Mével, C, Gillis, K.M., Allan, J.F., and Meyer, P.S. (Eds.), 1996Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 147

3. RARE EARTH ELEMENT CONSTRAINTS ON THE ORIGIN OF AMPHIBOLEIN GABBROIC ROCKS FROM SITE 894, HESS DEEP1

Kathryn M. Gillis2

ABSTRACT

The origin of amphibole in a sequence of gabbronoritic rocks recovered at Ocean Drilling Program Site 894 was evaluatedon the basis of textural and geochemical criteria. Major element compositions indicate that all the amphibole is metamorphicwhereas textural relations suggest that amphibole formed either by magmatic or metamorphic processes. The rare earth element(REE) content of amphibole, clinopyroxene, apatite, and zircon were determined by ion microprobe to assess these interpreta-tions. Amphibole shows broad range in REE concentrations (up to 1,000 times chondritic values) with (La/Sm)N ratios from0.1 to 1.4. There is no systematic relationship between REE content and textural type. The origin of amphibole with REE con-tents less than or equal to clinopyroxene cannot be distinguished by compositional criteria. By contrast, the REE content of allother amphibole can only be explained by magmatic processes. Calculated melts in equilibrium with igneous amphibole indi-cate that amphibole crystallized from melts with progressively higher REE contents; in some samples melts evolved withoutsignificant fractionation of the REEs whereas in other samples melts became light-REE-enriched. An increase in the total abun-dance of the REE melts is consistent with the crystallization of Plagioclase, clinopyroxene, and orthopyroxene from progres-sively more evolved melt. The light-REE-enriched melts may have been influenced by the crystallization of accessory phasesor by interaction with exsolved magmatic fluids. These results indicate that there was a continuum between magmatic andhydrothermal processes and that groundmass amphibole records a complex history of crystallization from a melt and itsexsolved magmatic fluids, followed by interaction with seawater-derived hydrothermal fluids.

INTRODUCTION

Amphibole may form as a late-stage magmatic phase, crystalliz-ing from chemically-evolved late-stage melts or exsolved magmaticfluids as well as a hydrothermal phase replacing pyroxene and olivineor filling fractures in oceanic plutonics (e.g., Mével, 1987). Distinc-tion between these origins has been evaluated using textural relation-ships, major and minor element compositions, and stable isotopiccompositions (e.g., Stakes et al., 1991). In some cases, determinationof the origin of amphibole is straightforward using a combination ofthese criteria. For example, brown, high-TiO2 (3^4 wt%), pargasiticamphibole that forms coronitic overgrowths around clinopyroxene isconsidered to be magmatic whereas green, Iow-Tiθ2 (<0.5 wt%) par-gasitic amphibole that fills fractures in mylonitic zones is consideredhydrothermal (e.g., Mével and Cannat, 1991). In most cases, howev-er, the origin of amphibole is ambiguous.

Development of reliable criteria to determine the origin of am-phibole is critical to our understanding of the magma-hydrothermaltransition in oceanic hydrothermal systems. Characterizing the distri-bution and abundance of magmatic amphibole will place importantconstraints on the evolution of late-stage melts and magmatic fluidsin oceanic plutonics. Moreover, determining the distribution of hy-drothermal amphibole is essential as this phase typically records theconditions at which hydrothermal fluids first penetrate the lowercrust.

During Ocean Drilling Program (ODP) Leg 147, two sites weredrilled within the Cocos-Nazca rift valley at Hess Deep to investigatethe evolution of crustal processes at the fast-spreading East PacificRise (EPR). At Site 894, a sequence of high-level gabbroic rocks wasrecovered in a series of seven holes that were drilled at the westernend of an intrarift ridge in ~l Ma crust. Amphibole is a common

'Mével, C, Gillis, K.M., Allan, J.F., and Meyer, P.S. (Eds.), 1996. Proc. ODP, Sci.Results, 147: College Station, TX (Ocean Drilling Program).

2 School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victo-ria, British Columbia, Canada V8W 2Y2. kgillis@postoffice.uvic.ca

phase in all lithologic units of Hole 894G, in the form of isolatedgrains intersertal to Plagioclase, rims on pyroxene, and fracture infill-ings. Although these textures may suggest that the amphibole formedeither by magmatic or metamorphic processes, the major composi-tions of all textural types indicate a metamorphic origin. To furtherinvestigate the origin of amphibole, I have determined the rare earthelement (REE) content of amphibole with a variety of textures insamples representative of all lithologic types and grain size. I willshow that the composition of amphibole records a complex evolutionthat involved chemically evolved-melts, exsolved magmatic fluids,and seawater-derived hydrothermal fluids.

IGNEOUS LITHOSTRATIGRAPHYAND METAMORPHIC CHARACTERISTICS

OF HOLE 894G

Hole 894G penetrated 154.5 meters below seafloor (mbsf), withan average recovery of 35.4%. The igneous rock types recovered in-clude gabbronorite, olivine gabbronorite, gabbro, olivine gabbro, Fe-Ti oxide gabbronorite, and basalt, in order of their abundance (Ship-board Scientific Party, 1993). Thirteen igneous lithostratigraphicunits were recognized on the basis of mineralogy, texture, and grainsize (Shipboard Scientific Party, 1993). Variation in texture and min-eralogy is common on a thin section and hand specimen scale suchthat equigranular gabbronorite occurs within poikiolitic gabbronoriteand coarse-grained gabbro. Between 100 and 120 mbsf, a magmaticfoliation, defined by the alignment of tabular Plagioclase grains, oc-curs within fine-grained gabbro and, less commonly, coarser grainedgabbronorite. Apatite and zircon locally occur in trace amounts (<O.lmodal%), associated with Fe-Ti oxides and brownish green amphi-bole in coarse-grained patches of gabbro and gabbronorite. The anor-thite content of Plagioclase (An) ranges from 50 to 70, although a fewsamples are more calcic with An contents up to 95 (Natland and Dick,this volume; Pedersen et al., this volume). The Mg# (Mg/Mg+Fe2+)of clinopyroxene and orthopyroxene range from 62 to 80 and from 63

59

K.M. GILLIS

to 73, respectively (Natland and Dick, this volume; Pedersen et al.,this volume). The variation in texture and grain size and the presenceof evolved rock types in the Site 894 plutonics are similar to high lev-el plutonics in many ophiolites (e.g., Pedersen, 1986).

The gabbroic rocks recovered at Hole 894G are variably altered toamphibolite to zeolite facies assemblages (Shipboard Scientific Par-ty, 1993; Früh-Green, this volume; Lécuyer and Gruau, this volume;Manning and MacLeod, this volume). Higher temperature assem-blages are commonly overprinted by lower temperature assemblagesand there is no systematic variation in either metamorphic grade ordegree of alteration with depth. Secondary minerals replace 20%-50% of the primary phases in most of the drillcore, except adjacent tomacroscopic fractures or Cataclastic zones where 50%-80% of thegroundmass phases are replaced. Several generations of veins cross-cut the gabbroic rocks, ranging from microfractures filled with am-phibole to macroscopic fractures filled with assemblages of amphi-bole, chlorite, prehnite, epidote, smectite, calcite, and zeolites.

METHODS, SAMPLE SELECTION,AND ANALYTICAL STRATEGY

Mineral compositions were determined by wavelength dispersivetechniques using a JEOL 733 electron microprobe equipped withTracor Northern software at Massachusetts Institute of Technology(MIT); standard Bence-Albee corrections and natural mineral stan-dards were used. Data collection was guided by backscattered elec-tron images.

Rare earth element concentrations were measured using a CamecaIMS 3f ion microprobe at Woods Hole Oceanographic Institution.Operating conditions were similar to those reported by Shimizu andLe Reox (1986) and Johnson et al. (1990). Concentrations were de-termined using empirical relationships between secondary ion inten-sity (relative to Si or Ca) and concentration (working curves) estab-lished for mineral standards. Trace element abundances for clinopy-roxene were calculated from intensity values, ratioed to 30Si, usingworking curves developed by Shimizu and Le Reox (1986). Interfer-ences of barium oxides and hydroxides with l5lEu and 153Eu wereconsidered to be negligible as Ba is highly incompatible in clinopy-roxene (Johnson and Dick, 1992). Trace element abundances for am-phibole were calculated using working curves developed by Sisson(1994). Because interferences of barium oxides and hydroxides with151Eu and 153Eu are likely in amphibole, there is no correction factorfor Eu; inconsistent data between analytical sessions precluded a cor-rection factor for Yb (Sisson, 1994). In the absence of working curvesfor zircon, REE concentrations were calculated using the clinopyrox-ene working curves. Trace element abundances for apatite were cal-culated from intensity values, ratioed to 44Ca, using correction factorsderived by Grandjean (1989). The spot size was -15-20 mm and theaccuracy and precision for clinopyroxene, amphibole, and apatite arebelieved to be ~10%-20% for the light REE, and ~10%-15% for themiddle and heavy REE. The accuracy of the zircon analyses is un-known.

Ten samples were selected from Hole 894G that spanned therange in rock type, grain size, and depth. The igneous characteristicsof these samples are given in Table 1. The major element composi-tion of the primary minerals in these samples indicate that they areslightly more chemically evolved than the average composition forHole 894G (see Natland and Dick, this volume). Amphibole grainswithin these samples were selected for ion probe analysis if their tex-tures or major and minor element compositions suggested a mag-matic or high temperature hydrothermal origin. Amphibole that filledfractures was avoided. Clinopyroxene grains were selected for ionprobe analysis to provide a comparison to amphibole or because itstexture and/or composition indicated a secondary origin.

