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

Larkman Nunatak (LAR) 06299 is a vesicular LL chondrite impact melt breccia that cooled rapidly (0.1-0.3 degrees C s-1) during crystallization. Ar-Ar data from the literature indicate that the impact event that formed this rock occurred approximately 1 Ga ago. About 30 vol% of the meteorite consists of a melt matrix containing faceted and intergrown mafic silicate grains (mainly 4-11 mu m size olivine phenocrysts) partially to completely surrounded by 2-20 mu m size patches of plagioclase. Suspended in the melt are 30-370 mu m size ellipsoidal to spheroidal metal-sulfide nodules (several hundred per thin section), many connected to 8-600 mu m 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 LAR 06299 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 LAR 06299, we obtain a lower limit of approximately 1.5 km for the radius of the LAR 06299 parent body. The body was probably substantially larger.