REE geochemistry of oldhamite-dominated clasts from the Norton County aubrite: Igneous origin of oldhamite

Geochimica el Cosmochimica Acta Vol. 58, pp. 449-458 Copyright Q 1994 Eke&r Science Ltd. Printed in U.S.A.

0016-7037/94/$6.00

+ .OO

REE geochemistry of oldhamite-dominated clasts from the Norton County aubrite: Igneous origin of oldhamite MAYA M. WHEELOCK, ‘-*

KLAUS KEIL,‘++ CHRISTINEFLOSS,~”G. J. TAYLOR,‘STand GHISLAINECROZAZ3 ’ Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87 131, USA

*Planetary Geosciences, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA 3Earth and Planetary Sciences Department and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63 130, USA (Received February 12, 1993; accepted in revisedform June 7, 1993)

Abstract-Oldhamitedominated

lithic clasts represent

a new igneous

lithology

of tbe aubrite

parent

body. They contain single crystals of oldhamite up to 2 cm in size, with inclusions of ferromagnesian alabandite, troilite, daubreelite, caswellsilverite, and Fe,Ni metal; they are usually in intimate contact with a silicate portion consisting of enstatite, forsterite, and/or plagioclase. Textural evidence for igneous origin includes apparent primary igneous grain boundaries between oldhamite and forsterite, coarse grain size, and the presence of round, droplet-like Mn-Fe-Mg-Cr-Na sulfide inclusions within oldhamite which appear to represent an immiscible sulfide liquid. We propose that the oldhamitedominated lithology formed during the melting and fractionation of enstatite chondrite-like precursor material and represents a locally CaS-rich facies. During melting, two mutually immiscible sulfide liquids-a Ca sulfide and a Mg-Fe-Mn-Cr-Na sulfide-formed in the silicate magma. Upon cooling, the immiscible sulfides crystallized, forming large oldhamite crystals containing inclusions of Mn-Fe-Mg-Cr-Na-bearing sulfides; forsterite, enstatite, and plagioclase crystallized from the surrounding silicate melt. At subsolidus temperatures, tiny ferromagnesian alabandite crystals exsolved from oldhamite. REE abundances in oldhamite are high (about 200X CI), but REE patterns are nearly identical within single crystals and from clast to clast, indicating equilibrium conditions. High REE abundances have been cited as evidence that oldhamite grains in aubrites are nebular relics. However, we find it difficult to imagine that the rather homogeneous REE patterns of oldhamite in the oldhamite-dominated lithology of Norton County are not the result of equilibration of the REEs with a silicate melt during formation of the igneous aubrites through parent body melting, differentiation, fractionation, and cooling, where peak temperatures of around 1450-l 500°C must have been reached. We conclude that oldhamite in the oldhamite-dominated lithology of Norton County is of igneous origin and that its REE abundances were established by equilibration with the aubrite silicate melt. INTRODUCTION

Our aim in undertaking this project was to characterize as completely as possible the entire suite of igneous lithologies that are preserved in aubrites. In our search for new lithologies, we examined thousands of hand specimens of Norton County, including hundreds of previously undescribed samples acquired by the Institute of Meteoritics at the University of New Mexico in 1987 with the purchase of the La Paz Collection. Inspection revealed that coarse-grained igneous clasts several centimeters in diameter are quite common and easily recognizable in hand specimen (except when severely shocked; in such cases, polycrystalline clasts are difficult to distinguish from large single enstatite crystals, due to their glassy appearance and lack of cleavage; REID and COHEN, 1967). The majority of these clasts are pyroxenites and are described by WHEELOCK( 1990). In addition, we discovered a new sulfide-dominated lithology whose major mineral is oldhamite (CaS) ( WHEELOCKet al., 1989). About a dozen loose fragments (2 mm to several centimeters in diameter) were found, and additional centimeter-sized fragments are visible on sawn surfaces of larger Norton County specimens. Twelve polished thin sections prepared from seven of these clasts are the subject of the present study, which involves optical microscopy and electron microprobe and ion microprobe analyses (see WHEELOCK, 1990, for specific section

AUBRITES(ENSTATITEACHONDRITES)~~~ highly reduced, essentially FeO-free, brecciated enstatite pyroxenites. It is now a widely held view that they are rocks of igneous parentage which were brecciated after crystallization and solidification on their parent body (e.g., WOLF et al., 1983; OKADA et al., 1988; KEIL, 1989). Although brecciated, aubrites contain a variety of lithic clasts with well-preserved igneous textures that testify to the igneous parentage of the aubrites and to the complex fractionation and crystallization history of the aubrite parent body. In the case of Norton County, the largest and most comprehensively studied aubrite, observed clast lithologies include orthopyroxenite, pyroxenite, feldspar-silica, and, possibly, dunite clasts ( OKADA et al., 1988). These primary clast lithologies are interpreted as the products of fractional crystallization from a magma that could be modelled reasonably by the CaO-MgO-A1203-Si02 system ( TAYLOR et al., 1988; LONGHI, 1987). * Present address: Department of Earth and Planetary Sciences, The Johns Hookins Universitv. Baltimore. MD 21218. USA. t Also assocjated with the %waii Cent& for Volcanology. * Present address: Max-Planck-Institut ftir Kernphysik, D-6900 Heidelberg, Germany.

