Geochemistry of lunar crustal rocks from breccia 67016 and the composition of the Moon

Geochemistry of lunar crustal rocks from breccia 67016 and the composition of the Moon

OO16-7037/92/$5.00+ .Do Gmhimica d Casmcchimica [email protected] 56, pp. 1013-1024 Copyrisht 0 1992Pergamon PIWpit.Printed in U.S.A. Geochemistry of lunar c...

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OO16-7037/92/$5.00+ .Do

Gmhimica d Casmcchimica [email protected] 56, pp. 1013-1024

Copyrisht 0 1992Pergamon PIWpit.Printed in U.S.A.

Geochemistry of lunar crustal rocks from breccia 67016 and the composition of the Moon* MARC D. NORMAN’++ and STUART Ross TAYLOR’s* ‘ResearchSchool of Earth Sciences, The Au~aii~

National Unive~ity, Canberra, ACT 260 1,Australia ‘Dept. of Nuclear Physics, Research School of Physical Sciences, The Australian National University (Received December 4, 1990; accepted in revi~~d~r~ December 6, 199 1)

Abstract--Global differentiation of the Moon has produced a plagioclase-rich crust overlying an u&ram&c mantle. We have conducted a major and trace element study of anorthositic clasts from an Apollo 16 breccia to investigate the geocbemical features of these hi~lan~ lithologies and their role in lunar crustal evolution. Samples analyzed for this study have aluminous, alkali-poor compositions, and varied Fe0 and MgO contents (A&O3 25-35%, NazO 0.3-l%, I&O < O.l%, Fe0 and MgO 0.5-80/o). Three compositional groups are recognized. One group is poor in m&c ~nstituen~ f A&O3 ;r 30%, Fe0 and MgO s 4% ), with the low abundances of lithophile trace elements typical of lunar anorthosites. The other two groups are more m&c (A1203 i 28%, MgO and Fe0 r 5%) and are distinguished from each other by FeO/ MgO ratios > 1 (ferroan noritic) and
teau (NORMAN, 198 I ) . The specific breccia we have sampled, 670 16, is a typical FFB, composed of dark melt-breccia clasts and white anortbositic clasts in a grey, fragmental matrix that is dominated by plagioclase (Fig. 2). We have studied the geochemistry of anorthositic clasts from 670 16 to investigate the role of these rock types in lunar crustal evolution. Petro~aphic descriptions and mineral data for these clasts are provided by NORMAN et al. ( 1991).

THE COMPOSITION AND origin of the Moon’s crust is one of the fundamental problems in p1anetaz-yscience. It is a primary crust, formed early in the Moon’s history by global differentiation, and is composed of about 70% plagioclase (TAYLOR, 1982).

The bulk com~sition

of the Moon’s crust is

reasonably well known, but exactly how it formed and the petrogenetic relationships among its various rock types remain areas of active study.

ANALYTICAL METHODS

The Apollo 16 mission sampled a central nearside lunar highlands site in detail. One of the objectives of the Apollo 16 mission was to sample the Descartes Formation, exposed on the northern margin of the site around North Ray Crater (Fig. 1f . The Descartes Formation was an immrtant sampling target because it is exposed across a broad region of the central nearside highlands and may represent ejecta from the Nectaris impact basin. The rocks around North Ray Crater are predominantly feldspathic fragmental breccias (FFBs), with bulk compositions and physical properties that link them with regional units exposed in the Descartes Mountains and the Kant Pla-

Major and trace element concentrations were determined in thirteen anorthositic clasts, one melt-breccia clast, and a bulk-rock split using spark-source mass spectrogmphy (SSMS ), inductively coupled plasma emission (ICP), and electron microprobe (EMP) (Tables 1,2). The clasts were extracted from the breccia at the NASA Lunar Curatorial Laboratory, further cleaned of adhering matrix under a laminar flow hoed in our laboratory, and powdered in an agate ball mill reserved for lunar samples. Major element concentrations for all splits were measured by energy dispersive electron microprobe analysis on fused beads that were prepared from a small amount ( lo-20 mg) of powder melted on a moly~enum strip under an argon atmosphere, mounted in epoxy and polished. For the ICP analyses, mixed acid (HF, HN03, HCIO,, HCL) dissolutions were carried out in a clean laboratory using distilled reagents. No detectable blank was found for any of the elements analyzed. A measure of the ICP analytical precision is indicated by the analyses of replicate sptits ( ,I6 A and B, and ,320 A and B, respectively, Table 2). Spark-source analyses uswl standard methods developed in this laboratory (TAYLOR and GORTON, 1977). Additionaldata for splits of 67016 and other FFBs are given by LINIXTROMand SALPAS( 1983 ) , LINDSTROM and

* Presented at the symposium for S. R. Taylor, “Origin and Evolution of Planetary Crusts,” held October l-2, 1990, at the Research School of Barth Sciences, ANU. + Presentaddress: PlanetaryGeosciences,DepartmentofGeology and Geophysics, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA. 1013

1014

M. D. Norman and S. R. Taylor

FIG _ I. Location maps showing the regional setting of the Apollo 16 landing site in relation to major geologic features, and a more detaikd view of the landing site (box). LM indicates the location of the landing module, and the numbers indicate station locations. North Ray Crater, at the base of Smoky Moun~in, was the principal sampling site of the

DescartesFormation. 67016 was collected from station I I.

~N~T~OM ( 1986), and other referenceslisted by RYDER and NOR(1980).