Many of the trace element analyses reported in Table 2 reflect thecomposition of more than one mineral phase due to fine-grained in-

tergrowths of hydrothermal minerals. REE analyses for amphibolegenerally represent zones within a grain with a uniform major ele-ment composition, but a few analyses included a mixture of high- andlow-Al2O3 amphibole. Clinopyroxene REE analyses may include cli-nopyroxene, exsolution lamellae of orthopyroxene, and minor am-phibole. The proportion of these phases for each analysis are reportedin the Appendix.

RESULTS

Clinopyroxene

Textural Relations

Clinopyroxene in the Hole 894G gabbroic rocks occurs as oiko-crysts and equigranular grains (Fig. 1A, C). It is variably altered toassemblages of amphibole, magnetite, and secondary clinopyroxene,with incipient alteration to amphibole being focused along grainboundaries and cleavages. Secondary clinopyroxene forms as isolat-ed blebs that lack exsolution textures within its magmatic precursoror along grain boundaries. It typically has higher birefingence incomparison to its precursor, and locally contains fluid inclusionstrapped during mineral growth.

Major and Minor Element Compositions

Clinopyroxene has a narrow range in Mg# (0.68-0.75) and showsconsiderable scatter in Na2O (0.05-0.42 wt%), TiO2 (0.1-1.0 wt%),A12O3 (0.1-2.5 wt%), MnO (0.05-0.50 wt%), and Cr2O3 (0.05-0.40wt%) content (Table 3). The wollanstonite component of an individ-ual grain may vary by 5-8 mol% without change in Mg# and minorelement content. The minor elements, Na2O, TiO2, A12O3, and Cr2O3,however, decrease sharply in concentration at wollanstonite (CaSiO3)contents >43 to 44 mol%. Where textural evidence for exsolution(i.e., presence of orthopyroxene lamellae in clinopyroxene) was ob-served using backscattered electron images, the wollanstonite con-tents were <44 mol%, however, these textural relations were oftenobscured by the alteration of clinopyroxene to amphibole. Based onthese textural and compositional criteria, the transition from magmat-ic to secondary clinopyroxene is estimated at wollanstonite contentsof 43-44 mol%, similar to other plutonic suites (Manning and Bird,1986; Stakes et al., 1991).

REE Contents

Clinopyroxene has a narrow range in REE content and chondrite-normalized profiles (Table 2; Fig. 2). The REE content of clinopyrox-ene typically ranges from 1 to 10 times and from 10 to 30 times thechondrite-normalized values for light and heavy REE, respectively.All clinopyroxene grains are light-REE-depleted with flat heavyREE; (La/Sm)N and (Dy/Er)N ratios range from 0.05 to 0.4 and from0.95 to 1.4, respectively (Fig. 2). Individual grains are generally ho-mogeneous in composition; however, most analyses were collectedaway from grain boundaries as these zones are preferentially altered.

A few clinopyroxene grains in Samples 147-894G-17R-1, 81-86cm (Piece 11), and 18R-1, 2-3 cm (Piece 1), have wollastonite con-tents > 44 mol% and are interpreted to be secondary in origin. Thesegrains do not show any systematic trends in their REE content. Forexample, in Sample 147-894G-18R-1, 2-3 cm (Piece 1), the REEabundances in secondary clinopyroxene are both depleted and similarto magmatic clinopyroxene (cf. analyses 6 and 8; Fig. 2P).

AmphiboleTextural Relations

Amphibole is a minor phase in all rock types throughout Hole894G, both in the groundmass and filling micro- and macroscopic

60

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

Table 1. Summary of igneous characteristics of analyzed samples, Hole 894G.

Core, section,interval (cm)

147-894G-6R-1, 112-1168R-1, 13-159R-3, 51-549R-3, 70-7612R-1, 117-12512R-3, 112-11512R-5, 50-5717R-1,23-2517R- 1,81-8618R-1,2-3

Pieceno.

1024

5C12D7A7B6111

Lithology

GabbronoriteGabbronoriteGabbroOxide gabbronoriteGabbroGabbronoriteGabbroGabbroGabbronoriteGabbronorite

Unitno.

6666

111111111111

Depth(mbsf)

55.9268.6377.5977.7894.9797.85

100.16126.03126.61130.92

Grainsize

mf to mm to cm to cm to cm toeftocf toef to mf to m

Igneous texture

Granular-intergranularPoikioliticIntergranularIntergranular-pegmatiticGranular-intergranularGranular-intergranularIntergranularPegmatiticGranular-poikioliticGranular-poikiolitic

Plag(An)

0.530.560.480.480.590.530.540.510.520.58

Cpx(Mg#)

0.700.72——

0.720.720.71—

0.710.75

Opx(Mg#)

—0.70———

0.69——

0.68—

Accessory phases

Fe-Ti oxidesFe-Ti oxides, apatiteFe-Ti oxides, apatiteFe-Ti oxides, zirconFe-Ti oxides, apatiteFe-Ti oxidesFe-Ti oxidesFe-Ti oxides, apatiteFe-Ti oxidesFe-Ti oxides

Notes: Depths are curated depths. Grain sizes are as follows: f = fine (<l mm), m = medium (1-5 mm), and c = coarse (5-30 mm). See Shipboard Scientific Party (1993) for definitionof textures and unit numbers. Plag (An) = average composition of primary Plagioclase. Cpx (Mg#) = average composition of clinopyroxene with wollastonite content <44 mol%.Dashes (—) = not determined.

fractures (Shipboard Scientific Party, 1993). The most common oc-currence of amphibole in the groundmass rims clinopyroxene in theform of fibrous, green grains (Fig. 1A) or granular, brownish greengrains that are optically zoned (Fig. 1C). Fibrous, green amphibolerims generally have a preferred orientation that parallel exsolutionlamellae, and are intergrown with very fine-grained, disseminatedmagnetite and relict clinopyroxene, suggesting that amphibole re-placed igneous clinopyroxene. Granular, brownish green amphibolerims on clinopyroxene are locally intergrown with Fe-Ti oxides andmay be partly replaced by fibrous green amphibole. Backscatteredimages show that granular rims are composed of amphibole and thatrelict clinopyroxene and disseminated magnetite only occur immedi-ately adjacent to clinopyroxene, suggesting that granular amphiboleonly partly replaced igneous clinopyroxene. Groundmass amphibolealso forms as isolated euhedral grains (Fig. IB) or clots of randomlyoriented grains (Fig. ID) that are intersertal to Plagioclase or adjacentto Fe-Ti oxides; these grains may be partly replaced by fibrous greenamphibole. This brownish green to green amphibole is most commonin gabbro intervals and coarse-grained patches of gabbro within gab-bronorite. Backscattered images show that isolated amphibole grainsare composed of amphibole with a range of composition but do notcontain relict clinopyroxene or disseminated magnetite, suggestingthat they do not replace primary minerals. Samples containing theseamphiboles have slightly more evolved primary mineral composi-tions than the Hole 894G average (Natland and Dick, this volume);coarse-grained patches that host accessory phases are the mostevolved of all (Table 1). Textural relations suggest that the onset ofamphibole crystallization postdated Plagioclase, clinopyroxene, andorthopyroxene, may or may not have been coeval with apatite, andpreceded zircon.

Major and Minor Element Compositions

Most of the amphibole in the gabbroic rocks of Hole 894G is cal-cic, with a range in composition from actinolite to magnesio-horn-blende (nomenclature after Leake, 1978). All amphibole grains havethe following characteristics: (1) CaO contents increase systematical-ly with increasing FeO and have a weak negative correlation withMgO; (2) MnO contents are generally <0.4 wt% with a few grains upto 1.4 wt%; (3) Cr2O3 contents are <0.15 to 0.35 wt%; (4) K2O con-tents are <0.6 wt%; and (5) F and Cl contents are <0.6 and <0.3 wt%,respectively (Table 4). The A1IV content of all occurrences systemat-ically increases with Site A occupancy and Ti content and falls withinthe compositional range of other samples from Hole 894G and HessDeep (Fig. 3). These compositions indicate a metamorphic origin atgreenschist to the transition to the amphibolite facies conditions(Liou et al, 1974).

REE Contents

Amphibole has a broad range in REE profiles. (La/Sm)N and(Dy/Er)N ratios range from 0.1 to 1.4 and from 0.08 to 1.6, respective-ly, and concentrations may vary within an individual sample by 1000times chondritic values (Fig. 2; Table 2). Two compositional groupsare distinguished by the abundance and shape of their REE profile.One group has REE profiles that mimic those of clinopyroxene withREE concentrations that are slightly depleted or enriched relative toclinopyroxene; (La/Sm)N and (Dy/Er)N ratios are <0.4 and >1.3, re-spectively (Figs. 2, 4A). Another group of amphiboles are enrichedin light REE relative to clinopyroxene with (La/Sm)N ratios >0.4 to1.4; concentrations are similar to the light-REE-depleted amphibole.Light-REE-depleted and -enriched amphibole cannot be distin-guished on the basis of major and minor element compositions ortextural types.

Individual amphibole grains may be homogeneous or show abroad range in REE content. In Sample 147-894G-6R-1,112-116 cm(Piece 10), the core of a granular, isolated grain (analyses 6 and 7;Fig. 2A; Table 2) is more enriched in REEs but is less aluminous thanits fibrous rim (analysis 8). In Sample 147-894G-12R-1,117-125 cm(Piece 12D), the core of an amphibole grain rimming clinopyroxeneis light-REE-enriched (analyses 1, 2, and 6), whereas toward the rimit is light-REE-depleted (analyses 3 and 16). There is no systematicvariation in the major element composition of this grain (see Appen-dix). In Sample 147-894G-12R-3, 112-115 cm (Piece 7A), an am-phibole grain rimming clinopyroxene is both light-REE-enriched andaluminous (analyses 5-7) and light-REE-depleted and more actino-litic (analysis 8; Table 2; Fig. 21) but without a systematic zonation.In Sample 147-894G-12R-5,50-59 cm (Piece 7B), the core of an iso-lated grain has a REE profile that mimics clinopyroxene (analysis 1;Table 2; Fig. 2J) whereas the rim is enriched in middle to heavy REE(analysis 2; Table 2; Fig. 2J). There is no zonation in major and minorelement composition.

