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Master of Science (MS)
DeBari, Susan M., 1962-
Straub, S. M. (Susanne M.)
Rusk, Brian G.
Magmas from the Izu Bonin rear-arc show a geochemical signature (medium- to high-K and light rare-earth element [LREE] enriched) that resembles the average composition of the continental crust. I investigate that continent-like signature by examining a suite of rear-arc-derived mafic to felsic tephras from International Ocean Discovery Program (IODP) Site U1437, which was drilled as part of IODP Expedition 350. Volcaniclastic glasses from Site U1437 comprise a compositional series from basalt to rhyolite, aged 1.1-4.4 Ma.
Site U1437 tephras record the timespan when the Izu-Bonin arc began rifting and provide a well-dated record of changing magmatic compositions through the transition into arc rifting. Considered with context provided by a comprehensive recontextualization of published analyses ofrocks, magmatic compositions are shown to vary in coherent chronological and spatial trends and can be classified into three series: LREE-depleted volcanic front series; flat pattern REE rift-related series; and LREE-enriched rear-arc seamount chain-type (RASC-type) series which includes Site U1437 glasses. Each series has a distinctive characteristic basalt type whose trace element and radiogenic isotope chemistry matches rhyolite type from the same region.
Geochemical modeling shows that fractional crystallization is a viable mechanism to generate2and oxygen isotope systematics show that some degree of open-system processes are involved in rhyolite petrogenesis; however there is no evidence for large-scale melting of pre-existing crustal sources. The >12.9 Ma crustal sources inferred to exist within the rear-arc region cannot be the source for rear-arc rhyolites because of differing La/Yb and ɛHf signatures. Thus, I favor a model where fractional crystallization is the differentiation mechanism involved in the generation of intraoceanic arc rhyolites in this region.
igneous petrology, Izu-Bonin arc, subduction zone, island arc, intracrustal differentiation, rhyolite, IODP, continental crust, fractional crystallization, tephra
Western Washington University
North Pacific Ocean
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Heywood, Luan J. (Luan Jean), "Diversity and Petrogenesis of Rhyolites from an Intra-oceanic Arc: Evidence from IODP Site U1437, Izu Bonin Rear-arc and Surrounding Area" (2018). WWU Graduate School Collection. 759.