Karson, J.A., Cannat, M., Miller, D.J., and Elthon, D. (Eds.), 1997Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 153

17. CUMULUS AND POSTCUMULUS CRYSTALLIZATION IN THE OCEANIC CRUST: MAJOR-AND TRACE-ELEMENT GEOCHEMISTRY OF LEG 153 GABBROIC ROCKS1

Kent Ross2 and Don Elthon2

ABSTRACT

Gabbroic rocks drilled at Sites 923 and 921 are cumulate rocks that crystallized from at least two distinct parental magmas.One parental magma was geochemically similar to basalts erupted near the Kane Fracture Zone. The second parental magmawas substantially more depleted in incompatible trace elements. Primitive gabbros at Site 923 and 921 have abundant poikiliticclinopyroxene more magnesian (Mg# 88-84) than should crystallize from most mid-ocean ridge basalts (MORBs) at low pres-sure. This observation suggests that the gabbros drilled during Leg 153 could have formed at elevated pressure (2-5 kbar) orthat they could have formed at low pressure from melts with compositions unlike erupted MORBs. Mass-balance modeling ofincompatible trace-element abundances in gabbroic whole rocks suggests that these rocks are adcumulate to orthocumulate,with contents of trapped liquid that vary from near zero to about 30%. Electron probe and ion probe data show enrichments ofincompatible trace elements in portions of clinopyroxene grains that are attributed to postcumulus crystal growth from or inter-action with evolving intercumulus melts. Trace-element-enriched zones in clinopyroxene commonly show little or no change inmajor-element compositions. The zirconium-bearing phase baddeleyite has been observed in 11 samples of the 55 gabbrosexamined in this study. Its occurrence in these rocks is anomalous, because it is expected that silica-saturated magmas will ulti-mately crystallize zircon (ZrSiO4) at extreme degrees of fractionation, and not baddeleyite. The origin of baddeleyite in theserocks is enigmatic. Hornblende, apatite, and ilmenite are also common accessory phases in these rocks, occurring in sampleswith refractory mineral compositions. The presence of these phases suggests extensive crystallization of trapped intercumulusliquid, and yet extreme major-element zoning of Plagioclase and mafic phases is not observed in rocks with late-stage phases.

INTRODUCTION

Leg 153 of the Ocean Drilling Program (ODP) recovered cores ofgabbroic rock at Sites 921 through 924 by drilling directly into gab-broic outcrops exposed along the western wall of the median valleyof the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK).Deep-seated plutonic rocks have been exposed on the seafloor be-cause of extension along a large, east-dipping, detachment structure.Detailed studies of mineral and rock compositions of suites of gab-broic rocks from Sites 921 and 923 have been undertaken to improveour understanding of the formation of the plutonic section of the oce-anic crust and of the evolution of mid-ocean ridge basalts. Studies ofgabbroic rocks from the oceanic crust are important because process-es that are inferred to influence the compositions of lavas erupted atspreading centers, such as crystal fractionation, crystal accumulation,in situ fractionation (Langmuir, 1989), and magma mixing, should bediscernible in the gabbroic plutonic rocks that make up the founda-tions of the oceanic crust, so that models of mid-ocean ridge basalt(MORB) evolution can be tested by studying the plutonic section.

In this paper, the term cumulate is used to refer to rocks that arecomprised of minerals that were separated, with varying degrees ofefficiency, from the magmas from which they crystallized. The pro-cess of cumulate formation results in plutonic rocks whose composi-tions are very different from those of their parental magmas, becausethe rocks consist largely of mixtures of cumulus phases. The separa-tion of crystals from magma can be accomplished by crystal settling,or by crystallization along the walls and floor of magma reservoirs,and the use of the term cumulate in this paper does not imply thatcrystal settling is the means by which the rocks investigated in this

'Karson, J.A., Cannat, M., Miller, D.J., and Elthon, D. (Eds.), 1997. Proc. ODP, Sci.Results, 153: College Station, TX (Ocean Drilling Program).

2Department of Chemistry, and the Texas Center for Superconductivity, Universityof Houston, Houston, TX 77204-5641, U.S.A. DKROSS@UH.EDU

study were formed. Cumulate rocks have been distinguished on thebasis of the quantity of trapped melt that they contain. Rocks thatcontain greater than 25% trapped melt are referred to as orthocumu-lates, those that contain from 7% to 25% trapped melt are mesocumu-lates, and those that contain less than 7% are adcumulates (for discus-sions of cumulus terminology and processes, see Wager et al., 1960;and Irvine, 1982). The pore spaces between cumulus mineral grainscan be filled in various ways. Crystal growth from trapped or evolv-ing interstitial liquid during cooling results in zoning of mineral phas-es. Trapping of interstitial melt leads to enrichment of the bulk rockin trace elements that are incompatible in the cumulus mineral grains.Overgrowth of rims similar in composition to the original cumulusgrains is known as adcumulus growth. Heteradcumulates are rocks inwhich postcumulus crystal growth formed phases other than the orig-inal cumulus phases, and these phases grew from a primitive melt thatwas not evolving, so that heteradcumulus grains are not zoned andhave primitive compositions. The amount of trapped liquid present incumulate rocks may be estimated using abundances of strongly in-compatible trace elements, although some assumption about the com-position of the trapped liquid must be made.

Gabbroic rocks recovered from the MARK area are of cumulateorigin, based on bulk-rock compositions, most of which are substan-tially depleted in incompatible trace elements relative to spatially as-sociated erupted lavas and to their parental melts, whose composi-tions have been inferred from clinopyroxene compositions in the gab-bros. The existence of these cumulate gabbros constitute prima facieevidence for the operation of crystal fractionation with effective sep-aration of melt from crystals. Gabbroic rocks drilled during Leg 153exhibit various features that are typical of cumulate rocks from basiclayered intrusions, such as grain-size layering, modal layering, phaselayering, and formation of poikilitic mineral grains (Cannat, Karson,Miller, et al., 1995). The physicochemical conditions prevailing dur-ing gabbroic rock crystallization at the MARK area resulted in the de-velopment of classic textural features typical of cumulate rock forma-tion.

333

K. ROSS, D. ELTHON

ANALYTICAL METHODS

Major- and minor-element mineral compositions were deter-mined by electron microprobe analysis. These data were collected us-ing the JEOL JXA-8600 Superprobe at the University of Houston.Polished thin sections were prepared from 55 gabbroic rock samplesfrom Sites 921,922, and 923. X-ray intensity data were reduced usingan on-line NORAN ZAF routine. The electron probe was operatedusing an accelerating voltage of 15 kV and a beam current of 30 nA.Six to nine spots on several grains for each major constituent phasewere analyzed. Pyroxene analyses were obtained using a defocusedbeam with a diameter of 25 µm, to attempt to determine magmatic(pre-exsolution) pyroxene compositions. Exsolution lamellae of low-calcium pyroxene in host augites are ubiquitous. Plagioclase was an-alyzed using a 10-µm defocused beam to prevent sodium migrationduring exposure to the electron beam. A 10-µm spot diameter wasalso used for analysis of apatites. Oxides and olivine were analyzedusing 1-µm spot diameter. Elements were calibrated using naturaland synthetic mineral and glass standards. Counting times were gen-erally set at 100 s for the peak, with 50-s counting times on back-ground positions on either side of the peak.

Instrumental neutron activation analysis (INAA) of whole-rockpowders and mineral separates from selected samples have been ob-tained using the laboratory at the NASA Johnson Space Center. Gam-ma-ray spectra were processed using the TEABAGS program (Lind-strom and Korotev, 1982). Whole-rock powders were prepared bychoosing a sawn piece of sample adjacent to the material from whichmineral separates were obtained. The sample was jawcrushed usinghardened steel plates and powdered in a SPEX mill using hardenedsteel container and balls. Plagioclase and clinopyroxene mineral sep-arates were obtained by jawcrushing ultrasonically cleaned pieces offresh sample, sieving the resulting crumbled rock, recovering the250- to 500-µm-sized fraction, magnetically separating initial plagio-clase, clinopyroxene, and olivine fractions, and finally careful hand-picking under a stereoscopic microscope. Separates of 50-100 mgwere obtained by this procedure and this material was then ultrason-ically cleaned in 1-N HCL for 5-10 min. The resulting mineral sep-arates were >99% pure.

Rare-earth element (REE) and trace-element compositions of cli-nopyroxene were obtained by secondary ion mass spectrometry at the

Woods Hole Oceanographic Institution in Dr. N. Shimizu's laborato-ry. Round, 1-in-diameter, thin-section mounts of selected gabbroicrocks were prepared for ion probe studies, so that clinopyroxenegrains could be analyzed in situ to permit evaluation of trace-elementzoning. Energy filtering suppressed molecular interferences (Shimi-zu and Hart, 1982). The ion probe excavates a pit that is about 20-25µm in diameter and about 25 µm deep. Given the scale of exsolutionin clinopyroxene common in these rocks, with exsolution lamellaethat are 1-3 µm wide of orthopyroxene in clinopyroxene hosts, it isunlikely that exsolution phenomena produce the scatter in ion probedata observed. The instrument collects ion intensity ratios repeatedly,over a period of about 25 min during REE analyses and 20 min forother trace-element analyses. The data presented are average resultsfrom these repeated measurements.

PETROGRAPHY AND MINERALOGY

Samples selected for this study consist of some combination ofPlagioclase, clinopyroxene, orthopyroxene, olivine, and/or oxideminerals (ilmenite and/or magnetite) as major or minor mineralogicconstituents. Trace constituents include apatite, brown pleochroicamphibole, zirconia, and sulfide minerals as likely or possible prima-ry disseminated trace minerals. These samples also include varioussecondary alteration phases, generally in minor abundances. Talc,magnetite, chlorite, and actinolite are among the most common alter-ation phases. The samples used were chosen based on their freshnessbecause primary geochemical and mineralogic features are the focusof this study.

Calculated primary modes, along with observed trace mineralsand rock names, for the Site 923 suite of samples are presented in Ta-ble 1. Trace minerals were determined by transmitted light microsco-py, reflected light microscopy, backscattered electron imaging, andenergy dispersive X-ray analysis. The modal abundances of olivine,clinopyroxene, Plagioclase, orthopyroxene, and oxide minerals,when present, were calculated on the basis of whole-rock and mineralabundances of FeO, CaO, Na2O, Cr, and Ni. Modal data were derivedfrom bulk-rock chemical data to permit the use of these modes inmodeling incompatible trace-element abundances in whole-rocksamples. The use of modes derived directly from bulk-rock chemical

Table 1. Calculated modal data for Hole 923A gabbros.

Core, section,interval (cm)

153-923A-2R-1.8-123R-1, 138-1414R-1,35-374R-1, 127-1315R-1, 58-635R-2, 0-55R-3, 0-66R-2, 18-237R-2, 34-408R-1, 61-658R-2, 96-1009R-1, 135-1419R-3, 10-1510R-2, 122-12711R-2, 129-13512R-1,24-2812R-1, 115-12013R-1,65-6914R-1,36-4015R-2, 114-11815R-3, 21-2516R-3, 5-916R-4, 17-21

Olivine

8.0

14.511.013.011.512.511.013.511.58.58.5

12.038.511.023.514.89.0

17.017.033.020.0

Clinopyroxene

32.045.024.028.524.530.531.541.0

5.031.533.033.034.023.033.023.0

9.529.914.512.013.77.0

20.0

Plagioclase

39.047.045.557.064.556.557.046.582.055.055.558.557.565.028.564.067.055.376.571.069.360.060.0

Orthopyroxene

14.0Tr

23.5

Tr

Tr

Tr

Oxides

15Tr7TrTrTrTrTr2TrTrTrTrTrTr2TrTrTrTrTrTrTr

Others

hbl, sulhbl, sulbad, ap, hblhbl, sul, aphbl, sulhbl, sulhbl, sulhbl, sulap, hbl, sulap, hbl, sulhbl, sulhbl, sulhbl, sulhbl, sulhbl, sulhbl, sulbad, hbl, sulhbl, sulhblhbl, sulap, hbl, sulbad, ap, hbl, sulbad, ap, hbl, sul

Rock type

Oxide gabbronoriteOlivine gabbroOxide gabbronoriteOlivine gabbroPoikilitic olivine gabbroOlivine gabbroOlivine gabbroOlivine gabbroPoikilitic olivine gabbroOlivine gabbroOlivine gabbroOlivine gabbroOlivine gabbroOlivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbroOlivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbroPoikilitic olivine gabbro

Notes: Phases listed in the "Others" column generally make up

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

data was deemed more appropriate than the use of point-countedmodes on thin sections because the latter might not be representativeof the aliquot from which INAA data was collected.

Modal abundances were calculated by mass balancing whole-rockcontents of FeO, CaO, Na2O, Cr, and Ni. The abundance of these el-ements were determined in whole rocks by INAA, and in mineralphases by electron probe or INAA. Modal abundances were calculat-ed interactively in a spreadsheet program, and the phases included inthe mass-balance calculations were constrained from petrographicobservations. The amount of Plagioclase in each sample is tightlyconstrained by the bulk-rock Na2O, because Plagioclase is the domi-nant reservoir of sodium in these rocks. FeO in the bulk-rock con-strains the relative proportions of mafic phases (clinopyroxene andolivine are the dominant iron-bearing phases in all rocks except thosefew that have abundant orthopyroxene or iron-titanium oxides).Na2O and FeO are measured with high precision by INAA (typical er-rors at the 2σ level are l%-2% relative for FeO and Na2O). CaO hasrelatively high errors measured by INAA (5%-10% relative) and Crand Ni can also be present in oxides and sulfides respectively, so thatmass-balance calculations were performed with misfits minimizedfor Na2O and FeO. Errors on modal data are estimated to be less than3% relative for samples with only olivine, Plagioclase, and clinopy-roxene as major constituent phases (all but two samples). In trappedliquid modeling discussed in a later section of this paper, calculatedmodes are used to model the trace-element abundances in theserocks. Inferred amounts of trapped melt in these samples are ratherinsensitive to errors in the modes, because melt is strongly enrichedin incompatible trace elements relative to the crystalline phases.

The Site 923 rocks are dominantly olivine gabbros, with minortroctolitic gabbros and oxide gabbronorites. Olivine gabbros in thissuite have been subdivided on the basis of the textures of clinopyrox-ene in these samples. In "olivine gabbro," clinopyroxene is a cumulusphase with numerous subhedral grains that rarely enclose other min-eral phases. In "poikilitic olivine gabbro" clinopyroxene enclosesPlagioclase grains, or is interstitial to euhedral or subhedral plagio-clase. This texture is commonly thought to suggest that clinopyrox-ene grew within pore spaces whose shape was defined by surround-ing Plagioclase grains. In hand specimen and in thin section, poikilit-ic clinopyroxene grains can be recognized because physicallyseparated grains are in optical continuity. In this suite, poikilitic cli-nopyroxene commonly grew with large inclusion-free cores (2-4cm) and margins intergrown with surrounding euhedral to subhedralcumulus Plagioclase grains. Primary clinopyroxene textures inpoikilitic rocks are illustrated in Cannat, Karson, Miller, et al. (1995,fig. 4, p. 183; fig. 14, p. 188; figs.l8A, B, p. 230; fig. 20A, p. 232).Reddish brown rims on poikilitic clinopyroxene are low in chromiumand enriched in TiO2. The cores are magnesian and chrome rich.Poikilitic grain textures have been viewed as evidence for melt trap-ping or postcumulus crystal growth because the inclusion of primarycumulus phases in poikilitic grains indicates that their growth oc-curred after the formation of cumulus phases. Haskin and Salpas(1992) have shown that poikilitic and interstitial mafic grains in Still-water anorthosites have geochemical characteristics, such as refrac-tory major-element compositions and high abundances of compatibleelements, similar to cumulus material. In the oceanic cumulates thatare the focus of the present study, it will be shown that this observa-tion holds; phases with postcumulus textures geochemically are likecumulus material, but have rims or zones that are enriched in incom-patible trace elements.

ELECTRON MICROPROBE RESULTS

Mean olivine compositions are presented in Table 2. At Site 923,olivine ranges in composition from Fo8 4 2 to Fo6 4 7 (forsterite [Fo] isthe molar ratio 100 × Mg/[Mg + Fe]) with NiO contents that varyfrom 0.21 to 0.05 wt%. Olivine at Sites 921 and 922 have composi-

tions ranging from Fo84 to Fo59. Crystallization modeling (using theprogram MIXNFRAC; Nielsen, 1988) indicates that about 50% per-fect fractional crystallization of a primitive Kane basaltic liquid(sample 649B 1 007F; data from Bryan et al., 1993) is required toproduce olivine compositions with this range of forsterite contents.Mean Plagioclase compositions are presented in Table 3. Mean anor-thite (An) contents in Plagioclase (molar ratio Ca/[Ca + Na + K])range from An74 to An44 at Site 923, from An73 to An40 at Site 921,and from An77 to An48 at Site 922. Fractional crystallization modelingsuggests that approximately 50% fractional crystallization is requiredto produce the observed range of anorthite values at Site 923. The es-timated amount of crystallization needed to produce the full range ofmineral compositions in these rocks is based on the assumption of asingle parental magma. In a later section of this paper, it will beshown that two distinct parental magmas were involved in the forma-tion of this suite. The degree of crystal fractionation of these magmaswill be discussed further in the section that follows.

One notable feature of Plagioclase compositions in these rocks isthe substantial variability within individual samples. Tables 2 and 3show standard deviations for forsterite contents in olivine and anor-thite contents in Plagioclase. It is apparent from the standard devia-tions that Plagioclase compositions are more variable than olivinecompositions within most samples. Analytical uncertainties based oncounting statistics on forsterite and anorthite ratios are 0.5 (2σ) sothose samples with standard deviations of Fo and An that exceed 1clearly do not have uniform mineral compositions. In most samples,olivine compositions are essentially uniform, but those of Plagioclaseare not. Variable Plagioclase compositions could result from postcu-mulus crystallization from intercumulus melts, or, in some cases, thePlagioclase grains deposited at the cumulus stage might have beenvariable.

The wide range of Plagioclase compositions in individual samplessuggests that postcumulus crystal growth during cooling of the crys-tal mush has deposited Plagioclase from a range of intercumulus meltcompositions. In two samples with oxide-rich veins (Samples 153-921D-5R-2, 0-4 cm, and 922A-3R-1, 23-27 cm) Plagioclase analy-ses within the oxide veins or proximal to oxide concentrations havemean An values that are 19 and 29 units lower in the vein, relative toPlagioclase distal from the oxide concentrations with no overlap invein and host Plagioclase compositions. The range of Plagioclasecompositions in most samples shows that the intercumulus liquid didnot evolve to the extreme degrees of crystallization required to crys-tallize Fe-Ti oxide minerals, apatites, or zirconium-bearing late-stagephases, yet these late-stage phases are found in many rocks. Ilmeniteoccurs as a disseminated accessory phase in every cumulate gabbro,even the most primitive samples. Accessory ilmenite and apatite areusually observed in interstitial spaces between cumulus grains, butthey have also been observed enclosed in magnesian mafic phases.

Mean forsterite in olivine is plotted vs. mean anorthite in plagio-clase in Figure 1. The data define a broad trend of decreasing forster-ite with decreasing anorthite. The scatter in this trend could reflect ei-ther variable parental magmas or varying extents of postcumuluscrystal growth from evolving intercumulus liquid. Fo in olivine isplotted vs. NiO in olivine in Figure 2. This plot shows a fractionationsequence, with a compositional gap between the residual mantle sam-ples drilled during Leg 153 and the most primitive gabbroic samplesthat indicates that the most primitive portion of the cumulate se-quence was not sampled by drilling in the gabbroic rocks. These oliv-ines are the products of fractional crystallization from magmas withMg# ranging from approximately 65 to 40.

Mg# in clinopyroxene (100 × Mg/[Mg + Fe']) is plotted vs. anor-thite in Plagioclase in the Leg 153 gabbroic rocks in Figure 3. Alsoshown are fields for other oceanic cumulate rocks. The data define atrend of decreasing An with decreasing Mg#. The trend of these datais similar to that from Leg 118 (Hebert et al., 1991) and the Mid-Cay-man Rise (Elthon, 1987), indicating that the Leg 153 samples crystal-lized from relatively high-Na MORB. Clinopyroxene mineral com-

335

K. ROSS, D. ELTHON

Table 2. Average olivine compositions in Leg 153 gabbros.

