HR: 0800h
AN: V31C-1451    [Abstracts]
TI: Mineral Compositions from the Hawaii Scientific Drilling Project (HSDP): Preliminary Results Part II - Plagioclase
AU: * Smart, C M
EM: chrismart@csufresno.edu
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93720 United States
AU: Polfer, K M
EM: k_polfer@hotmail.com
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93720 United States
AU: Putirka, K D
EM: kputirka@csufresno.edu
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2345 E. San Ramon Ave., MS/MH24, Fresno, CA 93720 United States
AB: As noted in part I, the drill core samples recovered from the HSDP provide a temporal view of Mauna Kea volcano, Hawaii. Presently, drill core samples extend to depths of 3000 meters, which represents an eruptive age of approximately 550 ka. This depth interval probably represents about 1/2 the life span of Mauna Kea, since the base of the crust is thought to lie some 6 km beneath the drill hole site. Our samples thus portray the latter half of Mauna Kea evolution. In part I we examined clinopyroxene grains; here we present an analysis of plagioclase compositions from the HSDP, and utilize new models that allow the calculation of crystallization depths and temperatures for plagioclase + liquid pairs (Putirka, 2004, Am Min, in press). As with clinopyroxenes (see part I), we apply a saturation surface model as a filter to test whether plagioclase grains are consistent with equilibrium with their whole rock compositions. Approximately 57 % of plagioclase grains pass this filter. Unlike clinopyroxene, plagioclase compositions change with time. Minimum depth estimates at any particular core depth (i.e., age) all indicate shallow level storage. But maximum crystallization depths increase with decreasing age of the edifice (decreasing depth in the HSDP core). Plots of An and Ab contents vs. core depth reveal similar systematics in mineral compositions with core depth, hence these temporal variations do not reflect liquid composition variations alone. The base of the core, at 3 km, should sample Mauna Kea near the height of its shield-building phase; in these flows all depth estimates are less than 10 km. Interestingly, maximum crystallization depths increase with time, and the greatest depth estimates and the largest depth range are observed for post-shield samples. We propose that plagioclase crystallization reflects a change in magma supply rate. It might be expected that the supply rate of magma at Mauna Kea should be near a maximum for samples recovered at 3000 m, and that supply rates would diminish, and eruption recurrence intervals increase, as one moves upward in the core. Given this expected change, plagioclase compositions may record a late-stage clogging of the magma conduit as recurrence intervals decline. During periods of high magma supply rates the conduit is open; temperatures are sufficiently high so that the plumbing system is rarely clogged, and plagioclase crystallization, which may occur only following sustained periods of storage, occurs only at the shallowest depths. As magma supply rates decline, recurrence intervals become too long to keep the conduit fully open. Magma is thus trapped at progressively greater depths, and stored long enough to achieve plagioclase saturation at depth. The progressive change in plagioclase depths thus reflects a dwindling magma supply rate as Mauna Kea moves off of the Hawaiian hot spot.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting