Md. Norman et al., FRAGMENTS OF ANCIENT LUNAR CRUST - PETROLOGY AND GEOCHEMISTRY OF FERROAN NORITIC ANORTHOSITES FROM THE DESCARTES REGION OF THE MOON, Geochimica et cosmochimica acta, 59(4), 1995, pp. 831-847
Clasts of ferroan noritic anorthosite from Apollo 16 breccia 67016 hav
e mineralogical and geochemical affinities with pristine ferroan anort
hosites and therefore may represent primary igneous rocks from the lun
ar crust. The textures of these clasts document a complex history invo
lving magmatic crystallization, brecciation, subsolidus recrystallizat
ion, and sulfide metasomatism. Nonetheless, the major, minor, and trac
e element compositions of their minerals, as determined by electron an
d proton microprobe, show that the clasts are essentially monomict and
belong to the ferroan anorthositic suite of lunar highlands rocks. Th
e low bulk Ni contents, low Ni/Co ratios, and ancient Sm-Nd isotopic a
ge (greater than or equal to 4.5 Ga; Alibert et al., 1994) of the clas
ts are consistent with this interpretation. Strontium and gallium cont
ents of plagioclase in the clasts show that their parental magma was d
epleted in Ga relative to Sr by a factor of about 30 relative to carbo
naceous chondrites. This depletion of Ga, which is both moderately vol
atile and moderately siderophile, probably reflects volatile loss from
the Moon rather than extraction of metal. The parental magma from whi
ch these clasts originally crystallized was enriched in Sr by about 4-
6X relative to proposed bulk Moon compositions, which implies either t
hat ferroan anorthosites crystallized late in the evolution of a lunar
magma ocean, or that their parental magma was a basaltic partial melt
. Bulk major and trace element compositions of these clasts are remark
ably similar to that of the average upper crust of the Moon, which sug
gests that igneous ferroan noritic anorthosite such as that found in t
he North Ray Crater breccias and soils may be a significant primary co
mponent of the lunar crust. The ferroan noritic anorthosite clasts in
67016 are unusual among lunar samples for their abundance of troilite,
and for the evidence of metasomatic introduction of volatile chalcoph
ile elements such as Cu, Zn, Ni, Sb, and Se. Anhydrous C-O-S-Cl vapors
are the most likely transport agent for these metals, which would be
more consistent with an indigenous lunar origin for the volatiles rath
er than a cometary or meteoritic source.