HR: 13:55h
AN: V53F-02 INVITED    [Abstracts]
TI: Radioisotopic Age Constraints on Crystallization, Crystal Inheritance, and Eruption of Coso's Pleistocene Rhyolites: Tracking the Evolution of a Silicic Magma System
AU: * Simon, J I
EM: simon@eps.berkeley.edu
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, 94709,
AU: * Simon, J I
EM: simon@eps.berkeley.edu
AF: Earth and Planetary Science, UCB, Berkeley, 94720,
AU: Vazquez, J A
EM: jvazquez@csun.edu
AF: Department of Geological Sciences, CSUN, Northridge, 91330,
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, 94709,
AU: Renne, P R
EM: prenne@bgc.org
AF: Earth and Planetary Science, UCB, Berkeley, 94720,
AU: Reid, M R
EM: Mary.Reid@nau.edu
AF: Department of Geology, NAU, Flagstaff, 86011,
AU: Schmitt, A K
AF: Earth and Space Sciences, UCLA, Los Angeles, 90095,
AB: Radioisotopic dating at Coso provides a direct assessment of the rates at which large silicic magma reservoirs develop and whether upper crustal reservoirs remain thermally viable for protracted amounts of time. We dated a representative suite of Coso extrusions (8 units) ranging in age from ~230 ka to ~56 ka using Ar/Ar techniques, with additional analyses forthcoming. Accurate eruption ages are required to track secular geochemical and petrological changes within the magma system. We also dated zircon from a granodiorite core sample from injection well 46A-19RD and in favorable cases we were able to separate and date accessory minerals (zircon and allanite) from the crystal-poor extrusions. Application of accessory mineral dating is a robust approach for quantifying the time scales associated with physicochemical changes in magmas reservoirs. For example, the age distribution and character of zircons from Coso's ~600 ka Devils Kitchen rhyolite led Miller and Wooden (2004) to suggest that it was emplaced in the crust as a near-solidus crystal mush over an ~200 ka period prior to eruption. Our U-Th ages for zircon and allanite obtained by ion microprobe analysis, when compared to their respective ~115 ka (Dome 24) and ~85 ka (Cactus Peak and Sugarloaf Mountain) Ar/Ar ages, yield near-eruption ages and little evidence of recycled zircon (or allanite) from their older Pleistocene predecessors. Likewise zircon ages in the granodiorite exhibit evidence for a single crystallization event at ~200 ka. However, U-Pb dating of a significant subset of zircon in Coso's ~85 ka rhyolites and in the granodiorite core yield Mesozoic ages between ca. 100 and 200 Ma. The latter includes grains that previously yielded U-Th isotope ages within error of secular equilibrium. Two end-member cases may account for the bimodal distribution of zircon ages and evidence for assimilation and crustal contamination of low temperature (≤800 °C) rhyolite, as well as secular changes in their trace element compositions. Post ~230 ka extrusions may have tapped a persistent and integrated magma chamber. If a single long-lived magma reservoir applies to Pleistocene Coso then trace element variations (e.g., La/Nd) track its chemical differentiation. Alternatively trace element changes may reflect separate magma generation events with distinct source components. The observed decrease in the La/Nd ratio of post ~115 ka extrusions, which is often caused by fractionation of allanite or monazite, can be used as a monitor of fractional crystallization differentiation. However this trend is also consistent with auto-assimilation of highly evolved intrusions. Increases in the La/Nd ratio in Coso's ~230 ka to ~115 ka rhyolites likely reflect an episode of magma replenishment by less evolved melt, but could be due to incorporation of allanite-rich cumulate materials. Fluctuating La/Nd ratios recorded by the succession of Coso rhyolite extrusions, therefore, either represents: (1) a single long-lived reservoir that incorporated and/ or was rejuvenated by a less evolved component starting at ~230 ka, then was contaminated by a more evolved component by ~115 ka, and finally incorporated Mesozoic crust, as indicated by the xenocryst ages showing up in the ~85 ka rhyolites, or (2) the existence of a number of distinct magma bodies that formed and evolved more or less independently.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting