EXOGENIC IMPACT MELT DROPLET FOUND IN A METEORITE ANALOG FOR THE SURFACE OF ASTEROID 4 VESTA

Among the most surprising findings by NASA’s Dawn Spacecraft during its recent orbital campaign around asteroid 4 Vesta, was the identification of hydrogen-bearing areas on the surface of this otherwise anhydrous body. These hydrogen-bearing areas have been interpreted to contain exogenic material implanted by impacts onto Vesta’s surface, the exogenic material resembles CM chondrite meteorites. Vesta is a nearly intact relict planetesimal that formed within a few million years of the formation of the first solids in our Solar System and before the formation of Earth. The howardite, eucrite, and diogenite (HED) meteorite clan likely originates from the asteroid 4 Vesta. Of the HED clan, the howardites (breccias composed of clasts with diverse origins) are the best meteorite analogs for most of Vesta’s surface. In one such howardite breccia—a meteorite found in the Grosvenor Mountains (GRO) in Antarctica, GRO 95574—we found an olivine-rich solidified melt droplet that in mineralogy and chemistry does not resemble the impact melts typically found in howardites. As determined by EMP and SIMS, the elemental and oxygen-three isotope signatures of the olivine microphenocrysts in this melt droplet are consistent with this clast forming from a melted primitive chondritic meteorite, most likely a CM chondrite. The Fe-rich composition of the glass in this melt droplet and its bulk silicate composition are also consistent with it forming from whole-rock impact melting of a CM chondrite rather than this droplet being an isolated chondrule. Unmelted CM chondrites contain 6.5-12.5 wt. % water equivalent hydrogen primarily as mineralogically bound OH in phyllosilicates; accordingly, if this droplet formed from a CM chondrite most of this water was degassed because no water was detected in our micro-Raman spectroscopic analyses of the glass in this clast (detection limit of 2 wt. % H₂O for these samples). The identification of this CM chondrite impact melt droplet in a vestan regolith analog complements findings by the Dawn spacecraft mission CM chondrite-like materials were emplaced into the specific areas on the surface of Vesta by impacts.