Geochimica
0 Pergamon
0016-7037/82/091591-I
er Cosmochimico Acra Vol. 46. pp. 1591 to 1601
Press Ltd. 1982. Printed in U.S.A.
1$03.00/O
Lunar anorthosite 60025, the petrogenesis of lunar anorthosites,
and the composition of the Moon
GRAHAM
Lunar
Curatorial
Laboratory,
Northrop
Services,
RYDER
Inc., P.O. Box 34416,
Houston,
TX. 77034,
U.S.A.
(Received January 29, 1982; accepted in revised form April 29, 1982)
Abstract-Systematic
variations of the mineral chemistry of ferroan anorthosite 60025, which is probably
a mixture of closely related materials,
suggest that lunar anorthosites
formed by strong fractional
crystallization
and near-perfect
adcumulate
growth, without trapping liquid. The parent liquid for the
most primitive samples was saturated
with olivine, plagioclase,
pigeonite, and chromite, and evolved to
one saturated with plagioclase, pigeonite, high-Ca pyroxene, and ilmenite. The parent liquid also had
a very low Na20 content, and combined with strong fractional crystallization this explains the steep
trend of anorthosites on an Mg* (atomic 100 X Mg/(Mg + Fe)) V. An diagram. The mineral and
chemical data for other anorthosites
are consistent with such a model. Near-perfect
adcumulation
can
occur if growth takes place at the crystal-liquid
interface without the physical accumulation
of crystals
grown elsewhere, and is encouraged
by the shifts in phase boundaries
with pressure.
Anorthosites
are probably the remnants of a crust floating on, and crystallizing
at the surface of, a
magma ocean originally of bulk Moon composition.
Mineralogical
and trace element data suggest that
the parental
liquid for the most primitive anorthosites
had previously crystallized
no plagioclase
and
some but perhaps very little pyroxene. Hence the bulk Moon appears to be similar to that proposed by
Ringwood (1976) but to have even lower alkalis, a subchondritic
Ca/Al ratio, and REE abundances
and patterns close to chondritic.
The mare basalt sources are not directly complementary
to the feldspathic crust, because experimental
and trace element data indicate that they are too magnesian
and
contain too much high-Ca pyroxene. Other crustal rocks, such as the Mg-suite samples, are not closely
related to anorthosites;
in addition to their chemical differences
they have a different crystallization
sequence: 01 - plag - px, in contrast with the 01- px - plag inferred for anorthosite
parental liquid
evolution.
LUNAR
ANORTHOSITES
are an important constituent
of the
lunar crust, as demonstrated
by the individual large
samples collected on the Apollo 15 and 16 missions,
and by the aluminous nature of highlands soils. However, their genesis is not well understood.
The recogn+ion of the highlands as feldspathic (Wood et al.,
1970; Smith et al., 1970) led to the generalized concept of a magma ocean from which plagioclase
floated to form the crust very early in lunar history.
The idea that the mare basalts (negative Eu anomalies) had mantle source regions which are mafic
cumulates complementary
to the floated anorthosites
(positive Eu anomalies) became, and still is, widely
accepted by lunar scientists (Taylor, 1982).
Lunar anorthosites
are cataclasized
and they
rarely have more than a few percent mafic minerals.
They are ferroan (Dowty et al., 1974) and are chemically and mineralogically
distinct from other highlands igneous rocks, which compose an Mg-suite
(Warner et al., 1976; Warren and Wasson, 1977)
(Fig. 1). This paper concentrates
on the petrogenesis
of the anorthosites,
without recourse to, or a requirement to explain, data for the Mg-suite. The genesis
of the Mg-suite,
at least some members of which
probably date back to the earliest differentiation
of
the Moon, is an important but separate problem. It
might ultimately be related to anorthosite genesis in
some complicated
way, but attempts
to show this
have so far been without complete success (e.g., LonANORTHOSITES
ghi and Boudreau,
1979). In fact the Mg-suite is
internally complex (James, 1982; Ryder, 1982) and
may constitute
several petrogenetic
schemes. The
model for anorthosite genesis suggested in this paper
is neither confirmed nor vitiated by the known characteristics of Mg-suite rocks.
On Fig. 1 the anorthosites
show a steep trend
which has a wide range of mafic mineral compositions
but a narrow range of plagioclase compositions (Mg*
= atomic 100 X Mg/(Mg
+ Fe)). However, individua/ anorthosite
samples have fairly narrow ranges
of mafic mineral compositions.
Those which do not
have narrow ranges have generally been interpreted
to be polymict (McCallum et al., 1975; James, 1980;
Kempa and James, 1982). The steep trend has been
attributed
to abundant cumulus, or suspended
and
assimilated, plagioclases and varied amounts of both
trapped liquid and cumulus mafic minerals
(e.g.,
Wood, 1975; Longhi and Boudreau,
1979; James,
1980). This explanation
appears to be capable of
explaining a similar trend in anorthosites
of the Stillwater intrusion (Raedeke and McCallum, 1979). The
validity of this explanation for the lunar anorthosites
has been questioned by Norman and Ryder (1979)
and Warren and Wasson (1980), in part because
several samples contain large mafic crystals which
cannot result from trapped liquid. Further, Haskin
et al. ( 198 I ) found that the differences in trace elements between splits of individual samples could not
be explained by mixtures of cumulate plagioclases
and trapped liquid.
1591
G. Ryder
1592
mol % Ab in PIW
80m
2
6
5
50.
5
b
z
_.
0
80-
FIG. 1. Mg* of mafics v. An of plagioclase
for lunar
highlands pristine rocks, adapted from Warren and Wasson
(1980). Data for 60025 from this study.
Little attempt
appears
to have been made to establish if there are any systematic variations among
anorthosites; most discussions are of individual samples or generalize for the group as a whole. In the
present study, I made a new, detailed study of sample
60025, in particular its mineralogy and mineral
chemistry, and established that there are systematic
variations and relationships among its minerals. Literature data on petrography and chemistry demonstrates that the same features exist for anorthosites
as a whole. The data strongly suggest that the anorthosites are samples of a suite of related near-perfect adcumulate rocks, i.e., all crystals in a given
unmixed sample crystallized from a liquid of constant
composition and temperature and are unzoned; little
or no liquid was trapped. The rocks crystallized successively from a magma undergoing strong fractional
crystallization. The steep trend on Fig. 1 is a natural
result of the low-alkali content of the magma. The
data allow strong inferences to be made about the
parental liquid and its evolution, and about the bulk
composition of the Moon and its evolution. Mare
basalt sources are not directly complementary to the
anorthosites.
a reinspection of sample 60025 was made. lt proved
to contain clumps of green mafic minerals up to several millimeters wide and a few centimeters long, as
was suggested in the original Apollo 16 Sample Information Catalog (1972). Some of this material
was allocated, with plagioclase, for the geochronological study. At the same time samples were taken
for petrographic and microprobe analyses to support
the geochronological study.
60025 is a large (1836 g) sample of coarse-grained
cataclastic anorthosite. A slab was cut from it (Fig.
2) and all of the original allocations for chemistry,
petrography, and other studies were made from it.
Thin sections from this slab (99% plagioclase) contain iron-rich pyroxenes and lack olivine (Walker ef
al., 1973; Kushiro and Hodges, 1973; Dixon and
Papike, 1975; Takeda er al., 1979). An undocumented chip, rich in mafic minerals and which was
loose in the same sample return container as 60025,
was made into thin sections. Apparently from a lack
of conviction that this chip was really a part of 60025,
no data were reported for it until Warren and Wasson
( 1978) presented mineral analyses. These authors did
not allude to the possibility that this sample might
not be from 60025 nor to the clear difference in the
mafic mineral chemistry (olivines and pyroxenes,
more Mg-rich) from the data previously reported for
60025. The green mafic mineral clumps are on the
end of the sample opposite from the slab (Fig. 2).
For this study, I have made microprobe analyses of
mafic minerals, plagioclase, and oxide minerals in the
clumps; in a thin section serial to that which was
analyzed by Warren and Wasson (1978); and in thin
sections from the slab. The immediate objectives
were to ascertain the composition of the mafic clots.
FEZRROANANORTHOSITE 60025
Lunar anorthosites have extremely primitive 87Sr/
‘??Grratios and so are believed to have been formed
at the very beginning of lunar evolution, approximately 4.5 billion years ago. However, no mineral
isochron has thus far been obtained, mainly because
the mafic minerals are so small and rare. It was in
response to a request for material to obtain an internal Sm-Nd isochron on a ferroan anorthosite that
FIG. 2. Original splitting of 60025: the matic clumps were
at one end and not sampled. Pieces labelled “T” were made
into thin sections, those labelled “G” had grains picked for
grain mounts.
Lunar anorthosite 60025
the total variation of mineral compositions, and the
homogeneity of individual grains. A fuller presentation of the data, including pyroxene minor element
chemistry and oxide analyses, will be given elsewhere
(in prep.). Photomicrographs of the mafic chip and
of picked mafic grains are shown in Figs. 3 and 4.
60025 MAFIC MINERAL
VARIATIONS
The pyroxene analyses were mainly for nine elements,
with deadtime, background, and Bence-Albee matrix corrections. Some were for three elements (Ca, Mg, and Fe)
with an empirical correction procedure, and these agreed
with the former for the same grains to about half mole
percent En. The olivine data were almost all three elements
(Ca, Mg, Fe) with similar correction procedures; a few were
re-analyzed for nine elements and agreed to within one mole
percent Fo. The major element data are shown in Figs. 5
to 7.
The mafic clumps consist of olivine and low-Ca pyroxene,
individually at least 2 mm across (the size of the picked
fragments). Olivine is apparently slightly more abundant,
but the distinction of olivine and pyroxene cannot be made
macroscopically. The pyroxenes are inverted pigeonites: the
augite lamellae are in two directions (Fig. 4) and the bulk
compositions of the grains contain about 8 mol % Wo. The
olivines are homogeneous, Fo~.~-~,.,,.The pyroxenes are individually homogeneous (except for the exsolution lamellae)
but show significant small differences between grains (Fig.
S), wtth a range of En68-72.
The mafic chip which was also analyzed by Warren and
Wasson (1978) contains mafic grains (fractured aggregates) nearly a centimeter across (Fig. 3), with individual
relics up to 4 mm across. They are olivines and pyroxenes
which have compositions identical with those in the mafic
clumps, hence the chip is undoubtedly a part of 60025. (A
systematic difference of a few mol percent in both En and
Fo between the analyses reported here and those of Warren
and Wasson (1978) is apparent.) In addition, the chip contains small pyroxenes and a few olivines which are more
Fe-rich. The pyroxenes include both high-Ca and low-Ca
varieties, and the compositional range extends to that of
pyroxenes in the slab (Fig. 6). Olivine has a much more
restricted range. Individual grains of both olivine (Fig. 7)
and pyroxene are quite homogeneous.
Thus 60025 contains clumps of Mg-rich pyroxenes and
olivines and small grains of more Fe-rich pyroxenes, both
high-Ca and low-Ca. The total range is close to that reported for the ferroan anorthosite group as a whole. While
each grain is homogeneous, the differences between grains
show that metamorphic equilibration has not taken place
over a distance of more than a millimeter, at least since the
rock was cataclasized. Because of their grain size, at least
the Mg-rich mafics did not crystallize from trapped liquid
or exsolve from plagioclase; they must be cumulate or adcumulate.
60025 PLAGIOCLASE VARIATIONS
A survey was made for Na, K, and Ca in many plagioclases, and several grains were separately analyzed for Si,
Al, Ca, Na, K, Fe, and Mg. Probes were operated at 15
Kv accelerating potential and 20-30 nA sample current,
with the beam focused at approximately 3 pm. Backgrounds
were taken on each point in the seven-element analyses.
Standards were feldspars except for Fe and Mg, for which
a clinopyroxene was used. Data were corrected for deadtime, backgrounds, and matrix effects (Bence-Albee). To
avoid secondary fluorescence (Longhi et al., 1976), no analyses were made within 100 pm of any malic or oxide mineral
in the seven-element analyses. In the seven-element analyses
counting for 180 seconds gave 300-1000 total counts above
1593
background for Fe, Mg, and K, and about 2000 counts
above background for Na. Consistency of the data for Mg
for the slab sample with that of Hansen et al. (1979) indicates that the third order CaK, peak is not contributing
to the Mg count, and this conclusion is supported by analyses of the anorthite glass standard which have MgO
- 0.01%. On each grain or area of a grain analyzed, up
to six such counts were made within a few hundred microns
and averaged for the data plotted in Fig. 8. For such point
sets, precisions (1 u) are less than 5% for Na, 20% for Fe
and Mg, and 30% for K. On a given point, individual 30
second counts have 1crof 5- 10% for all elements. The albite
content is calculated directly from Na as was done by Hansen et al. (1979).
Most of the analyses were made on the thin section of
the undocumented mafic chip, the mafic mineral study having shown it to be a part of 60025. Plagioclases from the
slab and from the malic clumps were also analyzed. The
total range of compositions for single analyses in the threeelement survey is Ab1.77-4.35r
with the majority in the range
Ab3.7-4.0.Individual grains are homogeneous.
There is a conspicuous correlation of both Fe0 and Mg*
in plagioclase with the Ab content (Fig. 8). In contrast,
MgO shows scatter around a fairly constant abundance,
and K20 (not shown) has no systematic variation. The data
from the slab samples analyzed are consistent with those
of Hansen et al. (1979) except that their Mg* value is
lower. There is possibly some systematic error in the data
presented here, and I cannot claim the accuracy of Hansen
et al. (1979), who used different and more appropriate standards for Mg and Fe in plagioclase.
SPATIAL ASSOCIATIONS AND MINERAL
CHEMISTRY CORRELATIONS
Although there is a wide variation of mineral compositions, they are not randomly distributed in the rock. In the
slab sample, all plagioclase grains are sodic (-Ab,)
and
the mafic grains, lacking olivine, are iron-rich (Fig. 6). In
contrast, the most Mg-rich mafic grains are adjacent to the
most calcic plagioclases: in the one multimineralic picked
grain analyzed (depicted in Fig. 4b) pyroxene En,*, olivine
Foe,, and plagioclase Abz are present. In the mafic chip,
several textural zones occur (Fig. 3). A consists of cataclasized olivine (FOG,)in coarse grains, with some pyroxene
(En,,). C consists mainly of a single 4 mm pyroxene (En,),
with minor olivine (Fo& and some plagioclase (Abz-&.
D is similar to the slab and consists mainly of coarse plagioclase (Ab,) with minor Fe-rich pyroxenes, and lacks
olivine. B, in contrast, mainly consists of finely ground-up
plagioclases and mafic minerals, with a variety of compositions. Therefore, with the exception of zone B, there are
correlations among the types and compositions of mafic
minerals, and between the compositions of mafic minerals
and plagioclases. These are plotted on Fig. 1, using data
from the slab, the analyzed multimineralic picked grain,
and zones C and D from the mafic chip. The trend is positive
as in normal igneous rocks, but is steep, not quite vertical
(Fig. I). That this slope is positive has not previously been
demonstrated for a lunar anorthosite, although it was suggested by Dixon and Papike (1975) for a group of several
anorthosites. The scatter for anorthosites in Fig. 1 probably
results from a combination of varied-quality analyses, different methods of calculating the albite content of plagioclases, and the unrecognized polymict nature of some of
the samples. (There is also the possibility that different
anorthosites fall on different, roughly parallel, lines.) A further correlation, if the spatial inferences are correct, is that
of Mg* plagioclase with Mg* mafic minerals.
PETROCENESIS
OF 60025
The mafic minerals in 60025 are not accidental
inclusions, because their compositions vary system-
FIG. 3. Thin section 60025,21 from matic chip. a) Crossed polarizers.
b) Plane light, with textura:
zones outlined. Dark pattern in zone D is plucked plagioclase.
not mafic mineral. c) Map showing maltcharacteristics
of the four textural zones.
atically with plagioclase compositions.
The range in
compositions
suggests that the assemblage
was not
produced by metamorphism,
and that 60025 has not
significantly equilibrated
between grains in the subsolidus. At least the large mafic mineral grains did
not exsolve from the plagioclases during exsolution.
Lunar anorthosite 60025
mainly -z .2mm/
FIG. 3. (Continued).
and there is no evidence that the small grains did.
Hansen et al. (1979) suggested re-equilibration
to
be the cause of the calculated KEi”s (Fig. 9) being
less than the 0.49 experimentally
determined for lowTi mare basalts by Longhi et al. (1976). In contrast,
I conclude that the low values approaching
those of
are those of igneous processes. The only sigK&i:LMg
nificant subsolidus mineralogical
changes were inversion and exsolution of pigeonite.
Given an igneous origin and the inferred mineralogical correlations,
the phase variations show a response to an evolving silicate liquid. That liquid was
multiply saturated: plagioclase crystallized
continuously, and at first (Ab,) was accompanied
by pigeonite, olivine, and chromite,
and later (Ab,) by
pigeonite, high-Ca pyroxene, and traces of silica and
ilmenite. Olivine, which ceased to crystallize,
must
1595
have been, or become, in a reaction relationship
with
the liquid. The data are consistent with those from
other anorthosites
in which primary high-Ca pyroxene is much later and in minor quantities compared
with low-Ca pyroxene and olivine; olivine much m;re
Fe-rich than Faso has only rarely been reported.
The mafic mineral clumps in 60025 are of a size
such that they are unlikely to be crystallized trapped
liquid, but similar-sized
grains appear to have been
produced by cumulate
and adcumulate
growth in
terrestrial
intrusions. The clumps also contain aluminous chromite;
their mafic mineral grains have
small compositional
differences;
and they are associated with the fenst sodic plagioclases. None of these
characteristics
is consistent with trapped liquid, nor
indeed with much reaction with the melt at all. The
large mafic minerals would have crystallized
from
a liquid with Mg* - .40, so even the more Fe-rich
mafic grains (Mg* crystals - .50) are too magnesian
to be simply trapped liquid; and silica, phosphates,
and other minor phases are absent or extremely rare.
Haskin et al. (1981) concluded that the rare earth
chemistry of ferroan anorthosites,
including that of
several samples from the 60025 slab, is consistent
with only extremely small quantities of trapped liquid, and the mafic minerals are more abundant than
can be accounted for by these small quantities (this
is so even if 60025 is metamorphosed
as long as it
did not melt). Thus, the mineral compositions
in
60025 were a product of adcumulus
growth in response to a changing liquid composition,
but that
liquid was never trapped very much, and re-equilibration between liquid and crystals did not take
place.
FIG. 4. a) Picked low-Ca pyroxene grain showing exsolution lamellae. Crossed polarizers. Width
- 1.2 mm. b) Picked multimineralic grain. Crossed polarizers. Width -0.7 mm.
1596
G. Ryder
\r
l
0
0
z
2
.
‘.\ ..w
l
Y
Y
Y
Y
72
Yol%
Y
\
68
10
En
might provide a means for partial homogenizatron
(James, pers. comm.), a means lacking in the lunar
case. However, individual plutonic samples of the
Mg-suite have homogeneous mineral compositions.
including those samples which contain evidence (zucon, phosphates, etc.) for trapped liquid. At least one
class of lunar rocks, the feldspathic impactites ur
granulites (Warner et al., 1977) was substantially
equilibrated in the subsolidus. It seems likely that
60025 is polymict, but not generally mixed on a fine
scale, and all the pieces are related magmatically.
Longhi ( 1977) calculated mineral fractiunul crystallization paths for several estimated bulk Moon
compositions. For those with low alkali contents, the
trend on an Mg* (mafic) is. .4b (plagioclase) dia
gram following plagioclase crystallization 1s steep
and similar to that observed in 60025 (and indeed
the whole anorthosite suite). This is perfectfy logical:
in this sequence, there is twice as much Na in the
latest plagioclases (Ab > 4 mole %) as in the earliest
ones (Ab < 2 mole %). Assuming a constant DNa for
plagioclase-liquid, the liquid must have undergone
more than 50% crystallization, enough to allow the
mafic minerals to evolve from Mg* approximately
70 to approximately 50. Thus, the steep slope is 2
natural consequence of the low alkali content, and
does not require trapped liquid. The similarity of the
Stillwater and lunar trends is illusory, or at least the
trends result from different mechanisms. Longhi and
Boudreau (1979) have invoked the suspension and
assimilation of plagioclase to create the steep slope
for anorthosites on the Mg* 11.An diagram. This
model effectively uncouples plagioclase (equilibrium)
from mafic mineral (fractional) crystallization and
would not necessarily produce the mineralogical car.,
relations observed in 60025. That model invokes
trapped liquid which is not apparent in lunar anorthosites.
FIG. 5. Homogeneity and distinction of magnesian pyroxenes in 60025 analyzed in this study a) dashed lines
encompass analyses of individual grains picked from mafic
clumps b) each portion represents a single grain from a thin
section of the undocumented chip. Dashed circle represents
the size of an individual point in Fig. 6.
Individual hand samples from the Stillwater intrusion show much less variation of mafic mineral
chemistry than 60025, usually only a few mol percent
En or Fo, although plagioclase shows variations of
up to 10 mol percent An (Raedeke and McCallum,
1980). While it is possible that this homogeneity in
part results from metamorphism, McCallum (pers.
comm.) believes it to be primary. A greater variation
of up to approximately 10 mol percent Fo, En, and
An exists in those individual samples of the Skaergaard which show independent evidence for trapped
liquid (Hoover, pers. comm., 1981). Trapped liquid
can produce large ranges of mineral composition over
very small distances, but in the terrestrial case, a
hydrous phase present during subsolidus cooling
FIG. 6. Compositions of pyroxenes in 60025. Circles (thus
study): solid, host phase; open, exsolved phase. Tie lines
connect points from individual grains. Dashed areas enclose
previous data from the slab published by Walker et cri.
(1973). Hodges and Kushiro (1973), Dixon and F’apikc
( 1975), and Takeda et al. (1979).
Lunar
anorthosite
1597
60025
FIG. 7. Compositions of olivines in 60025 analyzed in this study. Each vertical stroke is a point analysis;
each horizontal band is an individual grain. Replicates
are analyses on a single grain taken at different
times in the study. Marjalahti
(the standard)
analyses shown to same scale to compare homogeneity.
ADCUMULUS
GROWTH
AND CHEMISTRY
For adcumulate
growth to be the case of lunar
anorthosites
in general, a plot of the calculated liquid
Mg* (henceforth
Mg:) against calculated liquid Sm
from individual samples should show a correlation.
The Sm in the liquid should be proportional
to that
in the bulk rock if most of the Sm is one mineral
*20
,,,,,,,,,
-
,,,,,,,,,
FeOwt%
I,,,
’
*l5-
a’.
:
00
40 -
*
:
.I
V
I
phase, as it is in most analyzed splits of lunar anorthosites.