MAN

RESULTS Major Elements

All splits analyzed for this study have aluminous, alkalipoor compositions (AllO 24-35%, NazO 0.3-l%, l&O < 0.1%) typicai of feldspathic material fi-om the lunar highlands. Fe0 and MgO contents are variable (OS-SW), depending on the modal abundances of mafic minerals present. Three compositional groups can be recognized based on AlFe-Mg relations (Fig. 3; also LINDSTROMand SALPAS,1983). One group has A1203 2 30%, Fe0 and MgO I 4%, and FeO/ MgO - 1, This group includes samples of mafic-poor anorthosite, the bulk rock, and most of the melt breccias. Some of the mafic-poor anorthosite clasts are relatively sodic, with Na*O ranging up to 1.2 wt% compared to more typical values for lunar ferroan anorthosites of 0.2-0.5% Na20 (Fig. 4). These clasts are not as sodic as the alkali anorthosites found

at the Apollo 12 and 14 sites (WARREN et al., 1983). Sodic anorthosites have not been recognized in 67016 by previous studies and may constitute a new type of lunar crustal rock (NORMAN et al., 1991). The other two compositional groups comprise more mafic anorthositic clasts (A&OX5 28%, MgO and Fe0 2 5% ) , and are distinguished from each other by their FeO/MgO ratios (Fig. 3). One of these groups is more ferroan, with Fe0 > MgO. The other is more magn~ian, with MgO > Fe0 Based on their modal and normative mineralogy, the ferroan mafic-emiched ciasts could be classified as anorthositic norites or anorthositic gabbro-norites, and the magnesian m&c-enriched clasts could be classified as troctolitic anorthosites. We wilf use the terms ‘ferroan nor&c and ‘magnesian troctolitic,’ respectively, to describe these two compositional groups. Our bulk-rock split has a major element composition intermediate between the ano~hositic clasts represented by the three groups, suggesting they all contributed to the matrix. Mixing models for several FFBs ( LINDSTROMand SALPAS,

Geochemistry of the primordial crust of the Moon

FIG. 1. (Continued)

1983) suggest that these three lithologies (mafrc-poor anorthosite, ferroan anorthositic norite, and magnesian troctolitic anorthosite) are predominant components in all of the KREEP-poor FFBs. The dark melt-breccia clasts in 67016 cluster near the bulk rock composition. Lithophile Trace Elements Abundances of most lithopbile elements in the anorthositic clasts are at the low end of the compositional range observed in rocks from the lunar highlands (TAYLOR, 1982). As pointed out by LINDSTROMand LINDSTROM( 1986)) the three groups of anorthositic clasts recognized by their major element compositions (ma8c-poor, ferroan noritic, magnesian troctolitic) can also be distinguished by their trace element compositions.

The mafic-poor anorthosites tend to have the lowest trace element concentrations except for those elements (e.g., Sr and Eu) which enter plagioclase (Fig. 5, 6). Sr, Ba, and Eu concentrations tend to be highest in the sodic varieties (Fig. 6). Li concentrations appear to be well correlated with Na (Fig. 6)) but note that the only two mafrc-poor anorthosites for which we have Li data are sodic varieties. Pristine maficpoor ferroan anorthosites such as 60025,60215, and 6 1016 typically contain only l-2 ppm Li, although few data are available (PHILPOTTS et al., 1973; HUBBARD et al., 1974; LINDSTROMet al., 1977; STEELEet al., 1980; ROSE et al., 1973, 1975). A positive correlation of Li and Na in lunar plagioclase was also found by STEELE et al. ( 1980). Rare earth element (REE) patterns in the most mafic-poor clasts are slightly LREE-enriched with the large positive Eu anomaly typical of lunar anorthosites (Fig. 7 ). Samples classified as

1016

M. D. Norman and S. R. Taylor

FIG. 2. A photo of lunar breccia 67016 showing the dark melt-breccia clasts and white anorthositic clasts in a grey fragmental matrix. The orientation block is a one-inch cube. NASA photo S8 l-26045.

“ma&-poor” based on their major elements (i.e., 230% A&OX, ~4% FeO, Fe0 = MgO) but with l-4% Fe0 have generally higher concentrations of trace elements compared to the most mafic-poor examples (Figs. 5, 7). Ferroan noritic clasts typically have incompatible element concentrations and REE patterns comparable to the “maficpoor” clasts with l-4% Fe0 but are distinguished principally by their high concentrations of Sc and, to a lesser degree, Cr (Fig. 8), which are contained mainly in clinopyroxene. The magnesian troctolitic clasts have among the highest concentrations of incompatible trace elements observed for the three groups of anorthositic clasts and are slightly more LREEenriched than the ferroan clasts (Figs. 5, 7 ). Our split ,16 of bulk rock 670 16 contains the highest concentrations of REE observed in our study, although other incompatible trace elements (e.g., Th, Li) are not as enriched (Figs. 5, 6, 7). Mixing models suggest that a rare, trace element-rich lithology occasionally found as clasts in 670 16 and other FFBs contributes a significant amount of the trace element budget to the bulk rock (LINDSTROM and SALPAS, 1983 ). Our sample of melt breccia contains REE and other trace element abundances intermediate between the most mafic-poor anorthositic clasts and the mafic-enriched clasts

(Fig. 7), although other melt breccias in 67016 are compositionally closer to the bulk rock ( LINDSTROM and SALPAS, 1983). Siderophile Trace Elements The ferroan noritic and mafic-poor anorthositic clasts have low concentrations of Ni and Co, and Ni/Co substantially less than the chondritic ratio (~40 ppm Ni, ~10 ppm Co, Ni/Co 16; Fig. 9). Many of these clasts probably would qualify as “pristine” (i.e., uncontaminated by siderophile elements from meteorites; WARREN and WASSON, 1977, 1978 ). The magnesian troctolitic clasts have considerably higher levels of siderophile elements, with some clasts recording up to 750 ppm Ni (Fig. 9). There is a general positive correlation of siderophile and incompatible element abundances in the 670 16 anorthositic clasts, reflecting the higher abundances of both groups of elements in the magnesian troctolitic clasts (Figs. 5. 9). Two different solutions prepared from the bulk-rock powder ( ,I6 A and B) gave widely different Ni and Co abundances (56 I vs. 75 ppm Ni). We suspect this reflects a heterogeneous distribution of metal fragments in the sample, which is sup-