Core, section,interval (cm)

15 3-923 A-3R-1, 138-1414R-1, 127-1315R-1,58-635R-2,0-55R-3, 0-66R-2, 18-237R-2, 34-408R-1, 61-658R-2, 96-1009R-1, 135-1419R-3, 10-1510R-2, 122-12711R-2, 129-13512R-1,24-2812R-1, 115-12013R-1,65-6914R-1, 36-4015R-2, 114-11815R-3, 21-2516R-3, 5-916R-4, 17-21

153-921A-2R-2, 12-15

153-921B-2R-1, 120-1242R-2, 89-933R-1, 144-1473R-2, 71-754R-1,52-574R-2, 25-304R-2, 122-1264R-3, 90-964R-4, 1-5

153-921C-2R-2, 70-753R-1, 108-110

153-921D-2R-1, 13-185R-2, 0-4

153-921E-2R-2, 8-123R-1, 19-243R-1, 81-855R-2, 19-245R-3, 23-266R-1, 101-1067R-2, 12-177R-4, 68-72

153-922A-2R-5, 125-129

153-922B-1W-1, 37-402R-1, 17-21

n

777677

777777

6

67777

7

767667767

78

37

76777777

7

77

SiO2

37.8636.8737.9737.2837.6837.5538.6138.0237.2638.3737.5337.8539.8738.7338.7937.6438.5838.9738.7339.2638.77

36.78

37.3038.3237.9238.2139.2639.3638.2739.3439.40

38.9837.53

37.1137.07

37.1436.2337.9339.2138.6438.9237.4239.04

39.33

39.7539.31

FeO

26.2730.6726.9427.5428.7925.3022.7524.9426.0324.2727.5525.5515.0019.5917.7026.4623.4019.5319.6618.7520.85

32.27

30.0623.8726.3221.5519.6817.9621.6916.6116.66

20.5624.67

29.4529.16

29.9834.8224.6916.9119.5716.3124.5819.80

19.40

15.3116.66

MnO

0.420.480.410.440.450.400.370.400.400.370.400.390.240.310.290.400.390.320.310.320.35

0.52

0.480.370.420.330.320.310.360.290.28

0.330.40

0.480.55

0.480.560.410.280.310.280.390.33

0.33

0.240.29

NiO

0.080.050.070.070.060.080.150.090.080.080.060.070.210.150.170.080.120.140.130.140.13

0.04

0.060.080.070.090.140.160.130.180.19

0.140.12

0.060.07

0.070.050.090.160.130.140.120.13

0.16

0.210.17

MgO

35.8031.5735.6534.5134.2436.5638.9136.3535.8937.6234.4236.0044.8041.0743.1136.1738.4841.2141.0241.8639.86

30.90

32.7037.9435.8739.2941.2642.3438.9443.4143.18

40.5336.65

32.8033.19

32.6628.6037.0243.0840.5843.3136.3740.43

41.73

44.9843.72

CaO

0.040.050.050.040.040.040.050.050.040.060.040.040.060.050.060.050.050.050.050.060.12

0.06

0.040.040.060.040.060.060.060.060.05

0.060.05

0.040.06

0.050.060.030.050.040.050.060.07

0.05

0.040.06

Total

100.4799.69

101.0999.88

101.2699.93

100.8499.8599.70

100.77100.0099.90

100.1899.90

100.12100.80101.02100.2299.90

100.39100.08

100.57

100.64100.62100.6699.51

100.72100.1999.4599.8999.76

100.6099.42

99.94100.10

100.38100.32100.1799.6999.2799.0198.9499.80

101.00

100.53100.21

Fo

70.64.770.269.167.972.075.372.271.173.469.071.584.278.981.970.974.679.078.879.977.3

63.1

66.073.970.876.578.980.876.282.382.2

77.872.6

66.567.0

66.059.472.882.078.782.672.578.5

79.3

84.082.4

1 s.d.

±0.5±0.4±0.2±0.3±0.2±0.2±0.3±0.7±0.3±0.3±0.2+ 0.4±0.6±1.0±0.1±0.2±0.3±0.2±0.3±0.6±0.6

±0.3

±0.4±0.6±0.3±0.4±0.8±0.3±0.7±0.4±0.2

±0.4±2.2

±0.1±3.7

±1.3+ 0.3±0.5±0.2+ 0.7±0.5±2.4±0.3

±1.7

±0.3±4.6

Notes: All data are given in wt%; "«" is the number of analyses per sample; s.d. = standard deviation. Fo = forsterite (100 • Mg/[Mg + Fe'], molar).

positions for Site 923 and 921 gabbroic rocks are shown in Table 4.Mg# ranges from 88 to 70 in clinopyroxene. In samples with magne-sian clinopyroxene, the texture of clinopyroxene grains is distinctive,with large, green, poikilitic clinopyroxene partly or entirely enclos-ing grains of Plagioclase. These samples are classed as poikilitic oli-vine gabbros, and in some thin sections most or all clinopyroxenegrains share one optical orientation. In some cases, poikilitic clinopy-roxene grains have large (up to 6 cm) cores that are free of inclusions,with margins that enclose Plagioclase grains. It is not clear that thistexture is entirely postcumulus. Rare nuclei of cumulus clinopyrox-ene could have continued to grow during the postcumulus stage, re-sulting in poikilitic margins. The origin of these magnesian poikiliticclinopyroxene grains is enigmatic, and not apparent from texturesalone. Their textures are commonly thought to suggest that they arepostcumulus, and yet they are the most primitive clinopyroxenegrains in this suite of gabbros, with the highest Mg# and Cr contentsand the lowest incompatible-element abundances. These grains areheterogeneous with respect to Cr and Ti contents, but the variation in

Mg# is usually restricted. We have obtained a complete data set forclinopyroxene at Site 923, but have yet to complete analyses of cli-nopyroxene in samples from Site 921. Ferric iron contents in clinopy-roxene, calculated on the basis of stoichiometry and charge balance,generally make up less than 10% of the iron present in the clinopy-roxene. Crystallization modeling (using the program of Nielsen,1988) indicates that 50% perfect fractional crystallization is requiredto produce a change in Mg# in clinopyroxene of 18 units. Nielsen'smodel run on a primitive Kane basalt (sample 649B 1 007F; datafrom Bryan et al., 1993), however, produces an initial clinopyroxenewith an Mg# of 82. As noted previously, in these rocks, clinopyrox-ene Mg# extends to 88. This problem will be discussed further in alater section.

The very primitive nature of magnesian clinopyroxene in poikil-itic olivine gabbro is shown in Figure 4 by the high Cr2O3 contents inpyroxenes. Despite the compositional gap in Mg# between clinopy-roxene in residual mantle and in the early gabbros, the chrome con-tents of some clinopyroxene grains in primitive gabbros reach values

336

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

Table 3. Average Plagioclase compositions in Leg 153 gabbros.

Core, section,interval (cm)

153-923 A-2R-1,8-123R-1, 138-1414R-1, 35-374R-1, 127-1315R-1,58-635R-2, 0-55R-3, 0-66R-2, 18-237R-2, 34-408R-1,61-658R-2, 96-1009R-1, 135-1419R-3, 10-1510R-2, 122-12711R-2, 129-13512R-1,24-2812R-1, 115-12013R-1,65-6914R-1,36-4015R-2, 114-11815R-3, 21-2516R-3, 5-916R-4, 17-21

153-921A-2R-1, 127-1312R-2, 12-15

153-921B-2R-1, 120-1242R-2, 89-933R-1, 144-1473R-2, 71-754R-1, 52-574R-2, 25-304R-2, 122-1264R-3, 90-964R-4, 1-5

153-921C-2R-2, 70-753R-1, 108-110

153-921D-2R-1, 13-182R-1, 67-715R-2, 0-45R-2, 0-4 ox

153-921E-2R-1, 100-1052R-2, 8-123R-1, 19-243R-1, 81-855R-2, 19-245R-3, 32-365R-3, 32-366R-1, 101-1067R-2, 12-177R-4, 68-72

153-922A-2R-5, 125-1293R-1, 23-27 ox3R-1,23-27

153-922B-1W-1, 37^02R-2, 17-21

n

999

9998

189

8

888899

9

89

98999

998

89

887

988996

999

8?:1

88

SiO2

56.4353.3456.3353.3453.9654.0053.8352.5151.1852.2853.1953.0053.4252.6448.9549.9449.6752.3151.9050.3349.9651.9651.76

53.8654.51

53.9853.0653.6652.3849.9449.6650.0549.6648.95

51.8149.88

52.7155.4252.7557.51

52.4552.8155.3051.9751.4350.6151.2451.3451.6950.32

48.4155.7048.30

48.9048.38

A12O3

27.5729.8728.0629.4329.0429.1529.3329.6931.4229.8329.7629.9529.8230.2832.4732.0832.0530.1930.3131.7231.6931.0530.83

28.7428.57

29.1429.8129.0730.4031.8431.6931.7032.0332.36

30.6631.41

29.5428.0629.8926.53

29.6330.0127.7230.6030.8631.2831.1230.9031.0331.24

33.0427.9733.12

32.2132.44

FeO

0.330.220.240.280.310.290.330.270.280.270.260.310.340.310.200.250.310.230.280.230.250.250.23

0.340.32

0.300.200.370.160.190.300.400.300.33

0.290.33

0.340.300.270.35

0.260.290.220.190.250.260.260.280.320.23

0.180.300.25

0.170.13

MgO

0.040.040.020.040.040.030.040.030.030.030.030.030.050.050.050.030.050.030.020.050.030.040.03

0.050.04

0.030.030.050.080.030.040.050.050.05

0.040.04

0.040.040.030.04

0.030.030.020.020.040.040.040.060.050.04

0.020.020.03

0.030.04

CaO

9.1712.099.48

11.4411.3111.5911.5311.8713.7712.2111.9912.4712.1412.4214.9914.3814.5412.3912.5714.2614.1913.5512.93

10.9110.60

11.2011.9111.1012.0914.0714.2513.8914.4114.86

12.7413.63

11.6910.0212.008.21

11.8612.109.84

12.6313.2613.7013.2913.2013.3513.63

15.569.86

15.70

15.0915.38

Na2O

6.354.676.194.995.044.934.954.693.634.524.704.444.584.342.953.293.224.464.323.383.423.844.16

5.295.53

5.204.825.274.633.573.423.533.363.05

4.313.76

4.845.754.676.84

4.794.615.914.313.983.734.003.983.853.73

2.635.892.59

2.882.70

K2O

0.090.050.080.070.050.050.050.040.040.060.040.040.040.040.020.020.030.040.050.020.020.030.04

0.070.07

0.090.050.070.040.040.050.040.040.04

0.070.05

0.060.070.070.13

0.070.060.100.050.030.040.040.030.050.03

0.030.090.03

0.050.04

Total

99.98100.28100.4099.5999.75

100.04100.0699.10

100.3599.2099.97

100.24100.39100.0899.6399.9999.8799.6599.4599.9999.56

100.7299.98

99.2699.64

99.9499.8899.5999.7899.6899.4199.6699.8599.64

99.9299.10

99.2299.6699.6899.61

99.0999.9199.1199.7799.8599.6699.9999.79

100.3499.22

99.8799.83

100.02

99.3399.11

An

44.158.745.655.755.256.356.258.270.959.758.460.759.361.173.670.671.360.461.569.969.666.063.1

53.151.2

54.157.553.658.968.469.568.370.272.7

61.866.5

57.049.058.439.6

57.659.047.761.764.767.064.664.665.566.7

76.648.177.0

74.175.7

ls.d.

