INV10d-0002
Magmatic diversity as an indicator of changing tectonics in the post-Oligocene Izu-Bonin Arc

Tuesday, 15 December 2020: 18:20
Luan Heywood1, Kimberly Wurth1, Susan M DeBari2, James B Gill3, Susanne M Straub4, Julie Christin Belo5 and Ricardo Daniel Escobar-Burciaga2, (1)Western Washington University, Bellingham, United States, (2)Western Washington University, Bellingham, WA, United States, (3)Univ Califonia Santa Cruz, Santa Cruz, CA, United States, (4)Lamont Doherty Earth Observato, Palisades, NY, United States, (5)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Abstract:
Intraoceanic arcs are important sites for their large range of erupted magma types, despite their comparatively thin crustal structures. Long-term tracking of spatial and temporal shifts in magma geochemistry can reveal coherent trends that document changing geodynamics.

Heywood et al., 2020 presented a re-contextualisation of magmas erupted recently (<7 Ma) in the Izu-Bonin arc segment between and west of Aogashima and Torishima. We examined new and published analyses of volcanic rocks obtained via dredging and ROV diving, with new data from regionally derived glass shards obtained at International Ocean Discovery Program (IODP) Site U1437. We showed that volcanic rocks can be separated into three magma series, where each corresponds broadly to an across-arc region: volcanic front series (La/Yb < 1.2), rift-related series (La/Yb = 1.2-2.2), and rear-arc seamount chain (RASC)-type series (La/Yb > 2.2).

Using this framework, we examined trace element data from regionally-derived clinopyroxenes from Site U1437. By using partition coefficients confirmed with coexisting fresh glass and clinopyroxene from <4.4 Ma Site U1437 ash layers, we used clinopyroxenes to infer the trace element characteristics of calculated liquids from older parts of the U1437 core (which lack fresh glass). In this way, clinopyroxenes can help create a history of rear-arc melt compositions spanning 0-15.4 Ma.

Results indicate that prior to 9 Ma, the dominant magmatic type at Site U1437 showed rift-related chemistry (average La/Yb = 2.0), which transitioned to a RASC-type signature (average La/Yb = 3.6) between 4.4-9 Ma. This suggests that the primary locus of rift-related magmatism has migrated eastward over time, which we interpret as accompanying slab roll-back.

Heywood, L. et al. (2020). Across-arc diversity in rhyolites from an intra-oceanic arc: Evidence from IODP Site U1437, Izu-Bonin rear-arc and surrounding area. Geochemistry, Geophysics, Geosystems, 21.

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