60. THREE GABBROS FROM DSDP LEG 37, SITE 334: THEIR PETROGRAPHYAND PYROXENE MINERALOGY

R.E. Hill, Division of Mineralogy, CSIRO, Floreat Park, Western Australia

INTRODUCTION

Three samples of gabbro drilled from the ocean floorat Site 334 are described in this report. The sampleidentifications and exact positions in the core are listedin Table 1, together with the weights of the samples ob-tained and their specific gravities (S.G.).

Similar mineralogy characterizes the gabbros fromCores 21 and 23. These samples consist essentially ofplagioclase feldspar, clinopyroxene, and orthopyrox-ene. Partial hydrothermal alteration of clinopyroxeneto pale green actinolitic amphibole has occurred inSample 23-1, 113-119 cm. The pyroxenes from bothsamples exhibit well-developed exsolution textures andhave compositions (see later) indicative of relativelyhigh temperatures of crystallization.

Late-stage magmatic green-brown hornblende,prehnite, chlorite, and tremolite characterize the heavi-ly altered pyroxene- and plagioclase-bearing gabbrofrom Core 22, which, as a result, has a specific gravitylower than the other samples (Table 1).

Core

212223

Section

111

TABLE 1Sample Identification

Sampled At(cm)

52-585-6 (#1)113-119 (#12)

Sample

HIHIHI

wt.(g)

25.512.833.07

S.G.(g/cc)

3.062'.842.94

Because detailed spatial, petrographic, and min-eralogical relationships among the three gabbros arenot known to this author, comparative petrologicalrelationships cannot be discussed in detail. However,limited comparisons are made in an attempt to throwsome light onto certain aspects of the crystallizationhistories of the rocks as reflected by the pyroxenephases. Also, comparisons are made between thepyroxenes from the Site 334 gabbros and those fromsimilar rocks, particularly those from continentallayered intrusives.

PETROGRAPHY

Sample 21-1, 52-58 cm; Hypersthene GabbroThis rock is a coarse-grained holocrystalline gabbro

with a hypidiomorphic granular texture and is com-posed essentially of plagioclase feldspar (38%),orthopyroxene (22%), and clinopyroxene (40%).

Plagioclase feldspar occurs as subhedral to anhedraltabular to equant crystals up to 3 mm in size. Complex

albite, Carlsbad, and pericline twinning are ubiquitous.The crystals are strained as evidenced by shadowy ex-tinction and by distorted and fractured twin lamellae.They are subtly normally zoned with compositionsranging from Anss to Anro (electron microprobe).

Bronzite orthopyroxene is present as subhedral toanhedral grains with a maximum size of 4 mm. Thecrystals exhibit faint pleochroism from pale pink togreen and are moderately cleaved and fractured. Theycommonly include small subhedra of twinnedplagioclase and partly mantle grains of clinopyroxene(Plate 1, Figure 1). Extremely fine, closely spaced ex-solution lamellae of the Bushveld type, parallel to (100)of the host are observable in thin section in grains ofsuitable orientation. In other grains this exsolution isobserved as a fine diffuse lamellar texture parallel to(100) when viewed under crossed nicols, particularlynear extinction.

Augitic clinopyroxene, with an average size of 4 mm,generally closely resembles orthopyroxene in itsmorphology. It also occurs as subordinate large tabularsubhedral phenocrysts up to 1 cm long. All grains con-tain exsolution lamellae of Ca-poor pyroxene alignedparallel to (100) and (001) (Plate 1, Figure 2). The (001)lamellae are larger, reaching 5 µm in width. The (100)lamellae predominate and are extremely fine and close-ly spaced. The crystals are well cleaved and are com-monly twinned about the (100) plane (Plate 1, Figure3). The compositions of both pyroxenes are listed inTable 2 and discussed later in this report.

Sample 22-1, 5-6 cm; Hydrothermally AlteredHornblende Gabbro

The rock is an extensively altered, coarse-grained,hornblende gabbro consisting of plagioclase, clino-pyroxene, amphibole, chlorite, and prehnite. It exhibitsa pronounced cataclastic texture.