±0.9+ 2.4±0.9±3.4±1.6±1.4±1.8±3.1±6.0±5.1±2.1±1.4±2.6±1.6±4.8±4.2±5.1±4.1±6.8±3.7±4.7±2.8±4.5

±3.7±3.5

±1.4±1.1±0.4±2.0±4.1±3.0±2.3±2.5±4.4

±4.9±4.3

±4.2±1.2±3.7+ 0.4

±4.7±2.2±4.5±5.3±1.5+ 2.0±3.2+ 2.6+ 5.6±5.4

±3.4±4.2+ 3.3

+ 2.8±3.1

Notes: All data are given in wt%; "n" is the number of analyses per sample; s.d. = standard deviation. An is anorthite, (100 • Ca/[Ca + Na + K], molar units). In samples with primaryoxide veins, "ox" adjacent to sample name indicates that these analyses were within the vein or proximal to oxides, the second line with the same sample name is the average ofanalyses obtained distal to the oxide vein.

as high as those in the residual mantle samples. This feature suggestsminimal fractionation of Cr spinel from these magmas before crystal-lization of these samples.

Figure 5 shows Mg# vs. Na2O in clinopyroxene. All samples, ex-cept one, plot on a trend of moderately increasing sodium with de-creasing Mg#. The lone outlier on this plot is a sample (Sample 153-923A-7R-2, 34-40 cm) in which clinopyroxene is remarkably en-riched in Zr and other trace elements (see below). Aside from thissample, it appears from this plot that this suite could plausibly repre-sent crystal fractionation from a single parental magma. It will be

shown, however, on the basis of ion probe data for clinopyroxene inthese gabbroic rocks that heterogeneity in parental magma trace-ele-ment composition is required. Shown for comparison in Figure 5 arefields for other suites of oceanic cumulates, including the Mid-Cay-man Rise (Elthon, 1987), Leg 118 (Hebert et al., 1991), and Deep SeaDrilling Project Site 334 (Hodges and Papike, 1976; Ross and Elthon,1993). Sodium oxide in clinopyroxene in Leg 153 gabbros is similarto the lowermost contents in Leg 118 and Mid-Cayman Rise gabbros.Inferred parental magma compositions will be discussed in a latersection.

337

K. ROSS, D. ELTHON

80

roc<

75-

70-

65-

60-

55-

50

Site 921

Site 923

»o .•©

•o

o

60 65 70 75

Forsterite

80 85

Figure 1. Mean forsterite in olivine vs. mean anorthite in Plagioclase in Leg153 gabbroic rocks.

0.5

α> 0.4-c

O 0.3-c

S 0.2-

8 MJ

0.0

•

D

O

Site 923

Site 921

Site 920

Leg 153peri dot ites~

Leg 153gabbroic rocks

55 65 75

Forsterite85 95

Figure 2. Mean forsterite in olivine vs. NiO in olivine in Leg 153 gabbrosand in residual peridotites from Site 920.

In contrast to the plot of Mg# vs. Na2O in clinopyroxene shown inFigure 5, which showed essentially a single trend, at least two distincttrends are clear in Figure 6, a plot of Mg# vs. TiO2 in clinopyroxene.The bulk of the poikilitic olivine gabbros define a trend with steeplyincreasing TiO2 as Mg# decreases, and a second, more gently increas-ing trend is defined by the olivine gabbros with cumulus-textured cli-nopyroxene and by the oxide gabbronorites. These trends are labeled"postcumulus trend" and "cumulus trend," respectively, in Figure 6.Clinopyroxenes with the lowest TiO2 in poikilitic olivine gabbros fallat or near the intersection of these two trends. From these data, it isinferred that melt in the interstitial pore space of rocks with postcu-mulus clinopyroxene followed an evolutionary path distinct from thattaken by the main magma, which deposited rocks that define the "cu-mulus trend." It appears that evolving, crystallizing, interstitial meltis, to some degree, buffered in major-element contents by continuingequilibration with magnesian cumulus mafic phases, whereas incom-patible trace-element contents in pore melts (such as Ti) are enrichedby melt-mass reduction as the pore melt crystallizes. In samples thatshow Ti-enrichment in poikilitic clinopyroxene, TiO2 varies by a fac-tor of >3, whereas Mg# varies by 3-4 units. Zoning profiles and core-rim pairs in clinopyroxene show that Mg# varies minimally, whereasTiO2 increases in the rim by a factor of 3 to 4. It is possible that poikil-itic clinopyroxene grains originally had greater variability in Mg#,

α>

roc<

75

Mg# in CPX

85 95

Figure 3. Mean anorthite in Plagioclase vs. Mg# in clinopyroxene (CPX) in

Leg 153 gabbros and other suites of oceanic cumulates. SWIR is the South-

west Indian Ridge (data from Meyer et al., 1989), MCR is the Mid-Cayman

Rise (data from Elthon, 1987), Site 334 data are from Hodges and Papike

(1976) and Ross and Elthon (1993), and Leg 118 data are from Hebert et al.,

(1991).

and that subsolidus equilibration by diffusion of Fe and Mg withinclinopyroxene grains has reduced this variability, whereas tetravalentspecies such as Ti cannot readily diffuse over the distances requiredto smooth out their compositional variations. Evolution of pore meltsby crystallization of intercumulus phases, with buffering of major-element ratios such as Fe/Mg by equilibration with mafic cumulusphases, could explain the apparent premature appearance of ilmenitein rocks with very magnesian cumulus olivine and clinopyroxenecompositions.

Compositions of orthopyroxene in Leg 153 gabbros are listed inTable 5. Orthopyroxene is an abundant phase in oxide gabbronorites(Samples 153-923A-2R-1, 8-12 cm, and 4R-1, 35-37 cm; see Table1 for modal data) and is an accessory phase, occurring as rims on oli-vine grains in some other cumulate rocks at Site 923. Mean Mg# inorthopyroxene ranges from 75.3 to 64.4.

Table 6 shows mean compositions of oxide phases in these rocks.Data are presented for disseminated ilmenite that occurs in olivinegabbro and poikilitic olivine gabbro. A mixture of exsolved ilmeniteand magnetite occurs in oxide-rich veins or layers that have oxide-gabbronorite mineralogy. Samples with magnetite compositions re-ported in Table 6 are oxide-rich samples. Those with ilmenite but nomagnetite are samples in which oxide minerals are disseminated ac-cessory phases, rather than essential major constituents of the crystal-lizing assemblage. In some cases, oxide-rich samples clearly areveins that crosscut fabrics or layering in host gabbroic rocks. In othercases, it is unclear whether these samples represent veins or oxide-gabbro cumulate layers. Zirconia, presumably the monoclinic formbaddeleyite (Smith and Newkirk, 1965), has been observed as a traceaccessory phase («1%) in eleven samples in this suite of gabbroicrocks. Baddeleyite has been reported associated with ilmenite in oce-anic gabbroic rocks from the Mathematician Ridge (Batiza and Van-ko, 1985). Natural occurrences of baddeleyite are reviewed in Hag-gerty (1987). Baddeleyite has been reported in gabbroic rocks in theRhum intrusion by Williams (1978), in kimberlites by Scatena-Wachel and Jones (1984), and in gabbroic rocks from Axel HeibergIsland and in Hawaiian alkali basalts by Keil and Fricker (1974).Compositions of baddeleyite in two samples are shown in Table 6.Titanium and hafnium are the dominant substituting cations in thisphase. Baddeleyite in this suite is invariably observed in associationwith ilmenite, but does not exhibit a readily recognizable exsolutiontexture. Baddeleyite could have formed as an exsolution productfrom ilmenite, as a late-stage phase crystallizing from trapped melt,

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

Table 4. Average clinopyroxene compositions in Leg 153 gabbros.

Core, section,interval (cm)

153-923A-2R-1, 8-123R-1, 138-1414R-1,35-374R-1, 127-1315R-1,58-635R-2, 0-55R-3, 0-66R-2, 18-237R-2, 34-408R-1, 61-658R-2, 96-1009R-1, 135-1419R-3, 10-1510R-2, 122-12711R-2, 129-13512R-1,24-2812R-1, 115-12013R-1, 65-6914R-1, 36-4015R-2, 114-11815R-3, 21-2516R-3, 5-916R-4, 17-21

153-921A-2R-2, 12-15

153-921B-3R-2, 71-754R-1,52-574R-2, 122-1264R-3, 90-96

153-921C-2R-2, 70-753R-1, 108-110

153-921E-5R-2, 19-246R-1, 101-1067R-2, 12-17

n

6

6

7

769

9

888886

78

8

8889

87

98

SiO2

51.7352.2052.0452.2552.2551.9851.9052.7351.0652.4352.3552.4252.0852.2352.2252.0452.0451.8750.6751.7951.5852.1151.56

51.68

52.1652.3351.9851.97

52.3951.88

52.5452.5251.60

TiO2

0.730.690.730.730.660.700.640.530.930.630.730.570.620.530.520.521.060.601.050.740.550.720.67

0.72

0.590.390.410.52

0.550.52

0.430.420.47

A12O3

2.292.582.232.562.572.542.572.633.272.682.482.822.662.773.683.653.293.083.293.103.432.923.14

2.58

2.633.583.293.38

3.203.51

3.023.093.41

Cr2O3

0.010.120.020.050.050.070.040.180.350.110.100.170.130.161.070.990.120.470.130.420.750.120.40

0.04

0.170.970.740.72

0.600.88

0.320.410.58

FeO

10.937.82

10.547.767.767.928.296.476.507.377.517.188.107.594.354.485.517.197.465.685.235.815.70

8.39

6.354.474.714.64

4.875.36

5.104.815.13

MnO

0.350.240.400.240.240.240.240.190.210.220.220.200.230.200.150.150.170.200.230.180.170.190.18

0.26

0.200.150.150.15

0.150.17

0.170.150.15

MgO

14.0016.3513.8815.2516.5615.4415.9217.2315.2716.4215.9517.0416.3416.7717.3516.9716.3616.0315.2716.1116.7417.0516.07

15.74

17.1417.1717.4217.60

17.1016.83

17.7017.4417.37

CaO

19.5619.5920.1320.1419.5420.6120.0119.2521.3119.8620.5719.3419.5119.4920.3820.8021.0220.3820.6220.9520.6620.6221.39

19.77

19.8120.2320.1320.37

20.2119.90

19.7920.4520.24

Na2O

0.450.370.440.400.370.420.390.350.530.370.350.370.380.350.380.360.370.390.390.380.400.330.37

0.39

0.340.380.350.34

0.360.38

0.330.330.36

NiO

0.010.010.010.010.010.010.010.010.020.010.020.020.010.020.040.030.020.020.020.020.020.020.03

0.01

0.010.030.020.03

0.020.03

0.030.030.02

Total

100.0699.97

100.4299.39

100.0199.93

100.0199.5799.45

100.10100.28100.13100.06100.11100.1499.9999.96

100.2399.1399.3799.5399.8999.51

99.58

99.4099.7099.2099.72

99.4599.46

99.4399.6599.33

Mg#

69.678.970.177.879.277.777.482.680.779.979.180.978.379.887.787.184.179.978.583.585.183.983.4

77.0

82.887.2

87.1

86.284.9

86.186.685.8

1 s.d.

±0.82±0.80±0.90±1.18±1.29±1.22±1.19±1.91±0.93±1.64±1.96±1.34±2.21±1.46±0.79±1.51±1.53±2.82±3.05±0.60± 1.41±0.68±1.95

±3.53

±0.44±1.29±2.45±1.02

+ 1.61±3.50

±0.44±0.90±1.85

Notes: All data are given in wt%; "n" is the number of analyses per sample; s.d. = standard deviation. Mg# is 100 Mg/(Mg + Fe), with all iron as FeO.

1.50

Q.O

1.25-

1.00-

0.75-

0.50-

0.25-

0.00

Site 923• Poikilitic olivine gabbroa Olivine gabbroO Oxide gabbronorite

Site 921Δ Poikilitic olivine gabbroA Olivine gabbro

Site 920Leg 153 peridotites

α α

65 70 75 80 85

Mg# in CPX

90 95

Figure 4. Mg# vs. Cr2O3 (wt%) in clinopyroxene (CPX) in Leg 153 gabbroic

rocks and residual peridotites.

or by crystallizing from Zr-, P- and Fe-enriched immiscible meltformed by unmixing of very evolved intercumulus melt. Apatitecompositions are reported in Table 7. Apatite is a trace phase in allsamples in which it occurs, except in some oxide gabbronorites,where its modal abundance can reach levels that suggest apatite is acumulus phase. Apatites consist mainly of solid solutions of fluorap-atite and hydroxyapatite, with generally low levels of Cl~. Thosesamples with somewhat higher chlorine were recovered at Site 922.The holes at Site 922 recovered rock that generally have much higherdegrees of hydrothermal alteration than rocks from Sites 921 and

0.2-

0.0

Leg 118

DSDP Site 334

MCR

50 60 90 10070 80

Mg# CPX

Figure 5. Mg# vs. Na2O (wt%) in clinopyroxene (CPX) in Leg 153 gabbroic

rocks and other oceanic cumulates. Data sources are as in Figure 4.

923, and it is possible that the high chlorine in these apatites was at-tained during intense alteration.

INAA AND ION PROBE RESULTS

We obtained INAA data for gabbro whole rocks from Sites 921 to923, and for clinopyroxene and Plagioclase separates from selectedsamples (mostly from Site 923). INAA data for clinopyroxene sepa-rates are shown in Table 8. Plagioclase data appear in Table 9 and

339

K. ROSS, D. ELTHON

whole-rock data are in Table 10. Chondrite-normalized REE patternsof clinopyroxene separates are displayed in Figure 7, Plagioclase sep-arates in Figure 8, and whole-rock patterns in Figures 9A-9G. Themost striking feature of rare-earth abundances in mineral separates isthe great variability within this suite. La in clinopyroxene varies by afactor of 11, and by a factor of 8 in Plagioclase in rocks in which theMg# in clinopyroxene varies from 88 to 70. Heavy REE abundancesare also quite variable in clinopyroxene mineral separates, rangingfrom 5 to 30 × chondrite. As discussed in the previous section, the ex-tent of crystallization required to produce the observed changes inmajor-element compositions of minerals in these gabbroic rocks is50%, assuming fractionation of a single parental magma. Perfectlyincompatible-element concentrations would increase in relatedevolving melts and crystals formed from these melts by a factor ofabout 2 over this extent of crystallization, yet these minerals vary bya factor of 10. Thus, either these rocks are not all related by simplefractional crystallization to a single parental melt, or postcumulusprocesses in these rocks have enriched some rocks and minerals in in-compatible elements, or indeed some combination of these effectscould have operated. A critical problem in using cumulate rocks tomake inferences about melt compositions is to distinguish geochem-ical variations that reflect primary cumulus stage processes fromthose that are caused by postcumulus effects. The data in this papershow that any attempt to resolve this issue requires the trace-elementmicroanalytical techniques of the ion probe. Only by examining in-trasample and intragrain variations in mineral compositions can thisproblem be resolved.

Comparing trace-element abundances in clinopyroxene deter-mined by ion probe to the bulk analyses determined by INAA showsthat clinopyroxenes in individual samples have a range of composi-tions that extends to values substantially lower than the bulk analy-

1.25-

XJj 1.00-.E

2 °•75•

0.50-

0.25-

Site 923• Poikilitic oliv. gabbro

α Olivine gabbro

O Oxide gabbronorite

Site 921Δ Poikilitic oliv. gabbro

A Olivine gabbro

cumulus trend

post-cumulus trend• \\

65 70 75 80

Mg# in CPX

85 90

Figure 6. Mg# vs. TiO2 (wt%) in clinopyroxene (CPX) in Leg 153 gabbroic

rocks. The "cumulus trend" is consistent with fractional crystallization; the

"postcumulus trend" is defined by samples in which clinopyroxene has inter-

cumulus or poikilitic textures, and is attributed to crystallization from evolv-

ing intercumulus melts buffered at high Mg# by interaction with cumulus

crystals (further discussion in text).

ses. This suggests that the bulk analyses cannot be used to adequatelycharacterize the parental magmas of these rocks. Growth of trace-el-ement-enriched rims or zones on cumulus minerals leads to an overallincrease in incompatible trace-element concentrations in bulk-miner-al separates and in bulk-rock analyses. For example, in Sample 153-923A-12R-1, 24-28 cm, a poikilitic olivine gabbro, spot analyses ofLa in clinopyroxene vary from 0.05 to 0.19 ppm, and spot analyses ofZr vary from 5 to 14 ppm; the bulk analysis of La in clinopyroxene is0.12 ppm. Note that some core analyses are enriched by a factor of>2 in REE and Zr relative to the most depleted spots. Trace-elementenrichments occur not only at the rims but also in some cores. If aconstant D value for clinopyroxene/melt partitioning of La is as-sumed, the parental melt calculated from the bulk analysis would,therefore, overestimate the La contents of the most primitive meltthat crystallized this sample by a factor of more than 2. Comparisonof ion probe and INAA data for those other samples for which wehave both data types shows similar enrichments in some spot analy-ses and in the overall bulk analyses. It appears that attempting to es-timate parental melts from this suite using bulk analyses produces up-per limits on the parental melt composition, rather than an accuratemeasure of the original parental melt. In some samples, bulk sepa-rates are only slightly enriched relative to the more depleted group ofion probe analyses, but to derive the best estimates of parental meltcompositions, we use ion probe data. We attribute the large variabil-ity of trace-element contents within individual samples and grains toevolution of intercumulus melt, rather than to large variations in orig-inal parental melt compositions.

As with titanium in clinopyroxene, Zr and REEs in clinopyroxenevary by factors of 2-5 in individual samples, whereas major-elementvariations such as Mg# change by amounts that are consistent withonly 10%-20% crystallization of intercumulus melt. As noted above,buffering of intercumulus melt major-element compositions inhibitsthe deposition of low-Mg# clinopyroxene in these pore spaces,whereas incompatible trace elements are free to vary by much largerextents. For bulk clinopyroxene separates, those samples with La,,

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

Table 6. Average oxide mineral compositions in Leg 153 gabbros.

Core, section,interval (cm)

Ilmenite:153-923A-

2 R - 1 , 8 - 1 24R-1 ,35 -375R-3, 0 -612R-1,24-2816R-4, 17-21

153-921B-3R-1 ,33-363R_1, 144-1474R-4, 1-5

153-921E-3R-1, 81-85

153-921D-5R-2, 0 - 4

153-922A-3R-1 ,23-27

Magnetite:153-923A-

2R-1, 8 -124R-1, 35-37

153-921B-3R-1, 33-36

153-921D-5R-2, 0 - 4

153-922A-3R-1 ,23-27

Core, section,

interval (cm)

Zirconia:153-923A-

16R-4, 17-21

153-921E-3R-1, 81-85

n

55558

477

6

5

4

33

3

4

3

n

3

4

SiO2

0000.090

000

0

0

0

0.060.01

0.04

0.03

0.08

SiO2

0

0

TiO2

47.6548.8647.7049.5650.32

49.7546.3150.26

51.06

48.06

48.20

6.506.57

10.76

7.78

7.14

TiO2

1.13

2.14

A12O3

0.290.220.180.120.15

0.160.160.14

0.15

0.15

0.21

3.963.31

4.92

3.14

3.35

ZrO2

97.21

95.94

Cr2O3

0.010.010.060.240.07

0.010.070.37

0.08

0.01

0

0.120.11

0.13

0.01

0.09

HfO2

1.82

1.48

ZrO2

0.050.030.050.030.04

0.090.050.04

0.02

0.07

0.09

0.010

A12O3

0.06

0.07

FeO<

48.4948.9646.9043.3144.40

47.0248.3840.49

42.95

48.32

48.81

83.2385.1

79.25

83.36

84.75

Cr2O3

0.01

0

MnO

0.861.080.632.331.61

0.922.730.84

0.77

1.62

1.77

0.180.23

0.29

0.45

0.32

FeO

0.23

0.52

CaO

000.010.030.02

000.02

0

0

0

0.010

MnO

0.01

0.01

MgO

0.430.292.792.742.42

0.490.666.11

3.76

0.59

0.15

0.440.37

0.57

1.19

0.25

MgO

0.01

0.05

Recalculated

FeO

41.2442.3437.3037.4139.30

42.9837.7333.45

38.44

40.56

41.33

29.7831.33

29.8

29.11

30.61

CaO

0.02

0.01

Fe2O3

8.067.35

10.676.565.67

4.4911.847.82

5.01

8.62

8.31

59.3259.68

54.89

60.22

60.09

Total

100.50

100.22

Total

98.59100.1899.3999.1199.60

98.8999.5599.05

99.29

99.68

100.06

100.38101.61

101.40

101.93

101.93

Ilm

88.789.678.278.682.2

91.8

68.7

79.8

86.1

87.7

Pyro

1.862.311.354.973.41

2.015.871.76

1.61

3.46

3.81

Geik

1.661.10

10.4310.259.03

1.852.51

22.34

13.9

2.26

0.55

Hem

7.797.00

10.066.205.34

4.3111.337.22

4.69

8.22

7.93

Notes: Data are given as wt%; "n" is the number of analyses per sample. Ilmenite analyses are recast into the following components: Ilm = ilmenite (FeTiO3), Pyro = pyrophanite(MnTiO3), Geik = geikelite (MgTiO3), and Hem = hematite (Fe2O3). Iron is recalculated as FeO and Fe2O3 based on 3 oxygens per unit formula in ilmenite and 4 oxygens in mag-netite.