Amphibole grains within an individual sample show the samerange in REE content as in individual grains. In Sample 147-894G-8R-1, 13-15 cm (Piece 2), an amphibole grain that rims clinopyrox-ene and an isolated amphibole grain both have chondrite-normalizedprofiles (Fig. 2C) that mimic clinopyroxene (Fig. 2D). The major el-ement composition of these grains are uniform and similar, with A1IV

contents from 0.57 to 1.0, and Mg#s from 0.68 to 0.76. In Sample147-894G-9R-3, 71-76 cm (Piece 5A), two isolated amphibolegrains were analyzed. One grain from this sample is surrounded byPlagioclase (analysis 4 and 5), and the other grain (analyses 7 and 8)is intergrown with Fe-Ti oxides (analyses 7 and 8 occur within 100and 500 µm of the Fe-Ti oxides, respectively). Both grains have sim-ilar REE profiles, however, the grain surrounded by Plagioclase is

61

Table 2. Rare-earth-element data for amphibole, clinopyroxene, apatite, and zircon.

Analysisno. Mineral Comments L.a Ce Nd Sm Eu Dy Er Yb La/Sm Dy/Er La/Er

147-894G-6R-1, 112-116 cm (Piece 10):1 Amp Rim clinopyroxene (3)2 Amp Rim clinopyroxene (3)3 Cpx4 Amp Rim clinopyroxene (3)6 Amp Isolated grain, same grain as (7,8)7 Amp Isolated grain, same grain as (7,8)8 Amp Isolated grain, same grain as (7,9)9 Cpx Same grain as (9,10)

10 Cpx Same grain as (9,11)11 Cpx Same grain as (9,10)

147-894G-8R-1, 13-15 cm (Piece 2):1 Cpx Same grain as (2,3)2 Cpx Same grain as (1,3)3 Cpx Same grain as (1,2)5 Cpx7 Amp Rim clinopyroxene, same grain as (8), next to apatite (12)8 Amp Rim clinopyroxene, same grain as (7), next to apatite (12)

10 Amp Isolated grain12 Apatite Next to amphibole (7,8)

147-894G-9R-3, 51-54 cm (Piece 4):1 Amp Groundmass clot2 Amp Groundmass clot3 Amp Groundmass clot4 Amp Groundmass clot

147-894G-9R-3, 70-76 cm (Piece 5C):Groundmass clotGroundmass clotIsolated grain, same grain as (5)Isolated grain, same grain as (4)Isolated grain, same grain as (7)Isolated grain, same grain as (8)Isolated grain

Isolated grain, next to zircon

Isolated grain, next to zirconIsolated grain, next to zircon

-125 cm (Piece 12D):Rim clinopyroxene, same grain as (2,3,6,16)Rim clinopyroxene, same grain as (1,3,6,16)Rim clinopyroxene, same grain as (1,2,6,16)Rim clinopyroxene, same grain as (1,2,316)Groundmass clotIsolated grainGroundmass clot, same grain as (11, 12, 13)Groundmass clot, same grain as (10, 12, 13)Groundmass clot, same grain as (10, 11, 13)Groundmass clot, same grain as (10, 11, 12)Same grain as (15)Same grain as (14)Rim clinopyroxene, same grain as (1,2,3,6)

1245

7

9

AmpAmpAmpAmpAmpAmpAmpZircon

394G-9R-3, 70-10 Zircon1112131415

B94123679

1011121314151617

AmpZirconZirconAmpAmp

3-12R-1, 11AmpAmpAmpAmpAmpAmpAmpAmpAmpAmpCpxCpxAmpApatite

3.23.20.93.65.14.71.22.01.92.1

1.71.31.62.31.31.21.0

128.0

2.52.21.92.5

3.44.91.54.4

56.76.11.50.7

0.21.40.41.41.41.1

14.0142.0

2.243.8

0.71.13.01.62.02.11.51.57.7

241

13.012.63.0

14.325.521.06.89.67.48.7

7.86.37.09.36.85.25.2

449.0

16.513.911.616.2

20.232.710.023.2

316.040.011.311.5

15.59.3

11.611.611.68.9

55.2612.3

11.4198.4

5.17.79.57.57.96.76.66.3

34.5935

13.911.12.8

14.722.714.17.8

11.77.69.0

9.47.58.19.17.16.46.3

393.0

21.818.215.620.3

27.051.815.236.5

313.057.116.711.4

9.017.28.3

15.323.014.8

44.6325.2

11.2113.4

11.111.68.17.4

10.66.69.28.4

29.8909

4.81.36.07.83.93.66.34.14.8

5.03.84.24.63.13.23.2

94.7

10.47.95.78.1

10.821.4

6.817.398.623.2

7.415.9

23.98.9

15.714.611.07.9

13.267.9

4.628.1

6.04.42.42.23.92.74.65.9

10.5233

——0.3

—

1.21.01.0

1.10.80.81.1—

7.4

—

—

—

—

—

1.11.0

4.82.55.95.93.42.8

10.57.28.5

8.56.46.47.35.65.25.2

87.4

20.114.010.114.5

17.230.912.731.9

10438.515.0

238

60218.9

46714920.714.4

23.495.7

7.736.714.27.44.42.37.03.88.98.5

15.5238

5.13.81.44.74.72.72.35.94.14.6

4.83.84.04.72.93.03.0

31.2

10.56.64.96.9

9.315.46.4

15.151.617.78.1

386

9958.2

861247

10.67.2

13.351.0

3.617.86.63.12.41.33.82.05.15.26.2

109

——

1.4

—

5.94.45.1

4.63.53.94.4

—

—23.1

—

—

—

——

679

1564

1373420

—

—

—

—

—5.25.1

86.8

0.350.420.410.370.410.750.220.200.28

0.220.220.240.310.270.220.191.35

0.150.180.210.20

0.200.140.140.160.360.170.130.03

0.010.100.020.060.080.08

0.671.310.300.980.070.160.780.450.320.480.210.150.460.65

0.820.821.180.630.820.820.821.151.13

1.161.111.041.011.261.131.151.80

1.251.381.351.37

1.211.321.301.381.321.421.210.40

0.401.510.360.391.281.31

1.161.231.381.351.401.601.201.181.211.241.151.071.641.40

0.420.420.570.430.600.741.180.370.230.3 1

0.250.240.280.320.310.260.232.77

0.160.230.260.25

0.250.220.160.200.740.230.130.00

0.000.110.000.000.090.10

0.721.890.401.670.070.260.830.850.360.690.210.190.842.21

Table 2 (continued).

Analysisno. Mineral Comments l.a Ce Nd Eu Dy Er Yb La/Sm Dy/Er La/Er

147-894G-12R-3, 112-115 cm (Piece 7A):CpxAmpAmpAmpAmpAmpAmp

Groundmass clotGroundmass clotRim clinopyroxene, same grain as (6,7,8)Rim clinopyroxene, same grain as (5,7,8)Rim clinopyroxene, same grain as (5,6,8)Rim clinopyroxene, same grain as (5,6,7)

147-894G-12R-5, 50-57 cm (Piece 7B):1 Amp Groundmass clot, same grain as (2)2 Amp Groundmass clot, same grain as (1)3 Amp Groundmass clot4 Amp Groundmass clot5 Amp Groundmass clot7 Cpx Same grain as (8)8 Cpx Same grain as (7)

147-894G-17R-1, 23-27 cm (Piece 6):1 Amp Rim clinopyroxene; same grain as (2), next to apatite (9,10)2 Amp Rim clinopyroxene; same grain as (1), next to apatite (9,10)3 Amp Groundmass clot4 Amp Groundmass clot5 Amp Rim clinopyroxene, same grain as (6,7,8)6 Amp Rim clinopyroxene, same grain as (5,7,8)7 Amp Rim clinopyroxene, same grain as (5,6,8)8 Amp Rim clinopyroxene, same grain as (5,6,7)9 Apatite Same grain as (10)

10 Apatite Same grain as (9)

147-894G-17R-1, 81-86 cm (Piece 11):1 Amp Rim clinopyroxene (4,5), same grain as (2,3)2 Amp Rim clinopyroxene (4,5), same grain as (1,3)3 Amp Rim clinopyroxene (4,5), same grain as (1,2)4 Cpx Same grain as (5)5 Cpx Same grain as (4)6 Cpx Same grain as (7,9)7 Cpx Same grain as (6,9)8 Cpx Same grain as (6,8)9 Cpx Same grain as (10)

10 Cpx Same grain as (9)14 Amp Rim clinopyroxene, same grain as (15)15 Amp Rim clinopyroxene, same grain as (14)

147-894G-18R-1, 2-3 cm (Piece 1):1 Cpx Same grain as (2)2 Cpx Same grain as (1)3 Amp Rim clinopyroxene (4,6)4 Cpx Same grain as (6)6 Cpx Same grain as (5)8 Cpx Same grain as (13)

