, Bach, and Furnes, 1996). The proposed

change from sill emplacement to extrusive-basement within the lower igneous subunit

(4B) cored on Leg 205 is not resolvable with available data; additional trace element

analyses will be used to address this question. However, available shipboard

geochemical data indicate that the subunit can be distinguished from the majority of

regional (CNS and EPR 5-10°N latitude) spreading center basalts on the basis of trace

element ratios.

Source and Melt Characteristics:

Partial melting and combined partial melting plus fractional crystallization

processes were modeled in order to determine mechanisms that could replicate the

incompatible trace elements ratios of the gabbros; ratios of highly incompatible trace

element were used because they are relatively resistant to major changes during

igneous processes such as melting (except very small degrees) and fractional

crystallization and they are well characterized by INAA (in general, uncertainties for La

and Sm are ~1%, Hf ~2-3%, and Ta <8% at sub-ppm concentrations; Table 1). Visually

estimated modal abundances of major phenocrysts are used in the fractional

crystallization modeling; additional modeling with normative mineral abundances shows

little effect on fractional crystallization paths. Samples from the CNS and EPR generally

have Hf/Ta > 10 with (La/Sm)N ~0.5-1.5 and plot at the lower right hand side of Fig.

11A. The Galapagos Island data have Hf/Ta < 5, as do the Leg 170 gabbros, with

(La/Sm)N ~1-2.5. The Isla Floreana “main series” basalts (Fig. 11A), which are part of

the Galapagos Islands, have anomalously high (La/Sm)N up to ~7; these are briefly

discussed below.

The results illustrated in Fig. 11A suggest that simple (modal) batch melting of

primitive mantle (PM) alone cannot explain the ratios of Leg 170 gabbros because the


modeled liquids have higher (La/Sm)N for a given Hf/Ta (Fig. 11A, B, see figure caption

for model parameters). Subsequent fractional crystallization modeling of these partial

melts indicates that crystallizing phases would have reduced (La/Sm)N at nearly

constant Hf/Ta; this process could explain some of the ratios observed for the gabbros.

However, this is considered unlikely because the modeled fractional crystallization array

(light blue line in Fig. 11B) is at a high angle relative to the trend formed by partial

melting of primitive mantle, whereas the Leg 170 samples (like the Galapagos Island

data) form a shallower trend. Also, the residual liquids (pink line in Fig 12B) would have

a much higher (La/Sm)N than is observed in any of the regional basalts. The distinct

Hf/Ta ratios in groups 1 and 2 suggest that the differences between the Leg 170

gabbros are best explained as the result of similar degree partial batch melts of variably

enriched mantle sources.

The slightly enriched source of the gabbros was modeled as a mixture of

depleted upper mantle (EPR MORB-source) and an enriched mantle (OIB-source);

determination of end-member compositions is described below. Mixing calculations

show that small additions of enriched material have moderate effects on the Hf/Ta ratios

of initially depleted material; the radius of curvature of the mixing line is ~6. For

example, ~15% addition of enriched end-member material to the depleted end-member

is adequate to achieve primitive mantle (PM) ratios; this is largely due to the low Ta

abundance of the depleted end-member (0.14ppm). Thus, a mixture of depleted

material with small volumetric addition of enriched material will itself appear enriched for

the elements considered here. Assuming little variation due to fractional crystallization,

modeling calculations indicate that group 1 gabbros are consistent with ~9% partial

batch melt from a mixture of ~28% hotspot end-member and ~72% MORB-source end-

member (Fig. 11B). Group 2 gabbros are consistent with ~20% melt of a slightly less

enriched mixture (24% / 76%). This modeling solution is not unique, and other possible

scenarios can explain the data. The most probable alternative explanation is mixing of

variously depleted and enriched partial melts, as opposed to source mixing prior to

melting modeled here. The calculated trend of residual solids from partial melting of PM

is consistent with the observed regional spreading center basalt data (red line with

circles in Fig. 11A), which may support the notion of source mixing prior to melting. This


discussion is based on incompatible trace element ratios from Leg 170 gabbros.

Testing of these proposed mixing and melting relationships between groups 1 and 2 can

be extended to Leg 205 once trace element data, and ultimately isotopic data, are

available for Leg 205 material.

Enriched and depleted mantle compositions used in mixing calculations were

extrapolated from an analysis of published geochemical data from hotspot (the

Galapagos Islands and hotspot tracks) and spreading center (EPR between 5-10°N

latitude and CNS) products. Published studies have variously argued for three to five

end-member source compositions to explain the observed ranges and heterogeneities

in isotopic composition of regional volcanic products (White et al, 1993; Kurz and Geist,

1999; Harpp and White, 2001; Blichert-Toft and White, 2001). In terms of incompatible

trace elements, these end-member source compositions reduce to: 1) depleted, MORB-

source, 2) enriched, OIB-source, and 3) a high incompatible trace element (ITE) source.

The ITE-rich source is believed to be metasomatized mantle, possibly formed through

ancient recycling of subducted crust (Bow, 1978; Bow and Geist, 1992; Geist, 1992;

Harpp and White, 2001; Blichert-Toft and White, 2001), and is most prevalent in the

anomalous Isla Floreana “main series” basalts (Fig. 11A; c.f., Bow and Giest, 1992). If

the ITE-rich source is a unique mantle end-member, there is no evidence for significant

involvement of the high ITE end-member in the gabbros (extrapolated to be (La/Sm)N

~7, Hf/Ta ~1; (La/Sm)N ~10 from Bow and Geist, 1992; Fig. 11A). However, very small

(<1%) partial melts of a source compositionally similar to PM are able to reproduce

trends observed in the Hf/Ta and (La/Sm)N ratios of the most ITE-rich (i.e., the Isla

Floreana “main series”) samples (Fig 11A).

The comparison made here shows the greater similarity of Leg 170 (and

presumably Leg 205) gabbros incompatible trace element ratios to the hotspot end-

member than to the depleted mantle; the volumetric contribution of depleted end-

member is likely greater (see above). The gabbros are geochemically similar to some

of the most enriched CNS and EPR basalts (a minority of the population), suggesting

regional influence of the hotspot source and smearing of the end-member signals, an

idea that has been pointed out elsewhere (White et al, 1993, Harpp and White, 2001;

Blichert-Toft and White, 2001 and references herein).


Depth of Melting:

Incompatible trace element ratios indicate a strong similarity between the Leg

170 gabbros and Galapagos Islands basalts, but there is geochemical evidence for

different depths of magma generation. A mixing line was constructed in (Sm/Yb)N vs.

(La/Sm)N space to determine possible regional mixing and melting relationships (Fig.

12). Enriched and depleted end-member ratios were chosen from published analyses

of Galapagos Island basalt samples that best represent regional variation (Harpp and

White, 2001); this mixing line therefore represents hypothetical mixtures of plume and

ambient upper mantle, from which partial melts are generated. The Galapagos Islands

typically have a higher (Sm/Yb)N for a given (La/Sm)N compared to Leg 170 gabbros,

which may indicate a longer melting interval in the garnet stability field for hotspot

products because Yb is highly compatible in garnet (see figure caption for Fig. 11). In

the absence of garnet, clinopyroxene dominates the bulk partition coefficient, and Sm is

more compatible than La in clinopyroxene. Thus, higher La/Sm for a given Sm/Yb, as

observed in Leg 170 gabbros and the majority of spreading center basalts, suggests

shallower melting processes.

The gabbros have higher (La/Sm)N and (Sm/Yb)N ratios than the majority of the

spreading center basalts, probably reflecting their relative enrichments. However,

distinctions can also be made between the gabbros: group 1 gabbros have

characteristically higher (La/Sm)N and (Sm/Yb)N ratios than group 2; this may also

reflect differential enrichment. An alternative explanation of the gabbros elevated

(La/Sm)N and (Sm/Yb)N ratios is that they simply were not directly derived from mantle

sources used here and to model regional variations. Polybaric melting, whereby melting

begins in the garnet field and ends in the spinel field, is also consistent with the

observed ratios but cannot be distinguished from melting in the spinel field alone.

Leg 170 data clearly do not fall on the mixing line. If source mixing is the cause

of most of the differences in (La/Sm)N and (Sm/Yb)N ratios, than mixing most likely

occurred before appreciable igneous processes, which can alter the ratios. Difficulties

in rigorous fractionation normalization of the data prohibit a quantification of the role of

fractionation crystallization in creating the gabbros higher (La/Sm)N at a given (Sm/Yb)N,


but the effects of fractional crystallization can be considered. Fractional crystallization

of magmas is expected to be most important in the relatively shallow (i.e., crustal)

plagioclase stability field; therefore, only clinopyroxene (in the absence of garnet and

spinel) is available to fractionate La from Sm (the effect on Sm/Yb is negligible). The

differences between the gabbro groups are best explained by >5% partial melts (c.f.

White et al, 1993; Harpp and White, 2001), the majority occurring in the spinel field, of a

partially enriched source with trace element ratios intermediate between hotspot and

depleted mantle end-member. Fractional crystallization overprints these processes,

and residual liquid paths would mimic smaller degrees of partial melting, as shown in

the differences between whole rock and glass analyses of CNS (Fig. 12). These issues

will be more fully addressed once additional major and trace element analyses are

completed.

Emplacement and Timing of Groups 1 and 2:

The igneous units cored on Legs 170 and 205 share many textural,

mineralogical, and major element geochemical similarities (Morris and Villinger et al,

2003, in press). However, differences in petrography and incompatible trace element

abundances and ratios allow a subdivision of these gabbros. In this section, the issue

of physical emplacement of these distinct groups is addressed. Group 1 rocks are

found in Hole 1039C (Leg 170) except for the basal few meters and as subunit 4A in

Hole 1253A (Leg 205), while group 2 is found in the lowermost drilled gabbros in Hole

1039C, the entire Hole 1040C (Leg 170), and as subunit 4B in Hole 1253A (Fig. 5). The

change from group 1- to group 2-type geochemistry must occur within Hole 1039C.

This change, near the base (~443mbsf between cores 10R-3 and 11R-1), is marked by

the occurrence of an increased ROP, which implies the presence of softer sediment

horizons (Kimura, Silver, and Blum et al, 1997). Although core recovery was poor

(~35%) and drilling was halted ~6m below this geochemical demarcation, available data

are consistent with two geochemically separate magma injections. Group 2-type rocks

are found in Holes 1039C and 1040C, ~4km distant, while group 1-type rocks are

identified only in Holes 1039C and 1253A, ~1km distant, indicating that enriched group

1-type rocks may be more geographically limited (Fig. 5). That group 1 rocks were


cored at different depths in closely spaced holes argues that the unit is not a simple

tabular body (Kimura, Silver, and Blum et al, 1997).

Plate reconstruction of the <16.5Ma gabbroic sills results in a paleolocation

significantly closer to the CNS and Galapagos Island (Fig. 4). Meschede and

Barckhausen et al (2001) place the ~24Ma EPR crust, presumed to underlie the location

of Legs 170 and 205, nearly atop the plume at ~16-15Ma. At approximately the same

time, the N-S trending transform faults that offset the Costa Rica and Ecuador Rifts from

the CNS (see Fig. 1 for locations) may have been located close to the Galapagos

hotspot (Meschede and Barckhausen, 2001). Ito et al (1997) show that crustal

production rates were higher when the CNS was closer to the plume. As the CNS

migrated north and approached the plume, the resulting higher crustal production rates

presumably led to higher rates of magma intrusion, a possible plutonic analogy to the

seamount volcanism that occurs on the Cocos plate between the locations of Legs 170

and 205 and the Cocos Ridge. Furthermore, Legs 170 and 205 are ~30km north of the

EPR-CNS triple junction trace, where spreading at the CNS ended after a southward

ridge jump ~19.5 Ma. It is conceivable that this abandoned spreading center may have

provided a zone of previous weakness that was exploited as it migrated over the plume

~16Ma, possibly with proximal transform faults acting as magma conduits. The gabbros

cored by Leg 170 and 205 may represent samples of episodic intrusion across the triple

junction trace that tapped the geochemically enriched hotspot domain. This scenario is

consistent with the intermediate trace element geochemistry of the gabbros and the

evidence of multiple magma injections.

Conclusions: Gabbros cored from ODP Legs 170 and 205 appear to derive from a depleted

mantle that has been variably enriched by the addition of Galapagos-type material as

inferred from a mantle-mixing array constructed from ranges in ratios of incompatible

trace element ratios. Data from the majority of oceanic igneous basalts of the eastern

Pacific margin offshore Costa Rica, including the Cocos-Nazca spreading center, East

Pacific Rise, and Galapagos Islands can also be reproduced through modeling of

moderate degrees of batch melting of this same range of hybrid mantles. The simplest


explanation for the observed range in incompatible trace element ratios of the regional

basalts is mixing between enriched plume and depleted asthenosphere end-members

followed by moderate degrees of partial melting (~10% or more) dominantly in the

garnet (Galapagos Islands) or spinel (CNS, EPR, Legs 170 and presumably 205)

stability fields. Trace element ratios of spreading center basalts are consistent with an

origin related to the residue of partial batch melting of a primitive mantle source or,

alternatively, moderate degrees of melting (probably >15%) from this residue; a more

developed treatment of the effects crystal fractionation requires additional major and

trace element data.

Leg 170 gabbros are more enriched than most of the spreading center basalts

and are similar to less enriched Galapagos Island basalts. Mixing calculations illustrate

the volumetric dominance of depleted over enriched materials in the gabbros, despite

exhibiting trace element ratios indistinguishable from many enriched Galapagos Island

basalts. Two groups of gabbros revealed by trace element ratios appear to sample

episodes of intrusive activity originating near the CNS during an interval of elevated

plume-ridge interaction, possibly exploiting a previous zone of crustal weakness such as

the trace of the CNS-EPR triple junction or a transform fault offsetting nearby rifts.

Thus, the gabbros may represent the intrusive equivalent of geochemically enriched

hotspot related seamount volcanism offshore central Costa Rica. That the Galapagos

Islands are between 30-90% enriched suggests a mixing mechanism where variable but

large amounts of depleted ambient asthenospheric mantle can be incorporated over

long time scales.


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Table 1: Instrumental Neutron Activation Analysis of Leg 170 Holes 1039C and 1040C gabbros. Sample identifier gives core-section and interval number. One sigma precision for the elements FeO, Na2O, Sc, Cr, Co, La, Ce, Sm, Eu, Yb, Hf is generally less than 3%; precision for the elements CaO, Tb, Lu, Ta, Th, and U is between 3-10%; 10-30% for Zr, Sr, Ba, and Nd. Values preceded by “<” are upper limits. Precision values are estimates based on counting statistics and laboratory experience with analyzing each element (Korotev, 1996). Accuracy is based on comparison between measured and certified values of JGb-1, Geologic Society of Japan gabbro. Results are similar for reference materials GSM-1 and NBS-668 (not shown), which have fewer certified values. Measured and certified values agree within the quoted one-sigma precision levels, except Zr, Sm, Hf, Ta, and Th, which are all within estimated two-sigma analytical error, and Yb, whose certified values may be in error (see text for details). Uncertainties for the trace element ratios (La/Sm)N, Hf/Ta, and Sm/Lu are 0.03 (group 1 and 2), 0.2 (group 1) and 0.4 (group 2), and 0.3 (groups 1 and 2). Table 2: Comparison between the analyses of standard reference materials for XRF (shipboard Leg 170) and INAA (Washington University in St. Louis) with ICP-AES (shipboard Leg 205). Major element data agree within 5%, except P2O5 and K2O, between XRF and ICP-AES analyses; available trace element data generally agree to within 5%, except Zr. Major element analysis is limited with INAA (gamma-ray spectroscopy), but FeO, CaO, and Na2O show good agreement with ICP-AES data; available trace element data show very good agreement between Sc and Cr (<1% difference), agreement within 10% for Sr and Ba, and poor agreement for Zr. Leg 205 shipboard values for Fe2O3 were converted to FeO for comparison with INAA values. X-ray fluorescence and ICP-AES analyses are indistinguishable at the one-sigma level for the elements Ti, Al, Fe, K, Mg, Mn, Ca, Na, P, Ni, and Y (two-sigma for Si, Cr, and Sr, and three-sigma for V and Zr). The elements Fe, Ca, Na, Sc, Cr, and Ba are indistinguishable at the one-sigma level, while Sr and Zr are within two-sigma, for INAA and ICP-AES analyses. Fig. 1: Bathymetric map of the eastern central Pacific showing the location of the Leg 170/205 and 206 drilling area, Middle America Trench off Costa Rica, Cocos-Nazca spreading center (CNS), and the Galapagos Islands. The dotted line denotes the location of the “rough smooth boundary”; solid lines indicate the locations of the East Pacific Risa and the Cocs-Nazca spreading center. Also shown are the Costa Rica Rift, Cocos Ridge, Nicoya Peninsula, and the Osa Peninsula The map is modified after Vannucchi et al (in press) based on a data compilation of C. Ranero. Fig. 2: Legs 170 and 205 Costa Rica drilling area (red box) and isochrons derived from seafloor magnetic anomalies (Barckhausen et al, 2001). Numbers indicate crustal age in million years. The fracture zone trace divides the first generation CNS crust from the EPR crust, while the triple junction trace separates crust formed at the second generation CNS and the EPR. The ridge jump records the change from CNS-1 to CNS-