1017

Geochemistry of the primordial crust of the Moon Majors are by fused bead electron Table 1. Major and trace element data for splits of breccia 67016. microprobe (EMP), normalized to 100%. Traces are by spark source mass spectrometry (SSMS). Wt is the allocated weisht of the split. Split: ,16 ,54 ,318 ,320 ,322 ,326/E ,330 ,332 ,334 ,338 ,339 ,343 ,346 ,349 wt (g) 2.203 0.204 0.176 0.920 0.301 1.577 0.587 0.091 0.510 0.276 0.053 0.518 0.511 0.303 FM MP MB FM FM MM MM MP Rock tVDe* BR MP MP MM MP FM BMP wt% 44.43 44.69 45.23 44.09 46.48 45.31 45.54 45.71 45.27 44.16 44.89 44.10 45.10 45.25 SiO2 0.38 0.43 0.13 nd 0.40 0.16 0.15 0.25 TiOZ 0.36 0.04 0.18 0.23 0.05 0.55 30.36 24.08 31.21 33.75 24.43 26.22 27.02 27.09 32.55 34.61 32.53 28.47 34.29 25.00 A1203 8.65 7.64 4.47 Fe0 3.81 3.18 2.03 6.56 4.67 1.53 0.77 1.78 4.11 7.42 0.48 MnO 0.05 0.12 nd nd nd 0.09 nd nd nd nd nd 0.07 nd 0.10 5.79 3.79 2.53 1.48 5.80 5.30 7.60 7.71 1.22 1.05 1.71 6.04 0.41 5.57 MgO CaO 16.60 15.81 17.34 18.62 15.21 15.81 14.80 14.68 18.33 18.86 18.04 16.42 18.65 15.66 Na2O 0.52 0.31 0.33 nd nd 0.27 0.28 0.28 0.73 0.47 0.78 0.44 0.93 0.31 0.03 0.06 nd nd nd 0.02 nd 0.06 nd 0.04 0.08 0.04 0.08 0.04 K20 SSMS ppm La Ce Pr Nd Sm EU Gd Tb

6.00 2.25 2.15 1.20 1.74 2.04 4.82 3.88 2.65 3.33 1.43 3.50 1.59 5.43 4.21 4.51 10.23 6.35 1.27 2.14 14.95 5.21 2.00 8.39 7.78 8.49 0.71 0.58 1.28 0.83 0.14 1.08 1.84 0.74 0.26 0.59 1.06 0.92 0.22 1.24 2.83 5.72 a.05 3.44 3.67 2.83 4.40 3.98 0.53 4.09 4.61 0.89 1.86 1.02 1.04 0.36 0.84 0.88 1.53 1.08 1.07 0.12 1.03 1.22 0.21 0.77 0.89 0.74 0.77 1.05 1.00 1.68 1.05 1.65 0.87 2.12 0.95 0.92 1.28 2.13 1.17 0.39 1.15 0.97 1.64 1.16 1.25 0.13 1.11 1.41 0.2 0.24 0.21 0.07 0.18 0.32 0.22 0.20 0.02 0.21 0.27 0.37 0.23 0.04 1.22 2.13 2.51 1.48 1.33 0.43 1.64 1.48 1.23 0.14 1.29 1.81 0.22 w 0.34 0.29 0.09 0.36 0.28 0.47 0.34 0.26 0.03 0.26 0.40 HO 0.52 0.04 1.03 0.25 1.08 0.85 1.42 1.00 0.70 0.08 0.7 Er 1.50 0.84 1.19 0.12 0.25 0.92 1.46 1.15 0.71 Yb 1.33 0.97 0.86 1.03 0.07 0.68 1.13 0.14 0.12 0.04 0.19 u 0.15 0.06 nd 0.04 0.11 0.07 0.01 0.11 0.20 nd Th 0.21 0.47 0.10 0.14 0.13 0.70 0.41 0.26 0.41 0.76 0.61 0.04 0.04 0.27 0.72 1.45 0.33 Hf 1.34 0.54 0.82 0.65 0.86 0.19 0.81 1.09 0.12 Nb 5.0 1.6 1.7 0.7 1.6 2.4 4.4 2.6 1.5 0.9 2.3 3.2 0.9 24 29 60 8 47 ZK 78 19 10 30 39 13 38 5 Ba 79 29 31 17 29 29 71 61 43 33 85 56 49 *BR = bulk rock, FM = ferroan mafic-rich anorthosite, MM = magnesian mafic-rich anorthosite, MP = mafic-poor anorthosite, MB = melt breccia. nd = not determined

Table 2. Major and trace element data for splits A and 0 indicate analyses of independent Sample ,16A ,165 ,320A ,320B ,322 MB FM BR MB Rock type* BR wt.% Ti02 0.488 0.491 0.116 0.115 0.464 Al203 29.92 30.48 34.46 34.24 24.57 Fe0 4.78 3.69 1.97 1.97 7.94 MnO 0.054 0.055 0.034 0.033 0.112 Mg'=' 3.79 3.81 1.64 1.72 5.82 CaO 17.27 17.44 19.17 19.13 15.78 Na20 0.617 0.686 0.432 0.430 0.370 K20 0.050 0.047 0.009 0.012 0.018 CrzO3 0.039 0.028 0.039 0.013 0.131 ppm Ti Mn Na

2923

of 67016 by ICP. replicate solutions. ,326/E ,330 ,334 MM FM MP 0.486 25.64 7.96 0.104 5.42 16.04 0.388 0.020 0.083