449

450

M. M. Wheelock et al.

AlternativeIy, ~s~bution coefficients may be correct, and our observed REE abundances in oldhamite may reflect local subsolidus re-equilibration (DICKINSONet al., 199 1). ANALYTICAL PROCEDURES Numerous polished thin sections (WHEELOCK, 1990) were examined optically in a petrographic microscope in transmitted and

FIG. 1. Hand specimen photographs of oldhamite-dominated, igneous clasts from the Norton County aubrite. (a) Clast Nl5707.

Dark material (center) is oldhamite covered with (pistachio-green) terrestrial weathering rind; white is forsterite and enstatite. Clast is

2.4 cm in longest dimension. (b) cleavage surface of clast L-SO. Black is (reddish chestnut-brown) fresh oldhamite; gray (pistachiogreen) and white materials on oldhamite and along oldhamite cleavages are terrestrial weathering products; translucent white grain in lower left is forsterite; other white material is enstatite. Clast is 3.5 cm in longest dimension.

reflected light. Major and minor elements were analyzed in silicates, sulfides, and metallic Fe.Ni with a JEOL 733 electron micronrobe operated at I5 keV and 20 nA beam current. A defocussed beEtmof 5-7 pm was used for the analysis of plagioclase and some caswellsilverite and oldhamite grains; a focussed beam of l-2 pm was used for all other minerals. Matrix corrections were performed using a Bence-Albeeprocedure for silicates (BENCEand ALBEE, 1968) and a Phi-Rho-Z mocedure for sulfides and metal ~POUCHOUand PICHOIR,1984a;b), with reference to natural and synthetic standards of well-known compositions. REE abundances of individual oldhamite, alabandite, and plagioclase grains, as well as of the weathering products of oldhamite, were measured in situ in carbon-coated, doubly polished thin sections with a modified Cameca IMS-3f ion microprobe. Energy hhering was used to correct for molecular inte~e~n~ (for details, see ZINNERand CROZAZ,1986a,b). Care was taken prior to ion microprobe analysis to optically select inclusion-free areas of mineral grains for analysis, in an attempt to avoid beam overlap with undesirable phases which could result in misleading data. These grains were then bombarded with an O- primary beam with currents of 2-20 nA (in rare cases, up to 30 nA). The resulting beam sputter-pits range from approximately 5-50 pm in diameter. Most measurements cycled through a mass list twenty times: the counts for each cycle were inspected individually in order to further ensure against contamination by submicroscopic inclusions or neighboring grains during analysis. To compute REE concentrations, raw counts were compared with a known standard using an appropriate sensitivity factor, and then scaled to a reference element (a major element that had been analyzed previously in the electron microprobe). For silicates, sensitivity factors determined from synthetic glass standards were used, with SiOa as a reference oxide ~ZINNERand CROZAZ, 1986b). For nonsilicates, sensitivity factors were determined using a perovskite standard ( FAHEY et al., 1987). Reference elements for nonsilicates are Ca (oldhamite and its weathering products) and Mg (ferromagnesian alabandite). Reported errors are 10 errors due to counting statistics only. Errors in absolute abundances of REEs are difficult to estimate, since ion yields for some of these minerals may differ from those of the standards used. However, ion yields for a variety of mineral standa& have been shown to vary by less than 30% (LUNDBERGet al., 1988I, and relative ion yields for REEs in those minerals are virtuallv identical.

PETROGRAPHY AND ELECTRON MICROPROBE ANALYSIS Sutfide

numbers). We show that oldhamite and associated sulfides in these clasts are not nebular relicts but are igneous in origin and probably grew to their centimeter sizes (Fig. 1) from an aubrite magma ocean. We confirm earlier results ( LAR~MER and GANAPATHY, 1987; LUNDBERG and CROZAZ, 1988; HEAVILON et al., 1989; LUNDBERG et at., 1989,1991; WHEELOCK et al., 1989, 1990, WHEELOCK, 1990, FLOSS and CROZAZ, 1990, 1991; FLOSS et al., 1990; LQDDERS and PALME, 1991; KURAT et al., 1992) that oldhamite isthe major

carrier of the REEs in enstatite meteorites. REE abundances in oldhamite clasts are in equilibrium with each other; if this reflects equilib~um with the aubrite magma, then experimentally determined sulfide liquid/sili~te liquid ~~bution coefficients are far too low (LADDERSand PALME,1989,199O; DICKINSON et al., 1990a,b,c, 1991; LODDERS et al., 1990).

