Glass veins in the unequilibrated eucrite Yamato 82202

Abstract

The unequilibrated eucrite Yamato 82202 (Y82202) contains a network of relatively thick (up to 1 mm in width) glass veins. The host of the meteorite represents a monomict breccia composed of volcanic rock that crystallized > 4.3 Ga ago as a lava flow on the surface of 4 Vesta. The veins formed 3.90 ± 0.04 Ga ago, probably as a result of frictional melting associated with impact, under conditions of low effective oxygen fugacity and higher sulfur fugacity. The glass contains disordered structural elements characteristic of pyroxene and feldspar, reminiscent of the eucritic target lithology. The unequilibrated pyroxenes of the volcanic host and the pristine character of the glass indicate that this meteoritic material did not experience significant thermal metamorphism after initial crystallization of the lava. Hence, it was not affected by regional metamorphism caused by burial to a significant depth or by long-term contact metamorphism associated with a thick lava flow, large intrusion, or hot layer of impact ejecta. The meteorite resided at a shallow depth (though not at the surface) on 4 Vesta or on one of the vestoids until it was ejected and traveled to Earth, probably with other HED materials that have 36Ar exposure ages of ∼13 Ma. These data suggest that the surface of the asteroid 4 Vesta has significant, though probably small, proportions of glass, as well as unequilibrated volcanic rock.