Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta ～4.50 Ga ago

摘要

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 ~(39)Ar-~(40)Ar 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 the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals (<1.5 mm), and in a large assemblage of tridymite-plagioclase-pyroxene about 4 mm in size. One large opaque assemblage (～1.4 mm in size) is a decomposition product of Cr-ulvospinel and consists of zoned Ti-chromite, ilmenite and minor Fe-metal. The opaques are often rimmed by Fe-rich olivine and pigeonite with some chemical variations, indicating disequilibrium with the surrounding type 5 pyroxenes. Ca-phosphate with low REE abundances, and the LREE signatures observed in plagioclase suggest that melt was present during metamorphism. This indicates that EET90020 was reheated above the subsolidus temperature of eucrites of ～1060 ℃, 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 ℃/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 ℃, 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. ~(39)Ar-~(40) Ar 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). Rsetting of the Sm-Nd ages at 4.51 ± 0.04 Ga appears to be closely realted to the remelting of Caphosphates. 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 ℃ to above the subsolidus temperature of eucrits ～ 1060 ℃.