Portions

Modal analysis of the sulfide portion of a typical clast of the oldhamite-dominated lithology (sample L50; thin sections UNM 930 and 943; 2000 points counted) shows it to consist of 86.6 ~01%oldhamite (CaS), with inclusions (Figs. 2-4) of 9.7 ~01%ferromagnesian alabandite [ (Mn,M~Fe)S] , 2.15 ~01% troilite (FeS), 0.85 ~01% daubreelite (Fe@&), 0.65 ~01%caswellsilverite (NaCrSz), and 0.10 ~01% metallic Fe,Ni. Oldhamite is coarsegrained, with single crystals ranging up to 2 cm in size (Fig. 1). Identification of oldhamite is confirmed by XRD analysis; measured d-spacings (in order ofdecreasing intensity: 2.842,2.011, 1.643, 1.279, 1.422, A) agree well with pubfished values. In hand specimen, oldhamite is pinkish chestnut-brown on a fresh surface but is more commonly covered by a pistachio-green weathering rind. It

Oldhamite of igneous origin in Norton County

451

weathering of oldhamite in the terrestrial environment, even in recently repolished sections; similar analytical difficulties have been reported by others, e.g., LARIMERand GANAPATHY ( 1987). Ferromagnesian alabandite is rather uniform in composition (Table 1) in different textural settings; and titanium-bearing troilite, caswellsilverite, Sibearing metallic Fe,Ni, and daubreelite have compositions within the ranges reported previously for lithologies in Norton County (e.g., KEIL and FREDRIKSSON,1963; OKADA et al., 1988). Silicate Portions The silicate portions of the oldhamite-dominated clasts consist mostly of essentially FeO-free forsterite, with some FeO-free enstatite and plagioclase. Forsterite grains are coarsegrained (typically 5-8 mm in size), rounded, and may be partially enclosed by oldhamite (Fig. 2 ) . Plagioclase usually occurs as smaller grains interstitial to forsterite and enstatite, or rimming the sulfide portion. At least one plagioclase grain contains subhedral inclusions of sulfides, including oldhamite. In some cases, plagioclase and sulfide inclusions are found in forsterite. One forsterite grain (in section UNM 944) contains “veins” of sulfides, plagioclase, and clinoenstatite. The “veins,” or linear inclusions, are up to 4 mm long and straight. The clinoenstatite resembles optically the clino-ortho inter-

FIG. 2. Photomicrograph of thin section UNM 943 (from sample L-50b) of an oldhamitedominated igneous clast in the Norton County aubrite. Gray main mass is oldhamite; black blobs are inclusions of ferromagnesian alabandite, other sulfides, and Fe,Ni metal. Triangular clear gram (upper right) and large, clear, round grain (bottom center) are forsterite. Transmitted light; vertical dimension is about 7.5 mm.

is transparent pink in thin section, is isotropic, and displays excellent cleavage along ( 100) (Figs. lb and 4). The textures of the inclusions within oldhamite vary with inclusion sizes (Figs. 2-4). Tiny euhedral alabandite crystals are ubiquitous, 2-10 pm in size, and are usually monomineralic (Fig. 4). Their crystallographic orientation appears to be controlled by the host oldhamite, so they probably formed by solid-state exsolution. Round, drop-like inclusions are commonly S- 100 pm in diameter (Fig. 3), are polymineralic and may consist of intergrowths of ferromagnesian alabandite, troilite, daubreelite, and caswellsilverite. The largest inclusions are round, elongate, or angular objects up to 5 mm long and often are enclosed incompletely by the host oldhamite grain ( Fig. 2 ) . Neither the polymineralic nor the large, elongate to angular inclusions occur along preferred crystallographic planes of the host oldhamite. This, and their droplet to elongate-angular shapes suggest that these inclusions represent immiscible sulfide liquids. It should be noted that two oldhamite-dominated clasts also contain large, discrete grains of metallic Fe,Ni (one containing schreibersite) that are l-8 mm in diameter. These grains do not occur as inclusions in oldhamite; the metallic Fe,Ni usually occurs on the boundary between the sulfide and silicate portions. Electron microprobe analyses of oldhamite consistently give low totals between about 9 1S-95 wt% (Table 1). We can only surmise that this is due to rapid oxidation and

FIG. 3. Backscattered electron SEM image of sulfide inclusions (round, bright) in oldhamite (dark gray) from an oldhamite-dominated igneous clast in the Norton County aubrite. Inclusion phases are ferromagnesian alabandite (smooth, medium gray), troilite (white), daubreelite (pale gray-white lamellae intergrown with troihte and dark caswellsilverite), caswellsilverite (darkest, with lamellar habit): plucked areas are black. Vertical dimension is 240 pm.

M. M. Wheelock et al.

452

Table 2. *venI*e.sandTB”@zsin canpositionof 4 typicalplagioclrses(column1); ramphgioclsrcinclusicms in forsmite(columm2.3); andplegioclasc“car ~boundpyu,mc~c~i(eolumn4).fmmo~~i~s cleatsin theNmtcmCountym~britc. Oxidesin wt. S; end membersin ml. %.

Oxides

1

2

3

SiO2

60.0 (59.1 - 60.6)

53.8

60.6

66.2

Al203

26.0 (25.8 - 26.1)

29.2

24.9

22.2

C&J

6.5 (6.4 - 6.6)

11.5

6.3

2.77

Na20

8.4 (8.2

4.9

8.0

9.5

K20

0.m (0.19 - 0.21)

0.28

0.26

0.33

Taal

101.1

99.7

100.11

lW.95

Ab

69.2 (68.4 - 69.5)

43.2

68.7

84.8

8.4)

4

(k

1.1 (1.0 - 1.1)

I.6

1.5

1.9

An

29.7 (29.4 - 30.5)

55.2

29.9

13.3

reverse zoning, with albite-rich cores. This plagioclase occurs in the vicinity of Ca-bearing and Na-bearing sulfides (oldhamite and caswellsilverite). It is possible that reverse zoning is a peculiar by-product of the partitioning behavior of Ca and Na between sulfide and plagioclase under reducing conditions, which has not been investigated experimentally. Weathering Effects FIG. 4. Backscatteredelectron SEM image of tiny, euhedral ferromagnesian alabandite exsolution crystals (light gray) in oldhamite [main phase, with cleavage along ( loo)] of an oldhamite-dominated igneous clast in the Norton County aubrite. Vertical dimension is 250 pm.

growths observed in enstatite from the Shallowater aubrite ( KEIL et al., 1989). Whether the clinoenstatite formed due to shock or by rapid quenching from the protoenstatite stability field is uncertain, Although the sample displays shock effects, these linear inclusions occur in relatively unshocked regions of the forsterite crystal, are not related to breccia zones, and, thus, do not appear to be shock veins. Most plagioclase grains are uniform in composition and vary between Ab66_r0(Table 2). Extreme compositions range from Ab4, (in an inclusion in forsterite) to Abss (near a plagioclase-metal grain boundary). Several grains exhibit slight

Alabandite

Oldhamite weathers readily in the terrestrial environment, and all oldhamite-dominated clasts have portions that have suffered moderate to severe weathering. There are two distinct occurrences of weathering products ( WHEELOCKet al., 1990). One, probably vaterite, occurs as a spherulitic rind coating the surfaces of every clast and apparently grew from materials transported in solution (OK ADA et al., 198 1). In the other occurrence, an intergrowth of blade-shaped minerals (probably bassanite and/or portlandite) replaced oldhamite in situ. In addition, mixtures of these weathering products occur as transparent white or yellow-green veins along cleavage surfaces in oldhamite, on fractures, and on grain boundaries. Clear, microcrystalline material also fills euhedral cavities in oldhamite. Energy dispersive X-ray analysis identified Ca and S in varying proportions; some materials visibly degrade under electron bombardment. These observations are consistent with the tentative identification of portlandite, vaterite, and bassanite (OK ADA et al., 198 1). Despite weathering, all clasts

with oldhamite larger than about 2 mm in size contain cores of fresh oldhamite. The original textures and grain contacts in some clasts are obscured by the weathering products of oldhamite that invaded grain boundaries and by the fact that some sulfide and metallic Fe,Ni inclusions were plucked during thin section preparation. Nevertheless, considering all the clasts and sections studied, unambiguous primary grain contacts have been confirmed between all of the above mineral phases.

EklO&ilt

Oldtmmltc

Mg

0.53 (0.44-0.60)

13.4 (12.4-16.2)

ca

51.6 (51.0-52.2)

0.35 (0.16-0.89)

c!r

n.d.

0.13 (0.04-0.47)

Mn

0.73 (0.59-0.87)

28.1 (26.9-30.9)

Shock Effects

Fc

n.d.

15.4 (9.8-17.1)

s

39.8 (38.5-41.6)

42.9 (40.5-44.5)

Taal

92.7

lW.28

The oldhamite-dominated clasts exhibit a variety of shock features. Section UNM 944 may represent the most severely shocked clast. Shock effects are heterogeneously distributed, with some areas showing no effects; i.e., they have olivine

n.d. = notdnencd

Oldhamite of igneous origin in Norton County

in oldhamite have not been calibrated; however, the suite of shock effects in olivine suggests peak shock pressures of the order of 18-25 GPa ( ST~FFLER et al., 1988).

with uniform optical extinction and undisturbed regions in the sulfide portion. Shock effects in the sulfides include displacements along cleavage (as evidenced by truncated or displaced sulfide inclusions), kink bands (in which oldhamite has been rotated with respect to the unshocked portion), and fractures. In forsterite, unshocked areas grade into breccia zones, where forsterite has been fragmented in situ, with fragments rotated up to 20 degrees (in the plane of the section). The breccia zones are bordered by sulfide veins which are apparent products of incipient shock melting. One breccia zone in forsterite extends into a highly disturbed shear zone in the neighboring oldhamite, along which a ferromagnesian alabandite inclusion, has been displaced. There is no evidence of shock-melted silicates, mosaicized forsterite, or general blackening by remobilized sulfides. The presence of clinoenstatite may or may not be attributable to shock. Shock effects

I

I

I

01

La Ce Pr Nd

I

I

I

I

I

I

I

I

I

I

I

ION MICROPROBE

ANALYSIS

REE abundances of oldhamite, ferromagnesian alabandite, and plagioclase from the oldhamite-dominated clasts were measured, REEs are near or below detection limits (about lo-20 ppb) in troilite, daubreelite, caswellsilverite, metallic Fe,Ni, enstatite, and forsterite. Oldhamite crystals have flat to bow-shaped REE patterns, with abundances of about 200 X CI and with negative Eu anomalies (Fig. 5a,b, Table 3). A series of four measurements of the entire suite of REEs on a 2-cm-sized oldhamite single crystal of clast L-50, from spots ranging from the edge of the

