Heterogeneities in the magma chambers of monogenetic volcanoes revealed by melt inclusions from the Higashi-Izu Monogenetic Volcanic Field, Japan
Abstract
The Higashi-Izu Monogenetic Volcanic Field (HIMVF) consists of 70 subaerial monogenetic volcanoes on the north-eastern coast of the Izu Peninsula, Japan, and a further 50 submarine monogenetic volcanoes that extend offshore to the island of Izu-Oshima, a volcano on the volcanic front of the Izu-Bonin (IB) Arc. The HIMVF has been active from 0.3 Ma until the present day. The transition from polygenetic volcanism to monogenetic volcanism on the Izu Peninsula around this time is believed to be related to a change in the stress field after the Izu Peninsula tectonic block began to collide with the Honshu Arc (Hasebe et al., 2001, B. Volcanol., 63, 377-86). Major, trace and volatile (H2O, CO2, S, Cl) element concentrations have been measured in 116 inclusions hosted in olivines from scoria erupted by two of the basaltic monogenetic volcanoes, Sukumoyama and Takatsukayama. The volcanoes are located 5 km apart, trending across the IB Arc, and erupted at ~0.27 Ma, early in the history of activity in the HIMVF. The geochemical characteristics of the inclusions have led to the following findings. Firstly, the inclusions, consistent with whole rocks data from the HIMVF, indicate that the HIMVF has a geochemical affinity with the IB rear-arc, being more enriched in large-ion lithophile elements (LILE) high field strength elements (HFSE) and rare earth elements (REE) than the volcanic front. Secondly, some of the inclusions preserve compositions that have not been previously seen in rocks from the IB Arc. Most of the inclusions are normal IB low TiO2 (0.81 to 1.18 wt.%) high Al2O3 (15.67 to 19.34 wt.%) (low TiO2-HABs), however, 20 inclusions have high TiO2 (1.11 to 2.19 wt.%), low Al2O3 (11.75 to 14.04 wt.%) (high TiO2-LABs), and three have been found to contain high TiO2 (1.34 to 1.61 wt.%) and high Al2O3 (16.06 to 19.14 wt.%). There are cases where a high TiO2-LAB inclusion occurs in the same host crystal as low TiO2-HAB inclusions. The Takatsukayama high TiO2-LABs are also characterized by a uniform enrichment in trace elements, with the exception of Sr, compared to their low TiO2-HAB counterparts and both types of inclusion at Sukumoyama. These distinct inclusion compositions are interpreted to reflect melts with different crystallization histories existing in close proximity to one another within the same magma chamber. The common low TiO2-HABs are generated by the more usual crystallization of olivine ± clinopyroxene, whereas the high TiO2-LABs can be related to low TiO2-HABs in the same host crystal by the additional crystallization of extensive amounts of plagioclase feldspar (~40 %). Finally, despite being only a few kilometres apart, the volcanoes still reveal a cross-arc variation. Although the melt inclusions from both volcanoes exhibit rear-arc geochemical characteristics, inclusions from Sukumoyama, irrespective of the compositional group, have lower K2O and Th/Nb, and higher Ba/Th. This indicates that they have a slightly greater contribution from a shallow fluid component and thus a slightly stronger flavour of the volcanic front than those from Takatsukayama.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V43A2560W
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1043 GEOCHEMISTRY / Fluid and melt inclusion geochemistry