Supercooling on the lunar surface: a review of analogue information

Abstract

The crystallization of the principal constituent minerals, olivine, pyroxene and plagioclase, in dendritic or skeletal forms, is much more characteristic of lunar than of terrestrial igneous rocks. This type of crystallization is found in lunar rocks of such varied composition as olivine-normative mare basalt (12009) and spinel troctolite (62295). Olivine and pyroxene often occur as skeletal phenocrysts and the stage at which they crystallized is of crucial significance to interpretations of the genesis and cooling history of the porphyritic lavas. Furthermore, there is a widespread occurrence of glass and of immature, radiate crystallization, particularly of highly zoned pyroxenes and zoned plagioclase. Most of these characteristics are a response to a range of supercooling and rapid crystal-growth conditions that are typically lunar. The research value of direct comparison with terrestrial analogues is severely restricted, and the value of controlled cooling-rate experiments, with melts of appropriate compositions, is correspondingly enhanced. This need for information on the rapid consolidation of the lavas extruded on the Moon, and of impact liquids, has stimulated investigations of the phase petrology of supercooled melts. The evidence adduced from these analogue approaches is reviewed. Recently published research results based on this evidence have provided new guide-lines to interpreting crystallization on the lunar surface.