The plagioclase feldspar is present as large (up to 1cm) fractured subhedral to anhedral crystals, withabraded and sutured margins. The crystals exhibitevidence of extensive strain, being characterized bypronounced undulatory extinctions and bent and frac-tured twin lamellae. Twinning is not ubiquitous but,where present, is quite complex. Smaller twinnedanhedra of plagioclase occur as granular mosaics in theinterstices between larger abraded feldspars, togetherwith grains of altered clinopyroxene and blocky sec-ondary amphibole (Plate 1, Figure 4). The large feld-spar crystals commonly contain many trains of fluid in-clusions and are often associated with and partly al-tered to fine-grained prehnite.

Clinopyroxene, containing visible exsolutionlamellae of Ca-poor pyroxene aligned parallel to the

763

R. E. HILL

(100) plane, occurs as ragged anhedral remnants up to 1cm long and is extensively replaced by a mixture of palegreen pleochroic, fibrous amphibole and darker green-brown chlorite. The heavily altered clinopyroxene iscommonly mantled by pleochroic dark green-brown,late magmatic hornblende which also occurs as finergrained blocky, occasionally twinned, subhedra fillingfractures and occurring in the interstices between thecoarser crystalline phases. Some anhedral felted massesof very pale green fibrous amphibole are thought to bepseudomorphous after primary orthopyroxene,although no fresh orthopyroxene was observed in thinsection.

Prehnitization has been extensive. Veins of prehniteup to 0.3 mm in diameter cut all other minerals, andlarge patches, presumably once feldspar, are now oc-cupied by an equigranular mosaic of this prehnite. Theprehnite is commonly accompanied by brownish non-pleochroic chlorite. Very fine grained acicular tremolite(X-ray identification) also occurs as a secondary phase,filling vugs within fractures accompanying the chloriteand prehnite.

Sample 23-1, 113-119 cm; Hypersthene Gabbro

The rock is a coarse-grained holocrystalline gabbrowith hypidiomorphic granular texture consisting ofplagioclase feldspar (52%), clinopyroxene (20%),orthopyroxene (20%), and amphibole (8%).

The plagioclas.e feldspar occurs as subhedral toanhedral crystals up to 3 mm in size. The crystals arevery slightly normally zoned with compositions rangingfrom An78 to An75 (electron microprobe). They exhibitcomplex twinning and are characterized by sutured andabraded grain boundaries, bent and fractured twinlamellae, and stress-induced undulatory extinction.

Hypersthene is present as large anhedral crystals upto 6 mm across, slightly pleochroic from pale pink togreen. Both Bushveld and Stillwater types of ex-solution of clinopyroxene are ubiquitous. Very fine,closely spaced exsolution lamellae of the Bushveld typeoccur aligned in planes parallel to the (100) plane of thehost, and more primitive widely spaced vermiformstringers and patches of Stillwater-type clinopyroxeneup to 20 µm wide are crudely aligned parallel to (001) insome crystals, and in others are seemingly random(Plate 2, Figures, 1, 2). Cataclastic abrasion of thecrystals is evidenced in places by their irregularmargins. Limonite staining is visible on fractures whichtraverse the crystals.

Clinopyroxene occurs as colorless subhedral toanhedral crystals up to 8 mm across. The crystals con-tain abundant exsolution lamellae of Ca-poor pyroxenealigned in two directions, the most common being up to15 µm wide, paralleling the (001) plane of the host(Plate 2, Figures 1, 2). Some crystals include small sub-hedral to euhedral grains of plagioclase feldspar.

Augitic clinopyroxene crystals are irregularly man-tled and sometimes completely replaced by a pale greenpleochroic actinolitic amphibole. These deutericovergrowths of amphibole extend into fractures as veinfillings and cut across all other phases.

The chemical compositions of the pyroxenes are list-ed in Table 2.

CHEMICAL STUDY OF PYROXENES

Reliable quantitative electron microprobe analysesof the pyroxene phases have been obtained from therelatively fresh gabbros from Cores 21 and 23. Par-ticular grains were selected for analyses on the basis oforientation, size of lamellae, and freedom from incipi-ent alteration and fractures.

Where possible, both host and lamellae have beenanalyzed using a stationary focused electron beam. Thebulk compositons of the grains were determined using arastered focused beam.

The analysis of exsolution lamellae has been restrict-ed to those lamellae paralleling the basal plane of thehost phases, i.e. (001), since these lamellae were by farthe larger of the two varieties in all crystals. Analysesreported herein are from lamellae over 10 µm wide inorder to overcome phase boundary fluorescence effectswhich are particularly significantly for the analysis forcalcium in hypersthene lamellae (Reed and Long, 1963;Binns, et al., 1963; Boyd and Brown, 1969). No attempthas been made to correct for this effect in this study,and as a result the calcium contents presented hereinmay be slightly high (~O.l wt % Ca).

The electron microprobe used was a M.A.C. 400Smodel with a takeoff angle of 38.5°. The instrumentwas operated at 19.3 kv with sample currents of around10 nA for all analyses. Sealers were operated in thefixed-time mode, counts being made over a period of 20sec. Metal standards were used for all elements exceptaluminum, magnesium, and calcium. For these, AI2O3,MgO, and apatite, respectively, were used as stand-ards. The data were corrected using the computer pro-gram, MAGIC IV, of Colby (1971).

The analytical data, together with calculated for-mulas (cations per six oxygens) are listed in Tables 2and 3. The compositions of coexisting phases are por-trayed in Figure 1.

Both the Ca-rich and Ca-poor pyroxenes in the sam-ple from Core 21, Section 1, are more magnesian thantheir counterparts from Core 23, Section 1. This limitsdeductions concerning the petrologic and geneticrelationships between the two gabbros, suggesting thatthe gabbro from Core 21 may be the earlier differen-tiate. This also suggested by the higher anorthite con-tent of the plagioclase feldspar in the former sample(Anβs-w compared to An75-7β).

As expected, the Ca-poor pyroxenes have higherFe/Mg ratios and lower contents of alumina andtitania than the coexisting Ca-rich pyroxenes (Tables 2and 3).

The Ca-rich pyroxenes from both gabbros are augitessimilar in composition to those from a porphyriticolivine gabbro in Hawaii (Muir and Tilley, 1957) and tothe high-temperature, first-pyroxene separates from theLower Layered Series of the Jimberlana Dyke(Campbell and Borley, 1974). However, the analyzedpyroxenes contain less calcium than pyroxenes with a

764

THREE GABBROS

TABLE 2Electron Microprobe Analysesa and Ionic Formulas

SiO2

A12O3

TiO2

FeOMgOCaOCoO

NiO

Si

AlAlTiFe

MgCaCoNi

MgFe

Ca

Orthopyroxene

Bulk

54.671.360.11

13.1228.38

2.35—

-

1.96'

0.040.02^0.000.391.510.09

_

2.00

'2.01

75.8819.604.52

Host

54.971.410.08

13.2227.85

2.360.080.04

Clinopyroxene

Bulk

52.712.34

0.157.55

18.02

19.23-

-

Ionic Formula per Six Oxygens

1.97'

0.030.03^0.000.401.490.090.00

o.oo>

2.00

'2.01

75.2520.204.55

1.93

0.070.030.010.230.990.76

—

-

0.02"

0.01

o.or0.000.010.020.02

>

2.00

'2.02

50.0011.6138.38

Host

51.732.560.376.02

15.8722.88

0.280.28

1.92'

0.080.03 ;

0.010.190.88

0.910.01

0.01-

2.00

'2.04

44.449.60

45.96

aExpressed as normalized weight percent oxide. Data are for the bulkorthopyroxene and clinopyroxene phases and the host components fromthe Core 21, Section 1 gabbro sampled at 52-58 cm. Atomic proportionsof Mg, Fe, and Ca normalized to 100% are also presented. Bulk composi-tions were determined using a focused electron beam rastered over anarea 100 µm square. Standard deviations, S. are presented with selectedionic formulas.

similar Mg/Fe ratio from the Bushveld (Atkins, 1969)and Skaergaard (Brown, 1957) magmas (Figure 1).

The Ca-poor pyroxene from Core 21, Section 1 isbronzite and that from the Core 23 gabbro is pigeonite(Tables 3 and 4). The bronzite is similar in compositionto the Hawaiian and Jimberlana orthopyroxenes. Thepigeonite is similar in composition to the first pigeonitededuced to have crystallized from the Jimberlana Dyke,but is higher in both magnesium and calcium than pub-lished analyses and compositional estimates of the firstpigeonite to crystallize after the cessation of bronzite inother layered intrusions (Figure 1).

The high magnesium content of the pigeonite in-dicates a relatively high temperature of crystallizationfor the Site 334 gabbros. Compatible with this are therelatively high calcium content of the bronzite and thelow calcium content in the clinopyroxenes or morespecifically, the narrow subsolidus miscibility gap as ex-hibited by the coexisting bulk compositions in Figure 1.

The subsolidus exsolution textures displayed in thepyroxenes from each rock are summarized below.

'(001) lamellae and (100) lamellae are the terminologies used hereinto describe lamellae essentially parallel to the (001) and (100) planesof the host phase. Detailed crystallographic orientations have notbeen carried out to ascertain the exact crystallographic orientations.

1) Core 21, Section 1: augite containing (001)1 and(100)' lamellae + bronzite containing (100) lamellae.

2) Core 23, Section 1: augite containing (001) and(100) lamellae + inverted pigeonite containing (001)and (100) lamellae.

The presence of basal plane (001) exsolution lamellaein both of the Mg-rich augites suggests that theselamellae exsolved as the monoclinic phase pigeonite,and that crystallization of these pyroxenes took placeabove the monoclinic-orthorhombic inversion curve(Poldervaart and Hess, 1951; Hess, 1960). A corollaryto this is that the expected Ca-poor pyroxene crystalliz-ing in conjunction with the augites should also bepigeonitic.

These conclusions are not negated by the augite-pigeonite pair of the Core 23 gabbro. As implied, sub-solidus cooling of the Core 23 gabbro has resulted inthe inversion of the pigeonitic Ca-poor pyroxene toorthorhombic hypersthene with concomitant exsolu-tion and a decrease in its calcium content (Figure 1).The basal-plane exsolution lamellae are of high-calciumaugite similar in composition to the host component ofthe Ca-rich pyroxene. The (001) lamellae exsolved fromthe Ca-rich pyroxene are hypersthene with a composi-tion slightly more iron rich than the host of the Ca-poorphase (Figure 1, Table 3).

765

R. E. HILL

Diopside Ca{ Hedenbergite

Mg

Core 21 Section 1Core 23 Section 1Jimberiana DykeSkaergaardBushveldUwekahuna LaccolithField for early JimberlanaCa•rich Pyroxenes

FeATOMIC %

Figure 1. The coexisting Ca-rich and Ca-poor pyroxenes from the gabbros of Core 21, Section 1, 52-58 cm and Core 23,Section 1, 113-119 cm, plotted on the pyroxene quadrilateral. Compositions are plotted of bulk pyroxenes B, hosts H, andincluded lamellae L. Plotted for comparison are the early pyroxene trends from the Jimberlana (Campbell and Borley,1974); Skaergaard (Brown, 1957); and Bushveld (Atkins, 1969) intrusions. Coexisting Ca-rich and Ca-poor pyroxenes fromthe "porphyriticgabbro," Uwekahuna\ Laccolith, Kilauea (Muir and Tilley, 1957) are also plotted.

Similar subsolidus cooling relationships werepresented by Boyd et al. (1969) in their study of thecompositions of the hosts and lamellae comprisingcoexisting pyroxenes richer in iron from the BushveldIntrusion. These authors confirmed previous studies in-cluding those of Bown and Gay (1960) and Binns et al.(1963) that the exsolution lamellae aligned parallel to(001) in the Ca-rich pyroxene are now clino-hypersthene not orthorhombic hypersthene (invertedpigeonite) as was previously expected, Poldervaart andHess (1951). The consistently low calcium content of allanalyzed lamellae parallel to (001) of the augites fromthe Core 23, Section 1 gabbro (Table 3) and the lack ofdetectable secondary exsolution within the lamellaesuggest strongly that the lamellae may also beclinohypersthene which have failed to invert to theorthorhombic phase because of structural constraintsimposed by the host augite. These lamellae would havemaintained compositional equilibrium with the hostdown to relatively low temperatures by ease of ionicmigration.

In contrast to the Core 23 pyroxenes, those from theCore 21 gabbro are somewhat enigmatic. No other ex-amples of truly igneous Ca-rich pyroxenes of this com-position which contain basal plane exsolution lamellaeare known to the author, including those from the com-positionally compatible Jimberlana Dyke. All othercrystals are believed to have exsolved orthopyroxeneonly, paralleling or closely paralleling the (100)crystallographic direction, a feature compatible withcrystallization below the clinopyroxene-orthopyroxeneinversion loop. The coexisting Ca-poor pyroxene fromCore 21 is an orthorhombic bronzite containing only(100) exsolution lamellae, and there is no evidence tosuggest that it was at any stage a pigeonite. The com-positional relationships between the Ca-rich pyroxene

766

and its host component are similar to those of the Core23 gabbro. Although the basal plane exsolutionlamellae are too narrow to provide reliable analyses, apoint on the extension of the tie-line joining host andbulk compositions towards the expected compositionsof the exsolved phase lies to the iron side of the coexist-ing orthopyroxene-bulk clinopyroxene join, alsosimilar to the Core 23 gabbro (Figure 1). It is possiblethat these exsolution lamellae, like those in the Core 23gabbro, are monoclinic.

The orthorhombic bronzite contains many fine,closely spaced (100) exsolution lamellae visible only insections parallel to (010) of the host. There are toomany lamellae for all of these to be diopsidic augite incomposition, since the bronzite contains only 2.35 wt %CaO, (Table 1). Also, the average compositions of thehost component are almost identical to bulk com-positions, perhaps a function of the closeness of the ex-solution lamellae. It is suggested that the observed ex-solution texture may be the result of some subtlecalcium redistribution during structural adjustments ofthe Ca-poor pyroxene on cooling.

Coexisting Mg-rich metamorphosed pyroxenes fromthe core of an ultramafic inclusion in the BelchertownComplex, Massachusetts, comprise Ca-rich augiteswhich contain, as well as others, basal-plane lamellae ofclinopyroxene, and Ca-poor orthorhombic bronzitessimilar to those from the Core 21 gabbro, Jaffe et al.(1975). Jaffe et al. suggest two possible explanations forthis enigmatic relationship:

1) Under conditions where orthorhombic pyroxeneis stable, metastable monoclinic lamellae can nucleatein augite; and

2) Monoclinic lamellae are stable again at lowertemperatures. They favor the former explanation onstructural grounds for the Belchertown pyroxenes, and

THREE GABBROS

this is a not unlikely reason for the Core 21 pyroxenesas well.

The subtle noncolinearity of the bulk, host, andlamellae compositions for the coexisting pyroxenesfrom each rock, as plotted in Figure 1, may in part bedue to uncertainties in the microprobe analyses, butmay also suggest that interphase equilibration ceased atsome temperature after crystallization and that eachphase subsequently acted essentially as a closed systemin which intraphase exsolution and equilibration wasmaintained to lower temperatures. To what extent eachof these factors contributes to the noncolinearity is notknown. The presence of extensive exsolution, a productof slow cooling, doubtless suggests that at least somepostcrystallization subsolidus interphase reequilibra-tion must have occurred. However, for example, thedifferences in slope in Figure 1 between the tie-line join-ing the bulk compositions of the pyroxenes in the Core23 gabbro and those joining the respective host andlamellae components may reflect to some degree thetemperature interval between the cessation of inter-phase subsolidus equilibration and the cessation of ex-solution at lower temperatures. If this be so, concomi-tant changes in the distribution of iron and magnesiumbetween the two pyroxenes must be indicative of atemperature drop.

The simple ideal formula for the distribution coef-ficient KD (Kretz, 1963) where

KCa-poor/Ca-rich

D (Mg-Fe)

X MgSiθ3 1-XMgCaSi2O6

1-XMgSiθ3 XMgCaSi2O 6

and XMgSiCb and XMgCaSi2θ& are mole fractions,has been used to produce the coefficients listed in Table4.

The data listed in Table 4 are consistent with theabove discussion. The average value of KD around 0.90for the bulk pyroxenes is indicative of a hightemperature of equilibration. The lower values averag-ing around 0.69 for the host-lamellae pairs from bothrocks are compatible with relatively low temperaturesof equilibration. The value of 0.69 for the Core 21 gab-bro was determined using the compositions of the hostcomponent of the Ca-rich pyroxene and that oftheoretical hypersthene lamellae containing a similarcalcium content to the coexisting bronzite but lying onthe extension of the line joining the host and bulk Ca-rich pyroxene compositions (Figure 1).

The value of KD around 0.69 for the host-lamellaecomponents is consistent with that determined ex-

TABLE 3Electron Microprobe Analysesa and Ionic Formulas

SiO2

A12O3

TiO2

FeO

MgO

CaO

CoO

NiO

Orthopyroxene

Bulk

53.05

1.69

0.22

15.83

23.40

5.80—

Host

53.93

0.96

0.23

17.89

25.66

1.19

0.07

0.07

Lamellae

52.14

1.54

0.37

7.83

15.28

22.70

0.07

0.07

Clinopyroxene

Bulk

51.76

2.54

0.23

10.15

16.74

18.58

-

_

Host

51.49

1.91

0.41

7.87

15.50

22.82

0.00

0.00

LamellaeClinohypersthene

53.72

0.90

0.15

19.19

24.78

1.18

0.04

0.04

Ionic Formula per Six Oxygens

Si

AlAl

Ti

Fe

Mg

Ca

Co

Ni

Mg

Fe

Ca

1.94

0.06

o.or0.01

0.48

1.28

0.23_

64

24

11

2.00

'2.01

.32

.12

.56

1.97

0.03

o.or0.01

0.55

1.40

0.05

0.00

0.00

2.00

-2.02

70.00

27.50

2.50

1.94

0.06

0.01

0.01

0.24

0.85

0.91

0.00

0.00

0.01

0.01

o.or0.00

0.01

0.01

0.01

0.00

0.00.

2.00

-2.02

42.50

12.00

45.50

1.921 2.000.08J0.03

0.01

0.32

0.93

0.74

—

'2.03

46.73

16.08

37.19

1.92 | 2.000.08 J0.00^

0.01

0.25

0.86

0.91

0.00

0.00,

-2.03

42.57

12.38

45.05

1.97

0.030.01 ;

0.00

0.59

1.36

0.05

0.00

0.00,

2.00

-2.01

68.00

29.50

2.50

aExpressed as normalized weight percent oxide. Data are for the bulk Ca-poor and Ca-rich pyroxenes and theirhost and lamellae components from the Core 23, Section 1 gabbro sampled at 113-119 cm. Atomic proportionsof Mg, Fe, and Ca normalized to 100% are also presented. Bulk compositions were determined using a focusedelectron beam rastered over an area 140 µm square. Standard deviations, S, are presented with selected ionicformulas.

767

R. E. HILL

TABLE 4Distribution Coefficients KQ of Fe and Mg in

Coexisting Ca-Rich and Ca-Poor Pyroxenes

Sample Coexisting Pair

Core 21, Section 1 Bulk clinopyroxene 0.90Bulk orthopyroxeneClinopyroxene host 0.69Theoretical hypersthene lamellae

Core 23, Section 1 Bulk clinopyroxene 0.92Bulk pigeoniteClinopyroxene host 0.67Hypersthene lamellaeOrthopyroxene host 0.72Augite lamellae

perimentally by Lindsley et al. (1974) for coexisting Ca-Mg-Fe pyroxenes at 810°C. The compositions of thehost augites from both gabbros lie to the calcium-richor lower temperature side of the 810°C isotherm on thepyroxene solvus as determined by these authors. Thedata suggest, however, that exsolution continued downto at least 800°C. A more specific temperature for thecessation of exsolution and indeed an approximatefigure for the crystallization of the pyroxene cannot besuggested since variables such as pressure and composi-tion (including magma) have not been taken into ac-count and there are still marked discrepancies betweendata from natural pyroxenes and phase relation studies.

There is little doubt, however, that the Site 334 gab-bros crystallized at high temperatures and underwent aslow cooling which was continued to relatively lowtemperatures with continued extensive subsolidus ex-solution and intraphase equilibration.

REFERENCESAtkins, F.B., 1969. Pyroxenes of the Bushveld Intrusion,

South Africa: J. Petrol., v. 10, p. 222-249.

Binns, R.A., Long, J.V.P., and Reed, S.J.B., 1963. Somenaturally occurring members of the clinoenstatite-clino-ferrosilite mineral series: Nature, v. 198, p. 777-778.

Bown, M.G. and Gay, P., 1960. An X-ray study of exsolutionphenomena in the Skaergaard pyroxenes: Mineral. Mag.,v. 32, p. 379-388.

Boyd, F.R. and Brown, G.M., 1969. Electron-probe study ofexsolution in pyroxenes: Carnegie Inst. Wash. Yearbook,v. 66, p. 353-359.

Brown, G.M., 1957. Pyroxenes from the early and middlestages of fractionation of the Skaergaard intrusion, EastGreenland: Mineral. Mag., v. 31, p. 511-543.

Campbell, I.H. and Borley, G.D., 1974. The geochemistry ofpyroxenes from the lower layered series of the Jimberlanaintrusion, Western Australia: Contrib. Mineral. Petrol.,v. 47, p. 281-297.

Colby, J.W., 1971. MAGIC IV—a new improved version ofMAGIC: Sixth Natl. Conf. Electron Probe Analysis Proc.

Hess, H.H., 1960. Stillwater igneous complex, Montana:Geol. Soc. Am. Mem. 80.

Jaffe, H.W., Robinson, P., and Tracy, R.J., 1975. Orientationof pigeonite exsolution lamellae in metamorphic augite:Correlation with composition and calculated optimalphase boundaries: Am. Mineral., v. 60, p. 9-28.

Kretz, R., 1963. Distribution of magnesium and iron betweenorthopyroxene and calcic pyroxene in natural mineralassemblages: J. Geol., v. 71, p. 773-785.

Lindsley, D.H., King, Jr., H.H., and Turnock, A.C., 1974.Compositions of synthetic augite and hypersthene coex-isting at 810°C: Application to pyroxenes from lunarhighland rocks: Geophys. Res. Lett., v. 1, p. 134-136.

Muir, I.D. and Tilley, C.E., 1957. Contributions to thepetrology of Hawaiian basalts. I. The picrite-basalts ofKilauea: Am. J. Sci., v. 255, p. 241-253.

Poldervaart, A. and Hess, H.H., 1951. Pyroxenes in thecrystallization of basaltic magma: J. Geol., v. 59, p. 472-489.

Reed, S.J.B. and Long, J.V.P., 1963. Electron probe measure-ments near phase boundaries. In Pattee, H.H., Cosslett,V.E., and Engström, A. (Eds.), X-ray optics and X-raymicroanalysis: New York (Academic Press), p. 317-327.

768

R. E. HILL

PLATE 1

Figures 1-3 Hypersthene gabbro from Site 334, Core 21, Sec-tion 1, sampled at 52-58 cm.1. Large orthopyroxene grain, lower third of pic-ture, partly enclosing and occurring as rims onanhedral clinopyroxene crystals. Crossed nicols.2. Subhedral orthopyroxene, upper right (exsolu-tion lamellae not visible), and clinopyroxene, up-per and lower left, exhibiting both (100) and (001)hypersthene exsolution lamellae. Crossed nicols.3. Twinned clinopyroxene crystal containing both(100) and (001) hypersthene exsolution lamellae.Crossed nicols.

Figure 4 Hornblende gabbro from Site 334, Core 22, Sec-tion 1, sampled at 5-6 cm (#1). Twinned plagio-clase anhedra, altered clinopyroxene, and blockydeuteric amphibole forming a fine-grained inter-granular mosaic between two large abradedplagioclase crystals. Crossed nicols.

770

THREE GABBROS

PLATE 1

771

R. E. HILL

PLATE 2Hypersthene gabbro from Site 334, Core 23, Section 1,

sampled at 113-119 cm (#12).

Figure 1 A typical field of view under crossed nicols ex-hibiting anhedral clinopyroxene, upper left andlower right, partly mantled by secondary fibrousamphibole; anhedral orthopyroxene, center of pic-ture, and plagioclase feldspar. Crossed nicols.

Figure 2 Orthopyroxene with coarse (001) exsolutionlamellae, left side of picture. The (001) lamellae arenot visible in this orientation. Clinopyroxene,right side of picture, containing coarse continu-ous (001) lamellae and fine discontinuous (100)lamellae of hypersthene. Crossed nicols.

772

PLATE 2

THREE GABBROS

0.5 mm

773