Basalt Geochemistry of the Endeavour Segment, Juan de Fuca Ridge

J. Woodcock, J. Gill, J. Kela, : Earth Sciences, UCSC, Santa Cruz CA 95064P. Michael: University of Tulsa, Tulsa, OK 74104F. Ramos: Geological Sciences, CWU, Ellensburg, WA 98926

References CitedAsimow P., Dixon J., and Langmuir C. (2004) A hydrous melting and fractionation model for mid-ocean ridge basalts: applications to the Mid-Atlantic Ridge near the Azores. Geochem. Geophys. Geosyst. 5(1).

Cousens B., Blenkinsop J., and Franklin J. (2002) Lead isotope systematics of sulfide minerals in the Middle Valley hydrothermal system, northern Juan de Fuca Ridge, Geochem. Geophys. Geosyst. 3(5).

Klein E. (2003) Geochemistry of the igneous oceanic crust. Treatise of Geochemistry, chapter 3.13, 433-463.

Klein E. and Langmuir C. (1987) Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. J. Geophys. Res. 92, 8089-8115.

Ligi M., Bonatti E., Cipriani A., and Ottolini L. (2005) Water-rich basalts at mid-ocean ridge cold spots. Nature 434, 66-69.

Niu Y., Regelous M., Wendt J. I., Batiza R., and O'Hara M. J. (2002) Geochemistry of near-EPR seamounts: Importance of source vs. process and the origin of enriched mantle component, Earth and Planetary Science Letters, 199, 327-345.

Palme H. and O'Neil H. St. C. (2003) Cosmochemical estimates of mantle composition. Treatise of Geochemistry, chapter 2.01, 1-38.

Hydrothermal implications: Pb isotopes are available for sulfide samples from several of the hydrothermal vent fields at Endeavour (Fig. 6 and

Labonte et al., Poster V31D-0622 this meeting). Comparing data for basalts and sulfides leads to several conclusions. First,

almost all of the sulfides at Endeavour lie along the same chord in Pb isotope space as the basalts. The sulfides are not

displaced to higher 207Pb/204Pb and 208Pb/204Pb as at Middle Valley, the next segment north (cf. Cousens et al., 2002).

Consequently, there is no evidence for sediment-derived Pb in Endeavour sulfides. Second, the sulfides display a similar

along-axis variation as the basalts, with higher average 206Pb/204Pb in the south than north. This reflects the greater

abundance of FOZO group basalts in the south. Third, it may be possible to use basalt chemistry to constrain

hydrothermal recharge zones. For example, sulfides at Salty Dawg have lower 206Pb/204Pb than at other Endeavour

vent fields. This might be explained by more recharge in the west than east because the surface basalts in the east, near

the vent field, are E and N1 types (with higher 206Pb/204Pb) whereas there is more T3 (with lower 206Pb/204Pb) in the

west than elsewhere in the axial valley. If Pb in the vent fauna is similar to the Pb in the sulfides, then mantle

heterogeneity is being passed on directly to the animals - mantle to microbes.

Geological implications: All six basalt types have erupted in close spatial proximity within the axial valley despite the presence of an Axial

Magma Chamber (AMC) currently. The greatest diversity is near the west wall of the valley at the edge of the AMC.

Because the basalt types are not related to one another by differentiation processes within the AMC, successive lava

flows seem to sample different mantle sources. Therefore, magmas must frequently bypass the AMC, or the residence

time of magma in the AMC is short, or the chambers themselves are short-lived at this intermediate spreading rate ridge.

Future U-series disequiibria studies should clarify and quantify this conclusion.

The FOZO group (N1, T1, T2) has erupted only in the axial valley and especially near and south of the Main Endeavour

vent field. The FOZO component may, therefore, be non-uniformly distributed in the source and may melt out during

axial upwelling because it is not tapped off-axis.

In contrast, isotopically depleted T3 basalts dominate the lower western flank. Even more isotopically depleted N-

MORB is present even farther off-axis as very low-K N-MORB. Although the degree of melting beneath the flanks usually is

not high enough to produce N-MORB, the source which advects to the west is isotopically depleted.

The conventional group of basalts (N2 and E) can erupt anywhere and is the most abundant. Apart from the west wall,

basalts at the shallowest portion of the axial valley (where the thermal flux may be greatest), from High Rise to Sasquatch,

are mostly N2 and E with ~8% MgO. The east flank is paved solely by slightly more evolved E. N2 is restricted to the west

half of the axial valley where higher percent melts seem able to get through without mixing. Whether the conventional

group reflects a range in percent melting of a source that is a mixture of depleted and FOZO components, or reflects a

different kind of enriched source component, remains unknown.

Trace element and Pb isotope systematics: N, T, and E basalts become increasingly enriched in many incompatible elements in that order as K/Ti

increases. La/Yb ratios increase about 3-fold and Zr/Nb ratios decrease (Fig. 4). However, only N2 basalts are

LREE-depleted, and only T3 basalts have Zr/Nb ratios as high as EPR basalts relative to La/Yb. All the rest of

Endeavour seems to be more Nb-enriched relative to LREE and Zr than at the EPR. Although T and E basalts

overlap in La/Yb versus Zr/Nb space, they define separate groups and slopes with T3 being the most Zr-rich, T1

the most La-rich, E the most Nb-rich, and T2 in between and along the same chord as N1 (Figs. 4 and 5).

All Endeavour basalts lie on one chord in Pb isotope space that is displaced below the NHRL and at a

shallower slope (Fig. 6). The high 206Pb/204Pb end is similar isotopically to FOZO. Surprisingly, 206Pb/204Pb

varies with HFSE and SiO2 rather than K/Ti and, therefore, the conventional subdivisions of MORB. N2 and E

types have similar isotopic compositions of Pb, Sr, and Nd (not shown). In contrast, T3 basalts have lower 206Pb/204Pb whereas N1, T1, and T2 have higher 206Pb/204Pb.

In all cases, these traits are independent of differentiation. They are maintained from MgO contents ~8%

down to ~6%. Thus far we recognize little evidence of mixing between basalt types though we expect it.

Overall the data define three clusters:

o The isotopically depleted group (T3). T3 basalts are closest to the N-MORB source in 206Pb/204Pb. They

have the least positive Nb and Th spikes (lowest Nb/La and Th/La ratios) but greatest Zr and Hf spikes (highest

Hf/Sm ratios). They alone lie on the EPR trend for La/Yb and Zr/Nb. They have the lowest Ti8 of the T-types and

overlap E and N2.

o The conventional group (N2 and E). These are most common basalts at Endeavour. N2 and E basalts share

the high SiO2 and low Ti8 of T3. N2 is the closest that one gets to typical N-MORB at Endeavour in terms of Fe8

and Na8 for the water depth, and LREE-depletion. However, even it has higher Nb relative to REE and Zr than at

the EPR. E basalts rise to higher La/Yb, Nb/Zr, and Zr/Hf along with higher K/Ti. Positive Nb spikes reach their

maxima in some E-basalts. All isotopes are constant at intermediate 206Pb/204Pb.

o The FOZO group (N1, T1, and T2). These three basalt types have lower Si, higher Ti8, and higher 206Pb/204Pb than any of the above. These traits are greatest in T1 and least in N1. As with the conventional

group, there is an increase in La/Yb, Hf/Sm, Nb/Zr, and Zr/Hf as K/Ti rises, resulting in overlap between the

conventional and FOZO groups in these parameters.

Once all isotope data are available we hope to separate the effects of differentiation, melting, and source

composition as contributing factors to differences within and between these three groups.

Major elements: Six chemically distinct types of basalt have been recognized on the basis of microprobe analyses of glass

shards for all samples (Table 1; Fig. 2). They are classified first as N, T, or E-MORB using K2O/TiO2 with T-MORBs

having ratios of 0.15-0.25, N-MORB being lower, and E-MORB higher. The N and T-MORB groups are divided

further into N1, T1, and T2 which have lower SiO2 and higher FeO* and TiO2 than N2 and T3, respectively. T1

has the highest TiO2 and lowest SiO2 (and CaO). Only type N2 has typical Na8 and Fe8 for the water depth at

Endeavour (Fig. 3). The other N and T types extend along the typical Pacific 'local array' toward more Na-rich

and Fe-poor compositions, whereas the E type extends to lower values for both Na8 and Fe8 as is typical in

ridge segments affected by islands or adjacent large transforms (Asimow et al., 2004; Ligi et al., 2005).

Provisional models of fractional crystallization using Petrolog provide best fits to each separate basalt type

when water contents are <0.5 wt.% and pressure is <2 kb - i.e., within the Axial Magma Chamber - except for T1

for which higher pressure differentiation is required to explain the lower CaO by earlier fractionation of Ca-rich

clinopyroxene.

