The University of Arizona

Mineralogy and petrology of the LaPaz Icefield lunar mare basaltic meteorites

K. Righter, S. J. Collins, A. D. Brandon

Abstract


Five basaltic meteorites from the LaPaz ice field are paired on the basis of their mineralogy and texture, and represent a unique basalt type distinct from those in the Apollo or Luna sample collections. LaPaz Icefield (LAP) 02205, LAP 02224, LAP 02226, LAP 02436 and LAP 03632 all contain plagioclase, pyroxene, ilmenite, spinel, olivine, and minor troilite, metal, phosphate, baddeleyite and silica (cristobalite). Brown glassy melt veins are ubiquitous and cross the primary igneous texture. Plagioclase, the major mineral and occurring as laths in a subophitic texture, is of narrow compositional range, from An85-89. Pyroxene, also a major mineral, is strongly zoned, from augite and pigeonite cores to very iron-rich rims. Ilmenite laths comprise approximately 3-5% of the basalts. Spinels show a large compositional range, comparable to that documented in Apollo 15 basalts, indicating an early chromite-rich stage followed by an intermediate to late stage with Cr-rich ulvspinel. Relatively large, subhedral to skeletal olivine crystals (Fo46-62) are sparse, and are too Forich to be in equilibrium with the bulk rock, indicating that these are xenocrysts rather than phenocrysts. The presence of melt veins with a similar composition to the bulk rock, maskelynitized plagioclase feldspar, and metastable cristobalite indicate that these rocks underwent significant shock, between 30 and 50 GPa. Calculated oxygen fugacity, using spinel-ilmenite-iron metal equilibria, is within the range defined by previous studies of lunar materials. The bulk composition (low MgO) and low calculated temperatures, together with modelling calculations, indicate an origin by fractional crystallization of a more primitive low TiO2 parent liquid similar to Apollo 12 olivine basalt.

Keywords


basaltic Meteorites;Fractional crystallization;Antarctic meteorites;Mare basalt

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