0.281 27.73 4.94 0.057 7.08 15.63 0.584 0.031 0.061

0.258 32.82 1.51 0.027 1.22 18.67 0.938 0.043 0.017

2.34 5.39 0.67 3.16 0.88 0.74 1.13 0.21 1.39 0.31 0.87 0.89 0.06 0.21 0.62 2.8 29 31

,343 MM

,346 MP

,349 FM

0.231 29.52 3.89 0.052 5.89 17.08 0.564 0.031 0.048

0.063 34.91 0.45 q.008 0.42 18.79 1.197 0.069 0.002

0.567 24.03 8.02 0.104 5.55 15.62 0.393 0.015 0.128

2944 697 688 2784 2916 1681 1548 1384 375 3396 420 429 267 259 866 808 442 209 404 63 806 4575 5087 3205 3189 2741 2875 4336 6958 4186 8880 2917 K 417 388 74 101 145 164 259 356 256 574 122 Cr 270 192 265 92 896 570 419 115 323 16 875 Li 6.6 6.3 3.8 3.9 3.8 3.9 5.9 7.0 5.1 8.7 3.5 Sr 187 184 159 166 135 137 172 277 170 321 140 Ba 72 72 14 15 26 25 54 43 46 43 27 Zr 66 69 9 8 25 25 55 12 39 3 28 Y 17 18 2 2 10 9 14 8 10 1 10 SC 7.5 7.5 4.2 4.1 20.5 17.5 6.4 6.7 6.7 0.9 17.8 V 12.6 12.5 8.5 8.1 25.6 23.5 19.0 7.2 17.6 1.5 21.7 Ni 561 75 7 6 25 37 360 6 118 1 36 co 94 15 4 7 4 7 39 3 17 nd 8 Be 0.64 0.65 0.12 0.13 0.25 0.23 0.61 0.44 0.47 0.36 0.27 CU 3 2 0.5 nd 18 28 1 nd nd nd 23 *BR = bulk rock, FM = ferroan mafic-rich anorthosite, MM = magnesian mafic-rich anorthosite, MP = mafic-poor anorthosite, MB = melt breccia. nd = not determined.

,351 0.541 MP 45.19 0.05 34.26 0.46 nd 0.41 18.59 0.94 0.08

1.4 1.1 0.12 0.52 0.14 0.75 0.17 0.03 0.18 0.04 0.11 0.12 nd 0.09 0.28 0.9 5 24

M. D. Norman and S. R. Taylor

101%

d

1.5 -



67016

0

0 ’

Magnesian Tmctalitic

1.0 fh ppm 0 0.5 -

.a.

0.0

23

x*x

AA 30

: . b 25

0

0

Ferrcan Notitic

fIl

0 x

Mafic-poor A x *#

x

x

l

t



27

L

29

31

%

x

x AA * .a*,

33

35

FIG. 5. Plot of A&O, vs. Th concentmtions for splits of 67016. Symbots and data sources same as for Fig. 3. Magnesian troctolitic clasts have higher Th abundances than ferroan noritic clasts, precluding a relationship by igneous fractionation.

0

x

x

A1203 wt%

Magnesian Troctolitic

*Ax Ax 0 Mafic-poor x

A1203 wl%

Ferroan Noritic

00 0

O

.

DISCUSSION 200

2

4 MgOWWf

6

10

FIG. 3. The con~ntmtions of MgO, A1203, and Fe0 in splits of 67016 are used to define the three-foid subdivision into mafic-poor, ferroan no&c, and magnesian troctolitic samples used in this paper. Mafic-poor splits have Fe0 and MgO less than 4%, A1203 a 30%, and Fe0 = MgO. Malic-enriched anorthositic clasts have Fe0 and MgO > 4% and A&O3 5 29%. The mafic-enriched clasts are subdivided into ferroan noritic and magnesian troctolitic varieties based on FeO/MgO ratios > 1and < 1, respectively. Symbols: ferroan noritic clasts (black dots), magnesian troctolitic clasts (white dots), maficpoor anorthositic ciasts (black triangles), melt-breccia ciasts (crosses), and a sample of the 670 16bulk-breccia matrix (square). Data from this study and LINDSTROM and SALPAS ( 1983 f .

ported by the somewhat higher Fe abundance in the Ni-rich split (Table 2). In contrast, replicate analyses of our melt-

The textures and mineral compositions of anorthositic clasts in 67016 and other FFBs su8gest a complex history probably involving several igneous and metamorphic episodes (ASHWAL, 1975; BICKEL and WARNER, 1978; NORMAN, 1981; LINDSTROMand LINDSTROM, 1986). In this section we discuss some of the implications of the geochemical data for petrogenetic relations among the 67016 anorthositic clasts and for the evolution of the lunar crust.

Li PPm

breccia clast showed consistent and quite low abundances of Ni and Co (Table 2). Melt breccias in 67016 are typically low in Ni and Co (Fig. 9), which is somewhat surprising considering there is no doubt that they are quickly cooled impact melts.

li,~~F~~ ~1 u.1

~20 Wt%

0.5

0.7

0.9

1.1

EU

0.8 -

Magneeian Troctolitic 0.6 Ferroan 0.4 - Noritic

. 0.2’ 20

0.3

0 Y

A q

0 x.$

00 @a30

A

l

I.

@ ’

25

0 . .

I

30

Mafic poor

Ppm

” x kr

.

A

-

. ’

35



Al203 wt% FTC. 4. Plot of AlzOs vs. NazO concentrations for splits of 67016. Symbols and data sources same as for Fig. 3. Some m&c-poor anorthositic clasts have considerably higher Na contents than typical ferroan anorthosites.

FIG. 6. Plot of Na20 vs. Li and Eu concentrations for splits of 67016. Sodic varieties of ma&poor anorthosite tend to have higher concentrations of Eu, Sr, Ba, and Li. Symbols and data sources same as Fig. 3.

1019

Geochemistry of the primordial crust of the Moon

OxSorp

100

x

x

Ni ppm

0.

xx

l

0

10

xx

Ferroan noritic

x

A

AX

F

:: = &

!

Magnesian troctolitic clasts

.

i

Mafic-poor

I’20

_i

!p;~

A

14

.

I



25

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.

An . 35



“.‘I”..I”“I’-

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0

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12 0

0

100

10 -

0 0

x x

NilCo

:: $ 10

0

6 _ 0

0

4-

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l



0

l

’ ’ ’ ” LW3CePrb.U

’ I ’ ’ ” ’ SmEuGdlb Cy HoEr

I

’ ’ Yb

o-“““““““” 20

7. Chondrite-normalized rare earth element (REE) patterns for the three groups of anorthositic clasts (mahc-poor, ferroan noritic, magnesian troctolitic) and splits of bulk breccia and melt breccia analyzed for this study. FIG.

0. SC

0

x

15 -

wm

’ x I

Mafic poor

-

x A

2.I’

r

00

*

6f ” 1 8 c

x

x 25

30

35

A1203 wt% FIG. 9. Plots of A120, vs. Ni concentrations and Ni/Co ratios for splits of 670 16. Symbols and data sources are the same as for Fig. 3. Magnesian troctolitic clasts have the highest Ni concentrations and Ni/Co ratios observed in this study. These high values suggest meteoritic contamination in at least some of these clasts. The ferroan noritic clasts and the mafic-poor anorthositic clasts have lower Ni concentrations and Ni/Co ratios. For comparison, pristine lunar highlands rocks typically have Ni concentrations < 80 ppm and Ni/ Co ratios c 6. One matic-poor anorthosite sample from LINDSTROM and SALPAS( 1983) with 5.6 ppm Ni, 0.31 ppm Co, and Ni/Co 18.1 (split R) is not plotted on the lower diagram to expand the scale.

10 -

.

Magnesian b”x, x troctolitic [email protected] Ooc xx 8 x XA

5-

n 'i0

1200

25 ..’

30

I

600

Ferroan l 0 noritic

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020 l

Magnesian troctolitic 0

0

200 -

Mafic-poor 20

1

x

600

0

35

’ . . 1 .

1000 -

Cr ppm

:

Mafic-poor Ai.

.“‘~‘.‘..“‘.‘1.

-“.‘.“.“‘.As25

30

A *

x*

AA

.

35

Al203 wt?b FIG. 8. Plots of AhO3 vs. SC and Cr concentrations for splits of 67016. Symbols and data sources same as for Fig. 3. Note the enrichment of troth Sc and Cr in the ferroan noritic clasts suggesting accumulation of pyroxene.

Mineral compositions of the ferroan noritic clasts match those of pristine ferroan anorthosites (NORMAN, 198 1; NORMAN et al., 199 1) , suggesting a petrogenetic affinity between these rock types. Enrichments of SC, Cr, Ti, and the REE in the ferroan noritic clasts would be consistent with a greater abundance of cumulate pyroxene in the mafic-emiched clasts. Magnesian troctolitic clasts in 67016 have mineral compositions very similar to those of pristine Mg-suite troctolites, but the high Ni concentrations and Ni/Co of these clasts suggest that they have been contaminated by meteoritic siderophiles. The ferroan and magnesian clasts in 67016 appear to be another manifestation of the well-known dichotomy between ferroan anorthosites and Mg-suite troctolites and norites (WARREN et al., 199 1, and references therein), with the important distinction of the mafic-enriched nature of the noritic clasts relative to pristine ferroan anorthosites (FAN). We suggest that the ferroan noritic rocks discussed here could be a more abundant igneous rock type in the lunar crust than is generally recognized. Ferroan noritic clasts in 670 16 have several characteristics commonly found in the pristine rocks, including relatively low Ni and Co contents, low Ni/Co, and homogeneous mineral compositions identical

M. D. Norman and S. R. Taylor

1020

to mafic-poor FAN and so plausibly represent a true igneous rock composition and not a mechanical mixture. Such a rock type may represent the mafic-enriched component in polymitt highlands breccias, soils, and impact melts ( LINDSTROM and LINDSTROM,1986; KOROTEVand HASKIN, 1988) and could be a more important rock type in the lunar crust than is apparent from lithologic abundances in the current collection of pristine rocks. The overall compositional similarity between the bulk composition of the lunar crust (TAYLOR, 1982) and the ferroan noritic clasts in 67016 is notable. The bulk composition of the lunar crust commonly is considered to reflect a heterogeneous mixture of rock types with KREEP an important carrier of incompatible trace elements (JAMES,1980; STOFFLERet al., 1985; WARREN, 1985; DAVIS and SPUDIS, 1985; LUCEYand HAWKE, 1988; KOROTEVand HASKIN, 1988 ). Are the trace element compositions of either the magnesian troctolitic or the ferroan noritic clasts reflecting a mixture of KREEP with m&c-poor anorthosites? We think the answer is no for several reasons. Figure 10 depicts the elemental abundance patterns of average ferroan noritic, magnesian troctolitic, and mafic-poor anorthositic clasts from 67016, a typical pattern for Apollo 16 KREEP derived from the poikilitic impact melts 603 15, 62235, and 65015 (see summary in RYDER and NORMAN, 1980) and calculated mixtures of mafic-poor anorthosite with KREEP, all normalized to the average lunar crustal composition (TAYLOR, 1982). This simple calculation shows that although a simple mixture of KREEP plus mafic-poor an-

orthosite can account for the concentrations of many incompatible trace elements (e.g., Ba, Zr, Hf, Nb, Th, U, and the LREE) in the ferroan noritic and magnesian troctolitic clasts, such a mixture cannot reproduce the major element compositions, the shape of the HREE patterns, and the compatible trace element concentrations (e.g., SC, Cr, V, Ni) of these clasts. Similar arguments apply to a model involving the addition of KREEP to the ferroan noritic clasts in an attempt to create the magnesian troctolitic clasts. The excess of compatible elements in the mafic-enriched clasts relative to the mafic-poor anorthosite + KREEP mixtures indicates either that they incorporated a significant amount of the well-known but elusive “Mg-component” required by mixing models of other lunar breccias and soils (e.g., SCHONFIELDand MEYER, 1972; TAYLORand BENCE, 1975; WANKE et al., 1977; WASSON et al., 1977; RYDER, 1979; GARRISONand TAYLOR, 1980) or that they are themselves an important carrier of this component. The suggestion that mafic-enriched anorthosites contributed a significant amount of ferromagnesian material to polymict lunar breccias and soils has been made before ( KOROTEVand HASIUN,1988; LINDSTROMand SALPAS,I983 ) , but the importance of these rock types in the Moon’s crust has not been widely recognized due to their paucity in the lunar rock collection. Further arguments against a mixing model involving KREEP to account for the trace element enrichments of the 670 16 mafic-enriched anorthosite clasts derive from the variations in Li abundances observed in these clasts. Li is poten-

67016 Apollo 16 KREEP

AlCaNaEuSrBaNbHf

ZrTh

U LaCePrNdSmEuGdTbDyHoErYbV

FeCrScTiMgNi

FIG. 10. Concentrations of several elements normalized to average lunar crustal values (TAYLOR, 1982) for representative compositions of ma&z-poor anorthosite (squares), ferroan noritic clasts (black dots), and magnesian troctolitic clasts (white dots) from 67016. Also plotted are normalized compositions ofApollo 16 KREEP (crosses) and calculated binary mixtures of ma&-poor anorthosite with Apollo 16 KREEP (dashed lines). Percentages indicate the amount of KREEP in these mixtures. Although such a model can replicate the abundances of many incompatible trace elements in the noritic and troctolitic clasts, it cannot account for the major element compositions, the slope of the HREE, and the compatible trace element abundances (i.e., V, Cr, SC, Ti) of these clasts. Note the close correspondence of the compositions of the 67016 matic-enriched clasts with those of the average lunar crust.

1021

Geochemistry of the primordial crust of the Moon

tially a useful but little-used trace element in lunar studies. It is the least volatile of the alkalis and is at least moderately compatible in many igneous systems. The equilibrium condensation temperature of Li has been calculated as 10901225 K, depending on the assumed pressure ( WAI and WASSON, 1977 ) , and it appears to have followed the Mg-silicates rather than the refractory elements during early nebular evolution (DREIBUS et al., 1976). There is also a considerable dam base of isotope dilution and radiochemical neutron activation analyses of lunar material available in the literature. Figure 11 illustrates the variation of Li concentration with alumina content in the 670 16 clasts compared to that for the Apollo 16 impact melts and the compositional trends expected for mixtures of typical mafic-poor ferroan anorthosite (FAN), ferroan anorthositic norite, and KREEP. The compositions of the ferroan noritic clasts fall well away from the FAN + KREEP mixing line, showing that the mafic-enriched clasts cannot be simply mixtures of FAN + KREEP. In the impact melts, alumina and Li concentrations are negatively correlated, varying from more aluminous compositions relatively poor in Li to less aluminous compositions relatively rich in Li. None of the Apollo 16 impact melts appear to be two-component mixtures of mafic-poor ferroan anorthosite (FAN) and KREEP. All of the impact melts require at least

100

67016

:

Apollo 16

Ll wm

Magnesian

Matic-poor

10 ; ched anorthosites

: Magma . (&an

LilYb

i basalts KREEP

1,

Mantle Evolution

0.1

I.

1

1

1 10

100

Li ppm

FIG. 12. An outline of the Li vs. Li/Yb systematics for the lunar crust and mantle. Symbols are: KREEP-poor lunar crustal rocks (black dots), Apollo 15 green glass (GG ) , Apollo 12 and 15 low-Ti mare basalts (white squares), Apollo 11 and 17 high-Ti mare basalts (white circles), and KREEP-rich breccias from Apollo 14 and 16 (+). KREEP-poor crustal rocks include anorthositic clasts from feldspathic fragmental hreccias 670 16,674 15, and 67455, and large samples of mafic-poor anorthosites 60025, 60215, and 61016. The complementary compositions of lunar crustal and mantle-derived rocks are thought to reflect the general trends of an evolving magma ocean. The intersection of the crustal and mantle trends may provide an estimate of the magma ocean composition (box). Data from sources listed in Fig. 10 plus LINDSTROM et al. (1977), WINZERet al. (1977), PHILPOTTSet al. ( 1972, 1974), RHODES and HUBBARD (1973), RHODESet al. (1976), !SCHNETZLER and PHILPOTTS(1971) SHlHeta1.(1975),TAYLORetal.(1972),TERAetal.(l970),WANKE et al. (1970, 1971, 1972, 1977), and JOVANOVICand REED (1976).