New data and methods: We report new trace element and Pb isotope data for about 30 representative basalts from the axial valley

and flanks of the Endeavour Axial Ridge Volcano segment of the Juan de Fuca Ridge (Fig. 1). The segment is

only 10 km along-axis and extends ~2 km off-axis (~50 Ka at 6 cm/y full spreading rate). About 250 basalts were

collected using the ROV Tiburon in 2002 and 2004 with high spatial accuracy. About half the samples studied

here are glass shards; the rest are variably crystallized whole rocks. All were hand-picked and cleaned with HCl;

the aliquots for Pb isotopes were cleaned again using HBr. Trace element concentrations were measured using

solution HR-ICPMS. Data were double-corrected for instrumental drift and in most cases are precise to within

0.5-2% 2σ at rock concentrations >10 ppb. Pb isotope ratios were measured using a Neptune MC-ICPMS and Tl-

spiking. Results for NBS 981 are 206Pb/204Pb = 16.929 ±0.002 (2σ external reproducibility), 207Pb/204Pb =

15.483 ±0.002, and 208Pb/204Pb = 36.671 ±0.005. Results for BHVO-2 are 206Pb/204Pb = 18.627 ±0.002, 207Pb/204Pb = 15.534 ±0.001, and 208Pb/204Pb = 38.227 ±0.003.

Abstract: New trace element and Pb isotope data confirm that a wide variety of basalt types occur in close spatial

proximity at the Endeavour ISS "Bulls Eye". This variety cannot be related by differentiation in an axial magma

chamber, and requires heterogeneous sources along and across axis within the ISS and far from hot spots and

major transforms. Pb isotopes do not correlate with the K/Ti ratio as expected. The resulting spatial pattern in

basalt geochemistry constrains the location and longevity of melt lenses beneath the axial valley and flanks,

and appears to control the source of Pb in hydrothermal vents.

Figure 6. Pb has the greatest isotopic variation at Endeavour and varies by basalt type. As for other NE

Pacific basalts, 207Pb/204Pb and 208Pb/204Pb ratios are very low relative to 206Pb/204Pb. The average

Endeavour sulfide datum is the mean of eight unpublished TIMS analyses of samples from High Rise

chimneys. The Middle Valley fields are from Cousens et al. (2002).

Figure 5. Representative REE normalized to Primitive Mantle (Palme and O'Neil, 2003) for basalts with 7-8% MgO. Average N-

and E-MORB for EPR at 10-14°N are from Niu et al. (2002). Note that all Endeavour basalts have lower HREE concentrations and

steeper HREE slopes then for EPR N-MORB and most are even lower than EPR E-MORB. Only N2 is sub-parallel to EPR N-MORB

apart from La-Ce-enrichment in N2. Patterns cross at Endeavour even within the enriched types with T2 being shallowest, T1

steepest, and T3 and E intermediate. The negative Ce and Eu anomalies seem robust between two laboratories.

Figure 4. Each basalt type has a different combination of these and other trace element

ratios. Only T3 is similar to EPR MORB from Niu et al. (2002); other types are Nb-enriched. Only

N2 is LREE-depleted and even it lacks the usual strong depletion in La-Ce (see Figure 5). The

overall variation in basalt trace element geochemistry at Endeavour is large for such a small

area and is mantle-derived.

Figure 3. Fe8 and Na8 values are

calculated following Klein and

Langmuir (1987) and compared to

global and EPR MORB data from

Klein (2003), Only N2 has values

typical of the water depth at

Endeavour. Displacements from

N2 are attributed to heterogeneity

in the source composition and

melting processes rather than

potential temperature, but the

relationship to isotopic differences

between basalt types is not yet

explained.

Figure 2. TiO2 vs. MgO

for samples analyzed for

trace elements. Note

that the Conventional

Group (N2 and E types)

has the lowest TiO2, and

T1 the highest. N1, T2,

and T3 are intermediate

and similar. T2 and T3

differ from each other in

SiO2 and FeO*.

Figure 1. The Endeavour ISS showing ROV Tiburon dive tracks and basalt types. EMS300 bathymetry is from D. Kelley, UW. The T-MORB definition used here has K2O/TiO2 = 0.15-0.25. See text, table, and figures for other chemical differences between basalt types.

