What's up? Preservation of gravitational direction in the Larkman Nunatak 06299 LL impact melt breccia

Abstract

Larkman Nunatak (LAR) 06299 is a vesicular LL chondrite impact melt breccia that cooled rapidly (0.1-0.3°Cs-1) during crystallization. Ar-Ar data from the literature indicate that the impact event that formed this rock occurred approximately 1Ga ago. About 30vol% of the meteorite consists of a melt matrix containing faceted and intergrown mafic silicate grains (mainly 4-11μm size olivine phenocrysts) partially to completely surrounded by 2-20μm size patches of plagioclase. Suspended in the melt are 30-370μm size ellipsoidal to spheroidal metal-sulfide nodules (several hundred per thin section), many connected to 8-600μm size ellipsoidal to spheroidal vesicles. Most of the metal-sulfide nodules contain a large oblate metallic Fe-Ni bleb at one end of the nodule. For approximately 90% of the nodules, the metal blebs are aligned on the same side of the nodules; for approximately 80% of the nodules that are adjacent to vesicles, the vesicles are attached to the opposite end of the nodules from the oblate metal blebs. Most of the oblate metal blebs themselves are flattened in a direction perpendicular to the long axis of the nodule/vesicle. These features result from alignment in the gravitational field on the LL parent asteroid, making LAR06299 the first known chondrite to indicate gravitational direction. Using reasonable estimates of the cooling rate, viscosity of the metal-sulfide melt, and asteroid density, as well as the observed sizes of constituent phases in LAR06299, we obtain a lower limit of approximately 1.5km for the radius of the LAR06299 parent body. The body was probably substantially larger. © The Meteoritical Society, 2011.