THE ROLE OF SPINELS IN THE PETROGENESIS OF LUNAR MARE BASALTS

One of the most unusual aspects of lunar mare basalts is the abundance of Ti- and Cr-rich oxide minerals. Among these, the most intriguing are spinel and ilmenite as they are useful petrogenetic indicators that can be sensitive to parameters such as fO₂ of crystallization and cooling rate. This study focuses on the complicated chemistry and paragenesis of spinel phases in mare basalts.

A suite of Apollo 12 basalt thin sections was selected at Johnson Space Center, with spinels displaying a variety of interesting textures. Spinels present in lunar mare basalts are an effective solid solution of chromite, ulvöspinel, and hercynite, with the absence of Fe³⁺. Three major spinel occurrences have been observed in this suite of mare basalts: 1) Blue, early-formed chromite typically as inclusions in either olivine or pyroxene; 2) Tan, late-stage ulvöspinel generally found in groundmass; and 3) Composite spinel grains, occurring in all silicate associations and consisting of chromite with ulvöspinel overgrowths. The chemical and textural diversity observed in spinels is a function of their position in the crystallization sequence. These spinels display two types of compositional zonations: 1) chemically gradational contacts, and 2) chemically sharp contacts with a compositional gap between chromite and ulvöspinel end-members. The sharp contacts may reflect breaks during silicate crystallization.

Comparison of spinels with compositions of coexisting silicates, reveals that cooling rate, small shifts in fO₂, and bulk rock chemistry are the major controls on spinel chemistry and texture. Observed spinel and silicate compositions compared to those modeled by the fractional crystallization modeling program, MAGFOX [2], relate Cr # to degree of fractional crystallization. The textural classification scheme of Goodrich et al. (2003) for olivine-phyric Martian shergottites has also been incorporated into this study of lunar basalts. Oxide minerals are important indicators of petrogenetic conditions because they crystallize throughout most of the cooling history of a rock, and are sensitive to physiochemical changes during paragenesis.

References [1] Goodrich et al. (2003) LPSC XXXIV; [2] Longhi (1991) Amer. Min.