2 spreading. Convergence direction and rate (arrow) (DeMets et al, 1990) and arc volcanoes (triangles) are also shown. Note the segmentation of the volcanic arc and the offset in Wadati-Benioff zone isodepths at the QSC. FS = Fisher Seamount, QSC = Quesada Sharp Contortion. Fig. 3: A) Regional map showing the locations of igneous complexes associated with Galapagos volcanism (from Hoernle et al, 2002). Red circle marks the location of Legs 170 and 205. B) Predicted ages of hotspot products on the Galápagos hotspot traces. Black lines indicate Galápagos age trend; gray lines represent age trends from a second, smaller proposed hotspot centered about 500km northeast of the Galapagos hotspot (from Meschede and Barckhausen, 2001). Galapagos overprinting ages are in good agreement with K-Ar ages from basalts from Malpelo Island (Hoernle et al, 2002) and Cocos Ridge dredges (Werner et al, 1999). The presently subducting plate offshore Costa Rica was overprinted by Galapagos hotspot volcanism ~12-14.5Ma. Red circle marks the location of Legs 170 and 205. Fig. 4: Projection of the location of oceanic crust cored during Legs 170 and 205 from 12-16.5Ma, illustrating the proximity to the CNS and Galapagos hotspot. The Leg 170 gabbroic sill is intruded into ~16.5Ma sediments. The projection was performed through parameterizing the EPR and CNS spreading rates and directions through time. The locations of Sites 504 and 896 south of the Costa Rica Rift are also shown. The slowing rate of the dominantly E-W spreading EPR during this time interval results in the slight curve of the projection. Fig. 5: A) Migrated multichannel seismic profile BGR-99-44 (in Morris and Villinger et al, 2003, in press) across the Middle America Trench. Red lines = Leg 205 Sites 1253, 1254, and 1255. Thin black lines = Site 1039, 1040, and 1043 (Leg 170) locations (Kimura, Silver, and Blum, et al, 1997). CMP = common mid point. B) Schematic igneous stratigraphic column consistent with data and illustrating preferred interpretations. The change from group 1 to group 2 type gabbroic rocks occurs within the base of Hole 1039C. This is also the location of sediment horizons inferred from drilling operations aboard Leg 170 (Kimura, Silver, and Blum et al, 1997). Subunits 4A and 4B refer to stratigraphic units used during Legs 170 and 205. Group 1 gabbros are relatively more enriched, less altered, and geographically limited compared to group 2 gabbros. Fig. 6: (Na2O + K2O) vs. SiO2 showing the subalkaline nature of the Leg 170 (Holes 1039C and 1040C) and 205 (subunits 4A and 4B) gabbros, and their small variation in SiO2, in relation to regional volcanic products. The Galapagos Islands samples show a variable


geochemistry typical of ocean island basalts while the spreading center basaltic glasses, with a few exceptions, are generally more compositionally restricted and strongly subalkaline. GI = Galapagos Island basalts, CNS = Cocos-Nazca Spreading Center, EPR = East Pacific Rise (between 5-10°N latitude). Also shown are the schematic melt paths for partial melting and fractional crystallization processes assuming no alteration has occurred. The Galapagos Island basalts generally have higher abundances of K than spreading center basalts, which is consistent with typical alkaline ocean island basalts. Alteration is expected to increase the concentration of K. Fig. 7: A) AFM diagram showing the Fe-enrichment trend typical of ocean-floor tholeiitic rocks from (tholeiitic trend redrawn from Irvine and Baragar, 1971 separatiing the tholeiitic from calc-alkaline series), for the Leg 170 and 205 samples and the spreading center basalts. A slight alkaline enrichment trend is detectable in some of the Galapagos data. Only samples with SiO2 < 53 wt% are plotted. A = N2O + K2O, F = total Fe as FeO, M = MgO. B) SiO2 vs Mg# showing the small range in composition for the Legs 170 and 205 gabbros. Also plotted are samples from the Galapagos Islands, CNS whole rock basalts and glasses, and EPR basaltic glasses between 5-10°N latitude. Primary magmas are approximately Mg# = 0.7 (Roeder and Emslie, 1970) indicating that these gabbros do not represent original melts and have experienced fractional crystallization of phases with high Mg# (e.g., primarily olivine and/or clinopyroxene), as have the majority of spreading center basalts. Approximate fields for the augite, bytownite plagiclase, and olivine (Fo50) are also shown. Crystallization of a phase would move the residual liquid composition away from the composition of the phase. For a given SiO2 (i.e., degree of fractionation), the CNS basaltic glasses and EPR basalts generally showing higher Mg# than the Galapagos Island basalts and the Leg 170 and 205 gabbros; this is consistent with less fractionation of high-Mg# phases for spreading center basalts en route to surface. This relationship is less distinct for CNS whole rock basalts, also a result of fractional crystallization effects. Mg# = [Mg / (Mg + Fe+2)] recalculated from major element analyses of EPR, CNS, and Galapagos Island basalts, after assigning all Fe to Fe2+. Fig. 8: Exploded view of the normative nepheline-olivine-diopside-hypersthene-quartz basaltic tetrahedron projection after Thompson, 1984, showing the relationship of the Leg 170 and 205 gabbros to the fields of Galapagos Island basalts and MORB (redrawn after White et al, 1993). Galapagos Island basalts tend to cluster around the di-ol join, while MORB and the gabbros cluster near the center of the olivine tholeiite field, di-ol-hy. Fig. 9: A) Chondrite-normalized REE diagram of Leg 170, Holes 1039C and 1040C gabbros. The gabbros have been classified as group 1, with higher REE abundances, and group 2, with lower REE abundances; solid black lines are group averages. If the samples have similar parental composition, these differences are likely due to differences in


extent of partial melting; differences in source composition can also be the cause of this group separation. Group 1 includes all of the samples from Hole 1039C (Leg 170) except the two deepest cored samples; group 2 contains these lowest samples from Hole 1039C and all of the samples from Hole 1040C. Generally, these groups are characterized by modest LREE enrichment with HREE ~16x chondritic. Error bars on group averages (black lines) represent average analytical uncertainties for each group sample. Normalization values are from Lodders and Fegley, 1998. B) Comparison of Leg 170 gabbros with the ranges in Galapagos Island basalts (green) and spreading center basalts (yellow, CNS and EPR 5-10°N latitude). While within the ranges of each field, the gabbros are more similar to enriched Galapagos Islands, having similar normalized abundances and (La/Sm)N >1. Galapagos Island basalt data are from Harpp and White, 2001. Spreading center basalt data are from Laschek, 1985 and Allan, 1996. Normalization values are from Lodders and Fegley, 1998. Fig. 10: A) Variations in CaO/Al2O3 vs. Mg# appear to be consistent with fractionation of primarily olivine with lesser amounts of plagioclase (bytownite= An70-90) and clinopyroxene (augite) from a more primary melt, but the scatter suggest a complicated history, especially for Leg 205 samples. Fractionation of bytownnite and augite are inferred from petrographic analyses (see text for details). Primary magmas are thought to have CaO/Al2O3 ~0.77 and Mg# > 0.7. Higher CaO/Al2O3 and lower Mg# suggest fractionation of olivine; therefore, these gabbros do not represent melts of primary magma. B) V and Ti are highly compatible in ilmenite (Fe2+TiO3) and titanomagnetite [Fe2+(Fe3+,Ti)2O4], although they are incompatible in the bulk system (the abundances of both V and TiO2 increase with increasing Zr and decreasing Mg#, not shown). The presence of these phases controls the residual liquid composition. The majority of Hole 1039C and all of subunit 4A form a linear trend; the remaining samples form another clear trend. Comagmatic liquids are expected to have a correlated trend, and the distinct trends may suggest different source compositions or, alternatively, separate magma generation events originating from the same source. The group that includes subunit 4B has lower concentrations of TiO2 at a given V compared to the group that includes subunit 4A, suggesting different parent compositions. R2 values for the trends are for samples from Leg 205 subunit 4A and 4B only. Fig. 11: A) Hf/Ta vs. (La/Sm)N ratios for EPR basaltic glasses (5-10° N latitude) and CNS whole rock basalts fields with Galapagos Island basalts and the gabbros cored from Leg 170, illustrating the relative importance of mixing, partial melting, and fractional crystallization. Trajectories for batch melting of primitive mantle (PM) and subsequent fractional crystallization (shown here after 3% batch melting of PM) suggest that the gabbro field cannot be simply related to these processes. Instead, mixing between an enriched, OIB-type [high (La/Sm)N- low Hf/Ta] endmember and a depleted (MORB-source) type [low (La/Sm)N- high Hf/Ta] endmember is suggested. Numbers next to the


