HR: 09:45h
AN: V51E-08 INVITED     [Abstracts]
TI: On the Itinerant History of Crystals in Magma Reservoirs
AU: * Reid, M R
EM: mary.reid@nau.edu
AF: Northern Arizona University, Dept. Geology, Flagstaff, AZ 86011
AU: Cooper, K M
EM: kmcooper@u.washington.edu
AF: U. Washington, Dept. Earth Space Sci., Seattle, WA 98195
AU: Vazquez, J A
EM: jvazquez@csun.edu
AF: CSU Northridge, Dept. Geol. Sci., Northridge, CA 91330
AU: Simon, J I
EM: jisimon@ucla.edu
AF: UCLA, Dept. Earth Space Sci., Los Angeles, CA 90095-1567
AB: The storage times of magma systems have been imaged by a variety of geophysical and geochemical approaches, each of which provides different insights because each is necessarily biased in some fashion. Perhaps the most fundamental bias is the predominance of magma storage records based on extrusive rocks. This, in turn, implies some bias towards imaging of the most-fluid portions of a magma reservoir. Factors that may affect the probability of eruption and therefore apparent storage intervals are the frequency and interplay between magma replenishment and magma arrest in the crust, the volatile content of the magma, and the tectonic regime of magmatic activity. In situ Pb and Th isotopic analyses of the accessory phases zircon and allanite from rhyolites show 1) that successive eruptions can apparently sample the same crystal populations and 2) that crystal growth may occur intermittently, separated by up to tens of k.y. These results provide evidence for discontinuous crystal growth and for the rejuvenation of growth at least in part by magma mixing and magma replenishment. Chemical analyses suggest that these same observations also broadly apply to the major mineral phases but the chronological details could differ if crystals are selectively preserved during magma ascent and/or mixing, and/or due to differential buoyancy between phases. Our work on the age and compositional zoning of allanite might be particularly revealing in this respect since the buoyancy of allanite is similar to those of major phases. Radiometric methods generally give older crystallization ages than those determined by kinetic considerations (e.g., CSD, diffusional relaxation). Accepting the kinetic ages at face value, crystallization appears to typically require $<$100 y. with a maximum duration of $\sims$1 k.y. for major phenocryst phases. In apparent corroboration of these timescales, many magmas have $^{226}$Ra excesses that are difficult to reconcile with magma storage times of $>$few k.y. $^{230}$Th-$^{226}$Ra ages for mineral {\it separates} are generally indicative of crystallization on timescales that are an order of magnitude greater than those based on kinetic considerations, while $^{238}$U-$^{230}$Th ages may be even another order of magnitude greater still. These observations can collectively be reconciled if "phenocryst" populations include some older crystals thatare not easily distinguished on petrographic grounds. Accessory phase dating indicates that "old" crystals may be derived from earlier intrusions as well as from country rocks and/or source areas. Eruptions may only evacuate a fraction of a magma reservoir. At the same time, magma reservoirs are rarely close to a steady-state balance between influx and efflux nor are they well-mixed. Thus crystals might carry-over from one eruption to next if they are suspended in the most-mobile liquid portions of the chamber or if they are re-entrained in liquid as the liquid-mush transition zone migrates in response to the thermal effects of recharge and/or eruption. The almost ubiquitous evidence for complex and protracted crystal records is especially notable if nucleation occurs largely in solidification fronts: in this case the crystals most susceptible to recycling would represent only the most-recent intervals of crystal growth. The duration of the radiometric crystal record, in contrast, appears to require more dynamic reservoir processes, involving active crystal suspension, and rapid and large migrations in the mush-liquid transition.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting