POST-CRYSTALLIZATION REHEATING AND PARTIAL MELTING OF EUCRITE EET90020 BY IMPACT INTO THE HOT CRUST OF ASTEROID 4VESTA ~4.50 GA AGO

Abstract

We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET90020. This is one of the few rare basaltic eucrites whose 39Ar-40Ar age has not been reset during impact bombardment on the HED parent body =<4 ga ago and, thus, should provide a unique opportunity to study the nature of early thermal events on its parent body (presumably asteroid 4vesta). hand specimen inspection shows that rock consists fine-grained and coarse-grained lithology. microscopy indicates lithology has granulitic texture, with coarser-grained area large opaque assemblage embedded in matrix. an igneous, subophitic texture. pyroxenes similar those type 5 eucrites (type pyroxene) experienced prolonged metamorphism after rapid crystallization from near-surface melt. however, minor mineral assemblages are unusual suggest complex history. tridymite occurs as laths, irregular crystals (<1.5 mm), tridymite-plagioclase-pyroxene about 4 mm size. one (~1.4 size) is decomposition product cr-ulvospinel zoned ti-chromite, ilmenite fe-metal. opaques often rimmed by fe-rich olivine pigeonite some chemical variations, indicating disequilibrium surrounding pyroxenes. ca-phosphate low ree abundances, lree signatures observed plagioclase melt was present during metamorphism. this eet90020 reheated above subsolidus temperature ~1060°C, causing partial melting. Tridymite, Cr-ulvospinel, and some pyroxene and plagioclase crystallized from the melt. The presence of unequilibrated phases related to opaques suggests cooling rates greater than several °C/day. The reheating event was too short to destroy the exsolution textures of the type 5 pyroxenes. The temperature of the rock just before the reheating could have been ~870°C, based on the two-pyroxene temperature of the type 5 pyroxenes. The absence of shock effects in plagioclases suggests that EET90020 did not experience shock events >1-5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39Ar-40Ar measurements show a relatively flat pattern and an age of 4.49+/-0.01 Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at 4.51 +/- 0.04 Ga appears to be closely related to the remelting of Ca-phosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvospinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages ~4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of ~ 870°C to above the subsolidus temperature of eucrites of ~1060°C.