Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum

Abstract

Electron probe study of the pyroxene phenocrysts in rock 12021 of the Apollo 12 sample reveals an extraordinary variety of zoning phenomena. These phenocrysts have cores of relatively homogeneous pigeonite which are mantled by Ca-rich pyroxenes; the mantles are zoned outward from about 20 mol per cent FeSiO3 at the core-mantle boundaries to 70 mol per cent FeSiO3 at the margins of the phenocrysts. The character of the normal zoning differs in each growth sector. Four different trends characterizing growth on (110), (100), (010), and parallel to c are described. Pronounced but erratic oscillatory effects are impressed on the normal zoning in all sectors. Normal zoning in lunar pyroxenes is generally more extensive than in terrestrial pyroxenes, probably because the lunar lavas were an order of magnitude more fluid than terrestrial basalts. Turbulent flow in lunar lavas may have caused some of the oscillatory effects. The cyclic nature of augite and pigeonite crystallization in these pyroxenes is probably kinetic in origin; conceivably it could reflect the presence of an augite-pigeonite peritectic. It is likely that further knowledge of the crystal structure of pigeonite at high temperatures will be required before a structural model can be formulated that will satisfactorily explain the details of the sector zoning. Coarse-grained pyroxferroite and subcalcic ferroaugite have crystallized in apparent ‘equilibrium’ in the final stages of formation of the groundmass in rock 12021. Nevertheless, the crystallization of the pyroxferroite was probably metastable relative to the assemblage ferroaugite+ fayalite+ tridymite. Data on the effect of Mg substitution on the stability of pyroxferroite will be needed to establish this point. We suggest that the pigeonite cores of the phenocrysts in rock 12021 crystallized under quiescent conditions, probably in a magma chamber below the lunar surface, and that the strongly zoned augite mantles crystallized during emplacement of the lava on the surface.