The isotopic record of lunar volcanism

Abstract

The timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon. A late, heavy bombardment produced most of the observable lunar basins, easily recognized stratigraphic markers and also may have influenced lunar volcanism. Most of the nearside basins seem to have been produced 3.8–4.0 Ga ago, during a period often referred to as the lunar cataclysm. The pre-cataclysm period was characterized by formation of the lunar crust, plutonic rocks within the crust, and pre-mare volcanism. Isotopic analysis of breccia clasts pre-dating the cataclysm has yielded an outline of this period. The isotopic record of post-cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking. Specifically, the isotopic data show that some lunar anorthosites formed ~4.45 Ga ago, consistent with early formation of an anorthositic lunar crust. Some “Mg-suite” norites, troctolites, and dunites also formed approximately contemporaneously with the oldest anorthosites, but others are up to 200–300 Ma younger. More evolved rocks (granites and quartz-monzodiorites) also formed between ~4.4 and ~4.0 Ga ago. The earliest known pre-mare volcanism at ~4.25 Ga ago included aluminous, mare-like basalts and an olivine gabbronorite similar to Apollo 12 basalts. KREEP basalt volcanism began at least ~4.1 Ga ago, approximately contemporaneously with that of the ancient, mare-like basalts, and continued during the cataclysm. Recorded instances of high-Ti mare basalt volcanism began during or immediately after the cataclysm. Low-Ti mare basalt volcanism is the most recent for which samples are available. The isotopic data record an early primary lunar differentiation during which mare basalt sources formed, probably by crystallization of cumulus minerals from a lunar magma ocean. The isotopic record of post-cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources, but more complex petrogenesis also has been suggested. More complex petrogenetic processes including assimilation and Assimilation-Fractional-Crystallization (AFC) seem to be required for pre-mare volcanics. The isotopic data contain a hint that petrogenesis of the oldest pre-mare volcanics may be linked to formation of the so-called “Procellarum Basin,” possibly the oldest and largest lunar basin. Such a basin would provide a geologic context for the pre-mare volcanics, and its formation may have been a defining moment in lunar history.