Table 7. Average apatite compositions in Leg 153 gabbroic rocks.

Core, section,interval (cm)

153-921D-2R-1, 13-182R-1 ,67-712R-1, 120-1245R-2, 0 - 4

153-921B-3R-1 ,33-364R-2, 25 -30

153-923A-4R-1 ,35 -377R-2, 34 -4016R-3, 5 -916R-4, 17-21

153-922A-2R-5, 125-1293R-1 ,23-27

153-922B-2R-1, 17-21

n

1555

45

3545

56

4

CaO

56.1555.3455.7955.21

55.9355.61

56.1555.8855.4555.39

55.4655.24

55.09

P2O5

42.3841.9742.5442.22

42.4142.13

42.5942.2242.5142.18

42.2041.94

41.90

F

2.011.312.072.17

2.141.32

1.762.101.872.18

1.371.27

1.42

Cl

0.140.180.080.04

0.060.53

0.170.080.090.13

0.400.77

0.75

SiO2

0.110.270.120.19

0.130.13

0.080.170.090.21

0.210.19

0.21

MgO

0.120.160.060.09

0.060.09

0.050.090.180.08

0.070.04

0.04

SrO

0.030.020.010.02

0.020.01

0.010.010.010.01

0.010.03

0.02

Na2O

0.030.030.020.06

0.030.04

0.030.020.110.05

0.030.06

0.06

La2O3

00.010.020.03

0.010.01

0.030.010.040.02

0.020.03

0.02

Ce2O3

0.100.070.100.12

0.100.05

0.070.090.080.14

0.150.11

0.17

H2O

0.811.120.790.75

0.771.03

0.920.780.890.72

1.040.98

0.92

Total

101.88100.48101.60100.89

101.66100.94

101.86101.45101.31101.10

100.95100.66

100.59

O (= F, Cl)

0.880.590.890.92

0.910.67

0.780.900.810.95

0.670.71

0.77

Total

101.0099.89

100.7199.97

100.75100.27

101.08100.55100.50100.16

100.2899.95

99.82

X(F)

0.5280.3470.5440.574

0.5600.349

0.4610.5530.4920.576

0.3620.337

0.378

X(C1)

0.0200.0260.0120.005

0.0080.075

0.0240.0120.0120.018

0.0570.110

0.107

X(OH)

0.4520.6270.4440.421

0.4320.576

0.5150.4350.4960.405

0.5810.553

0.515

Notes: All data are given in wt%; "n" is the number of analyses per sample. H2O was calculated assuming F + Cl + OH = 1 per formula. O (= F, Cl) is the amount of oxygen subtractedfrom the total resulting from F, Cl balancing cation positive charges.

one, referred to as the "depleted series", had Lan of 1.5 to 3.5 × chon-drite, and the second melt, referred to as the "main series" gabbro par-ent, had 11 to 13 × chondrite at La. These relationships are displayedin Figure 10, which shows estimated main series and depleted seriesparental magmas and Kane basalts (data from Bryan et al., 1981).Large differences in REEs, Zr, Y, Hf, and (La/Sm)n are noted be-

tween these two magma series, but they appear to have only minordifferences in contents of Na, Sr, and Ti. Samples 153-923A- 12R-1,24-28 cm, and 11R-2, 129-135 cm, crystallized from a melt substan-tially more depleted in incompatible trace elements than did mostother gabbros in the Site 923 suite. There is a close match betweenour estimates of the parent of the main series gabbros and the erupted

341

K. ROSS, D. ELTHON

Table 8. INAA data for clinopyroxene mineral separates.

Core, section,interval (cm)

153-923A-3R-1, 138-1414R-1, 127-1315R-3, 0-67R-2, 34-408R-1, 61-6510R-2, 122-12711R-2, 129-13512R-1,24-2815R-2, 114-11816R-4, 17-21

153-921A-2R-2, 12-15

153-921B-4R-3, 90-96

Na2O

0.4630.4870.4330.6580.4370.4320.3790.3550.4430.425

0.420

0.375

CaO

19.218.619.320.518.818.819.920.520.620.0

17.5

19.6

FeO

7.648.848.376.997.847.334.484.095.354.67

10.09

4.52

Se

105.7105.3107.3100.0104.1100.983.387.192.691.8

105.2

85.1

Cr

886387343

3760584877

7720797051305360

265

5120

Co

41.544.544.732.042.740.232.032.731.532.1

51.8

32.8

Ni

8414089

190150101278260184241

126

214

La

0.5351.25

0.4311.29

0.8770.3310.17

0.1170.3620.268

0.468

0.263

Ce

3.16.3

2.489.94.82.4

5.02.3

2.7

Sm

2.053.662.115.102.571.730.990.962.021.33

2.19

1.05

Eu

0.6610.8010.7050.8850.7140.6160.3880.367

0.510.455

0.72

0.379

Tb

0.671.03

0.6621.450.80

0.5720.2870.3020.5820.46

0.75

0.363

Yb

2.423.802.555.542.962.171.191.182.201.47

2.91

1.27

Lu

0.340.560.360.800.420.340.170.150.320.21

0.40

0.20

Zr

150

Hf

0.922.160.863.731.430.700.460.391.160.64

0.91

0.49

Notes: Analytical uncertainties are l%-2% for Na, Fe, Se, Cr, and Co, 3%-5% for La, Sm, Yb, and Lu, 5%-10% for Tb, Eu, and Hf, and 10%-25% for Ce, Ni, Ca, and Zr. Na2O, CaO,and FeO are in wt%. Other elements are in ppm.

Table 9. INAA data for Plagioclase mineral separates.

Core, section,interval (cm)

153-923A-3R-1, 138-1414R-1,35-374R-1, 127-1315R-3, 0-67R-2, 34-408R-1, 61-6510R-2, 122-12711R-2, 129-13512R-1,24-28*12R-1, 115-12015R-2, 114-11816R-4, 17-21

153-921A-2R-2, 12-15

153-921B-4R-3, 90-96

Na2O

4.926.085.224.953.524.654.552.863.323.183.393.59

5.36

3.2

CaO

11.59.9

11.311.213.911.912.013.813.814.313.713.9

11.2

14.1

FeO

0.4580.3630.5320.5910.4160.517

0.520.6940.6660.5150.4190.465

0.552

0.548

Se

0.1570.2820.1220.1210.1050.1200.1700.3260.3410.2470.1460.186

0.126

0.173

Cr

1.21.61.60.81.30.81.1

11.55.20.62.41.8

0.7

2.8

Co

0.963.281.021.130.640.880.771.381.070.780.760.94

1.01

1.12

Sr

287292271283210283266206236222229217

262

247

La

0.701.351.72

0.4171.23

0.9570.39

0.3790.4480.2770.4170.37

0.64

0.488

Cc

1.403.913.691.173.032.310.951.011.050.771.170.90

1.34

1.27

Sm

0.1010.76

0.2760.1320.3210.1720.1030.1230.1450.1220.1320.136

0.139

0.154

Eu

0.5071.4150.8040.3670.5640.5390.5030.3820.4120.3350.3670.401

0.686

0.389

Tb

0.0090.1220.0290.0130.0330.0150.0150.0130.0400.0180.0130.013

0.018

0.018

Notes: * This Plagioclase separate is contaminated with a Zr-rich phase that appears to have concentrated heavy rare-earth elements.Se, Cr, and Co, 3%-5% for La, Sm, and Sr, 5% for Eu, and 15%-30% for Ce Rr Ca, Tb,

Yb

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

Table 10. INAA data for Leg 153 gabbroic whole rocks.

Core, section,interval (cm)

153-923A-2R-1, 8-123R-1, 138-1414R-1, 35-374R-1, 127-1315R-1, 58-635R-2, 0-55R-3, 0-66R-2, 18-237R-2, 34-408R-1, 61-658R-2, 96-1009R-1, 135-1419R-3, 10-1510R-2, 122-12711R-2, 129-13512R-1,24-2812R-1, 115-12013R-1,65-6914R-1,36-4015R-2, 114-11815R-3, 21-2516R-3, 5-916R-4, 17-21

153-921A-2R-1, 127-1312R-2, 12-15

153-921B-2R-1, 120-1242R-2, 89-933R-1, 144-1473R-2, 71-754R-1, 52-574R-2, 25-304R-2, 122-1264R-3, 90-964R-4, 1-5

153-921C-2R-2, 70-753R-1, 108-110

153-921D-2R-1, 13-182R-1, 67-715R-2, 0-4

153-921E-2R-1, 100-1052R-2, 8-123R-1, 19-243R-1, 81-855R-2, 19-245R-3, 32-366R-1, 101-1067R-2, 12-177R-4, 68-72

153-922A-2R-3, 15-182R-5, 125-1293R-1,23-27

153-922B-1W-1,37-402R-1, 17-212R-2, 88-92

Na2O

2.632.372.932.983.352.922.962.313.012.622.762.712.762.920.962.222.172.553.362.442.402.222.43

3.142.81

3.162.763.012.421.862.181.991.642.10

1.901.88

2.443.762.39

2.173.482.931.972.262.471.791.991.68

1.442.712.22

1.812.084.06

CaO

10.515.29.8

12.411.812.513.013.812.513.713.014.113.813.510.814.611.212.512.412.112.29.1

11.0

13.013.0

11.713.611.714.014.39.57.5

12.912.1

12.812.2

12.810.413.4

14.112.211.313.913.514.110.613.914.9

7.612.111.0

11.89.7

10.6

FeO

16.355.58

12.356.785.096.186.125.964.005.845.644.595.345.067.154.834.786.043.234.114.166.304.93

5.977.29

5.924.826.084.794.775.749.284.713.41

5.957.3

7.276.34

10.36

7.24.859.046.874.03

3.75.536.365.35

11.6412.744.14

4.256.979.61

Se

53.754.744.836.223.140.436.345.713.237.934.439.941.332.733.535.510.133.38.6

13.314.58.5

16.6

47.046.5

31.337.532.738.534.27.46.5

24.32.5

32.228.4

47.232.338.5

53.327.647.247.426.629.819.943.846.7

11.344.84.3

15.88.1

26.5

Cr

58.442653.3

170.994.3

135.9148.3

455159.1

258246315277253

23761979

121.5938

6251064859.2558

137.6143.9

118.3233.2

90.4393

1911117.4106.1156750.1

15711520

26189.1716

327160.9143.9

546540702514

1130804

117.870.5

145.3

621192

73.6

Co

48.238.456.140.735.439.639.742.630.143.138.331.731.933.573.136.043.742.326.940.133.054.939.6