partial melt and residual solid lines of PM refer to the percentage of partial melting. The pathway differences in modal fractional melting and modal and non-modal batch melting are small, although degrees of melting vary slightly along a given path. Non-modal fractional melting produces similar pathways only when the extent of melting is <1%; higher degree melts result in higher (La/Sm)N for a given Hf/Ta, as illustrated. Scatter within fields is likely due to differences in degrees of melting, crystallization, and/or source heterogeneity. The highest (La/Sm)N values of the Galapagos Islands are from the anomalous “main series” of Isla Floreana, thought to be related to partial melting of a high (La/Sm)N metasomatically-enriched source distinct from the other islands in the Galapagos (Bow and Geist, 1992; Harpp and White, 2001; see text for details). Error Leg 170 gabbros are (La/Sm)N ±0.03 (group 1and 2); Hf/Ta ratios are ±0.2 (group 1) and ±0.4 (group 2); see Fig, 11B for approximate error ellipse. Leg 170 data from INAA (this study) at Washington University. Galapagos Spreading Center data from Ridley et al, 1994; EPR (5-10° N latitude) data from Nui and Batiza, 1995 and references therein. Galapagos Island basalt data from Geist et al, 1995; Kurz and Geist, 1999; Naumann et al, 2002; Allan and Simkin, 2000; Reynolds and Geist, 1995; Baitis and Lindstrom, 1980. PM = primitive mantle from McDonough and Sun, 1995; DM = depleted mantle (represented by the spreading center sample with the lowest Hf/Ta and La/Sm ratios); WR = whole rock; normalization values for La and Sm from Lodders and Fegley, 1998. Whole rock analyses of CNS basalts were used due to the lack of basaltic glass data; resultant CNS data are scattered compared to EPR glass analyses, probably due to fractional crystallization effects, but remain representative of the regional spreading center field. Batch modal melting: melting modes of spinel lherzolite (peridotite) following Kinzler, 1997 = 53% ol, 27% opx, 17% cpx, and 3% Al-phase (spinel):

[ ])1( DFDC

C OL −+= where CL is the concentration of the liquid, CO is the concentration

of the original solid (e.g., PM), F is the fraction of melt produced, and D is the bulk distribution coefficient of the original solid, and

[ ])1( DFDDC

C ORS −+= where CRS is the concentration of the residual solid.

Fractional crystallization: crystallization modes approximate volume % phenocrysts in gabbroic samples (Kimura, Silver, and Blum et al, 1997):

)1( −= DOL FCC where CL is concentration residual liquid and F is the fraction of melt

remaining, and

)1( −= DOI DFCC where CI is the concentration of the instantaneous solid.