Some anorthosites,
e.g., 15415, are certainly metamorphosed
(James, 1980) but this should
not affect the bulk chemistry unless partial melting
and migration of the melt took place. Figure 10 plots
Mgf, calculated
from the mafic mineral compositions, against bulk rock Sm, for all samples for which
there are appropriate
data. (Ideally,
this exercise
should be for pure plagioclase separates or for a constant pyroxene content.) For samples larger than 10
g there is a good correlation. The most aberrant samples are 62236 and 62237, which contain significant
amounts of mafic minerals which have been shown
to contain significant Sm (Haskin et al., 1981). The
correlation
slope is appropriate
for the adcumulate
model: for Mgt to evolve from 36 to 20 requires
approximately
60% of a cotectic liquid to have crystallized. Therefore, incompatible
Sm should have increased approximately
2.5 times, which is close to
MgOWt%
.15 0
c
0
8
0
0
VV
0
0
0
.60-
l
6
.o
*50 -
V
l -
: MS*
V
40s
’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 3.0
4.0
mol % Ab
26
’ ’ ’ ’ ’ ’ ’ ’ ’
FIG. 8. Minor elements in plagioclases
analyzed in this
study, except open symbol from Hansen et al. ( 1979). Triangles from the slab, all others from undocumented
chip.
40
wl*
60
60
Plagioclase
FIG. 9. Mg* of parent liquid, calculated from mafic mineral compositions, v. Mg* in plagioclase for several lunar
anorthosite
samples.
X’s are end-members
Adapted from Hansen
from this study.
et al. ( 1979).
1598
G. Ryder
-10
BULK
15
ROCK
Sm
ppm
FIG. 10. Mg* of parent liquid, calculated from mafic mineral compositions, v. Sm of rock for lunar
anorthosite samples, Solid symbols, samples >lO g, open symbols, samples ~10 g. All Mg* calculated
from low-Ca pyroxene except squares, from olivine. Dashed lines connect olivine and pyroxene calculations from a single sample. ?, scatter in data base. Data sources in Ryder and Norman ( 1979, 1980)
and Haskin et al. (1981). I) 65327 2) 65315 3) 65325 4) 60055 5) 64819 6) 60015 7) 15415 8) 1529:i
9)67455,30 IO)67035 11)62237 12)67635 13)62236 14)67636 15)60025 16)62255 17)67455.;i
18) 15362 19) 67637 20) 67455.32
what the bulk rock data show to have happened.
Because Eu is almost constant in the suite, Mgf also
correlates with Sm/Eu, and the increase of Sm/Eu
from the most primitive to the most evolved sample
requires substantial plagioclase separation. A trapped
liquid model, in contrast, does not require a correlation of Mgt with Sm, unless either there are no
cumulate mafics, and liquids evolved to different
stages are trapped in different rocks or the same
amount of cumulate mafics but different amounts of
a single liquid are trapped in different rocks. Neither
are likely or compatible with the data of Haskin et
al. (1981).
Bulk rock NazO abundances (almost exclusively
in plagioclase) have a wide range for rocks larger
than 10 g (Fig. I 1, which is a re-expression of the
Mg* v. Ab diagram), but trapped liquids would produce trivial changes in bulk Na,O. For the central
part of the diagram there is a reasonable correlation
of Na20 and Mgt; the mafic rocks 62236 and 62231
are aberrant and fall closer to the correlation line
when allowance is made for the mafic minerals. The
three samples to the right of the diagram are truly
aberrant (plagioclase analyses by Hansen et ul..
1979, confirm their sodic nature). They are either a
separate type of anorthosite or have been altered in
some way.
A refinement to these tests could be made by analyzing pure plagioclase separates, or by obtaining
pure plagioclase trace element analyses by extrapolation for those samples for which more than one
split has been analyzed (e.g., SC v. Sm; SC v. Y b).
If the strong fractional crystallization with near-perfect adcumulate growth model is correct, the COTrelations (such as in Figs. 10 and 11) should then be
very strong. In calculating parental liquids and their
evolution from bulk rock data, it is clear that the
presence of a few percent mafic minerals, which may
”
*
-20
-30
BULK
FIG. 11.
10.
-40
ROCK
Na20
50
-60
wt%
Mg* of parent liquid v. NarO of rock, analogous to Fig. 10. Symbols, data sources as Fig
Lunar anorthosite 60025
have only a small effect on the light REEs, may have
a large effect on the heavy REEs. If the distribution
coefficient for Yb between pigeonite and liquid is
similar to that proposed by Schnetzler and Philpotts
(1970) then the presence of 2% pyroxene in a rock
will increase Yb approximately
30% over than that
of pure plagioclase.
