Interior of the Moon: The presence of garnet in the primitive deep lunar mantle

Abstract

New trace element data have been generated by inductively coupled plasma‐mass spectrometry (ICP‐MS) for 65 high‐ and low‐Ti mare basalts from the Apollo 11, 12, 14, 15, and 17 landing sites. These data demonstrate that ratios of the high field strength elements and yttrium are generally chondritic in the mare source regions for the low‐Ti basalts but nonchondritic in the high‐Ti basalts and potassium, rare earth elements, and phosphorus (KREEP); such differences can be explained by addition and subtraction (respectively) of ilmenite. This interpretation is consistent with derivation of the basalts from lunar magma ocean (LMO) cumulates through which late stage ilmenite has sunk and mixed with earlier magma ocean products. Compositional differences between the mare basalts and volcanic glass beads suggest that at least some of the latter were derived from a garnet‐bearing source region. Siderophile and chalcophile element data suggest that the garnet‐bearing source escaped the LMO melting event and thus represents primitive Moon material. The remaining glasses are generally very low Ti (VLT) and appear to have been derived from early LMO cumulates (i.e., olivine and orthopyroxene, similar to the VLT basalts). The remaining glasses were derived from below the LMO cumulates, and as the magmas rose they underwent sequential assimilation (to varying degrees) initially of an ilmenite‐rich cumulate followed by a KREEP component. This study presents geochemical evidence for the existence of garnet in a mare source and demonstrates the likelihood that the LMO was not “whole Moon,” thus preserving primitive lunar material, which has acted as the source region for at least some of the volcanic glasses.