I

L

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

I

L

I

1

La Cs Pr Nd

1

I(

II

0’0

I

I

I3

I

Sm Eu Gd Tb Dy Ho Er Tm Y% Lu

1.i (d)

(a

100

100 0 \

453

t

100

I0.01 La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

FIG. 5. REE abundance patterns (relative to Cl; PALME et al., 198I) of minerals in oldhamite-dominated igneous clasts of the Norton County aubrite. (a) Analyses of four different spots on a 2-cm-sized single oldhamite crystal (clast L-SO, section UNM 943). (b) Average analyses of one to four spots of oldhamite from seven different clasts (L-50, UNM 943; NC-15707, UNM 981; NC-161 15, UNM 982; NC-17001, UNM 984; NC-17002, UNM 985; NC-17003, UNM 986; NC-17004, UNM 988). (c) Three analyses of ferromagnesian alabandite from two different clasts (clast L-50, UNM 943, points connected by lines; clast NC- 1700 I, UNM 990, points not connected by lines). (d) Plagicclase (clast NC-17001, UNM 990).

M. M. Wheelock et al.

454 T&e3 REE 13 analyses

con&nfs of typical r&lhamia

@ohxml4),

grains fcolumns I-3). average and ranges of

and ermr range (10, from cotmfing siaristics only) for lowest

and highest vahws (cohmm 5). for crystals in the oh%amitedominated

lithology of the

Norton County aubrite (in ppm).

La

Table 4. REE contents average

and ranges

of typical

fe~mngn~i~

of 5 analyses

(cohmm 4).

alabandifc

grains (columns

and error range (lo.

l-3).

from counting

rtatwtics only) for lowest and highest values (column 5). for grains in the oldhamitedominated lithology of the Norton County a&rite (in ppb:

= hclow detection limit).

coiumn

I

2

3

4

5

Sample

NCl7001

L50

NC17WI

NC17004

I33

NC17001

PTSU

UNM-990

UNM-943

UNW986

UNM-988

UNW943

wit@90

SW

AlALAB

A5ALAB6

AZALAEI

A2Gw12

AKXDg

AlGW9

Ia

II

78

54(11-78)

4.2

23.3

ID0

207

170 (65 - 309)

5.8

32.7

31

3301-56)

2.0

8.4

217

159 (33.347)

3.8

23.9

23

86

42(12-86)

3.4

17.2

2

9

9

1.4

8.9

65

125

75(12 - 1%)

4.5

15.8

32

43

31(12-43)

1.7

4.7

Elemnt

41.2

41.7

43.4

44.3 (39.0 - 55.6)

0.8 - 2.R

ce

135.2

150.1

146.8

154(128-189)

2.5 - 6.3

PI

21.5

21.9

24.0

23.1 (18.1

0.8 - 1.5

Nd

2.1 - 4.7

Sm

2.1 - 4.4

Eu

0.2

30.2) 169)

103 12

117.2

135.1

140.2

135 (109

46.2

36.5

52.0

46 (38

4.5

3.1

4.1

4.0 (3.4

0.5

Gd

46.1

40.7

39 .o

42 (27 _ 54)

2.8 - 5.1

l%

8.0

6.8

9.0

7.8 (3.4

0.5 - 1.3

lb

50

27s

340

222 (.sO _ 340)

4.4

15.1

56.5

53.4

60.5

56(26-71)

1.4 - 3.1

Ho

20

93

101

70120.101)

2.4

7.8

57) 4.5)

10.2)

15.8)

16

12.3

12.0

13.0

12.1 (5.4

0.4 ^ 1.2

Er

130

320

365

294 (130 -365)

8.0 - 20.2

33.3

29.0

34.7

28.7 (15.7 - 36.0)

0.8 - 19

Tm

48

154

97

LOO(48 -154)

4.7

5.0

4.3

5.0

4.8 (2.7

0.3 - 0.5

Yb

615

1188

916

732(106-

30.6

31.4

26.8

35.7

31(21-

1.4 - 2.4

IA

106

272

178

182 (106 - 272)

3.9

3.4

4.3

4.2 (3.5

6.0) 36) 5.4)

1188)

