Surfing the Fra Mauro Shoreline: Highlands Crust at the Apollo 14 Site

Abstract

Plutonic rocks at the Apollo 14 site may be grouped into four lithologic suites. In order of relative abundance these are—magnesian suite, alkali suite, evolved lithologies, and ferroan anorthosites (FAN). Most of the samples described to date occur as clasts in lunar polymict breccias or in regolith breccias; these clasts rarely are more than a few cm in diameter. The magnesian suite (63 clasts) includes troctolite, anorthosite, norite, dunite, and harzburgite, all characterized by plagioclase ≈An95 and mafic minerals with Mg#s from 82 to 92. Alkali-suite rocks (21 clasts) and evolved rocks (10 clasts) generally have plagioclase An90 to An40, and mafic minerals with Mg#s from 82 to 40. Lithologies include anorthosite, norite, quartz monzodiorite, granite, and felsite. Ferroan anorthosites (5 clasts) have plagioclase ≈An96 and mafic minerals with Mg#s from 45 to 70. Plots of whole-rock [Mg/(Mg+Fe)] versus whole-rock [Ca/(Ca+Na+K)] show a distinct gap between rocks of the magnesian suite and rocks of the alkali suite, suggesting distinct parent magmas or distinct physical processes of formation. Chondrite-normalized REE patterns show that rocks of both the magnesian suite and alkali suite have similar ranges, despite the large difference in major-element chemistry. Most magnesian-suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Norites may contain significant trapped liquids, and some alkali norites may represent cumulate-enriched, near-liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include evolved partial cumulates related to alkali-suite fractionation (quartz monzodiorite), immiscible melts derived from these evolved magmas (granites), and impact melts of pre-existing granite (felsite). Recent models for the origin of the magnesian suite envision a komatiitic parent magma derived from early magma-ocean cumulates; these melts must assimilate plagiophile elements to form troctolites at low pressures, and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. As yet there are no plausible scenarios that can explain these unusual requirements. In contrast, alkali anorthosites and norites can be shown to form from crystallization of a KREEP-basalt parent magma similar to 15386, along with some magnesian norites and the evolved quartz monzodiorites. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystallization processes to form the alkali-suite cumulates.