The central focus of our work over the last four years has been the elucidation of processes and magma suites that formed the western highlands crust. Our work at the Apollo 14 site focused on the two largest groups of post-magma ocean igneous rocks, the Mg-suite and the alkali suite, using electron microprobe and ion microprobe studies of mineral chemistry, whole rock geochemistry, and new petrologic observations of existing samples. We have adopted the use of Moore County achondrite as a plagioclase standard for SIMS analysis (after Papike and others, 1996), which has allowed us to complete two major manuscripts on the REE geochemistry of the Mg-suite and alkali suite parent magmas (Shervais and McGee, 1998a, Geochimica Cosmochimica Acta, vol. 62, p. 3009-3023, and Shervais and McGee, 1999b, American Mineralogist, vol. 84, #5/6, p. 806-820). We have completed a major review of highland lithologies at the Apollo 14 site, including new whole rock major element geochemistry for nine highlands clasts, and integrating our new data on the Mg-suite and alkali suite parent magmas (Shervais and McGee, 1999a, Journal of Geophysical Research, vol. 104, #E3, p. 5891-5920). We have also published a shorter review of western highlands petrology as part of the Larry Taylor memorial (Shervais, 1999, International Geology Review, vol. 41, p. 141-153). These publications include our first attempts at constraining the Mg-suite and alkali suite parent magmas, and modeling their evolution. This work is still incomplete because it is not yet clear which forward modeling programs are most appropriate for lunar highlands compositions (McGee and Shervais, 1998a).
The Mg-suite is an enigma because rocks of this suite exhibit both refractory mineral chemistry (indicating a primitive parent magma) and high concentrations of KREEPy incompatible elements (indicating an evolved parent magma). Previous ion probe studies of this suite were confined to the relatively evolved norites (Papike et al, 1994, 1996). Our study is the first and only SIMS investigation to focus on primitive Mg-suite troctolites and anorthosites (Fo87-90, An94-96). Our data show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents Å1.5x to 2x high-K KREEP in concentration, and relative REE abundance patterns similar to KREEP. The data do not support models for crustal metasomatism to enrich the Mg-suite cumulates after formation, or models which call for a superKREEP parent to the troctolites and anorthosites (Shervais and McGee, 1998a).
We have also begun modeling Mg-suite formation and evolution. Our data, and previous work on this suite, suggest that Mg-suite parent magmas must have ultramagnesian komatiitic compositions that are relatively high in both Ca and Al (Hess, 1994; Shervais and McGee, 1999a). The most likely source of these magmas is partial melting of the primitive lunar interior, followed by buffering to high Mg contents in rising diapirs of early lunar magma ocean cumulates (Shervais and McGee, 1999a). We also suggest that the Mg suite may evolve along two distinct crystal lines of descent, depending on the depth of intrusion: deep crustal intrusions may form px-bearing troctolites and Mg anorthosites with high mg’, while shallow intrusions form the series dunite-troctolite-gabbronorite, with lower mg’ troctolites (Shervais and McGee, 1998a, 1999a).
The alkali suite has more evolved mineral compositions than the Mg-suite, but similar whole rock incompatible element concentrations. Our SIMS data show that plagioclase-rich cumulates of the alkali suite crystallized from magmas with high REE concentrations (Å0.7x to Å2.2x high-K KREEP) that were fractionated relative to high-K KREEP (La/Lu Å2x high-K KREEP), had small positive Eu anomalies relative to KREEP, and were enriched in plagiophile elements (Shervais and McGee, 1998b). The alkali suite parent magma may be related to the Mg-suite parent magma, but these magmas cannot be related by simple fractional crystallization as suggested by Snyder et al. (1995a). Our data suggest that the alkali suite parent magma may have originated as a KREEPy melt, but it was modified by anorthosite assimilation, fractionating the REE and enriching the resulting hybrid magma in Eu and other plagiophile elements (Shervais and McGee, 1998b, 1999a). This is the first published SIMS data for any alkali suite rocks (aside from various LPSC abstracts by the PI, and an abstract by Snyder et al, 1994).
We have developed a new model for the formation of alkali suite anorthosites and norites, based on our SIMS data and on fundamental petrologic observations, in which the assimilation of calcic anorthosite forces the crystallization of additional sodic plagioclase. In diopside-saturated ternary systems, assimilation of calcic plagioclase will force the hybrid melt into the plagioclase-only volume along isotherms that slope towards albite. Subsequent crystallization will result in plagioclase that is more sodic than crystals formed immediately prior to assimilation, and the volume of melt will decrease rapidly as the assimilated calcic plagioclase reacts with the liquid to form more sodic equilibrium feldspar (Shervais and McGee, 1998b,c, 1999a). Because of the low REE contents of ferroan anorthosite plagioclase, we conclude that the assimilant must have been older Mg-suite anorthosite or even troctolite, where the dense mafic phases would settle out of the system while plagioclase would float and digest slowly into the melt. We are currently preparing a manuscript for publication which develops this model more fully.
We have also found direct evidence
for magma mixing in one alkali suite anorthosite (Shervais and McGee, 1998d).
The mixing of primitive magma with a more evolved magma has distinct petrologic
manifestations that are easily distinguished from those produced by the
assimilation of crystalline rocks. In particular, the occurrence of reverse
zoning in early-formed crystals is characteristic of magma mixing, but
does not occur during assimilation because the assimilant cannot raise
the temperature of the hybrid magma above its pre-assimilation value. There
is some suggestion that the magma mixing we have observed involves mixing
of an Mg-suite magma into an alkali suite magma. Alternately, an evolved
alkali suite magma may have mixed with a primitive alkali suite parent
magma &emdash; perhaps during convective overturn of a zoned magma
chamber. Injection of hot primitive melt into the magma chamber may have
induced this convective overturn, with mixing between the primitive magma
and the evolved magma already resident in the chamber (Shervais and McGee,
1998d).