12 Amp Rim clinopyroxene (4,6)13 Cpx Same grain as (8)

1.31.51.7

21.815.413.31.1

1.42.51.92.32.62.32.1

7.638.04.82.38.64.74.61.1

318306

0.40.80.80.90.91.21.21.20.80.80.40.4

0.70.41.50.50.30.81.50.6

6.27.17.560.546.042.37.3

6.815.010.49.08.911.19.3

44.722031.516.349.423.228.78.5

12721268

3.23.44.03.54.24.54.54.83.84.12.12.1

2.92.54.91.91.03.54.52.4

8.08.510.236.728.232.113.1

8.020.612.68.78.911.110.8

50.025140.527.753.226.933.314.6

12951326

4.23.24.54.05.64.84.05.54.64.73.34.3

4.63.95.32.81.34.33.84.1

4.14.04.311.18.58.95.0

3.59.54.63.33.76.55.6

17.811314.011.120.211.313.51.1

365372

3.02.01.92.83.23.52.42.92.82.71.92.1

2.12.12.11.60.63.01.32.7

1.0———

——

—

—1.21.5

——

—

——

—

—0.70.90.80.60.80.60.8

—

0.70.9

0.60.30.7—0.4

5.87.37.514.112.515.67.5

5.521.36.87.06.810.510.2

29.226120.416.234.618.826.216.6375394

2.54.23.94.15.35.85.15.54.85.03.53.4

5.04.71.73.91.14.61.73.6

4.03.43.37.15.77.23.9

2.89.62.93.13.36.65.6

12.41269.98.314.68.312.67.1

181186

1.82.12.02.73.03.82.63.12.92.61.72.1

3.22.51.32.40.72.91.42.5

4.0———

——

—

—6.86.5

——

————132141

——2.33.13.62.42.92.82.7.——

2.82.8

2.10.72.8—2.1

0.20 0.940.230.241.231.140.940.14

0.250.160.250.440.440.220.23

0.270.210.220.130.270.260.220.090.550.52

.40

.48

.29

.44

.41

.27

.25

.44

.53

.48

.34

.05

.18

.54

.35

.35

.28

.55

.47

.36

.53

.33

.36

0.09 0.890.250.26

.32

.250.19 0.990.170.230.320.250.190.200.120.12

0.200.130.45 (0.190.280.160.73 (0.13 (

.16

.00

.30

.14

.09

.28

.36

.07

.02

.23).82.08.04.03182).95

0.220.300.342.071.831.240.19

0.340.170.440.510.540.240.25

0.420.200.330.190.400.380.250.111.191.11

13.2216.5116.0020.6222.7629.4121.4024.7021.7319.6712.2714.89

0.140.110.760.130.250.180.750.15

trW

no

1>i-

/—.

5*

Notes: Concentrations in parts per million (ppm). Numbers in parentheses indicate analysis number. La/Sm, Dy/Er, and La/Er ratios are chondrite-normalized, with chondritic values from Anders andGrevesse(1989).

K.M. GILLIS

1 mm

Figure 1. Photomicrographs showing the textures of amphibole analyzed for trace element content. A. Green, granular to fibrous amphibole rim on clinopyrox-ene in a gabbronorite (Sample 147-894G-17R-1, 81-86 cm, Piece 11). B. Isolated grain of amphibole is intersertal to Plagioclase in a gabbronorite (Sample 147-894G-12R-5, 50-59 cm, Piece 10). C. Brown amphibole rim on clinopyroxene in a gabbronorite, orthopyroxene is altered to green, fibrous amphibole, andigneous Plagioclase is turbid due to minute opaque inclusions (Sample 147-894G-12R-3, 112-115 cm, Piece 7A). D. Randomly oriented, green granularamphibole forms clots that are intersertal to Plagioclase in a weakly foliated gabbro (Sample 147-894G-12R-5, 50-57 cm, Piece 7B). C = clinopyroxene, O =orthopyroxene, P = Plagioclase, and A = amphibole.

Table 3. Representative clinopyroxene analyses, Hole 894G.

Core, section:Interval (cm):Piece no.:IP analysis no.:

SiO2TiO2A12O3MgOMnOFeOT

CaONa2OK2OCr2O3Total

WoEnFsMg#

6R-1112-116

103

52.850.211.35

12.950.20

11.8620.01

0.250,000.06

99.75

0.390.400.210.66

6R-1112-116

109

52.700.491.49

13.340.09

10.5220.52

0.280.000.34

99.76

0.400.420.180.69

8R-113-15

25

51.180.711.96

13.870.109.66

21.030.330.000.29

99.12

0.420.420.150.72

8R-113-15

22

52.600.711.74

14.690.12

11.4618.980.300.000.25

100.84

0.370.440.190.70

12R-1117-125

1215

52.200.481.43

14.750.04

10.2820.70

0.280.000.28

100.45

0.410.440.150.72

12R-550-57

7B8

51.440.761.84

14.200.08

10.9420.15

0.310.000.30

100.01

0.400.430.170.70

17R-181-86

117

52.300.621.51

13.610.319.49

22.720.220.000.13

100.90

0.450.400.150.72

17R-181-86

1110

53.150.771.95

13.720.21

10.5719.990.290.000.15

100.78

0.380.430.190.70

18R-12-3

14

53.110.371.02

14.960.237.16

23.540.200.000.21

100.80

0.460.440.100.79

18R-12-3

18

52.200.641.76

14.420.197.52

23.070.280.000.23

100.30

0.460.430.110.77

Notes: Major element composition in weight percent (wt%). FeOT = total iron as FeO. IP analysis number corresponds to analysis number in Table 2. Wo = wollanstonite, En = ensta-tite, and Fs = ferrosilite (calculated following the procedure given in Lindsley, 1983, and Lindsley and Anderson, 1983). Mg# = Mg/(Mg+Fe2+).

64

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

—i 1 1 1 1 1 1 1 r—A Amphibole(147-894G-6R-1,112-116cm,Piece 10)

i r

_ o Rimcpx(1)

_ g Rim cpx (2)

_O Rim cpx (4)

- X Isolated grain (6)

_ -I Isolated grain (7)

- A Isolated grain (8)

i i i i i i i i i i r

B Clinopyroxene (U7-894G-6R-1,112-116 cm, Piece 10)

. Wo 0.39 (3)

π Wo 0.40 (9)

e> Wo 0.41 (10)

_ * _ Wo0.42(11)

i

Dy La Ce Nd Sm Eu Dy

100

10

; I

: c

r

-

1 ^i

I 1

Amphibole

1 1

1 1

(147-894G-8R-1

I 1

1 1 1 1

,13-15 cm, Piece 2)

I I I I

I

frr

I

Rim opx (7)

g Rim cpx (8)

_ - o - .Isolated grain (10)

-

I I I I

i i r i i i i i i

D Clinopyroxene (147-894G-BR-1,13-15 cm, Piece 2)

. WnO.41 (1)

_u—Wo0.37(2)

Wr, 0.40 (3)

x _ .Wo 0.42 (5)

Dy Dy

Figure 2. Chondrite-normalized REE profiles of amphibole (A, C, E, F, G, I, J, L, M, O) and clinopyroxene (B, D, H, K, N, P). The occurrence of amphibole isfollowed by the analysis number (in parentheses); the average wollastonite content of clinopyroxene is given in mol%, followed by the analysis number (inparentheses). In all plots, each analysis has a separate symbol and analyses of the same grain have the same line. Note that Eu and Yb contents were not deter-mined for amphibole. Concentrations are normalized to Cl chondritic values using the elemental abundances of Anders and Grevesse (1989). cpx = clinopyrox-ene, gm = groundmass, and Wo = wollastonite content.

less enriched than the grain intergrown with Fe-Ti oxides (Fig. 2F).The grain surrounded by Plagioclase is less aluminous than the grainmixed with Fe-Ti oxides and has a higher Mg#. In Sample 147-894G-12R-3, 112-115 cm (Piece 7A), two grains in a groundmass clot(analyses 3 and 4) have similar REE contents to the amphibole rim-ming clinopyroxene (analysis 8; Fig. 21).

Amphibole that rims clinopyroxene has either a granular or fi-brous texture and may have a range in REE content. For example, fi-brous green amphibole in Sample 147-894G-17R-1,81-86 cm (Piece11) (analyses 1-3 and 14-15) has a uniform REE composition where-as brownish green amphibole in Sample 147-894G-12R-1, 117-125cm (Piece 12D), is zoned with both light-REE-depleted and -enrichedcompositions and a broad range in concentration (cf. analyses 1-6and 16).

As will be discussed in a later section, the presence of an accesso-ry phase(s) may influence the trace element content of amphibole.Apatite occurs adjacent to or within amphibole grains in Samples147-894G-8R-1,13-15 cm (Piece 2) (analyses 7 and 8), 12R-1, 112-117 cm (Piece 12D) (analyses 10-13), and 17R-1, 23-25 cm (Piece6) (analyses 1 and 2) (note that the analysis number of amphibole ad-jacent to apatite is given in parentheses). Amphibole grains in contactwith apatite are both light-REE-depleted and -enriched, with a rangein concentration. Amphibole adjacent to zircon in Sample 147-894G-9R-3, 70-76 cm (Piece 5C) (analyses 11, 14, and 15) has chondrite-normalized profiles similar to clinopyroxene.

Accessory Phases

Zircon

Zircon is a minor phase (<l modal%) in several intervals in Hole894G, occurring primarily in coarse-grained gabbronorites where itis associated with Fe-Ti oxides and amphibole. In Sample 147-894G-9R-3, 70-76 cm (Piece 5C), zircon is depleted in light REE and sig-nificantly enriched in heavy REE, with (La/Sm)N and (Dy/Er)N ratiosfrom 0.01 to 0.06 and 0.36 to 0.51, respectively (Fig. 5A).

Apatite

Apatite is a common minor phase associated with amphibole andFe-Ti oxides. It is Cl-rich and F-poor, with Cl and F contents from 1.3to 5.8 and <0.2 to 0.5 wt%, respectively. All grains have a concave-down REE pattern; (La/Sm)N and (Dy/Er)N ratios are 0.52 to 0.84 andfrom 1.33 to 1.80, respectively (Fig. 5B).