9.1

7.7 47.2 14.6

0.3 - 0.8

oldhamite crystal to its interior, show very little variability (Fig. 5a), indicating that even very large single crystals are homogeneous and unzoned in REEs (as well as in major and minor elements, as demonstrated by electron microprobe analyses). Analyses of oldhamite in seven distinct clasts indicate that variability between clasts is small as well (Fig. 5b; Table 3 ) , although there is a slightly larger range in REEs in oldhamite from different clasts than there is within a single oldhamite crystal (Fig. 5a,b). Several patterns show a slight depletion in the HREEs; this effect is most distinct in the somewhat anomalous pattern of clast NC- 16I I5 (Fig. Sb). Ferromagnesian alabandite inclusions in oldhamite also exhibit negative Eu anomalies, but they have much lower REE abundances than oldhamite and pronounced HREE enrichment trends (Fig. 5c; Table 4). As expected, plagioclase exhibits a large positive Eu anomaIy (Fig. 5d; Table 5). In fact, plagiocIase is the only mineral in Norton County with measurable REEs that does not have a pronounced negative Eu anomaly. It also is the only mineral to show a slight LREE enrichment trend (Fig. 5d; Table 5); HREE abundances are below detection limits. Since oldhamite is unstable in the terrestrial environment and weathers readily (OKADA et al., 198 I ), fresh oldhamite in enstatite achondrites and enstatite chondrites is relatively rare, and ion microprobe analyses of REEs of weathered oldhamite patches have been reported in the literature ( HEAVILON et al., 1989; LUNDBERGet al., 1989). Measurement of REEs in fresh oldhamite cores and associated weathered rims shows that, ~though the pattern shapes of weathered regions are always the same as those of the associated unweathered oldhamite, the REE abundances may differ significantly (FLOSS et al., 1990; FLOSS and CROZAZ, 1993). Norton

County, however, is an observed fall and suffered a relatively mild terrestrial weathering history (OK ADA et al., 198 1). We report here REE abundances measured in a single sample of fresh, unweathered oldhamite and in the two types of weathering products described above, namely bIaded minerals that grew to replace oldhamite in situ and a spherulitic weathering rind (probably vaterite) that grew from materials transported in solution (Fig. 6). We find that fresh, unweathered oldhamite and the bladed weathering mineral that grew on oldhamite in situ, have identical REE abundances as well as pattern shapes (Fig. 6). The vaterite that grew on oldhamite from materials transported in solution has much lower REE abundances than fresh oldhamite, but the shape of the pattern is approximately the same (Fig. 6). We conclude that REE

Table 5. REE cementsof typical plagioclase gains of 5 analyses (c&mm

fcoiumns I-3). average and ranges

4), and emn range (lo, from counting statistics only) for iowest

and highest values (column 5), for grains in the oldhamis-dominated Norton County aubrite (in ppb; REE nof listi,

lithology of the

and - = below detection bit).

calumil

1

1

3

Sample

NC15707

NC17001

NC16115

PTSY

UNM-981

UNM-986

UN&f-983

Spot

A3PLAGI

AlPLAG

MPLAGB

4

5

l3knwtIt La

35

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455

abundances (relative to the reference element Ca) in weathered Norton County oldhamite that has been replaced in situ are not significantly different (in absolute abundance or in pattern shape) from those of the original, unweathered oldhamite. (We point out, however, that all oldhamite REE measurements reported in this paper were obtained from fresh oldhamite) . DISCUSSION Bulk REE Abundances and Pattern Shapes in Aubrites are Dominated by Oldhamite

Our ion microprobe analyses of oldhamite in oldhamitedominated clasts yield REE abundances 50-300 X CI and in pyroxenitic clasts ( WHEELOCK, 1990) yield REE abunLa Cs Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm yb Lu

RG. 6. Comparison of REE abundances (relative to CI; PALME et al., I98 I ) of fresh and weathered oldhamite in an oldhamitedominated igneous clast (NC- 1700 1, UNM 984) of the Norton County aubrite. Upper solid pattern is for fresh oldhamite; dashed pattern is for weathered oldhamite that has been replaced in situ by terrestrial weathering products. Lower solid pattern is for the spherulitic weathering rind composed of vatetite that grew on oldhamite from solution.

oo,

.

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dances 40-800 X CL This confirms earlier data ( HEAVILON et al., 1989; WHEELOCK et al., 1989, 1990; FLOSS and CROZAZ, 1990, 1991; FLOSS et al., 1990; LODDERS and PALME, 1991; KURAT et al., 1992) that oldhamite is the main REE carrier in aubrites. We also show that alabandite (0.0410 X CI ) and plagioclase (below detection limit to 10 X CI) in oldhamite-dominated clasts, and diopside (0.2-9 X CI) in pyroxenitic clasts ( WHEELOCK, 1990) are minor REE car-

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FIG. 7. Comparison of bulk rock REE patterns (solid lines) for the Norton County aubrite ( MASUDA,1967; OKADA et al., 1988), with bulk rock patterns calculated from our ion microprobe analyses of REE abundances in individual minerals (dashed lines). Values in upper dashed pattern represent a bulk rock sample containing 0.6 wt% oldhamite with REE contents as for clast L-SO,assuming that no other significant REE carriers are present. Values in lower dashed pattern represent a bulk rock sample containing 2 wt% of a sulfide that is 96% alabandite and 4% oldhamite with REE abundances as in clast L-50, and assuming that no other significant REE carriers am present. The calculated REE patterns match the measured bulk rock REE patterns fairly well, suggesting that bulk REE abundances in Norton County (and other aubrites) are dominated by sulfides. Calculated REE patterns show slight HREE depletion, which is not seen in the measured bulk rock patterns. This may be due to REE contributions in bulk rock samples from some other phase, such as diopside.