31.341.1

35.333.343.134.937.250.976.641.530.7

44.454.4

3930.241.2

46.628.249.747.9

3527.553.743.639.9

64.643.532.9

39.543.629.7

Ni

K. ROSS, D. ELTHON

40

CA

Φ

-acooXQ.O

10-

1-

0.1

Leg153CPXin gabbros

LaCe SmEu Tb Yb Lu

Figure 7. Chondrite-normalized REE patterns of clinopyroxene (CPX) min-eral separates from Leg 153 gabbroic rocks. Two samples are from Holes921A and 92IB, the remaining samples were separated from Hole 923Arocks. Data are normalized to chondrite values of Anders and Ebihara (1982)multiplied by 1.27, as are all normalized REE data that appear in the figuresthat follow.

obtain the results of trapped melt and modal abundance models bywriting to K. R.). None of the models results in good fits for all mea-sured REEs. Calculating amounts of trapped melt needed to balancethe observed whole-rock REE abundances leads to the surprising re-sult that trapping melt in quantities sufficient to balance La in thewhole rock leaves large deficiencies in the model-rock heavy REEs(HREEs) for most samples. Models in which we minimized thesummed deviations for REEs result in La in model whole rocks thatexceeds the measured abundances well beyond analytical uncertain-ties. The amount of trapped melt calculated by balancing La is, inmost samples,

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

0.4La Ce Sm Eu Tb YbLu

o

ooDC

La Ce Sm Eu Tb Yb Lu La Ce Sm Eu Tb Yb Lu

Figure 9. Chondrite-normalized REE patterns of whole-rock gabbros from Leg 153. A. Hole 923A olivine gabbro. B. Hole 923A poikilitic olivine gabbro. C.Hole 923A oxide gabbronorite. D. Gabbroic rocks from Hole 921B. E. Hole 921 A, 921C, and 921D gabbroic rocks. F. Hole 921E gabbroic rocks. G. Site 922whole-rock REE patterns.

345

K. ROSS, D. ELTHON

Table 11. Ion probe data (in ppm) for clinopyroxene in gabbros.

Core, section,interval (cm)

153-923A-12R-1, 24-28 ccccrrr

153-923A-11R-2, 129-135 cccrcr

153-923A-15R-2, 114-118 ccrcc

153-923A-5R-1, 58-63 ccrc

153-923A-7R-2, 34-^0 cccccc

:r "c" indicates core c

Mg#

88.088.2

88.685.885.3

88.1

87.388.187.6

84.684.984.5

83.6

79.980.878.4

79.880.1

81.7

)f srain,

La

0.1860.1040.1090.0870.1520.0870.047

0.2160.1660.1140.3330.1370.233

0.4050.3190.4570.3670.524

0.3890.4100.5490.465

1.961.491.381.28

and "r"

Ce

0.840.650.650.450.870.420.32

1.000.930.801.700.751.25

1.961.882.642.283.13

1.732.163.052.11

11.339.166.425.42

Xd

1.841.381.721.112.011.101.05

1.961.821.702.901.672.53

2.893.094.503.665.11

2.943.385.193.28

18.1215.048.305.28

indicates rim. Ty

Sm

1.240.951.010.911.230.640.70

1.161.210.971.410.941.16

1.531.402.381.902.49

1.461.872.951.82

9.548.424.243.32

pical rel

Eu

0.430.410.500.440.530.390.39

0.570.510.440.540.450.54

0.590.490.560.650.68

0.700.650.930.71

1.171.101.080.47

Dy

2.511.822.111.722.581.311.20

2.332.422.023.312.143.14

2.682.593.833.354.27

3.333.465.003.19

15.3213.507.942.83

Er

1.271.181.181.141.880.801.00

1.151.331.211.691.101.70

1.581.452.091.832.62

1.972.172.871.90

8.117.414.462.28

Yb

1.501.161.251.101.930.800.79

1.321.391.262.061.321.99

1.601.612.322.082.87

2.052.362.902.07

9.928.835.322.68

Ti

220422351821

26801777

21251872204339992044

2767226641072163

338232904199

634754115701

59185742

V

233237215

259223

278302269334262

312235321295

363364405

481410470

439410

Cr

736272067440

48447490

67806716689951266477

8633606967931102

343274301

442618630

6751316

Sr

10.111.09.4

9.57.4

11.911.410.97.2

10.3

9.37.56.97.0

9.58.98.2

8.608.28.6

8.17.8

ative errors at these abundance levels for La and Ce are +15%—20%;

Y

9.59.67.9

14.98.9

10.612.410.618.310.5

13.613.520.115.9

17.418.725.5

79.768.835.1

68.376.5

Zr, Nd, Sm,

Zr

7.66.94.9

14.25.1

9.014.09.4

32.310.0

14

6421

141946

433389

68

281389

Eu, Er,10%; Ti, V, Cr are ± l%-2%; and Y and Sr are ± 5%.

Table 12. Inferred parental magmas of Hole 923A gabbros.50

Kane basaltMain series

gabbro parentDepleted seriesgabbro parent

D cpx/liqor pl/liq (Sr)

Mg#Na2OTiO2SrZrYHi"La.SmnYbn(La/Sm)n

62-632.8-3.01.1-1.5120-15090-12031-402.2-2.69.5-1317-1914-170.56-0.72

56-582.9-3.61.0-1.2514012030-362.3-2.611.0-13.021-2620-250.50-0.58

63-653.0-3.20.8-0.9512050-6517-211.5-1.81.5-3.710.0-15.09.5-130.17-0.30

0.270.120.402.00.120.480.2560.0950.320.400.30

Notes: Mg# is 100 Mg/(Mg + Fe), in molar units. Na2O and TiO2 are in wt%. Sr, Zr, Y,and Hf are in ppm. REEs are normalized to chondritic values from Anders and Ebi-hara (1982) multiplied by 1.27. Estimated parental melts are calculated by using thedepleted subset of ion probe clinopyroxene analyses from Sample 153-923A-12R-1, 24-28 cm, for the depleted series parent, and from Samples 153-923A-5R-1, 58-63 cm, and 15R-2, 114-118 cm, for the main series parent. Electron probe data forthese samples were used to derive estimates of parental melts for Na2O and TiO2.See text for further discussion; pi = Plagioclase, liq = liquid, cpx = clinopyroxene.

ppm Zr, while crystallizing clinopyroxene with Mg# 80. Titaniumoxide contents in clinopyroxene in this sample are uniformly high,rather than extending from the low values typical of most of the suite.This sample also falls well off the trend in Figure 5 (Mg# vs. Na2O inclinopyroxene) observed in the rest of the suite. Perhaps such a re-markably enriched melt, percolating through cumulates formed frommore conventional magmas, could explain the presence of baddeley-ite in so many of these rocks, and the presence of apatite in rocks withlimited major-element variations in mineral compositions.

DISCUSSION

Both Kane erupted lavas and the Leg 153 gabbroic rocks under in-vestigation here show features that are inconsistent with crystalliza-

346

enCD

+±"i_•σ

oO_CD

Q .

CO

10-

Main series

Kane lavasDepleted seriesgabbro parent

magma

LaCe Nd SmEu Tb Dy Er YbLu

Figure 10. REE field of model liquids in equilibrium with clinopyroxenefrom Leg 153 gabbros (calculated using the clinopyroxene/liquid distributioncoefficients from Grutzeck et al., 1974) and REE patterns of erupted Kanelavas (data from Bryan et al., 1981).

tion at the low pressures that would apply in crustal level magmachambers. In the gabbroic rocks, evidence that is inconsistent withlow-pressure crystallization consists of the high Mg# of clinopyrox-ene. Several experimental studies of Kane lavas (Tormey et al., 1987;Grove et al., 1990, 1992) have shown that at 1 atm, Kane melts willbegin to crystallize clinopyroxene at Mg# 82-83. In Leg 153 gabbrosfrom Site 923 and 921, many samples contain clinopyroxene withMg# ranging from 88 to 86. Modeling of crystallization of a primitiveKane lava using the MIXNFRAC program of Nielsen (1988) showsthat crystallization produces initial clinopyroxene with Mg# of 82,when MgO in the melt is at 6.2 wt%. In experiments on Kane Frac-ture Zone basalts (Tormey et al., 1987; Grove et al., 1990, 1992) cli-nopyroxene joins the crystallizing assemblage at 6.5 to 6.7 wt%

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

5-Leg153CPX

Main series

Site 334

6 9 12

FeO (wt%) in CPX

15

Figure 11. Sm (in ppm) in clinopyroxene vs. FeO (wt%) in clinopyroxene(CPX) in mineral separates from Leg 153 (determined by INAA). Samplesthat are believed to have crystallized from main series and depleted seriesmagmas are shown encircled. Two samples with much higher abundances ofSm probably crystallized from main series magmas, but are enriched in REEbecause of melt trapping. Shown for comparison are data from Site 334 ultra-mafic and gabbroic cumulates (Ross, 1994). Fractional crystallization pro-duces a nearly horizontal trend on this plot. The extreme variability in Leg153 gabbroic rocks suggests variable parental magmas and trace-elementenrichments resulting from crystallization of intercumulus liquid.

300

E 2501

α> 200qtnJSuo

o.c

100:

50:

0

Site 923 Plagioclase

Site 334Plagioclase

40 50 60 70

Anorthite

80 90 100

Figure 12. Sr in Plagioclase (INAA data) vs. An in Plagioclase (from electronprobe data). Site 334 data are from Ross (1994). Both data sets come fromsample suites where the Mg# of clinopyroxene ranges from 88 to 70. Cumu-late rocks record primary magmatic geochemical differences that result fromdifferent parental magma compositions. Leg 153 gabbros crystallized fromrelatively enriched N-MORB, and Site 334 cumulates crystallized fromstrongly depleted MORB.

MgO. In Figure 16, MgO vs. CaO in Kane glasses is shown. Themodel liquid line of descent predicted by Nielsen's program is shownby the curve for perfect fractional crystallization. The onset of cli-nopyroxene crystallization occurs at the inflection in this curve at ap-proximately 6.2 wt% MgO. Kane glasses show a trend of decreasingCaO beginning at about 8 wt% MgO. The boundary-layer fraction-ation trend shows the results of modeling a process of 50% crystalli-zation of liquids in a boundary layer at the magma-cumulate inter-face, with the evolved liquid produced repeatedly mixed back into themain magma body. This model could explain the MgO-CaO system-atics in these glasses, however, it does not predict the formation ofearly, magnesian clinopyroxene. We are not suggesting that Mg# inclinopyroxene and Fo in olivine are out of equilibrium, but that the

Φ+->"CT3Co

JZ

oüoer

25

10-

1-

0.1-

0.05

Site 923 olivine gabbro

Site 334 olivinegabbronorite

LaCe Sm Eu Tb YbLuFigure 13. REEs in Leg 153 olivine gabbros and Site 334 olivine gab-bronorite cumulates. Despite the complexities of cumulate rock crystalliza-tion, these rocks record differences in parental magma compositions.

