Post-caldera volcanism: in situ measurement of U-Pb age and oxygen isotoperatio in Pleistocene zircons from Yellowstone caldera

The Yellowstone Plateau volcanic field, the site of some of the largest kno wn silicic volcanic eruptions, is the present location of NE-migrating hots pot volcanic activity. Most volcanic rocks in the Yellow stone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km(3) of L ava Creek Tuff(LCT), have unusually low oxygen isotope ratios. Ion micropro be analysis of both U-Pb age and delta O-18 in zircons from these low-delta O-18 lavas reveals evidence of complex inheritance and remelting. A majori ty of analyzed zircons from low-delta O-18 lavas erupted inside the Yellows tone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5-0.4 Ma). These ages and the high-delta O-18 cores indicate that these lavas are largely derived from nearly total remelting of normal-delta O-18 Huckleberry Ridge Tuff (HRT) and other pre- LCT volcanic rocks. A post-HRT low-delta O-18 lava shows similar inheritanc e of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly pr oposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and U-Pb isotope ratios of individual crystals. (C) 2001 Elsevier Science B.V. All rights reserved.