10

noritic

KREEP Li ppm

10

1

15

..““.“I”“,““,’ 20

‘FAN 25 A1203 wt%

30

35

FIG. 11. Plots of A&O3 vs. Li concentrations for splits of 67016 compared to those of Apollo 16 impact melts (+). Symbols for the 67016 splits are the same as for Figure 3. Data from this study for the 67016 samples and HUBBARD et al. (1973, 1974), PALME et al. (1978), PHILPOTTSet al. ( 1973), and WANKE et al. (1975, 1976) for the impact melts. Solid lines indicate binary mixtures of typical ferroan anorthosite (FAN, 1.5 ppm Li, 35% Al*O,), ferroan noritic clasts from 67016 (3.8 ppm Li, 24.5% A1203), and a model KREEP composition (30 ppm Li, 16% Al,O,), with knots at 10% intervals. Mixtures of FAN + KREEP cannot produce the observed compositions of the impact melts or that of the ferroan noritic clasts. Many of the impact melts have compositions that fall along the ferroan noritic anorthosite + KREEP mixing trend, and all of the impact melt compositions require a component more mafic than FAN.

an additional component more mafrc than FAN, and many of the impact melt compositions scatter along the mixing line joining the ferroan noritic clasts and KREEP. This suggests that one or more matic-enriched components were important in the target stratigraphy of the Apollo 16 impact melts. The ferroan noritic clasts from 670 16 provide a possible example of this mafic-enriched component. The Li abundances of lunar rocks can be investigated in greater detail to understand further the place of mafic-enriched anorthositic material in lunar evolution. Figure 12 presents Li vs. Li/ Yb data for our 670 16 clasts, together with literature data for mafic-enriched anorthositic material from other FFBs, mafic-poor anorthosites, low-Ti mare basalts, high-Ti mare basalts, and KREEP-rich breccias. Li/Yb was chosen because Li appears to behave like a HREE in young terrestrial volcanic rocks (RYAN and LANGMUIR, 1987). Li/ Yb is constant in mid-ocean ridge basalts (RYAN and LANGMUIR,1987) and so provides a useful comparison between lunar and terrestrial rocks. This diagram (Fig. 12) demonstrates the complementary nature of the lunar crustal and mantle evolutionary trends. Compositions of the anorthositic crustal rocks range from mafic-rich varieties with Li contents of -3-6 ppm and Li / Yb of 2-5 to mafic-poor varieties with much higher Li/Yb and a fairly broad range of Li concentrations. The maficpoor anorthosites with low Li concentrations (Fig. 12) are typical pristine ferroan anorthosites (FAN), whereas the mafic-poor sodic varieties discovered in this study have con-

1022

M. D. Norman and S. R. Taylor

higher Li concentrations but Li/Yb ratios comparable to FAN. Apollo 12 and 15 low-Ti mare basalts have compositions that overlap those of mafic-enriched crustal rocks, and extend to somewhat higher Li abundances and lower Li / Yb values. High-Ti mare basalts from the Apollo 17 site typically have higher Li contents and lower Li/Yb ratios than the low-Ti mare basalts. Although KREEP is rarely (if ever) found as an igneous rock directly comparable to the mare lavas and highland cumulates, lunar breccias with high concentrations of incompatibIe trace elements and presumably dominated by the KREEP component have Li abundances about twice that of the high-Ti mare basalts, but relatively low Li/Yb ratios, comparable to those of high-Ti mare basalts ( Fig. 12 ) . The most primitive mantle-derived lunar material for which data are available, the Apollo 15 green glass, has a Li/Yb ratio virtually identical to those of the mafic-enriched anorthosites, although data for the green glasses are especially sparse. We suggest that the Li vs. Li/Yb relations of lunar highlands rocks and mare basalts (Fig. 12) track the evolution of the Moon’s crust and mantle during crystallization of the magma ocean. Mare basalt compositions would reflect the progression of their cumulate mantle sources toward higher Li abundances and lower Li/Yb values. Com~sitions of lunar crustai rocks extend to higher Li/Yb values complementary to the mare basalts. KREEP appears to be the ultimate product of lunar mantle evolution. We think it is significant that the most primitive lunar crustal and mantle compositions (i.e., the mafic-enriched anorthosites and the Apollo 15 green glass, respectively) have similar Li/Yb ratios, and we interpret the intersection of the crustal and mantle evolutionary trends as providing an estimate of the magma ocean (= bulk moon?) composition. This contention is justified at least in part by the observed dist~bution of Li between plagioclase and clinopyroxene as measured on mineral separates from the lunar impact melt rock 14310 (Fig. 13). Because 14310 was essentially a total melt, these data allow estimates of mineral-melt distribution coefficients for several elements, including Li, by ratioing the abundance of an element in a mineral phase to that of the whole rock. From these data, it can be seen that Li is unique among the elements analyzed in that its abundance in both plagioclase and clinopyroxene is similar to that of the whole rock, implying a distribution coefficient (Kd) near 1 for both phases, in contrast to terrestrial values (see later). This is an especially useful result, for a Kd of 1 for both plagioclase and clinopyroxene (both important phases in the lunar crust and mantle ) means that there will be little change in the concentration of this element during both crystahization and melting. In other words, for cumulates, rock compositions provide a direct estimate of their parental magma compositions, and lavas will image their sources. This differs somewhat from the terrestrial case. RYAN and LANGMUIR ( 1987) found that Li behaves like a moderately incompatible element with a Kd of 0.3-0.6 during fractional crystallization of mid-ocean ridge and ocean island basalts, but it is somewhat more compatible than this during melting. In a spine1 peridotite nodule, Li was found to be partitioned subequally among olivine, orthopyroxene, and clinopyroxene, whereas in a garnet peridotite Li was concentrated in the siderably

isolope dibtion dala oi Philpans et al. (1972) ’

Li

U

Rb

Sr

Zr

Ba Ce

Nd Sm

Eu

Dy

Er

Yb

FIG. 13. Concent~tions of various elements in plagioclase and pyroxene separates relative to the whole rock value for lunar impact melt rock 14310. Note that of these elements only Li has approximately equal concentrations in the plagioclase, pyroxene, and whole rock splits. Data from PHILPOTTS et al. ( 1972).