0.00160

Figure 14. Zr vs. P2O5 in gabbroic whole rocks from Leg 153 and in Kanelavas. Leg 153 Zr and P2O5 data are from shipboard XRF data (Cannat, Kar-son, Miller, et al., 1995).

300Sites 923 and 921 Plagioclase

trapped melt orlate-stage phases

Kane lavas

NSites 921 and 923gabbro whole rocks

Site 923 CPX

60 80 100 120 140 160 180 200

Zr (ppm)

Figure 15. Zr vs. Sr in Leg 153 gabbroic rocks, Kane lavas, and minerals ingabbroic rocks. Cumulate rocks should fall to the left of tie lines connectingclinopyroxene (CPX) and Plagioclase (olivine is very near zero in Zr and Srcontents). Clinopyroxene with enriched rims extends the clinopyroxene com-positions to high Zr values. Further discussion is in the text.

347

K. ROSS, D. ELTHON

12.0

11.0-

8.0

PFC

\

Kane glasses

Boundary layer fractionation

CPX

5.0 6.0 7.0

MgO (wt%)

8.0 9.0

Figure 16. MgO vs. CaO in Kane glasses (data from Bryan et al., 1993). Thetrend labeled PFC is perfect fractional crystallization model calculated usingthe program of Nielsen (1988). Boundary-layer fractionation model was cal-culated using the program "In Situ" of Nielsen and DeLong (1992). CaO inKane lavas decreases as MgO decreases at an earlier stage of fractionationthan predicted based on crystallization experiments using Kane basalts(Tormey et al., 1987; Grove et al., 1990, 1992).

earliest formed clinopyroxene is more magnesian than should crys-tallize from Kane melts at low pressure.

One potential problem with the suggestion that these rocks crys-tallized at moderate pressure is the observation that the samples thathave the highest Mg# in clinopyroxene also have trace-element abun-dances that show that their parental liquids were more depleted in in-compatible trace elements than Kane erupted basalts. The suggestionthat Mg# in clinopyroxene is too high for low-pressure fractionationis based on crystallization modeling of Kane basalts. The limited in-ferences that we can make about the major-element contents (Na2Oand TiO2; see Table 12) of the parental liquids of depleted seriesrocks suggest that their major-element compositions were similar tothe main series parent magma. Despite the marked difference in in-compatible trace-element contents of the two magma series, there isno evidence that there were large differences in major elements, andit is the major elements that determine when clinopyroxene begins tocrystallize. Figures 1 through 6 show that minerals in rocks formedfrom the two magma series lie on the same trend in major- and minor-element compositions, suggesting that there were insignificant differ-ences in the major-element compositions of their parental magmas.This observation suggests that it is appropriate to use modeling oferupted Kane basalts to make inferences about crystallization of thedepleted series gabbros. Among all published 1-atm experiments onMORB liquids, the only experiments that produced clinopyroxenemore magnesian than Mg# of 84 involved an unusual large-ion-litho-phile-enriched composition, or compositions that were doped withmagnesian diopside (Grove and Bryan, 1983).

Crystallization at elevated pressure could also account for theMgO-CaO trend defined by the Kane glasses (for examples of the ef-fects of changing pressure on liquid lines of descent in MgO-CaOplots, see Langmuir et al., 1992; and Elthon, 1993). Figure 17 (afterGrove et al., 1992) shows the position of Kane aphyric rocks (datafrom Bryan et al., 1981) on the clinopyroxene-olivine-quartzpseudoternary phase diagram, with the location of clinopyroxene-oli-vine-plagioclase saturated boundaries at 1 atm, 2 kbar, and 8 kbar.Kane glasses are saturated in clinopyroxene at 2 to 5 kbar, but not atlow pressure. So, both the erupted lavas and gabbroic rocks recoveredfrom the MARK area are not consistent with simple low-pressurecrystal fractionation, and an alternative model that could explain theglass trend (in situ crystallization, Langmuir 1989; also boundary-

Oliv.

Figure 17. Clinopyroxene-olivine-quartz (CPX-Oliv.-Qtz) pseudoternaryphase diagram showing the locations of clinopyroxene-olivine-plagioclasecosaturation boundaries at 1 atm, 2, and 8 kbar, based on studies by Tormeyet al. (1987) and Grove et al. (1990, 1992). Compositions of Kane lavas areconsistent with moderate-pressure crystallization at 2-4 kbar.

layer fractionation, Nielsen and Delong, 1992) does not account forthe magnesian clinopyroxene compositions in the gabbroic rocks.Figures 18A and 18B show Kane glasses and Leg 153 gabbroic rocksin Mg# vs. CaO, and MgO vs. CaO, respectively. Mg# in Kane glass-es was calculated assuming that Fe2+/total iron = 0.93. Mg# in gabbrowas calculated with all iron as FeO (the conclusion that many of theserocks are sufficiently Ca and Mg rich to lead to early decline of CaOin the Kane basalts is unaffected by the proportion of iron in the ferricstate in gabbroic rocks). On these plots, magnesian, high-CaO gab-broic cumulates have compositions that are appropriate to producethe observed trend in the glasses.

High Mg# clinopyroxenes are a common feature in gabbroic cu-mulates from the ocean basins. Mid-Cayman Rise gabbroic rocks(Elthon, 1987), Leg 118 gabbros (Hebert et al., 1991), Site 334 rocks(Hodges and Papike, 1976; Ross and Elthon, 1993), and the suite thatis the subject of the present study all contain high Mg# clinopyroxenethat would not crystallize from erupted mid-ocean ridge basalts atlow pressure. In some cases, the production of magnesian clinopy-roxene and orthopyroxene is the result of crystallization of liquids un-like erupted MORBs in major-element composition (Site 334; Rossand Elthon, 1993), but in samples from Leg 153, the evidence for theinvolvement of such unusual melts is not observed. The productionof high Mg# clinopyroxene in these gabbroic rocks is a fundamentalproblem in understanding the evolution of MORBs. The necessity ofearly clinopyroxene crystallization to produce the observed composi-tions of MORBs has long been recognized (Bender et al., 1978;Bence et al., 1979). The evolution of basaltic rocks from the Mid-Cayman Rise also requires early clinopyroxene crystallization (El-thon et al., 1995). Models for the widespread production of high Mg#clinopyroxene from MORB require either moderate- to high-pressurecrystallization or formation from melts unlike erupted MORB.

Observations of the Leg 153 gabbros that are difficult to reconcilewith high-pressure crystallization are the fact that, in all rocks withhigh-Mg# clinopyroxene, the poikilitic texture of clinopyroxene issuggestive of postcumulus crystallization, and the fact that these gab-bros have not undergone pervasive plastic deformation. If these rockscrystallized at high pressure and clinopyroxene crystallized early,then one might expect that clinopyroxene should have a cumulus tex-ture rather than being poikilitic. Also, if these rocks formed at 2-5kbar, the depth of formation would be placed at least at the base ofthe oceanic crust, and up to about 8 km below the base of the crust. Itis difficult to envision how such deeply formed rocks could be trans-ported, en masse, into the crust and up to the seafloor without under-

CUMULUS AND POSTCUMULUS CRYSTALLIZATION

16

14

13

O 12ü 11

10-

oa•

-

-

-

Kane glassesSite 921 gabbrosSite 923 gabbros

α

*

%×wr D

• α

4%11 D

aπ

Φ

B

•

α

π

B

D

O

45 55 65

Mg#

75 85

B 17

oCO

O

o Kane glassesD Site 921 gabbros

Site 923 gabbros

5 6 7 8 9 10 11 12 13 14 15 16 17

MgO (wt%)

Figure 18. A. Mg# vs. CaO in Kane lavas and gabbroic cumulates. Early

crystallization of magnesian, high-Ca cumulates drives melt CaO down at

early stages of fractional crystallization. B. MgO vs. CaO in Kane lavas and

Leg 153 gabbroic cumulates.

going pervasive plastic deformation. Discrete zones of high-temper-ature plastic deformation do, however, occur within the sections atSites 923 and 921.

The gabbros drilled at Site 923 crystallized from two parentalmagmas that are not related to one another by fractional crystalliza-tion. Presumably, these two parental magmas formed by varying de-grees of fractional melting of the suboceanic mantle, as envisioned inrecent models of MORB petrogenesis (Johnson et al., 1990). Themain series magmas are derivatives of relatively low-degree melts,whereas the depleted series magmas were formed by higher-degreemelting. The preservation of geochemical signatures related to thedifferent parental magmas that formed this suite shows that varying-degree fractional melts are not homogenized before delivery to thesite of gabbro formation.

CONCLUSIONS

Cumulate gabbroic rocks drilled during Leg 153 record the crystalfractionation of magmas. Bulk-rock incompatible trace-elementabundances and major-element mineral compositions clearly showthat these rocks are cumulates. Modeling of bulk-rock incompatibletrace elements suggests that the rocks include from about 1 % to 30%trapped melt. Evidence for variability in parental melt trace-element

compositions is revealed by trace-element compositions of clinopy-roxenes. Some bulk-mineral separates are enriched in trace elementsbecause of the growth of enriched rims or zones, so that estimates ofmagma compositions derived from these mineral separates are upperlimits on magma compositions. More accurate estimates are derivedby using the more depleted compositions determined by ion probe, asthese are unaffected or less affected by postcumulus enrichment pro-cesses. It appears that many of the gabbroic rocks studied here crys-tallized from melts geochemically similar to erupted Kane lavas, al-though some samples crystallized from melts more depleted in in-compatible trace elements. Mg# of clinopyroxene in some samplesare higher than would form from Kane melts at low pressure, or thanwould form in boundary-layer fractionation models. Moderate-pres-sure crystallization at 2-5 kbar could account for the formation ofthese magnesian clinopyroxene grains, although we cannot rule outthe involvement of melts unlike any erupted MORBs that could havecrystallized magnesian clinopyroxene at low pressure.

ACKNOWLEDGMENTS

Access to the INAA laboratory at Johnson Space Center was pro-vided by Dr. M. Lindstrom and assistance in data reduction was pro-vided Dr. D. Mittlefehldt. Assistance in sample preparation and col-lection of gamma-ray spectra was provided by Dr. D. Mittlefehldtand Rene Martinez. Access to the ion probe laboratory and assistancein the use of the instrument was provided by Dr. N. Shimizu. Supportfor this project was provided by the Joint Oceanographic Institutions.Excellent efforts by the ships crew and drilling team on the JOIDESResolution and by ODP technical support staff made this work possi-ble.

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