456

M. M. Wheelock et al.

riers. Enstatite in Norton County has REE abundances below our detection limits and at least three orders of magnitude lower than those of oldhamite (note that FLOSSet al., 1990, find LREE abundances below detection limits and HREE (Dy-Lu) abundances 0.02-0.2 X CI in enstatite of the Bishopville aubrite). All minerals with measurable REEs (except plagioclase) have negative Eu anomalies, and their patterns range from flat or slightly convexly bowed (as seen in oldhamite in oldhamite-dominated clasts), to upwardsloping HREE enrichment trends (in oldhamite in pyroxenites, as well as in alabandite and diopside) Bulk analyses of REEs have been performed on a variety of samples from Norton County, including brecciated matrix, igneous-textured clasts, and enstatite fractions (e.g., SCHMITT et al., 1963; MASUDA,1967: WOLF et al., 1983; STRAIT, 1983; OK ADA et al., 1988). Virtually all bulk REE patterns have negative Eu anomalies; are otherwise flat, slightly bowed (convexly or concavely), or HREE-enriched; and range in average abundances from roughly 0.1-2 X CI. It has been suggested previously that bulk REE abundances and pattern variability in aubrites in general (e.g., NEWSOMet al., 1986; KEIL, 1989; FLOSSet al., 1990) and in Norton County specifically ( WHEELOCKet al., 1989) are the result of admixture to bulk samples (which consist mostly of nearly REE-free enstatite) of variable amounts of oldhamite and other sulfides. Our data confirm this contention. For example, bulk REE abundance patterns of Norton County (MASUDA, 1967; OKADAet al., 1988) can, in one case, be matched by assuming that 0.6 wt% oldhamite of a composition, like that in sample L-50 (Fig. 5a,b), was present in the sample that had no other significant REE carriers (Fig. 7). In another case (Fig. 7), a bulk REE pattern (OKADA et al., 1988) can be matched by a sample with no other significant REE carriers but 2 wt% of sulfides that are 96% alabandite and 4% oldhamite of sample L-50 composition (Fig. 5a-c). Oldhamitedominated of Aubrites

Clasts are a New Igneous Lithology

The origin of oldhamite in enstatite meteorites is in dispute and has been the topic of extensive research and discussions for some time. For enstatite chondrites, many authors have suggested that oldhamite formed in the solar nebula and is a relict from that period. This conclusion is based on thermodynamic calculations which yield oldhamite as a hightemperature, refractory condensation product from a reduced solar nebula (C/O r 0.83; e.g., LARIMERand BARTHOLOMAY, 1979), as well as on the trace-element contents of oldhamite: LARIMER and GANAPATHY( 1987) and LODDERS and FEGLEY( 1992) have argued that the high concentrations of refractory trace elements in oldhamite, including the REEs and certain lithophile and chalcophile elements, suggest a nebular origin for this mineral. Similarly, the high and highly variable REE concentrations and shapes of REE patterns measured by ion microprobe techniques in individual oldhamite grains from low-petrologic-type enstatite chondrites have been cited as arguments for a nebular and relict origin of the phase (e.g., LUNDBERGet al., 1989, 1991), although some REE patterns of relict oldhamite in equilibrated enstatite chondrites may have been modified by metamorphic

equilibration and redistribution (e.g., FLOSS and CROZAZ, 1991). Aubrites are thought by most researchers to have formed by the melting and fractionation of enstatite chondrite-like precursor rocks, as suggested by mineralogical, chemical, and isotopic similarities between enstatite chondrites and aubrites (e.g., BRETT and KEIL, 1986; KEIL, 1989). Individual oldhamite grains in aubrites generally have highly variable REE abundances and patterns, many of which are similar to those observed in unequilibrated enstatite chondrites (e.g., FLOSS and CROZAZ, 1990, 199 1; FLOSS et al., 1990; LODDERSand PALME,199 1; KURAT et al., 1992), suggesting to these authors that oldhamite did not equilibrate with a single silicate magma. This, combined with the high melting temperature of 2798 K of pure CaS (CHASEet al., 1985), has prompted several authors to suggest that some oldhamite in aubrites is unmelted refractory residue from the enstatite chondrite-like precursor material of the aubrites (e.g., FLOSSand CROZAZ, 1990, 199 1; FLOSSet al., 1990; LODDERSand PALME, 199 1; KURAT et al., 1992). Thus, these authors suggest that the REE characteristics of this relict oldhamite survived the complex processes of melting and differentiation that formed the aubrites. This view has also been supported by LODDERS and PALME( 1989, 1990) and LODDERSet al. (1990), who argue that the discrepancies between experimentally determined REE sulfide liquid/silicate liquid distribution coefficients and the observed high concentrations of the REE in oldhamite can be explained best by assuming that the REEs were incorporated into oldhamite during condensation. We argue here that the oldhamite-dominated lithology in Norton County is igneous in origin and that, in this case, oldhamite is not a nebular relict but formed by igneous processes from immiscible sulfide melts ( WHEELOCK, 1990; WHEELOCKet al., 1989, 1990). We emphasize that we limit our discussion and arguments to the igneous origin of this particular lithology, although we do not dispute the suggestion of DICKINSON et al. ( 1990b, 199 1) that some oldhamite inclusions in other aubrite lithologies (such as pyroxenites) also formed by igneous processes from immiscible sulfide melts. We propose that the oldhamite-dominated lithology formed by the melting and fractionation of enstatite chondrite-like precursor material and represents a locally CaSrich facies. During melting, a Ca-rich immiscible sulfide melt which contained a Mn-Fe-Mg-Cr-Na sulfide as an additional immiscible melt formed in the silicate melt. Upon cooling, the immiscible sulfides crystallized, forming large oldhamite crystals with blobs of Mn-Fe-Mg-Cr-Na-bearing sulfides; forsterite, enstatite, and plagioclase crystallized from the surrounding silicate melt. At subsolidus temperatures, the tiny ferromagnesian alabandite crystals exsolved from oldhamite. We advance a number of arguments in favor of an igneous origin of the oldhamite-dominated lithology.

