Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts?

Vol.53, pp. 3331-3336 Persamon Presspit.Printedin U.S.A.

~i4-7037/89/$3.~ + .eo

Geochimica et Cosm~~~m~caAm Copyright

(4 1989

LETTER

Is plagioclase removal responsible for the negative Eu anomaly in the source regions of mare basalts? C. K. SHEARERand J. J. PAPIKE Institute for the Study of Mineral Deposits, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995, U.S.A. (Received Aprii 1 I, 1989; resubmitted in rev~~ed~rm ~e~tern~er 8, 1989; accepted in revisedform November 6, 1989)

Abstract-The nearly ubiquitous presence of a negative Eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean (LMO). Are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation? Crystal chemical rationale for REE substitution in pyroxene suggests that the combination of REE size and charge, M2 site characteristics of pyroxene, fO,, magma chemistry, and temperature may account for the negative Eu anomaly in the source region of some types of primitive, low TiOZ mare basalts. This origin for the negative Eu anomaly does not preclude the possibility of the LMO as many mare basalts still require prior plagioclase c~s~lli~tion and separation and/or hyb~dization involving a KREEP component. INTRODUCTION IT Is GENERALLYACCEPTEDthat the formation and crystallization of a lunar magma ocean (LMO) at 4.5 to 4.2 Ba was responsible for a moonwide differentiation event (WALKER et al., 1973; TAYLORand JAKES 1974; WOOD, 1975; TAYLOR, 1982). This event is thought to be reflected in the complementary REE patterns of the lunar crust (positive Eu anomaly, negative slope) and the source regions of mare basalts (negative Eu anomaly, positive slope) (TAYLOR and JAKES, 1974; TAYLOR, 1982; WAREN, 1985). The nearly ubiquitous presence of a negative ELIanomaly in mare basalts has been suggested to indicate prior separation and flotation of plagioclase from their source region (TAYLOR, 1982; LONGHI, 1978). However, the crystallization sequence among olivine, orthopyroxene, clinopyroxene, and plagioclase is dependent upon the LMO bulk composition (Longhi, 1978 ) , the highpressure phase relations ( LONGRI, 1978; KESSONand RINGWOOD,1977), and the thermal regime ofthe LMO ( SOLOMON and LONGHI, 1977; RINGWOOD, 1976; WALKERet al., 1975; HERBERT et al., 1977). Estimates of percent crystallization of LMO prior to plagioclase saturation range from 64% ( LONGHI, 1978) to 80% (BINDER, 1982, 1985; WANKE, et al., 1989). Therefore, the crystallization of some of the mare basalt source regions from the LMO may not have been synchronous with plagioclase saturation and flotation (WARREN and WASSON,1979; KESSONand RINGWOOD,1977; HOMES and KUSHIRO, 1974). Are there any mare basalts that were derived from a mantle source which did not experience prior plagioclase separation? We suggest that the negative anomaly in the source regions for some of the primitive, very low TiOZ mare basalts may ret&t pyroxene in the source and not the synchronous crystallization and flotation of plagioclase. Our arguments are based on predicted crystal chemical behavior of pyroxene at low oxygen fugacities and the REE characteristics of primitive mare basalts (represented by picritic 3331

volcanic glasses) that were presumably derived from the partial melting of a predominantly olivine-orthopyroxene mantle source. PYROXENE CRYSTAL ~~MIS~Y AND REE BEHAVIOR The distribution of REE between pyroxene and melt is a function of REE size and charge, M2 site characteristics,fOz, magma chemistry, and temperature. The crystal chemical rationale for REE substitution in pyroxene has been discussed by PAPIKEet al. ( 1988) and SHEARERet al. ( 1988a,b, 1989a). The pyroxene group minerals crystallize in a variety of space groups, but for our purposes only three are important: CZ/ c for augite, P2, /c for pigeon&e, and P&a for o~hop~oxene. These three different pyroxene structural topologies have a profound effect on the M2 site’s coordination and size. In orthopyroxene, as a direct result of the octahedral stacking sequence (CAMERONand PAPIKE, 198 I), the M2 site is constrained to be a relatively small octahedral site with very little ability to accommodate ions larger than Mn’+. However, the M2 site in augite and pigeonite has a great deal of flexibility to accommodate ions as large as Ca and Na in 8 c~rdination or as small as Mg in 6 coordination. Figure 1 illustrates the range of cation sizes that the M2 and Ml sites can accept, and the X-site for feldspars, for comparison. Based on these systematics, we predict that the REE in pyroxene are essentially restricted to the M2 site and that Eu’+ must be totally excluded from the M2 site of opx and is almost too large to fit into the expanded M2 site of augite. The effects of the M2 site characteristics on trivalent REE and Eu2+ substitutions into pyroxene have been documented (PAPIKE et al., 1988; SHEARERet al., 1988a, 1989a; SUN et al., 1974; GRUTZECK et al., 1973; MCKAY et al., 1986). These studies confirm crystal chemical predictions that ( 1) the Kgyx’L (distribution coefficient of REE between pyroxene and melt) for tri-

C. K. Shearer and J. J. Papike

3332

FELDSPAR

X-SITE Ca Na

Ba K

Sr*+ Eu*+

I 1

I 1 PYROXENE MgLi

M2 SITE

Fe*’ Mn*’

Ml Al 1

SITE

Ca Na

I I

I

III

PYROXENE

CATIONS

CATIONS

2+ Ti4*Cr3’Fe3$3* MgSc Fe h3 d

IIII

II

$++

Ill TRIVALENT Yb ~~ Dy

I

G&nNd Ce La

1111

II

0.5

REE 11

I

I

I

I

0.6

I

0.7

0.6

0.9

1.0

Sr2*Eu2+

1

I

1.1

I

1.2

I

1.4

1.3

IONIC RADII (A) VI COORDINATION (Shannon

and Prewitt,

1969)

FIG. 1. Comparison of ionic radii for the REE and Sr and ionic radii for cations in the pyroxene Ml and M2 and the feldspar X site.

valent REE increases with increasing Ca in the M2 site ( K$jpxJL < KgGiL < KguGiL),(2) pyroxene rejectsEu2+relative to trivalent REE, and (3)the rejection of Eu’+ is most pronounced in orthopyroxene and least in augite. The extent Eu ‘+ and Eu *+ substitutions into the pyroxene structure can be calculated using the partitioning similarities between Sr2+ and Eu*+ and between Sm3+ and Eu3+ ( PHILPOTTS, 1970). The calculated Eu 2+/ Eu 3+ is less than 0.1 for a pigeonite crystallizing from a pigeonite basalt (SHEARER et al., 1989a)

with a Eu 2+f Eu 3+ of approximately 3.0 (DRAKE, 1975) at anf02 of lo-l3 at 1200°C (SATO, et al., 1973; BENCEand PAPIKE, 1972 ) . The extent of the Eu in the M2 site in pyroxene is dependent upon temperature, j-0, (DRAKE, 1975 ), and magma composition (MORRIS and HASKIN, 1974; MORRIS et al., 1974) which dictates the availability of Eu ( Eu2+/Eu3+) in the melt. KLyxiLfor Eu2+, Eu3+, and Sm decrease with increasing temperature (MCKAY and WEILL, 1976; GRUTZECK et al., 1973; WEILL et al., 1974). The KD decreases much more abruptly for Eu2+ (as proxied for by Sr) than the KD for total Eu or Sm with increasing temperature (MCKAY and WEILL, 1976; WEILL et al., 1974). The decrease in Kg (at constant Kim) for augite with decreasing log f0, (and increasing Eu2+/Eu3+) has been demonstrated by SUN et al. ( 1974) at 1150°C and GRUTZECK et al. ( 1973) at 1265°C (Fig. 2). This is substantiated by comparisons of KE for pyroxenes with similar amounts of Ca in the M2 site and similar K”6” but that crystallized under different log f0, (MCKAY et al., 1986; SHEARERet al., 1989a) (Fig. 2). The comparison of the data of SUN et al. ( 1974) with that of GRUTZECK et al. ( 1973) also illustrated the relationship among f 02, temperature, and Kg in calcic clinopyroxene.

site

Based on crystal chemical arguments, orthopyroxene should show similar, but a much more exaggerated, relationship between Kp and log f02. That this exaggerated effect is not observed

in the few experimental

JO2 of the experiment

studies

is a result of the

or low detection limits for Eu (see

FIG. 2. Relations among Kg", fO,,and Eu*+ in melt. Eu’+/ (Eu*+ + Eu~“)~~~= was calculated from DRAKE ( 1975). DRAKE ( 1975) did not observe Eu*+/(Eu*+ + Eu3+) variation as a function of temperature between 1187 and 1300°C. Differences of -log D between Sm and Eu are from (A) SUNet al. ( 1974) at 1lSO”C,~B) GRUTZECK et al. (1973). WEILL et al. (1974) at 1265°C: and fC1 , approximate relationship extrapolated from pyroxenes of similar composition from MCKAYet al. ( 1986) (at -log f0, - 9) and I

SHEARERet al. (1989a)

(at -logf02

-

12.5).

3333

Negative Eu anomaly in the lunar crust

ADDITIONAL EVIDENCE). Similar problems existed for olivine between crystal chemical rational (KbRfiE Q Ki”““) and experimental determination ( Khp”” N Kl;jtREE) ( MCKAY and WEILL, 1976; SCHNETZLERand PHILPOTTS, 1970). Recent experimental studies by MCKAY (1986) indicated K~~PEE << K;REE for olivine. He attributed the results of the earlier studies to spuriously high LREE contents of olivine because of small crystal size. Crystal sizes in excess of 300 pm are required for measurements of KD as low as these for LREE in olivine (G. MCKAY, pers. commun., 1989).

LMO

4

Eu ANOMALY IN THE MANTLE SOURCE FOR MARE BASALTS We suggest that this combined crystal chemical-f02 effect resulted in a negative Eu anomaly in early mantle cumulates that formed prior to the removal of plagioclase in the LMO. These mantle cumulates could possibly be the source for some of the mare basalts. LMO models calculating the REE characteristics of the mantle source for primitive mare basalts must utilize distribution coefficients for Eu that reflectfOz, T, and Eu ‘+,fEU3+ of the LMO. Using distribution coefficients that reflect higher f0, does not test the basic hypothesis proposed here. In a simple comparison of the REE characteristics of (A), an olivine + orthopyroxene it clinopyroxene cumulate fractionated from LMO prior to plagioclase removal, with (B). an olivine + orthopyroxene cumulate fractionated from LMO with simul~neous remov~/flotation of plagioclase, we assume ( 1) a 3 X chondrite initial REE abundance for the LMO and 50% fractional crystallization of olivine (for A) or olivine + plagioclase (for B) prior to crystallization of an instantaneous cumulate unit of olivine + orthopyroxene rt clinopyroxene, (2) instantaneous cumulates with orthopyroxene = olivine and with variable clinopyroxene (O-50%), (3) distribution coefficiencies reflecting the impact of j”O, on the dist~bution of Eu between melt and pyroxene (GRUTZECK et al. 1973; extension of observations of SUN

Table

1:

I$wdues

used

in

caldaticns for Figures

the

3, 4, ard 5. OL

om

CSXl

cPx2

PUG .035

ce

. 00002

.Ol

.07

.24

sm

.00058

.06

.30

.50

,022

Eu

(.OOOS)

.04

.I0

.34

1.20

w

.OO3

.I0

.39

1.34

,019

M

.0194

.30

.53

1.82

.017

.27

.QO

2.8

2.8

.016

SC

*

OL: mFay (1986)with agpaiimtim for Eu

CPXL: Sm. El1f~Gmtzw.kPrqZ. shearerSt al. (1989) cPx2:

sm, m fron sun g& a. sheareret al. (1989)

PLXX

shih a-d schonfeld(1976)

(1973).Ce, &J, yb. Nonralized fron (1974)ce, w,

Yb.