clinopyroxene (RYAN and LANGMUIR, 1987). Neither of these nodules contained plagioclase as a major phase. Yb in both the lunar crustal and mantle rocks is probably mostly in pyroxene, which also has a Kd near 1 in melt rock 143 10 (Fig. 13). Yb is not partitioned strongly into plagioclase. This results in a fractionation of Li from Yb, producing the observed high Li/Yb ratios in the lunar mafic-poor anorthosites. Separation of high LijYb plagioclase results in lower LijYb values in lunar mantle cumulates, consistent with the concept that the lunar crust and mantle are complementary products of the crystallization of a magma ocean. The ubiquitous negative Eu anomalies in mare basalts are also consistent with the crystallization of plagioclase prior to, or coincident with, the formation of the cumulate mare basalt source regions. Assuming that the overlapping compositions of the malicenriched anorthosites and the Apollo 15 green glass images, the magma ocean composition provides values of Li = 3-5 ppm and LifYb = 4-6 for the magma ocean at the time these anorthosites and mare source regions formed. This estimate for the Li abundance is somewhat higher than previous values (cf. 0.83 ppm for the bulk moon by TAYLOR, 1982) but allows a consistent Yb concentration, especially if the Li value is near the lower end of our estimated range (e.g., Li = 3 ppm and Li/Yb = 5 predicts Yb = 0.6 ppm vs. bulk moon value of 0.61 ppm by TAYLOR, 1982 ). If these estimates for the lunar Li abundance and Li/Yb ratio are correct, then the lunar magma ocean seems to have been similar in composition to the earths mantle in terms of its Li f Yb ratio but contained about twice as much Li and Yb (Fig. 14). This result should be interpreted with caution in view of the differing terrestrial and lunar Kd. Li/Yb in the Earth and Moon appeal-s to be nearly a factor of two lower than the CI value (ANDERS and GREVESSE, 1989; WASSON and KALLEMEYN, 1988) but comparable to values observed

in CO, CV, and EL chondrites (Fig. 14). The cause of this variability in Li /Yb is not entirely clear, but the relative abundances of moderately volatile Li to refractory Yb may be related to volatility, or to variations during accretion in the proportions of Mg-silicates to more refractory condensates. Differentiated meteorites have a considerable range of Lil Yb ratios that also overlaps that of the Earth and Moon. Among the differentiated nieteorites, the eucrites have Li/

1023

Geochemistry of the primordial crust of the Moon 10

Acknowledgments-Malcolm McCulloch kindly provided access to

I ”

hisisotopelabfacilities fortherockdissolutions. MikeShelley, Nick

H,L, LL

Ware, andErika Kuchel lenttechnical assistance withtheICP,electron microprobe, and spark source analyses, respectively. Reviews of the manuscript by Odette James, Jeff Ryan, Larry Taylor, Jeff Taylor, and Klaus Keil are appreciated. We thank Frederick J. Doyle (National Space Science Data Center), Alenka Remet (Pacific Regional Planetary Data Center), and the JSC Planetary Materials Data Center for assistance with the landing site and sample photographs. Preparation of this manuscript was partially supported by NASA grant NAG 9-454 to Klaus Keil. This is contribution no. 652 of the Planetary Geosciences Division, and contribution no. 2703 of the School of Ocean and Earth Science and Technology.

LilYb

Editorial handling: S. M. McLennan 1

10

100

Li ppm FIG. 14. An outline of the Li vs. Li/Yb systematics for the Moon

compared to the Earth’s mantle, chondritic meteorites, and basaltic achondrites (black circles). Shown are fields for our estimate of the lunar magma ocean (MO, from Fig. 12), low-Ti and high-Ti mare basal& KREEP (Fig 12) terrestrial mid-ocean ridge basalts (MORB, RYAN and LANGMUIR,1987; RYAN, 199 1, pen. comm.) , and relatively primitive peridotites from Earth’s mantle (JAGOUTZ et al., 1979). Some high-Ti basalts and KREEP breccias have Li/Yb < 1 (see Fig. 12). Average compositions of the various classes of carbonaceous and ordinary chondrites ( WASSONand KALLEMEYN,1988) and an estimate of the eucrite parent body composition (EPB, PALME et al., 1978) are indicated by their initials. Enstatite chondrites plot off the diagram at Li 2.1 ppm, Li/Yb 13.1 (EH), and Li 0.6 ppm, Li/Yb 3.5 (EL), respectively. The lunar magma ocean, the terrestrial mantle, CV, CO, and EL chondrites all have similar Li/Yb ratios but different Li concentrations, precluding a unique match between the composition of the Moon and that of the Earth’s mantle.

Yb ratios similar to the Earth and Moon. By analogy with the Earth and Moon (Fig. 14)) the Li/Yb ratio of the eucrite parent body is likely to be higher than those of the basaltic partial melts (i.e., the eucrites themselves), especially considering the very high Li/Yb sometimes found in the complementary ultramafic meteorites. For example, diogenites can have Li/Yb up to -30 (data from WANKE et al., 1977). Estimates of the eucrite parent body composition ( PALME et al., 1978) suggest a Li/Yb ratio of 6.9, which is intermediate between the CI value and that of the Earth and Moon, although this result is model dependent. CONCLUSIONS 1) Ferroan ‘noritic anotthosite’ appears to be a more common igneous rock type in the crust of the Moon than previously recognized. It may be related to the more familiar malic-poor ferroan anorthosite by igneous differentiation. 2) Trace element compositions of igneous rocks from the lunar crust and mantle show complementary evolutionary trends, consistent with global differentiation in a magma ocean. 3 ) The relative abundances of Li, a moderately volatile element, and Yb, a refractory element, are similar in the Earth and Moon, but the absolute concentrations of both elements appear to be higher in the Moon. 4) Li/Yb in the Moon and the Earth is less than that of CI chondrites and the eucrite parent body, but similar to that of CO, CV, and EL chondrites.

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