Partition coefficient data (from the compilation of William White, Geochemistry, 1998, except * from Rollinson, 1993)):

end-member compositions (normalized, ppm)

ol opx cpx plag spinel garnet sp+gt enriched depleted La 8.8E-06 0.0056 0.052 0.082 0.01 0.0164 0.0132 74.9 16.7 Sm 0.000445 0.0085 0.462 0.033 0.0064 1.1 0.5532 46.2 24.7 Hf 0.001 0.021 0.223 *0.05 0.05 1.22 0.635 0.92 2.98 Ta 0.00005 0.015 0.013 0.11 0.06 0.11 0.085 1.59 0.14 Yb 0.0366 0.032 0.633 0.014 0.0076 3.88 1.9438 14.88 14.94 La/Sm 0.020 0.659 0.113 2.485 1.56 0.015 0.789 1.62 0.68 Hf/Ta 20 1.4 17.154 0.455 0.833 11.091 5.962 0.58 21.29 Sm/Yb 0.012 0.266 0.730 2.357 0.842 0.284 0.563 3.10 1.65 ol = olivine, opx = orthopyroxene, cpx = clinopyroxene, plag = plagioclase, sp + gt = 50% spinel + 50% garnet End-member concentrations and ratios used in mixing calculations (following Langmuir, 1978) are taken from a survey of published values. Melting and/or fractional crystallization in the garnet, rather than spinel, stability field results in compatibility of Sm and Hf in garnet, resulting in a shift of the partial melting paths towards the origin. However, other geochemical evidence argues against significant melting in the garnet stability field for the Leg 170 gabbros (see text for details). Further modeling (not shown here) with non-modal batch melting (melt-entering modes: ol= -0.06, opx= 0.28, cpx= 0.67, Al-phase= 0.11, of spinel lherzolite from Kinzler, 1997) cause little change to the partial melting and fractional crystallization trajectories. Fig. 12: (Sm/Yb)N vs. (La/Sm)N illustrating the relationship between Galapagos Island basalt, the Leg 170 gabbros, and regional spreading center basaltic glasses. The solid line represents a hypothetical plume-asthenosphere mixing line; enriched and depleted end-members are representative enriched and depleted samples from the Galapagos Islands from Harpp and White, 2001. A location along the mixing line represents a hypothetical mantle mixing line from which partial melts would be generated and extracted. Partial melts originating from the garnet field would have higher (Sm/Yb)N and would plot above the mixing line; melts generated in the spinel field would have higher (La/Sm)N and would plot to the right of the mixing line, as would residual liquids from fractional crystallization (see text for details). The effect of fractionation is illustrated by the relative locations of the CNS whole rock and basaltic glass data; whole rock data are more scattered but tend towards higher (La/Sm)N, suggesting shallow crustal fractional crystallization (see text for details). EM= enriched mantle; DM=


depleted mantle; PM= primitive mantle (McDonough and Sun, 1995); WR = whole rock; Galapagos Island, EPR, and CNS basalt and basaltic glass data extracted from the Petrological Database of the Ocean Floor, available from http://petdb.ldeo.columbia.edu/petdb/. Errors for Leg 170 gabbros are (La/Sm)N ± 0.03 and (Sm/Yb)N ± 0.3; approximate error ellipse also shown.


http://petdb.ldeo.columbia.edu/petdb/

05101520253035

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26

Tabl

e 1

Bria

n D

reye

r Pa

ge 4

6 6/

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03

123456

4547

4951

5355

SiO

2

(Na2O + K2O)

1039

1040

4A 4B GI

CN

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asse

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balk

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n

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Br

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Page

47

6/5/

2003

M70

8090

100

F

0

10

20

30

40

50

60

70

80

90

100

A

0

10

20

30

X

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X

XX

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X

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XX X

X X

X

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X X

XX

XX

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X

X

X

X

X

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X

X XXX

Legs

170

and

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land

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calc

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Figu

re 7

A

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8 6/

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3941434547495153

00.

10.

20.

30.

40.

50.

60.

70.

80.

9M

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1040

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di

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0 6/

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0.750.8

0.850.9

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40.

450.

50.

550.

60.

650.

7

Mg#

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061

R2 =

0.9

572

200

300

400

500

600

11.

52

2.5

3Ti

O2

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re 1

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ge 5

1 6/

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ian

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Page

52

6/5/

2003

(La/

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ain

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3 6/

5/20

03

0.51

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3.54

02

46

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solid

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%

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(% c

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anta

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s so

lid(%

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ng)

to d

eple

ted

man

tle

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re 1

1B

Bria

n D

reye

r Pa

ge 5

4 6/

5/20

03

0

0.51

1.52

2.53

3.5

00.

51

1.5

22.

5(L

a/Sm

)N

(Sm/Yb)N10

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GI_

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CN

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re 1

2

Bria

n D

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ge 5

5 6/

5/20

03

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