The coastal lowland of Minami Izu, Shizuoka Prefecture, Japan, has been inundated by tsunamis with wave heights of 5–6 m at least two times during the past 400 years. However, there have been no investigations of the frequencies or magnitudes of tsunami occurrences over long-term geological timescales in this area. We therefore conducted stratigraphic and paleoenviromental research on Holocene deposits based on a new outcrop and sediment cores (8–11 m long) from four sites in the lowland, examining the evidence for tsunami deposits. The Holocene sediments were deposited in a variety of coastal plain, shallow water, shoreface, backshore/dune, back marsh and floodplain. No any evidence for tsunami deposits was identified from the terrestrial deposits.
The whole-rock chemistries of the mafic rocks in the Moho Transition Zone, Fizh Massif, the Oman ophiolite have been analyzed by X-ray fluorescence spectrometry. The mafic rocks were classified into three texture types based on their microstructures: weakly deformed gabbro, moderately deformed gabbro and ultramylonites. There is no systematic variation of the whole-rock chemistries among the three texture types. The ultramylonites show various chemistries, although they consist mostly of very fine-grained matrix with minor relatively coarser porphyroclasts. It may suggest the effect of fluid-rock interaction during ductile shearing in the Moho Transition Zone or variation of the mafic rock chemical compositions.
A spinel-melt oxybarometer is established by combining the equation of Ariskin and Nikoraev (1996), describing the Fe2+-Fe3+ partitioning between spinel and mafic melt, and that of Sack et al. (1981), relating the Fe3+/Fe2+ ratio and oxygen fugacity (fO2) in silicate melt. Published experimental datasets for spinel-melt equilibria under controlled fO2 and temperature conditions were compiled, used for evaluating the reliability of the oxybarometer; the oxybarometer represents experimental fO2 conditions within ± 1 log unit (1σ). The oxybarometer was applied to spinel inclusions crystallized with olivine phenocrysts at liquidus temperatures in Cenozoic alkali basaltic magmas from the Higashi-Matsuura district, NW Kyushu, Japan. The results indicate that the redox states of the alkali basaltic magmas at liquidus temperatures were ca. 0.5 log unit below the quartz-magnetite-fayalite (QMF) buffer. The redox states are consistent with previously published results estimated from lithospheric mantle-derived spinel peridotite xenoliths from NW Kyushu, and are within the range of abyssal peridotites, suggesting that fO2 in upper mantle beneath the NW Kyushu back arc region was similar to that of MORB mantle.