Inter- and intra-volcano geochemical variations within the Higashi-Izu Monogenetic Volcano Field, Izu Peninsula, Japan
Abstract
The Higashi-Izu Monogenetic Volcano Field (HIMVF) consists of 70 subaerial monogenetic volcanoes on the eastern side of the Izu Peninsula, Japan, and a further 50 submarine monogenetic volcanoes that extend offshore towards the island of Izu-Oshima, a volcano on the volcanic front of the Izu-Bonin Arc. The HIMVF has been active from 0.3 Ma until the present day, and is related to a change in the stress field after the Izu Peninsula tectonic block began to collide with the Honshu Arc. This study examines olivine-hosted silicate melt inclusions erupted in scoria from two of the subaerial monogenetic volcanoes, Takatsukayama and Sukumoyama, that were active early in the history of the HIMVF (~0.27 Ma). Major element data (and volatile elements S and Cl) have been collected from 85 inclusions, from which subsets of 26 and 41 inclusions have been successfully analyzed for trace and volatile (H2O, CO2) elements, respectively. Overall the HIMVF inclusion data, together with whole rock data from previous studies, are more enriched in large-ion lithophile elements (LILE), high field strength elements (HFSE) and rare earth elements (REE) than the Izu-Bonin volcanic front, having a stronger geochemical affinity with the Izu-Bonin rear-arc. However, in detail there is a continuous compositional spectrum from more volcanic front-like to more rear-arc-like away from the volcanic front. This is detectable even over a few kilometers; with inclusions from Sukumoyama, 3.5 km closer to the volcanic front than Takatsukayama, having lower K2O and Th/Nb, and higher Ba/Th. The variation across the HIMVF is attributed to the melts being generated by a mixture of fluid and sediment melt components released from the downgoing slab, with the contribution from the fluid component decreasing across the HIMVF with distance from the volcanic front. The Takatsukayama and Sukumoyama inclusions also reveal an intra-volcano geochemical diversity. Most inclusions are typical Izu-Bonin low TiO2 (0.81 to 1.18 wt.%), high Al2O3 (15.88 to 19.34 wt.%) basalts (low TiO2-HABs), but at each volcano a few inclusions contain comparatively high TiO2 (1.11 to 2.19 wt.%) and low Al2O3 (11.75 to 14.04 wt.%), termed high TiO2-LABs, a composition that has not been seen elsewhere in the HIMVF or the Izu-Bonin Arc. The high TiO2-LABs are also characterized by higher K2O and LILE, except for Sr resulting in low Sr/Nd, HFSE and REE trace element concentrations, higher SiO2 content at a given MgO content, and lower CaO contents. In some cases the two types of inclusion have been trapped in the same host crystal, indicating that the heterogeneities they preserve exist in the magma chamber on a sub-crystal scale. As both groups exhibit the cross-HIMVF geochemical variability, we conclude that the heterogeneity does not reflect mantle processes or the true composition of melts in the magma chambers; instead we relate it to interstitial melts within the crystal mush of the magma chambers experiencing distinct crystallization histories. The high TiO2-LABs are generated by extensive crystallization of plagioclase in addition to the overall liquid line of descent (controlled by olivine ± clinopyroxene ± spinel crystallization) shown by the low TiO2-HAB melts. Unless they are trapped as inclusions in growing crystals, the high TiO2-LAB melts mix with and are overwhelmed by the more common low TiO2-HAB melts before eruption and are thus absent from the whole rock record.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V44C..07N
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1043 GEOCHEMISTRY / Fluid and melt inclusion geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry