Hidenao Hasegawa

Contemporaneous ascent of within-plate type and island-arc type magmas in the Beppu-Shimabara graben system, Kyushu island, Japan

Abstract Within-plate (WP) type and island-arc (IA) type magmas have erupted in north Kyushu and south Kyushu, respectively, during Quaternary time. The former is related to the eastern end of the East Asian extensional province and the latter to the subduction zone of the Philippine Sea plate. The Beppu–Shimabara graben system in the central part of Kyushu island is located in the area of overlap between them. Quaternary volcanic rock samples from the graben system can be chemically classified as WP- and IA-types by their multi-element patterns and positions on a Zr–Nb–Y discrimination diagram. Rocks of both type and of similar ages occur throughout the graben system. Furthermore, the rocks of WP-type give systematically higher 87Sr/86Sr ratios (0.7048–0.7049) than those of the associated IA-type (around 0.7040). The relations between 87Sr/86Sr and Nb/Y ratios of the former are clearly distinguished from those of the latter. These results demonstrate that this graben system is characterized by the contemporaneous ascents of WP- and IA-type magmas to the same Quaternary volcanic areas. Such rivalrous ascents of WP- and IA-type magmas seem to be a common feature of magmatism associated with graben in extensional provinces related to subduction zones of island arcs and their boundary areas.

A boundary between microbial gas and meta-thermogenic gas based on light hydrocarbon composition of natural gas.

Chemical compositions of methane (C1), ethane (C2), and propane (C3) of natural gases from their production wells in Japan were measured for clarifying mechanisms of their generations in combination with δ13Cc1 values. Natural gases having δ13Cc1 values of -70‰ to -62‰ give extremely high C1/(C2+C3) ratios of 1, 000 to 6, 900 and little C2 and C3 contents (lower than 0.1% and 0.05%, respectively), indicating that they are generated by microbial decomposition of organic matter in the formations. As δ13Cc1 values of natural gases increase from -60‰ to -50‰, their C1/(C2+C3) ratios decrease sharply from about 1, 000 up to 2, because of extreme increases of C2 and C3 contents up to about 10%. Furthermore, as their δ13Cc1 values increase from -40‰ to -30‰, their C1/(C2+C3) ratios increase again, showing that natural gases having δ13Cc1 values from -50‰to -40‰ give the lowest C1/(C2+C3) ratios. These results indicate the existence of metathermogenic gas proposed as an intermediate between microbial and thermogenic gases by Kita et al. (2001) and support their proposal that -40‰ of δ13Cc1 value can be defined as the boundary between meta-thermogenic gas (MTG) and thermogenic gas (TG) based on the relationship between δ13Cc1 values and N2/Ar ratios. Furthermore, the sharp decrease of C1/(C2+C3) ratios in the range of δ13Cc1 value from -60‰ to -50‰ indicates that -60‰ of δ13Cc1 value can be used as the boundary between microbial gas (MG) and meta-thermogenic gas (MTG). On the other hand, the relationship between C1/(C2+C3) ratios and δ13Cc1 values of natural gases is similar to that between their C1/C2 ratios and δ13Cc1 values. This indicates that C1/C2 ratio can be used instead of C1/(C2+C3) ratio for discussing their origins in combination with δ13Cc1 value.