A new method for dating late Quaternary submarine eruptions and its use to investigate magmatic and eruptive processes at island arc volcanoes

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New samples of submarine basaltic lavas from past ridge eruptions of Izu-Oshima were prepared for volatile analysis by FTIR. These include new pieces from the original rock collection as well as new samples collected by a recent cruise in the area. Samples have been prepared as wafers, as individual glass shards, and as melt inclusions. There is still some difficulty with low volatile concentrations giving large analytical uncertainties for thin samples. This large analytical uncertainty prevents these data being used for estimating eruption age from the change in sea level (eruption pressure) since the time of eruption. FTIR analysis is therefore continuing, with the aim of preparing suitably glassy samples of sufficient thickness for smaller analytical uncertainties. Meanwhile, magnetic paleointensity of a subset of samples has been analysed by collaborator Usui. By combining the magnetic paleointensity and geochemistry data for these samples, it is possible to constrain their eruption ages (without volatile data and sea level estimation, for now). The data suggest relatively young eruption ages, consistent with their stratigraphic relation to onland eruptions of known age. A manuscript on this topic has been submitted and is currently under revision.