Magmatic Sources and Processes Recorded by the Volcanics from the Izu Bonin Rear Arc

Abstract: The Izu rear-arc volcanism was investigated by IODP EXP350 in spring 2014. A total stratigraphy of1800 m was drilled, and contains volcaniclastic material within tuffaceous mudstone. This study willfocus on the explosive volcanism that erupted a 44 m thick unit dominated by lapilli-tuff and lapillistonebetween 682 and 727 mbsf that was deposited at approximately 4.3 Ma. Mineral chemistry and 87Sr/86Srisotope analyses has been conducted on plagioclase crystals with the objective of investigating themagmatic processes and magma sources recorded in the phenocrysts; themobarometric models wereutilised to infer conditions of magma storage. We present analyses of 93 individual plagioclase crystalsfrom 10 stratigraphically constrained rock samples and provide 645 new mineral spot analyses. Theplagioclase crystals exhibit a wide range of anorthite contents with a notably bimodal distribution. Thedominant population has ~An 20 to 40 (n=318) and a smaller population, with a left skewed distribution,has a peak at ~An 70 to 90 (n=155). Compositional variations in the plagioclase population as well asnormal and reverse zoning in individual phenocrysts reveals a complex mixing history. Stratigraphicrelationships between the samples indicate episodic recharge of a felsic magma chamber by a moremafic magma. There are 11 87Sr/86Sr analyses reported from 10 individual crystals returning valuesbetween 87Sr/86Sr = 0.703096 and 0.703380; 2SD on these analyses range from 0.000009 to 0.000024.These likely represent a low 87Sr/86Sr magma reflecting the ambient PSP mantle, and a magma whichhas experienced a higher degree of flux of subduction fluids resulting in an enriched 87Sr/86Sr signature.Thermobarometric modelling suggests mixing between these magmas likely occurred both in thelithospheric mantle and in a shallow crustal region; modelled crystallisation temperatures range from922 to 1175 (±36) °C and pressures range from 10 to 1590 (± 247) MPa.