1) The grain boundaries between forsterite and oldhamite appear to be igneous in nature; i.e., large oldhamite single crystals partially surround forsterite single crystals as though the former had been molten while forsterite was solid (Fig. 2). 2) Oldhamite in these clasts occurs as single crystals as large as 2 cm across (Fig. 1). It is hard to imagine that crystals

457

Oldhamite of igneous origin in Norton County

of that size would form by condensation from the nebula, but they can grow readily horn an immiscible sulfide melt. 3) Oldhamite single crystals contain round to irregularly shaped blebs of variable sizes of a Mn-Fe-Mg-Cr-Na sulfide assemblage which do not appear to occur along crystallographically preferred planes of the host oldhamite. These blebs appear to represent products of the crystallization of an immiscible Mn-Fe-Mg-Cr-Na sulfide melt within the CaS-rich melt. The possibility that these blebs exsolved from oldhamite (which formed from a single Ca-rich sulfide melt) cannot be dismissed entirely. Our interpretation is, however, consistent with the experimental observations of JONESand BOYNTON( 1983) that two distinct immiscible sulfides (in their case, one ironrich and one Ca-Mg-rich) formed in a simulated aubrite charge at temperatures between 1150- 1250°C. 4) Theoretical condensation temperatures show that oldhamite is more refractory than some of its inclusion phases, so it is unlikely that the observed assemblage could have condensed sequentially (or simultaneously) from the cooling solar nebula. For example, estimated condensation temperature of MnS is 700 K (GROSSMAN, 197 1) , whereas oldhamite condensation temperature is 1379 K at 10V3bars (LODDERSand FEGLEY, 1992). Thus, nearly pure CaS could not have condensed around millimeter-sized and larger blobs (solid or liquid) of lower temperature phases, such as ferromagnesian alabandite. 5) The highly variable REE abundances and patterns of individual oldhamite grains from aubrites have prompted a number of authors to suggest that oldhamite did not equilibrate with a single silicate magma and is not igneous in origin (e.g., FLOSSand CROZAZ, 1990, 199 1; FLOSSet al., 1990; LADDERS and PALME, 199 1; KURAT et al., 1992). These authors therefore suggested that some oldhamite is a relict phase and still has the REE abundances and patterns it inherited from the nebula. However, we find it difficult to imagine that the rather homogeneous REE patterns of oldhamite in the oldhamite-dominated lithology of Norton County are not the result of equilibration of the REEs with a silicate melt during formation of the igneous aubrites through parent body melting, differentiation, fractionation, and cooling, where peak temperatures of around 1450-l 500°C ( LONGHI, 1987) must have been reached. We conclude that oldhamite in the oldhamite-dominated lithology of Norton County is of igneous origin and that its REE abundances were established by equilibration with the aubrite silicate melt. One puzzling problem remains: the extreme REE enrichment observed in oldhamite implies that partition coefficients between immiscible oldhamite liquid and silicate liquid should be in the range of 100 to 1000. Recent experimental work, however, suggests values on the order of 0. 1- 10 ( LODDERSand PALME, 1989, 1990; DICKINSONet al., 1990a,b,c, 199 1; LODDERSet al., 1990). These experiments may not be applicable to aubrites because the bulk compositions of the experimental charges differ significantly from natural aubrite melts. Alternatively, DICKINSONet al. ( 199 1) suggest that in spite of the disparity between experimentally determined distribution coefficients and those inferred from analyses of nat-

Ural oldhamite, the experimentally determined distribution coefficients are correct. They argue that high REE abundances in oldhamite of aubrites, including that of the oldhamitedominated clasts, resulted not from primary crystallization but from local subsolidus equilibration during cooling after crystallization. Acknowledgments-We thank R. Jones for assistance with the X-ray diffraction work, T. Servilla for preparation of superb polished thin sections, K. Nichols for photography, and T. Ireland and J. Jones for constructive reviews. This work was supported in part by NASA grants NAG9-30, NAG9-454, and NAGW 328 I (K. Keil, P. I.) and NSF grant EAR8719528 (G. Crozaz, P.I.). One of us (MMW) was also supported in part by grants from the Student Research Allocations Committee and the Caswell Silver Foundation of the University of New Mexico. This is Planetary Geosciences Publication No. 733. and School of Ocean and Earth Science Publication No. 327 1. Editorial handling: S. R. Taylor

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