Nenmlized frm

---

CPX 1 CPX2

Fro. 3. Chondrite normalized REE patterns for olivine + opx rt:cpx cumulates (with variable cpx) without prior plagioclase crystatlization and with prior plagioclase crystalhzation ( IO-SO%). The 6X chondrite LMO composition is a result of 50% crystallization of olivine (+ plagioclase crystallization) of 3X chrondrite LMO. Fluctuations in Eu anomaly at 0% cpx cumulate is shown for (A) 10% plagioclase crystallization without anomalous behavior of Eu in opx, (B) 10% plagioctase crystallization with anomalous behavior of Eu in opx, and (C) 50% plagiocks-e crystallization. All during 50% crystaliiition of LMO.

et al., 1974; GRUTZECK et al., 1973; and SHEARERet al., 1988a, 1989a to orthopyroxene) (Table 1). Within the context of this simple comparison, fractional crystallization of olivine + orthopyroxene +- clinopyroxene prior to plagioclase saturation will result in cumulates with negative Eu anomalies and REE patterns with similarities to olivine-o~hopyroxene cumulates which crystallized synchronously with plagioclase crystallization and flotation (Fig. 3 ). With synchronous plagiodase crystallization, the extent of the Eu anomaly in the cumulates will still be affected by the behavior of Eu in the M2 site of pyroxene. The percentage of clinopyroxene has an effect on total REE abundance in the cumulate but does not dramatically affect the negative Eu anomaly. In addition, the data of SUN et al. ( 1974) and GRUTZECK et al. ( 1973 ) will give slightly different results in total REE abundance and extent of Eu anomaly. Are these olivine f orthopyroxene + clinopyroxene cumulates which crystallized prior to plagioclase saturation capable of producing negative Eu anomalies and REE abundances observed in some mare basalts and picritic glass beads? Using equilibrium batch melting equations, distribution coefficients in Table 1, and calculated REE patterns in Fig. 3, [Eu/Eu* IN, [Sm/CelN, [Sm],v, [Sm/YblN, and SC values can be calculated for melts derived from cumulates which experienced different percentages of plagioclase crystallization (O-SO%), cumulates with different proportions of clinopyroxene and different degrees of partial melting (Figs. 4 and 5 ) . In the calculated picritic melts produced by partial melting of the calculated cumulates, [ELI/ Eu * INand [ Sm], (an index of total REE abundance) are highest at low degrees of partial

C. K. Shearer and J. J. Papike

3334

0.6

FIG. 4. Calculated liquid [ Eu/Eu* INand [ SmlN as a function of prior plagioclasecrystallization, clinopyroxene in cumulate, and percent batch melting. Ranges for A-l 5 Green Volcanic Glass, A-17 VLT Volcanic Glass-Basalt are shown. Fields of high-Ti and low-Ti crystalline mare basalts are shown in insert. PLAG-A = melting of PLAG cumulate A (Fig. 3), PLAG-B = melting of PLAG cumulate B, PLAG-Cl = melting of PLAG cumulate C without anomalous behavior of Eu in opx, PLAG-C2 = melting of PLAG cumulate C with anomalous behavior of Eu in opx.

melting. Small degrees of partial melting (O-5%) of an instantaneous cumulate unit which did not experience prior plagioclase crystallization will have [ Eu/Eu * IN values of between I and .75 (Fig. 4a). In addition, the rather flat REE

A17 VLT 50REEN

01 0

’

.2

’

.4

’

.6

’

.8

0

1

GLASS

’

1.2

’

1.4

’

1.6

(Sm/Yb), FIG. 5. Calculated liquid [ Sm/YblN and SC (ppm) as a function of prior plagioclase crystallization, clinopyroxene in cumulate, and percent batch melting. Shaded region includes melts derived from cumulates PLAG-A through PLAG-C2 whereas PYX curve is for melts derived from cumulates which did not experience prior plagioclase crystallization.

pattern will have REE concentrations from approximately 2.5X to 5X chondrite. An instantaneous cumulate unit with prior plagioclase crystallization will produce melts with [ Eu/ Eu*]~ of between .94 and .44 (Fig. 4b). [ Sm/CelN values for all models are greater than 1. This contrasts with [Sm/ CelN values of less than or equal to 1 observed in most mare basalts. This contrast for all models (with and without plagioclase) is a result of selected LREE distribution coefficients. [ Sm/YblN and SC values are similar in all models. Based on trace element models of volcanic glasses and crystalline mare basal& the degree of partial melting reasonable for generation of mare basalts is less than 35% (e.g., BINDER, 1982, 1985; DELANO, 1986; HUGHES et al., 1988; MA et al., 1976; RINGWOOD and KESSON, 1976; TURCOTTE and AHERN, 1978 ) . How do lunar basaits compare to the model [ Eu/Eu* IN, [ Sm/CelN, [ Sm/YblN, and SC values? In Figs. 4 and 5, it is the VLT basalts and low Ti picritic glasses which more closely correspond to the models. The Apollo 15 green glasses (DELANO, 1986) and Apollo 17 VLT (very low-Ti) glasses (SHEARERet al., 1988c, 1989b,c) are probably the most appropriate glass compositions to invert to a mantle chemistry and mineralogy representing an olivine + orthopyroxene + clinopyroxene source (GROVE and LINDSLEY, 1978; DELANO and RINGWOOD, 1979; TAYLOR, 1982; SHEARERet al., 1989b; WENTWORTHet al., 1979). BINDER( 1982) however, suggests a shallower source than the previous studies and a source with up to 5% plagioclase. Assuming that these low Ti picritic glasses can be inverted to reflect mantle chemistry and mineralogy, the [ Eu/Eu * IN and [ Sm], for these picritic basalts fall into the range predicted by small degrees of partial melting (~5%) of an olivine + orthopyroxene ? clinopyroxene source which had not experienced prior plagioclase crystallization (Fig. 4). Our REE modeling results

Negative Eu anomaly in the lunar crust

for the picritic glasses contrasts with the A- 15 Green glass modeling results of MA et al. ( 198 1) who assumed a minor amount of plagioclase fractionation caused the Eu anomaly followed by deposition of an 01 + opx cumulate source prior to its subsequent partial melting. MA et al. ( 198 1) , however, used opx dist~bution coefficients which may not reflect the impact of,fO, on the behavior of Eu. Apollo 14 volcanic picritic glasses of all compositions and high Ti volcanic glass appear not to fit the model. The A- 14 glasses all have a minor to substantial KREEP signature, suggesting either assimilation or hybridization involving a KREEP component (SHEARERet al., 1988c, 1989b,c). The high Ti glasses may have been derived from a mantle source which crystallized synchronously with plagioclase crystallization or was involved in mantle hyb~di~tion (BINDER, 1982, 1985; DELANO, 1986; HUGHES et al., 1988). In conclusion, the origin of the negative Eu anomaly in very low TiOz mare basalts does not necessarily reflect a source region that crystallized synchronously with plagioclase crystallization and flotation. The crystal chemical-fOz effect in pymxenes may account for this negative Eu anomaly in the mantle source region for these mare basalts. This Eu effect is being more clearly documented, and much more rigorous models of basalt source regions are being tested by the coauthors in ongoing and future studies. The origin of the negative Eu anomaly in some types of mare basalts resulting from pyroxene in the source region does not preclude the possibility of the LMO, as many mare basalts still require prior plagioclase crystallization and/or hybridization involving a KREEP component. ADDITIONAL EVIDENCE During the review of this letter, preliminary experiments on Eu partitioning in low-Ca pyroxene substantiated the crystal chemical predictions made in this text (MCKAY et al., 1989). Compared to our conservative estimates of Eu opx/melt [KE/ly$?‘], = .67, MCKAY et al. ( 1989) suggested this ratio in low-Ca pyroxene is approximately 55 (measured Eu/Gd = 0.32). This difference would have the efkt of further decreasing the [ Eu/Eu* 1~ of a picritic melt derived by small degrees of partial melting of a cumulate with 0% cpx (Fig. 4). Acknowledgments-The authors appreciate the insightful reviews by Gordon McKay, Roman Schmitt, and an anonymous reviewer, In particular, we would like to thank Gordon McKay whose enthusiasm in the review process led to experiments on Eu pa~tioning in low Ca pyroxene. Additional modeling by James Brophy and Abhisit Basu on our initiai comments on this subject at the A-14 Workshop greatly assisted the authors. This research was supported by NASA under Grant 4-52000 (to J. J. Papike). Editorial handling: G. FAURE

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3335

BINDERA. B. ( 1985) The depths of the mare basalt source region. Proc. Lunar Planet. Sci. Conf lSth, C396-C404. CAMERONM. and PAPIKEJ. J. ( 198 1) Structural and chemical variations in pyroxenes. Amer. Mineral. 66, l-50. DELANO J. W. ( 1986) Pristine lunar glasses: Criteria, data and implications. Proc. Lunar Planet. Sci. Con_ I&h, D201-D2 13. DELANOJ. W. and RINGW#D A. E. ( 1979) Chemistry and possible origin of the Apollo 15 Green Glass. Lunar Planet. Sci. Conf X, 286-288. DRAKEM. J. ( 1975) The oxidation state ofeuropium as an indicator of oxygen fugacity. Geochim. Cosmochim. Acta 39,55-64. GROVET. L. and LINDSLEYD. H. I 1978 ) Comnositional variation and origin of lunar ultramafic green glass. Lunar Planet.Sci. Conf IX, 430-432. GRUTZECKM., KR~DELBAUGH S., and WEILL D. ( 1973) REE partitioning between diopside and silicate liquid. Eos 54, 1222. HERBERTF., DRAKEM. J., S&NET-TC. P., and WIS~R~HEN M. J. ( 1977) Thermal history of lunar magma ocean. Proc. Lunar Sci. Conf Sth, 573-582. HODGESF. N. and KUSHIROI. ( 1974) Apollo 17 petrology and experimental determination of differentiation sequences in model moon compositions. Proc. Lunar Sci. Conf 5th. 505-520. HUGHESS. S., DELANOJ. W., and SCHMITTR. A. (1988) Apollo 15 yellow-brown volcanic glass: Chemistry and petrogenetic relations to green volcanic glass and olivine-normalive mare basalt. Geochim. Cosmochim. Acta 52,2379-2392. KESSONW. E. and RINGWOODA. E. ( 1977) Further limits on the bulk com~sition of the moon. Proc. Lunar Sci. Conf 8th‘ 41 l432. L~NGHI J. ( 1978) Pyroxene stability and the composition of the lunar magma ocean. Proc. Lunar Planet. Sci. Conf 9th, 285-306. MA M.-S., MURALI A. V., and SCHMITTR. A. ( 1976) Chemical constraints for mare basalt genesis. Proc. Lunar Sci. Conf 7th, 1673-1695. MA M.-S., LUI Y.-G., and SCHMITTR. A. ( 1981) A chemical study of individual green glasses and brown glasses from t 5426; imphcations for their ~trogenesis. Prac. Lunar Sci’. Conf I23, 915933. MCKAY G. A. ( 1986) ~~s~l/liquid partitioning of REE in basaltic systems: Extreme fractionation of REE in olivine. Geochim. Cosmochim. Acta SO,69-79. MCKAY G. A. and WEILLD. F. ( 1976) The petrogenesis of KREEP. Proc. Lunar Sci. Conf 7th, 2427-2447. MCKAY G. A. and WEILLD. F. ( 1977) KREEP petrogenesis revisited. Proc. Lunar Sci. Canf 8th. 2339-2355. MCKAYG. A., WAGSTAFFJ., and YANGS.-R. ( 1986) Clinopyroxenes REE ~s~bution coefficients for she~ottites: The REE content of the Shergotty melt. Ge~h~rn. ~osm~h~rn. Acta SO,927-937. MCKAYG. A., WAGSTAFFJ., and LE L. ( 1989) The source of the mare basalt europium anomaly: REE distribution coefficients for pigeonite. LPI Workshopon Lunar PyroclasticGlasses(in press). MORRISR. V. and HASKINL. A. ( 1974) EPR measurement of the effect of glass composition on the oxidation state of europium. Geochim. Cosmochim. Acta 38, 1435-1445. MORRIS R. V., HASK~NL. A., BIGGARG. M., and O’HARA M. J. ( 1974) Measurement of the effects of temperature and partial pressure of oxygen on the oxidation state of europium in silicate glasses. Geachim. ~~smo~hirn.Acta 38, 1447-1459. PAPIKEJ. J., SHEARERC. K., SIMONS. B., and SH~MIZUN. ( 1988) Lunar pyroxenes: crystal chemical rationalization of REE zoning, pattern, shapes and abundances-An ion microprobe investigation. Lunar Planet. Sci. Conf XIX, 90 l-902. PHILFQ+~~X J. A. ( 1970) Redox estimation from a calculation of Eu*+ and Eu 3*concentrations in natural phases. Earth Planet.Sci. Lett. 9,257-268. RINGWWD A. E. ( 1976) Limits on the bulk compositionof the maan. Publication 1160, Research School of Earth Sciences, Austrian National University, Canberra. RINGWOODA. F. and KESSONS. E. ( 1976) A dynamic model for mare basalt petrogenesis. Proc. Lunar Sci. Conf 7th, 1697-1722. SATOM., HICKLINGN. L.. and MCLANEJ. E. ( 1973) Oxygen fugacity values of Apollo 12, 14, and 15 lunar samples and reduced state of lunar magmas. Proc. Lunar Sci. Conf 4th, 1061-1079.

3336

C. K. Shearer and J. J. Papike

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J. (1970) Partition coefficients of rare-earth elements between igneous matrix material and rockforming mineral phenocrysts, II. Geochim. Cosmochim. Acta 34, 33 I-340.

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SHEARERC. K., PAPIKEJ. J., SIMONS. B., and SHIMIZUN. ( 1988b) An ion microprobe study of the intra-crystalline behavior of REE and selected trace elements in pyroxenes from mare basalts with different cooling and crystallization histories, preliminary results.

SOLOMONS. C. and L~NGHI J. ( 1977) Magma oceanography 1: Thermal evolution. Proc. Lunar Sci. ConJ: 8th, 583-599. SUN C., WILLIAMSR. J., and SUN S. ( 1974) Distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid equilibria in oceanic ridge basalts: an experimental study. Geochim. Cosmochim. Acta 38, 1415-1433. TAYLORS. R. ( 1982) PlanetaryScience: A Lunar Perspective. Lunar and Planetary Institute, Houston. TAYLORS. R. and JAKESP. ( 1974) The geochemical evolution of the moon. Proc. Lunar Sci. Conf: 5th, 1287-1305. TURCOTTED. L. and AHERNJ. L. ( 1978) Magma production and migration within the moon. Proc. Lunar Planet. Sci. Con& 9th, 307-318.

Lunar Planet. Sci. Conj XZX, 107 I- 1072. SHEARERC. K., PAPIKEJ. J., SIMON S. B., SHIMIZUN., YURIMOTO H., and SUENO S. ( 1988~) An ion microprobe study of trace ele-

WALKERD., GROVET. L., LQNGHIJ., STOLPERE. M., and HAYS J. F. ( 1973) Origin of lunar feldspathic rocks. Earth Planet. Sci.

ments in Apollo 14 “Volcanic” glass beads and comparisons to mare basalts. In Workshop on Moon in Transition: Apollo 14, KREEP, and Evolved Lunar Rocks. pp. 77-8 1. SHEARERC. K., PAPIKEJ. J., SIMONS. B., and SHIMIZUN. ( 1989a) An ion microprobe study of the in&a-crystalline behavior of REE and selected trace elements in pyroxene from mare basalts with different cooling and crystallization histories. Geochim. Cosmo-

WALKER D., LONGHIJ., and HAYSJ. F. ( 1975) Differentiation of a

chim. Acta 53, 1041-1053.

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Letter 20, 325-336.

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