Detailed calculations
are not
possible without a better knowledge of the relevant
distribution
coefficients, and mineral separates data
for ferroan anorthosites
are highly desirable. All in
all, however, it is clear that the chemical variations
between anorthosites
are more systematic
than the
analysis of Haskin et al. (1981) suggests, and are
consistent with an adcumulate model. Indeed, for the
anorthosite
chemical
data, “Relative
amounts
of
FeO, Co, SC, Cr, and REE vary as would be expected
for different proportions
of plagioclase,
pyroxene,
olivine, chromite, . . . , if rather random proportions
of each phase were present instead of representative
portions of residual liquid or of accumulated
minerals” (Haskin et al., 198 1). In other words, an adcumulate model is compatible with the data.
ADCUMULUS
GROWTH, PHYSICAL SElTING,
AND PHASE EQUILIBRIA
Adcumulus growth for lunar anorthosites
has been
questioned
(e.g., Haskin et al., 1981) because terrestrial intrusions
rarely contain rocks devoid of
trapped liquid products, and because the elimination
of such trapped liquids from a cumulate pile is difficult. However, these objections more or less assume
that the crystals occur like a pile of dropped bricks
with liquid in between. This need not be appropriate
in a magma ocean, in which, once plagioclase had
started to crystallize
and float, slow growth might
take place dominantly at the crystal-liquid
interface;
hence there is no “cumulus”
pile or trapped liquid
to start with. Growth insulates already existing rocks
from reaction even though growth is slow. Further,
if a liquid is crystallizing
plagioclase on the cotectic
at even a shallow depth and convects upwards, phase
boundary shifts due to pressure put it slightly into
the plagioclase liquidus field, thus interface growth
is only of plagioclase, at least at first. The phase shifts
also stabilize olivine instead of pyroxene once the
cotectic is reached again; hence for a triply saturated
liquid, slight oscillations to and from the olivine-pyroxene-plagioclase
point resulting from shallow convection could explain the persistence of olivine over
several mol percent Fo rather than its reacting away.
However, only a small amount of olivine and pigeonite would crystallize at the interface, and most
might crystallize directly on the floor of the magma
system. Morse (1982) discusses the physical possibilities for adcumulate growth of lunar anorthosites.
ORIGIN OF PARENTAL LIQUID
The most primitive
anorthosites
(highest
Mg*)
crystallized
from a liquid at the olivine-plagioclase-
1599
pyroxene point (R in Morse, 1980; and hereafter)
in
the 01-Qu-An phase diagram. How did the liquid get
to R? The liquid probably had not crystallized
plagioclase prior to reaching R because calculated liquids do not show negative Eu anomalies. In fact they
show positive anomalies, which are explained by Longhi (1980) as resulting from plagioclase
being resorbed into the liquid, but are possibly artifacts resulting from our inadequate
knowledge of the Eu
distribution
coefficient. If the adcumulate
model is
more or less correct, then the constancy of Eu among
anorthosites
(see Haskin et al., 1981) suggests that
the partition coefficient DEU (wt. concentration
of Eu
in plagioclase/wt.
concentration
of Eu in liquid) is
actually - 1S, otherwise Eu would increase in successive rocks, assuming cotectic proportions
of plagioclase and mafic minerals crystallizing.
This DE”
is substantially
higher than the 1.0-l .2 value normally assumed, but is compatible
with some experimental
data for low oxygen fugacities
(Drake,
1975) and is probably within estimated errors of the
1.2 value (McKay,
1982). The higher D-value reduces or eliminates positive Eu anomalies calculated
for anorthosite
parent liquids. However, to allow
negative Eu anomalies in the parent liquid would
require even higher D-values. Thus the most primitive anorthosites
probably formed as soon as plagioclase started to crystallize from a magma ocean, if
an ocean is indeed the correct origin (see below). We
are more likely to have sampled thefirst anorthosites
to form than any others, because of flotation.
New studies by McKay (1982, and pers. comm.)
on the relevant crystal-liquid
distribution coefficients
and their application to anorthosite
genesis suggest
only a small, perhaps no, heavy REE depletion in the
parent liquid of primitive anorthosites
if no trapped
liquid is present (a small amount of trapped liquid
does produce a calculated
heavy REE depletion).
However, the data are suggestive of a heavy REE
depletion, which presumably
indicates the crystallization of some pigeonite prior to the liquid arriving
at point R, assuming chondritic
proportions
in the
original system. If so, the liquid travelled along the
olivine-pyroxene
peritectic (or cotectic?).
However,
this argument
is not strong, and without a firm
knowledge of the distribution of heavy REEs between
liquid, plagioclase, and pyroxene, the exact magnitude of the slope of the heavy REEs for the
liquid cannot be determined.
With strong fractional
crystallization,
olivine could not have persisted long
after any pyroxene started to crystallize, so perhaps
little pyroxene did crystallize prior to R. The total
amount of olivine crystallized
cannot be deduced
from the anorthosite
data alone. The crystallization
sequence
of the anorthosite
suite is then 01 opx? - pig - plag, which is different from that of
the Mg-suite samples (definitely 01 - plag - px and
which also includes spinel). In both suites high-Ca
pyroxene is very late.
Was the parent liquid of anorthosites
a magma
1600
G. Ryder
ocean? Two considerations suggest a positive answer
(Longhi, 1980; and pers. comm.). First, the required
complementary mafic cumulates have not been identified among samples, thus they are deeply buried,
implying a large scale system. Second, the anorthosites seem to be global in extent, although more
sampling, especially of the farside (approximately
30% Al,03 from remote sensing data) is needed to
verify this. Anorthosites may also be locally missing.
e.g., Apollo 17 site (Ryder, i 98 1).
If the anorthosites are from a magma ocean, its
original major element composition (and presumably
that of the bulk Moon) must be more similar to that
proposed by Ringwood (1976) than most other proposed compositions, i.e., low alkalis, and low A1203.
It could, of course, be richer or poorer in olivine than
proposed by Ringwood ( 1976) which does, however,
produce mafic minerals of appropriate Mg*. However, other details of the composition must be
changed, in particular lowering its alkali content.
According to the anorthosite data, the bulk composition of the Moon certainly could have, and is likely
to have, a chondritic REE pattern. It could also have
chondritic REE abundances, according to how much
olivine was crystallized before the point R was
reached. However, the late crystallization of high-Ca
pyroxene, and its rare occurrence among highlands
rocks, suggests a distinctly subchondritic Ca/Al ratio
of unknown cause.
COMPLEMENTARY MAFIC CUMULATES AND
MARE BASALT SOURCE REGIONS
Anorthosites crystallized from a cotectic liquid,
hence accumulations of olivine and pigeonite must
exist somewhere in the Moon. This cumulate sequence must have Mg* values ranging from approximately 70 to 50; more magnesian cumulates will not
have crystallized plagioclase (previous section). The
sequence must also lack high-Ca pyroxene except for
minor and very iron-rich ones. Most of the mare basalt sources, according to interpretations
of highpressure experimental data, have Mg* 75-85 (unless
the basalts are produced by -30% partial melting),
and are close to a three-phase olivine + low-Ca cpx.
+ high-Ca cpx. field (e.g., Green et al., 1975; Walker
et al., 1975). At least the high-Ti mare sources have
high-Ca clinopyroxene as a discrete phase, in some
cases residual, although in low-Ti mare sources the
high-Ca component is generally dissolved in a pigeonitic phase at multiple saturation. Trace elements, particularly the rare earths, strongly suggest
the influence of high-Ca clinopyroxene in many mare
sources (e.g., Nyquist et al., 1977). Hence, even
though mare sources have an intrinsic negative Eu
anomaly, they cannot be directly or simply complementary to the ferroan anorthosites. Because they
are also required to contain cumulus ilmenite in some
cases, they may have had a complex origin.
CONCLUSIONS
1) 60025 is a mixture of pieces from a related sequence of anorthosites.
2) This sequence was generated by near-perfect ad-
3)
4)
5)
6)
7)
cumulate growth during strong fractional crystallization.
The parent liquid of the most primtttve anorthosites was saturated with olivine ( -Fo,~), plagioclase ( -Ab2), pigeonite (-EnT2), and chromite, and evolved to one saturated with plagioclase (-AbJ,
pigeonite, high-Ca clinopyroxene.
and ilmenite.
The steep slope of anorthosites on an Mg* (mafits) v. Ab (plagioclase) diagram is a result of the
very low alkali content of the magma and of the
original magma ocean.
The bulk Moon had low A1203, a sub-chondritic
Ca/Al ratio, and REE abundances and patterns
which were probably close to chondritic.
Mare basalt sources are too magnesian and some
contain too much high-Ca clinopyroxene to be
directly or simply complementary to a floated anorthosite crust.
Mg-suite plutonic pristine rocks are not closely
related to anorthosites; their crystallization sequence of 01 - plag - px contrasts with the an
orthosites 01 - px -+ plag.
Acknowledgments-Reviews
by J. LonghI, ! Steele.
G. J. Taylor, D. Walker, and 1. McCallum, who do not
necessarily agree with any or all of the interpretations, contributed greatly to the clarity of the manuscript. This work
was supported by NASA contract NAS 9-15425 for the
operation of the Lunar Curatorial Facility
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