Discussion

The major element composition of amphibole in the Hole 894Ggabbroic rocks indicate a metamorphic origin. The coexistence oflow- and high-Al amphibole in many samples suggests formation attemperatures below the actinolite-hornblende immiscibility gap, atgreenschist (250°^50°C) to the transition to the amphibolite facies

65

K.M. GILLIS

—i 1 1 1 1 1 1 1 1 r

E Amphibole (147-894G-9R-1, 51-53 cm, Piece 6)

-Groundmass clot (1)

-Groundmassclot(2)

-Groundmassclot(3)

-Groundmass clot (4)

I I I I I I

Groundmass Clot (1)

GroundmassClot(2)

Isolated grain (4)

Isolated grain (5)

Isolated grain (6)

Isolated grain (7)

Isolated grain (β)

Isolated grain (11)

Isolated grain (15)

Dy La Ce Nd Sm Dy

: I I I I I I I I I I

I G Amphibole (147-894G-12R-1,117-125 cm, Piece 12D)

I I I I I I I I I I

: H Clinopyroxene(147-894G-12R-1,117-125cm,Piece12D)

_Wo0.41 (14)

-Wo 0.41 (15)

Figure 2 (continued).

(450°-600°C) conditions (Liou et al., 1974; Spear, 1980; Moody etal., 1983). By contrast, granular amphibole that is intersertal to pla-gioclase and rims clinopyroxene indicate an igneous origin, havingcrystallized from a melt or exsolved magmatic fluids. In the follow-ing sections, these interpretations for the formation of amphibole areevaluated using the REE data. The rationale for this approach is thatthe REEs are known to be relatively immobile at greenschist to am-phibole facies conditions (e.g., Thompson, 1983), implying that igne-ous amphibole could retain its magmatic REE signature during inter-action with seawater-derived hydrothermal fluids.

Role of Seawater derived Hydrothermal Fluids

Hydrothermal fluids venting at the seafloor today have distinctiveREE profiles, with light REE/heavy REE ratios >IO and large posi-tive Eu anomalies (e.g., Campbell et al., 1988). It is likely, however,that these hydrothermal fluids bear little resemblance to fluids deepin the crust. Seawater-derived hydrothermal fluids that reach >2 kmdepth below the seafloor must be very rock-dominated as field andtheoretical studies predict very low permeability in plutonic sequen-ces at subsolidus conditions (see summary in Rosenberg et al., 1993).Because the REE content of seawater is several orders of magnitudelower than oceanic crustal material (Elderfield and Greaves, 1982),seawater-derived hydrothermal fluids cannot be more REE-enrichedthan the rock through which it is passing. The partitioning of theREEs between amphibole and hydrothermal fluids (DamP-hydrothermal

fluids) has not been determined. Early experimental studies suggest thatj>i>nopyroxene-fluid are less than those for Dclin°vyroxem-me t (e.g., Cullers et

al., 1973). These conclusions may have no bearing on DamP"hydrothermal

n u i d s, however, as they were conducted with distilled water and the

transport of the REEs in aqueous fluids is strongly influenced by fluidcomposition and, in particular, the presence of Cl and F (Humphris,1984). If it is assumed that partition coefficients for amphibole-hydrothermal fluids are similar to those of hornblende-melt (e.g., Sis-son, 1994) and that the REE content of seawater-derived hydrother-mal fluids are less than or equal to rock values, it would not be possi-ble to produce REE enrichment greater than rock values. Based onthese arguments, amphibole grains with REE contents equal to or lessthan rock values can be considered hydrothermal in origin. Thesegrains include all textural types and span the range in major elementcomposition of Hole 894G amphibole. Amphibole veins from gab-broic rocks from the Mid-Atlantic Ridge that have the same major el-ement composition as the Hole 894G amphibole have REE contentsless than clinopyroxene but with similar profiles (K.M. Gillis, unpubl.data, 1993). Thus, it is likely that amphibole with the lowest REEcontents precipitated from a seawater-derived hydrothermal fluid.

Role of Melt Composition and Accessory Phases

Hornblende may crystallize from melts that range from basalt torhyolite in composition (Wones and Gilbert, 1982). For example, anexperimental study predicts that amphibole coexists with melts atpressures >2 kb where PH 2 O is >0.6 kb and temperatures are between850°C and 950°C (Spulber and Rutherford, 1983). Such studies alsopredict that melts become SiO2-rich at high degrees of crystallization(i.e., <30% melt), due to the crystallization of Fe-Ti oxides and, to alesser degree, amphibole (Dixon and Rutherford, 1979; Spulber andRutherford, 1983). At higher fluid pressure and oxygen fugacity, theupper stability limit of amphibole increases (e.g., Holloway andBurnham, 1972).

66

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

100

10

= I

: i-

-– Λ

1

1 1 1 1 1

Amphibole (147-894G-12R-3

I I I I I

I I

112-115 cm

— • _

I i

1 1

Piece 7A)

i i

, RrnunHtm« c | o t (3)

Q On

o - . Rir

>undmass clot (4)

ncpx(5)

- - x - Rimcpx(6)

(._ .Rir

4 Rir

: - ; ; •^

i i

n cpx (7)

n cpx (8)

-

1 i

—i 1 1 1 1 1 1 1 1 rJ Amphibole (147-894G-12R-5, 50-59 cm, Piece 7B)

. Isolated grain (1)

. Isolated grain (2)

.Groundmass clot (4)

- Groundmass clot (5)

J I

Dy Dy

100

10

1

: i

: K

-

i

I

CM

1

1 1 1 1 1 1 1 1 1

opyroxene ( 1 4 7 - 8 9 4 G - 1 2 R - 5 , 5 0 - 5 9 cm, Piece 7B)

I

, w n n « (7)

D Wn n an (H)

-

-

10* I I I I I I I I I I I T I

L Amphibole (147-894G-17R-1,23-25 cm, Piece 6)

icpx(8)

Dy Er Yb La Ce Nd Sm

Figure 2 (continued).

The partitioning of the REEs between amphibole and melt is de-pendent upon magma and mineral composition, pressure, and tem-perature (Green and Pearson, 1985; Adam et al., 1993; Sisson, 1994),although it has been suggested that magma composition has the mostprofound effect (Adam et al., 1993). Apparent distribution coeffi-cients (Ds) for the REE between amphibole-melt (DamPmelt) increase-10 times and become increasingly more convex over a composition-al range of basalt to high-silica rhyolite (Sisson, 1994).

Amphibole in the Hole 894G gabbroic rocks has a broad range inREE contents (Fig. 2). Some samples show a narrow range in REEcontent whereas other samples show a broad range in REE concen-trations (up to 1,000 times chondritic values) with (La/Sm)N ratiosfrom 0.1 to 1.4 (Fig. 2). Although there is no systematic relationshipbetween the REE content and texture of amphibole, the most light-REE-enriched amphibole is restricted to granular rims on clinopyrox-ene that occur within the most chemically evolved intervals. To eval-uate whether these variations may be explained by magmatic pro-cesses, the REE content of melts in equilibrium with amphibole hasbeen calculated for a range of melt compositions using the apparentDs of Sisson (1994) (Fig. 6A, B). A comparison of Figures 6A and6B shows that basaltic andesite melts would have higher total REEcontents, lower (La/Sm)N ratios, and similar (Dy/Er)N than dacitemelts in equilibrium with the same amphibole grain. These calcula-tions suggest that amphibole crystallized from melts with progres-sively higher REE contents; in some samples melts evolved withoutsignificant fractionation of the REE (e.g., Sample 147-894G-9R-1,71-76 cm, Piece 5C), whereas in other samples melts became light-REE-enriched (e.g., Sample 147-894G-12R-1, 117-125 cm, Piece12D). An increase in the total abundance of the REEs is consistentwith the crystallization of Plagioclase, clinopyroxene, and orthopy-

roxene from progressively more evolved melt (e.g., Pedersen andMalpas, 1984). The calculated melts with light-REE-enriched pro-files, however, cannot be produced from simple crystallization of theprimary igneous phases.

Crystallization of accessory phases will have a profound effect onmelt compositions as they strongly partition selective trace elements(Watson, 1980; Watson and Green, 1981; Irving and Frey, 1984). Thepartitioning of the REEs between apatite and melt is dependent pri-marily upon magma composition and temperature and is independentof pressure and H20 content of the melt (Watson and Green, 1981). Inthe more evolved intervals of the Hole 894G core, apatite occurs as aminor phase where it is intergrown with amphibole, Fe-Ti oxides,and, less commonly, zircon. Using the apatite REE contents fromHole 894G and DaPat i temel t determined by Watson and Green (1981),basaltic andesite and andesite melts in equilibrium with Hole 894Gapatite were calculated. These melts are similar to the calculated ba-saltic andesite and dacite melts in equilibrium with amphibole thathave the highest REE content, however, the light REE/heavy REE ra-tios of the melts in equilibrium with apatite are lower than those inequilibrium with amphibole (Fig. 6). Model calculations predict thatthe crystallization of apatite in trace amounts (e.g., -0.1 modal%)would prevent further increase in the REE contents and would in-crease the (La/Sm)N ratio of a melt (Pedersen and Malpas, 1984; Per-fit et al., 1983). Thus, it is unlikely that apatite crystallization couldproduce the most light-REE-enriched melts in equilibrium with am-phibole.

Zircon occurs as an accessory phase associated with Fe-Ti oxidesin the most evolved gabbroic intervals and probably postdates am-phibole crystallization. Precipitation of zircon will enrich and depletethe light and heavy REE content of a melt, respectively, and fraction-

67

K.M. GILLIS

I I I I I I I 1 I

M Amphibole (147-894G-17R-1, 81-86 em, Piece 11)

, Rimopx(2)

3 Rim opx (3)

i Rim opx (14)

<_ .Rimopx(15)

I N Clinopyroxene(147-894G-17R-1,81-86cm,Piece11)

I T . Wn 0.42 (4)

_β__Wo0.41 (5)

_ , Wo 0.43 (6)

_«_ .Wo 0.45 (7)

_ ( _ W o 0.42 (8)

_ , Wo 0.3B (9)

_ _ .Wo 0.39 (10)

La Ce Nd Sm Eu Dy

Amphibole (147-894G-18R-1, 2-3 cm, Piece 1)

- Rim cpx (3)

-Rim cpx (12)

I I I I I T I I I T

P Clinopyroxene (147-894G-18R-1, 2-3 cm, Piece 1)

-Wo 0.42 (1)

_Wo 0.42 (2)

_Wo 0.46 (4)

-Wo 0.46 (6)

-Wo 0.48 (8)

_Wo 0.46 (13)

Dy

Figure 2 (continued).

ate a melt such that both (La/Sm)N and (Dy/Er)N ratios would increase(Fig. 5B) (Watson, 1980). The impact of zircon crystallization onmelt REE contents, however, would be limited by the Zr content ofthe melt.

Calculated melts in equilibrium with amphibole and apatite over-lap the field of the most REE-enriched EPR basalts and are signifi-cantly more enriched in total REE contents than the Hole 894G gab-broic rocks (cf. Figs. 6, 7). An exception to this relationship is acoarse-grained, oxide gabbronorite interval in Section 147-894G-9R-3, which is enriched in total REE content and light REEs, has a largenegative Eu anomaly, and is depleted in heavy REE (Fig. 7; Pedersenet al., this volume). This evolved interval is similar to other fraction-ated plutonics from the Hess Deep scarps (J. Natland, pers. comm,1992) and ODP Hole 735B (J. Hertogen, pers. comm., 1993) whichare enriched in total REE content and have light-REE-depleted or-enriched profiles (Fig. 7). For comparison, ophiolitic plagiogranitesare enriched in REE and have flat to slightly light-REE-enriched pro-files (e.g., Aldiss, 1981; Pedersen and Malpas, 1984). These evolvedrock types, however, are not as REE enriched as the most fractionatedcalculated melts in equilibrium with amphibole and apatite.

Textural and compositions of Plagioclase, clinopyroxene, ortho-pyroxene, and olivine indicate that primitive basaltic melts becamemoderately fractionated during the initial stages of crystallization,followed by migration of highly fractionated interstitial meltsthrough the cumulus pile (Natland and Dick, this volume). Modelcalculations show that these interstitial melts were ferroandesiteswith enriched incompatible element contents (Natland and Dick, thisvolume; Pedersen et al., this volume). Although these predictions areconsistent with the calculated melts, the basaltic andesite and dacite

melts with light-REE-enriched profiles have higher total REE con-tents and are more light-REE-enriched than any known melt compo-sition. Evidence for these light-REE-enriched melts is restricted tomost evolved gabbronoritic samples where the light-REE-enrichedamphibole rims clinopyroxene. The high REE contents cannot be ex-plained by melt content because D a m p m e l t increases as melts becomemore siliceous (Sission, 1994). Moreover, it is unlikely that apatitecrystallization could produce these compositions as melts in equilib-rium with apatite are not as enriched in light REEs or total REE con-tent as the most fractionated melts in equilibrium with amphibole.

Role ofMagmatic Fluids

Amphibole may also crystallize from exsolved magmatic fluids.Melts with mid-ocean ridge basalt compositions only reach H2O sat-uration at high degrees of crystallization (-95%), due to the incom-patible nature of H2O and low H2O content of the melt (Burham,1979). Fluid inclusion data for the Hole 894G gabbroic rocks studiesshow that H2O-CO2-NaCl-±Fe-rich fluids were exsolved fromevolved interstitial melts (Kelley and Malpas, this volume). Evidencefor exsolved fluids is restricted to coarse-grained patches that havethe most evolved primary mineral compositions (Kelley and Malpas,this volume), similar to the occurrences of the light-REE-enrichedamphibole. If amphibole crystallized from fluids with compositionssimilar to those indicated by the fluid inclusions, the light-REE-en-riched amphibole may be explained by complexing with Cl~ as thelight REE are strongly complexed with Cl~ at high temperatures (Val-sami-Jones et al., 1994). As magmatic fluids were probably exsolvedat temperatures between 700°C and 800°C (Wyllie, 1977; Burnham,

68

Table 4. Average representative amphibole analyses, Hole 894G.

Core, section:Interval (cm):Piece no.:IP analysis no.:Occurrence:

SiO,TiO2A12O3Cr2O,F e O r

MnOMgOCaONa2OK2OcfFTotal

SiA1IV

A1VI

CrFe3+

Fe2 +

MnMgTiCaNaKClFMg#

6R-1112-116

102

Rim cpx

50.841.064.780.06

13.670.11

15.3211.520.900.260.200.25

98.97

7.330.670.140.0101.650.013.290.121.780.250.050.050.110.67

6R-1112-116

108(3)

Is grain

50.830.344.330.06

16.670.36

15.1610.490.810.190.210.25

99.70

7.310.640.100.010.311.700.043.250.041.620.230.030.030.080.66

8R-113-15

27(2)

Rim cpx

52.370.653.750.02

12.210.14

16.9211.970.610.090.110.15

98.98

7.410.550.070.000.301.140.013.570.071.820.170.020.030.070.76

9R-351-54

43(3)

Gm clot

53.480.401.170.02

16.510.28

14.9610.730.250.04—

97.84

7.790.180.020.000.061.950.033.250.041.670.070.010.000.000.62

12R-1117-125

12d2(2)

Rim cpx

55.630.191.060.07

12.140.28

17.1912.210.150.01

98.92

7.860.130.050.010.051.390.033.62i i , , :

1.850.040.000.000.000.72

I2R-1117-125

12d2(2)

Rim cpx

49.851.295.770.11

16.100.26

12.9711.580.990.32

99.25

12R-1117-125

12d11(3)

Gm clot

55.170.291.500.10

11.790.31

18.2111.710.210.050.070.13

99.52

12R-3112-115

7a4(4)

Gm clot

50.550.584.650.09

15.470.26

13.7811.980.610.120.190.11

98.41

12R-3112-115

7a7(3)

Rim cpx

48.021.416.640.07

14.170.23

14.4311.621.210.500.280.14

98.71

Cation proportions on the basis of 23 anhydrous O

7.200.800.180.010.291.650.022.790.141.790.280.060.000.000.63

7.730.250.000.010.151.230.043.800.031.760.060.010.020.060.76

7.330.670.120.010.331.550.022.980.061.860.180.020.050.050.66

6.951.050.080.010.431.280.013.110.151.800.330.090.070.060.71

12R-3112-115

7a8(3)

Rim cpx

54.820.141.450.04

12.470.19

17.0512.150.220.040.070.07

98.72

7.780.220.020.000.101.390.023.610.021.850.060.010.020.030.72

12R-550-57

7b1(2)

Is grain

50.640.374.640.01

16.030.26

13.9411.610.750.100.100.26

98.72

7.330.670.120.000.341.600.023.010.041.800.210.020.030.120.65

12R-550-57

7b2(4)

Is grain

52.890.412.610.02

15.550.22

15.8510.470.530.080.170.26

99.04

7.610.390.050.000.091.770.033.400.041.610.150.010.040.120.66

17R-123-27

65

Rim cpx

50.331.013.910.02

17.040.23

13.8310.580.770.37—

98.10

7.390.610.070.000.022.080.033.030.111.670.220.070.000.000.59

17R-123-27

68(3)

Rim cpx

53.530.241.840.03

15.800.25

14.7411.150.370.06—

98.01

7.780.220.090.000.051.860.033.190.031.740.100.010.000.000.63

18R-12-3

112(2)

Rim cpx

47.710.737.900.10

14.600.06

14.3512.061.670.100.270.30

99.85

6.831.170.160.010.511.240.003.060.081.850.460.020.070.130.71

Notes: Major element compositions in weight %. FeOT = total iron as FeO. IP analysis number corresponds to analysis number in Table 2. Number in parentheses = number of analyses. Compositions were recalculated from microprobe dataon the basis of 23 anhydrous oxygens. Fe3+ was calculated as the average of the maximum and minimum ferric iron content for solutions that satisfied charge balance constraints, assigning Ca to the M4 site and K to Site A (Robinsonet al., 1982). Mg# = Mg/(Mg+Fe2+). Rim cpx = rim clinopyroxene, Is grain = isolated grain, and Gm clot = groundmass clot.

K.M. GILLIS

B

O Groundmass clotIsolated grain

O Replacing clinopyroxene+ Rim clinopyroxene

Figure 3. Composition of amphibole in the samples analyzed for trace ele-ments. A. A1IV vs. (Na + K) in Site A. B. A1IV vs. Ti content. The shaded arearepresents the range in composition of amphibole from other Hole 894G andHess Deep samples (K.M. Gillis, unpubl. data).

1979; Kelley and Malpas, this volume), they cannot account for themajor element content of amphibole. Fluid inclusion data show, how-ever, that exsolved fluids migrated along microfractures and were notentrapped until they had cooled to ~300°C (Kelley and Malpas, thisvolume). Thus, it is plausible that amphibole crystallized from ex-solved magmatic fluids that had cooled to 300°-600°C. Although itis not possible to evaluate what proportion of the igneous amphiboleformed from magmatic fluids, it is the most viable explanation for thelight-REE-enriched amphibole.

SUMMARY

Textural and compositional data indicate that groundmass am-phibole in the Hole 894G gabbroic rocks record a complex history ofcrystallization that spans both igneous and metamorphic processes.The major element composition of amphibole indicates a metamor-phic origin whereas textural relations indicate both an igneous andmetamorphic origin. The REE data show no systematic variationwith either major element composition or textural type; however, am-phibole with the highest REE contents form as rims on clinopyrox-ene. Amphibole grains within an individual sample may have REEcontents less than or equal to clinopyroxene, show a continuous in-crease in REE content with no fractionation, or become light-REE-enriched with increasing REE content. The origin of amphibole withREE contents less than or equal to clinopyroxene cannot be distin-guished by compositional criteria. By contrast, amphibole with REE

S 0.8en

o Groundmass amphiboleG Amphibole rim on clinopyroxeneO Isolated grainX Apatite+ Zircon

oo

Gl D

<f> o

(Dy/Er)N

Figure 4. (Dy/Er)N vs. (La/Sm>N of amphibole. The shaded area indicates therange in composition of clinopyroxene.

contents greater than clinopyroxene could not have formed by inter-action with seawater-derived hydrothermal fluids and require an ig-neous origin. This would require that the major element compositionof igneous amphibole was reset by interaction with seawater-derivedhydrothermal fluids at subsolidus temperatures.

Textural and geochemical data for all igneous phases in the Hole894G gabbroic rocks indicate that evolved interstitial melts migratedthrough the cumulus pile during the latest stages of crystallization(Natland and Dick, this volume; Pedersen et al., this volume). Calcu-lated melt compositions in equilibrium with igneous amphibole showthat the REE content of late-stage melts progressively increased andthat the light REEs were locally enriched. Calculated melt composi-tions in equilibrium with apatite overlap the upper range of calculatedmelts in equilibrium amphibole but have lower light REE/heavy REEratios. Crystallization of apatite would increase the (La/Sm)N ratio ofa melt but could not produce the most fractionated melts. Enrichmentin the total REE content is consistent with the crystallization of pla-gioclase and the pyroxenes and slight increases in light REE contentmay reflect crystallization of trace apatite and zircon. These trendsare consistent with known compositions of evolved oceanic pluton-ics, however, they do not account for the light-REE-enriched melts.Alternatively, it is plausible that amphibole crystallized from mag-matic fluids that were exsolved from a melt during the final stages ofcrystallization. Fluid inclusion data show that H2O-CO2-NaCl-±Fe-rich fluids were present in the most evolved intervals of the Hole894G core (Kelley and Malpas, this volume), where light-REE-en-riched amphibole has been identified. As these fluids were trapped attemperatures between 300°C and 600°C, it may be predicted that am-phibole crystallized from cooled magmatic fluids, thus accountingfor the major element composition of amphibole. Based on the resultsof this and other studies of the Hole 894G core, there was a continu-um between magmatic and hydrothermal processes. Groundmassamphibole records a complex history of crystallization from anevolved melt and its exsolved magmatic fluids, followed by interac-tion with seawater-derived hydrothermal fluids.

ACKNOWLEDGMENTS

I would like to thank the many colleagues and friends who con-tributed to this paper. The Sedco and ODP crews of JOIDES Resolu-tion, C. Mével, my co-chief scientist, and J. Allan, our staff scientist,made Leg 147 memorable and successful. Discussions with H. Dick,J. Malpas, R. Pedersen, D. Kelley, N. Shimizu, J. Natland, and, inparticular, P. Meyer were invaluable. N. Shimizu and K. Burrhus pro-vided guidance in the collection of the trace element data; H. Dick, J.

70

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

Malpas, R. Pedersen, D. Kelley, and C. Manning shared samples and/or data; N. Chatterjee assisted with the microprobe analyses at MIT;and K. Sapp provided support in many aspects of this project. T. Sis-son is gratefully acknowledged for providing a preprint with theworking curves and distribution coefficients that were used in this pa-per.

REFERENCES

Adam, J., Green, T.H., and Sie, S.H., 1993. Proton microprobe determinedpartitioning of Rb, Sr, Ba, Y, Zr, Nb, and Ta between experimentally pro-duced amphiboles and silicate melts with variable F content. Chem.Geoi, 109:29-49.

Aldiss, D.T., 1981. Plagiogranites from the ocean crust and ophiolites. Na-ture, 289:577-578.

Anders, E., and Grevesse, N., 1989. Abundances of the elements: meteoriticand solar. Geochim. Cosmochim. Acta, 53:197-214.

Burnham, C.W., 1979. Magma and hydrothermal fluids. In Barnes, H.L.(Ed.), Geochemistry of Hydrothermal Ore Deposits: New York (Wiley),71-133.

Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., Edmond,J.M., Lawrence, J.R., Casey, J.F., Thompson, G., Humphris, S., Rona,P.A., and Karson, J.A., 1988. Chemistry of hot springs on the Mid-Atlan-tic Ridge. Nature, 335:514-519.

Dixon, S., and Rutherford, MJ., 1979. Plagiogranites as late-stage immisci-ble liquids in ophiolite and mid-ocean ridge suites: an experimentalstudy. Earth Planet. Sci. Lett, 45:45-60.

Elderfield, H., and Greaves, MJ., 1982. The rare earth elements in seawater.Nature, 296:214-219.

Gillis, K.M., Thompson, G., and Kelley, D.S., 1993. A view of the lowercrustal component of hydrothermal systems at the Mid-Atlantic Ridge. J.Geophys. Res., 98:19597-19619.

Grandjean, P., 1989. Les terres rares et la composition isotopique duneodyme dans les phosphates biogenes: tracers des processus paleo-oceanographiques et sedimentaires [Ph.D. thesis]. Univ. Nancy, France.

Green, T.H., and Pearson, NJ., 1985. Experimental determination of REEpartition coefficients between amphibole and basaltic to andesitic liquidsat high pressure. Geochim. Cosmochim. Acta, 49:1465-1468.

Holloway, J.R., and Burnham, C.W., 1972. Melting relations of basalt withequilibrium water pressure less than total pressure. J. Petrol, 13:1-29.

Humphris, S., 1984. The mobility of rare earth elements in the crust. InHenderson, P. (Ed.), Rare Earth Element Geochemistry: Amsterdam(Elsevier), 317-342.

Irving, A J., and Frey, F.A., 1984. Trace element abundances in megacrystsand their host basalt: constraints on partition coefficients and megacrystgenesis. Geochim. Cosmochim. Acta, 48:1201-1221.

Johnson, K.T.M., and Dick, H.J.B., 1992. Open system melting and temporaland spatial variation of peridotite and basalt at the Atlantis II FractureZone. J. Geophys. Res., 97:9219-9241.

Johnson, K.T.M., Dick, H.J.B., and Shimizu, N., 1990. Melting in the ocean-ic upper mantle: an ion microprobe study of diopsides in abyssal peridot-ites. J. Geophys. Res., 95:2661-2678.

Leake, B.E., 1978. Nomenclature of amphiboles. Am. Mineral., 63:1023-1052.

Lindsley, D.H., 1983. Pyroxene thermometry. Am. Mineral., 68:477-493.Lindsley, D.H., and Andersen, D.J., 1983. A two-pyroxene thermometer. J.

Geophys. Res., 88(Suppl.):A887-A906.Liou, J.G., Kuniyoshi, S., and Ito, K., 1974. Experimental studies of the

phase relations between greenschist and amphibolite in a basaltic system.Am. J. Sci., 274:613-632.

Manning, C.E., and Bird, D.K., 1986. Hydrothermal clinopyroxenes of theSkaergaard intrusion. Contrib. Mineral. Petrol., 92:437-447.

Mével, C , 1987. Evolution of oceanic gabbros from DSDP Leg 82: influenceof the fluid phase on metamorphic crystallizations. Earth Planet. Sci.Lett., 83:67-79.

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Nicolas, A., and Coleman, R.J. (Eds.), Ophiolite Genesis and Evolutionof the Oceanic Lithosphere. Petrol. Struct. Geol., 5:293-312.

Meyer, P.S., and Shibata, T., 1990. Complex zoning in Plagioclase feldsparsfrom ODP Site 648. In Detrick, R., Honnorez, J., Bryan, W.B., Juteau, T.,et al., Proc. ODP, Sci. Results, 106/109: College Station, TX (OceanDrilling Program), 123-142.

Moody, J.B., Meyer, D., and Jenkins, J.E., 1983. Experimental characteriza-tion of the greenschist/amphibolite boundary in mafic systems. Am. J.Sci., 283:48-92.

Pedersen, R.B., 1986. The nature and significance of magma chamber mar-gins in ophiolites: examples from the Norwegian Caledonides. EarthPlanet. Sci. Lett., 77:100-112.

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Watson, E.B., and Green, T.H., 1981. Apatite/liquid partition coefficients forthe rare earth elements and strontium. Earth Planet. Sci. Lett., 56:405-421.

Wones, D.R., and Gilbert, M.C., 1982. Amphiboles in the igneous environ-ment: introduction. In Veblen, D.R., and Ribbe, P.H. (Eds.), Amphiboles:Petrology and Experimental Phase Relations. Rev. Mineral., 9B:355-390.

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Date of initial receipt: 25 August 1994Date of acceptance: 2 March 1995Ms 147SR-012

71

K.M. GILLIS

1

: A

/V

Y

1 1 1

Apatite

1 1

1 o 147-894G-17R-1

D 147-894G-17R-1

- _ * _ .147-894G-8R-1,

_ . Λ 147-894G-12R-1

1 1

, 23-25 cm, Piece 6 (4)

, 23-25 cm, Piece 6 (5)

13-15 cm, Piece 2 (12)

117-125 cm, Piece 12D (17) "

~" - -o

I I I

Nd Sm Dy Er Yb

104

I I I I I I I I I

B Zircon (147-894G-9R-3, 71-76 cm, Piece 5C)

I I I I I I I I I I I I0.1

La CΘ Nd Sm Dy Er Yb

Figure 5. Chondrite-normalized REE profiles of the accessory phases apatite and zircon. Analysis numbers are listed in parentheses.

72

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

104

1000

104

1000

100

10

1000

100

I I ] I \ I I I I

-Sample 147-894G-12R-1, 117-125 cm, Piece 12DSample 147-894G-9R-1, 71-76 cm Piece 5C

La Ce Nd Sm Gd Dy Er

\ i i rB

-Sample 147-894G-12R-1, 117-125 cm, Piece 12DSample 147-894G-9R-1, 71-76 cm Piece 5C

La Ce Nd Sm Gd Dy Er

i i r i i i i i i i i i

-Tholeiitic andesite melt (dry)Andesite (5% H,O)

La Ce Nd Sm Gd Dy Er

Figure 6. Calculated melt compositions in equilibrium with amphibole from Samples 147-894G-9R-1, 71-76 cm (Piece 5C), and 12R-1, 117-125 cm (Piece12D). A. Basaltic andesite melts in equilibrium with amphibole. B. Dacite melts in equilibrium with amphibole. C. Calculated melt compositions in equilibriumwith apatite from Samples 147-894G-8R-1, 12-15 cm (Piece 2), 12R-1, 117-125 cm (Piece 12D), and 17R-1, 23-25 cm (Piece 6).

73

K.M. GILLIS

1000

100

10

_ . o Oxide gabbronorite (9R-3)_ g_ -Tonalite vein (Alvin 2218-1210)_ _<£ _ . Trondhjemite vein (735B, 53R-5, 40-43 cm)

EPR basalts (1-3°N)Site 894 gabbroic rocks

La Ce Nd Sm Eu Gd Dy Er Yb

Figure 7. Chondrite-normalized REE profiles of bulk

rocks showing the range in composition of gab-

bronorites from Hole 894G (Pedersen et al., this vol-

ume), EPR volcanics (N. Blum, pers. comm, 1992), and

felsic veins associated with oceanic plutonics (tonalite

vein from J. Natland, pers. comm, 1992; and

trondhjemite vein from J. Hertogen, pers. comm, 1993).

APPENDIX

Proportion of Phases and Average Mineral Compositions for Each Ion Microprobe Analysis

Analysisno. Mineral Comments Phases

100% amp100% amp30% amp100% amp100% amp100% amp100% amp<5% amp100% cpx20% amp

100% cpx100% cpx<5% amp<5% amp30% high-Al amp; 70% low-Al amp100% amp100% amp

100% amp100% amp100% amp100% amp

100% amp100% amp100% amp100% amp100% amp100% amp100% amp

Cpx(Wo)

—0.39————0.4

0.410.42

0.410.370.4

0.42——

———

——————

Cpx(Mg#)

—0.66————

0.690.70.71

0.720.70.710.72——

———

——————

Amp(AIIV)

0.690.67—0.9

0.420.410.640.26—

0.81

——1.19

1.0/0.650.570.8

0.210.210.180.22

0.460.480.280.220.540.610.18

Amp(Mg#)

0.6360.67—

0.710.650.660.660.71—

0.69

———

0.660.68/0.76

0.710.76

0.650.650.630.64

0.650.650.640.630.540.6

0.65

147-894G-6R-1, 112-116 cm (Piece 10):1 Amp Rim cpx (3)2 Amp Rim cpx (3)3 Cpx4 Amp Rim cpx (3)6 Amp Isolated grain, same grain as (7,8)7 Amp Isolated grain, same grain as (7,8)8 Amp Isolated grain, same grain as (7,9)9 Cpx Same grain as (9,10)10 Cpx Same grain as (9,11)11 Cpx Same grain as (9,10)

147-894G-8R-1, 13-15 cm (Piece 2)1 Cpx Same grain as (2,3)2 Cpx Same grain as (1,3)3 Cpx Same grain as (1,2)5 Cpx7 Amp Rim cpx, same grain as (8), next to apatite (12)S Amp Rim cpx, same grain as (7), next to apatite (l 2)10 Amp Isolated grain12 Apatite Next to amphibole (7,8)

147-894G-9R-3, 51-54 cm (Piece 4):1 Amp Groundmass clot2 Amp Groundmass clot3 Amp Groundmass clot4 Amp Groundmass clot

147-894G-9R-3, 70-76 cm (Piece 5C):Groundmass clotGroundmass clotIsolated grain, same grain as (5)Isolated grain, same grain as (4)Isolated grain, same grain as (7)Isolated grain, same grain as (8)Isolated grain

Isolated grain, next to zircon

147-894G-9R-3, 70-76 cm (Piece 5C):14 Amp Isolated grain, next to zircon15 Amp Isolated grain, next to zircon

147-894G-12R-1, 117-125 cm (Piece 12D):1 Amp Rim cpx, same grain as (2,3,6,16)2 Amp Rim cpx, same grain as (1,3,6,16)3 Amp Rim cpx, same grain as (1,2,6,16)

12456789I0111213

AmpAmpAmpAmpAmpAmpAmpZirconZirconAmpZirconZircon

100% amp

100% amp100% amp

High-Al amp with >50% low-Al ampHigh-Al amp with <10% low-Al amp50% high Al-amp, 50% low Al-amp

0.130.2

0.81/0.190.8/0.13

0.68

0.670.67

0.61/0.640.72/0.63

74

REE CONSTRAINTS ON AMPHIBOLE ORIGINS

APPENDIX (continued).

Analysisno.

6791011121314151617

Mineral

AmpAmpAmpAmpAmpAmpAmpCpxCpxAmpApatite

Comments

Rim cpx, same grain as (1,2,316)Groundmass clotIsolated grainGroundmass clotGroundmass clotGroundmass clotGroundmass clotSame grain as (15)Same grain as (14)Rim cpx, same grain as (1,2,3,6)

147-894G-12R-3, 112-115 cm (Piece 7A):2345678

CpxAmpAmpAmpAmpAmpAmp

Groundmass clotGroundmass clotRim cpx, same grain as (6,7,8)Rim cpx, same grain as (5,7,8)Rim cpx, same grain as (5,6,8)Rim cpx, same grain as (5,6,7)

147-894G-12R-5, 50-59 cm (Piece 7B):1234578

AmpAmpAmpAmpAmpCpxCpx

Groundmass clot, same grain as (2)Groundmass clot, same grain as (1)Groundmass clotGroundmass clotGroundmass clotSame grain as (8)Same grain as (7)

147-894G-17R-1, 23-27 cm (Piece 7):123

AmpAmpAmp

Rim cpx; same grain as (2), next to apatite (9,10)Rim cpx; same grain as (1), next to apatite (9,10)Groundmass clot

147-894G-12R-5, 50-59 cm (Piece 7B):4567

910

AmpAmpAmpAmpAmpApatiteApatite

147-894G-17R-1 ,81-É123456789101415

AmpAmpAmpCpxCpxCpxCpxCpxCpxCpxAmpAmp

Groundmass clotRim cpx, same grain as (6,7,8)Rim cpx, same grain as (5,7,8)Rim cpx, same grain as (5,6,8)Rim cpx, same grain as (5,6,7)Same grain as (10)Same grain as (9)

>6 cm (Piece 11):Rim cpx (4,5), same grain as (2,3)Rim cpx (4,5), same grain as (1,3)Rim cpx (4,5), same grain as (1,2)Same grain as (5)Same grain as (4)Same grain as (7,9)Same grain as (6,9)Same grain as (6,8)Same grain as (10)Same grain as (9)Rim cpx, same grain as (15)Rim cpx, same grain as (14)

147-894G-18R-1, 2-3 cm (Piece 1):12346

1213

CpxCpxAmpCpxCpxCpxAmpCpx

Same grain as (2)Same grain as (1)Rim cpx (4,6)Same grain as (6)Same grain as (5)Same grain as (13)Rim cpx (4,6)Same grain as (8)

Phases

100% amp50% high Al-amp, 50% low-Al amp100% amp100% amp100% amp100% amp100% amp100%cpx100% cpx

100% cpx100% amp100% amp100% amp100% amp100% amp100% amp

70% high-Al amp, 30% low-Al amp100% amp100% amp100% amp100% amp100% cpx100% cpx

100% amp100% amp100% amp

<5% low Al-amp100% amp50% high-Al amp, 50% low-Al amp100% amp100% amp

100% amp100% amp100% amp20% amp<5% amp100% cpx-10% amp-10% amp100% cpx100% cpx< 10% cpx

<5% amp<5% amp100% amp-10% amp100%cpx100% cpx100% amp<20% amp

Cpx(Wo)

———————

0.410.41

0.4——————

—————

0.420.4

———

—————

———

0.420.410.430.450.420.380.390.47—

0.420.41—

0.460.460.48—

0.46

Cpx(Mg#)

———————

0.720.72

0.72——————

—————0.70.7

———

—————

———

0.720.710.710.720.710.710.710.75—

0.750.75—

0.780.770.9—0.8

Amp(AP)

1.2/0.240.89

0.6/0.020.360.390.250.680.73—

0.331.06/0.64

0.670.861.021.05

1.07/022

0.67/0.280.39

0.780.74—

0.720.350.6

0.3/0.030.37

0.47/0.040.280.22

0.790.690.760.42——————

0.460.63

0.970.291.08———1.7

0.83

Amp(Mg#)

0.65/0.630.64

0.53/0.760.710.740.760.70.64—

0.780.66/0.64

0.660.740.720.7

0.65/0.72

0.66/0.680.66

0.670.67—

0.710.720.55

0.5/0.730.61

0.56/0.630.660.63

0.670.690.660.84——————

0.670.72

0.720.80.74———

0.710.75

Notes: Proportions of phases were determined from backscattered electron images, cpx = clinopyroxene, Wo = wollasonite content in mol%, and Amp = amphibole. Dash(—) = not present. Wo, Mg#, and Allv are average values. Average values of high- and low-Al amphibole are separated by a slash (/).

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