Table 1

37.600

37.800

38.000

38.200

38.400

38.600

38.800

39.000

39.200

18.300 18.400 18.500 18.600 18.700 18.800 18.900 19.000 19.100

206Pb/204Pb

208 P

b/20

4 Pb NHRL

Middle Valley Sulfides

Middle Valley Sediments

ET1T2T3N1N2Average Endeavour SulfidesAverage Middle Valley Basalt

5

10

20

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Yb Lu

Bas

alt /

Prim

tive

Man

tle

EPR E-MORB

EPR N-MORB

E

N1

N2

T1

T2

T3

EPR MORB

6.00

8.00

10.00

12.00

14.00

16.00

18.00

1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

La/Yb

Zr/N

b

E N1N2T1T2T3

Na 8

1.5

2.0

2.5

3.0

3.5

Fe86 7 8 9 10 11 12

Typical global array at -2200 m

T1T3

E

N1

T2

N2

EPR

Global MORB

1.00

1.25

1.50

1.75

2.00

2.25

2.50

5.50 6.00 6.50 7.00 7.50 8.00 8.50

MgO

TiO

2

E T1T2T3N1N2

GroupIsotopically

DepletedBasalt Type T3 N2 E N1 T2 T1SiO2 50.96 50.90 50.56 49.96 50.50 49.64Al2O3 14.86 14.67 15.17 15.36 14.97 15.75TiO2 1.65 1.40 1.39 1.42 1.65 1.95FeO 9.63 10.11 9.43 9.31 9.36 9.48MnO 0.20 0.17 0.17 0.19 0.20 0.19MgO 7.15 7.80 7.89 8.00 7.62 7.52CaO 11.63 12.22 12.25 11.89 11.80 10.81Na2O 2.97 2.39 2.44 2.78 2.78 3.00K2O 0.37 0.16 0.50 0.21 0.40 0.50P2O5 0.26 0.15 0.19 0.16 0.23 0.32Total 99.67 99.99 100.00 99.29 99.51 99.16K2O/TiO2 0.226 0.116 0.363 0.144 0.245 0.255La/Yb 3.16 1.45 3.39 1.97 3.49 4.02Zr/Nb 11.47 13.73 7.86 12.89 8.47 8.20Ni 58.0 65.6 64.6 100.0 73.3 127.8Cr 167 168 290 326 247 287V 295 288 266 262 280 275Sc 44.9 44.7 43.8 42.2 43.6 36.9Cs 0.053 0.03 0.07 0.04 0.09 0.10Rb 4.96 2.92 5.20 3.64 7.12 9.21Sr 228 108.2 214.0 146.8 195.6 225.4Ba 60.69 34.89 71.00 41.24 82.94 97.57Zr 128 74.3 103.0 92.2 118.2 153.8Hf 3.11 1.97 2.32 2.25 2.72 3.44Nb 11.2 5.41 13.1 7.15 14.0 18.8Ta 0.71 0.34 1.08 0.44 0.85 1.16Y 32.7 30.7 27.0 29.2 29.6 33.5Pb 0.820 0.403 0.480 0.491 0.854 0.842Th 0.63 0.36 0.76 0.45 0.91 1.19U 0.19 0.11 0.26 0.20 0.38 0.42La 9.3 4.14 8.52 5.20 9.19 11.87Ce 21.0 10.4 19.0 13.1 20.5 26.8Pr 3.00 1.65 2.63 2.02 2.83 3.70Nd 13.8 8.35 11.88 10.01 13.04 16.65Sm 4.04 2.85 3.32 3.17 3.69 4.47Eu 1.40 1.02 1.19 1.15 1.28 1.49Gd 4.80 3.87 4.00 4.13 4.49 5.19Tb 0.823 0.72 0.70 0.74 0.80 0.90Dy 5.18 4.73 4.42 4.71 4.94 5.45Ho 1.07 1.07 0.97 1.02 1.03 1.17Er 3.10 3.04 2.69 2.86 2.87 3.23Tm 0.453 0.45 0.40 0.41 0.41 0.46Yb 2.94 2.85 2.51 2.64 2.63 2.95Lu 0.467 0.42 0.37 0.39 0.38 0.4287Sr/86Sr 0.702412 0.702450 0.702448 0.702477143Nd/144Nd 0.513131 0.513114206Pb/204Pb 18.447 18.598 18.569 18.736 18.852 18.900207Pb/204Pb 15.465 15.488 15.493 15.492 15.497 15.492208Pb/204Pb 37.763 37.915 37.896 38.040 38.150 38.158

Conventional FOZO

500 0 500 1000 Meters

DEPTH (meters)

-1900 -2000 -2100 -2110 -2120 -2130 -2140 -2150 -2160 -2 170 -2180 -2190 -2200 -2220 -2240 -2260 -2280 -2300 -2800

N

129°8' 129°7' 129°6' 129°5' 129°4'

129°8' 129°7' 129°6' 129°5' 129°4'

47°56'47°57'

47°58'47°59'

48°00'48°0

0'47

°59'

47°5

8'47

°57'

47°5

6'

ET1T2T3N1N2

Dive TracksContours (100m)Contours (10m)Vent Fields

LEGEND

SasquatchSasquatch

Salty DawgSalty Dawg

High RiseHigh Rise

Main EndeavourMain Endeavour

MothraMothra