JpGU-AGU Joint Meeting 2017

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-GC Geochemistry

[S-GC52] [EE] Volatile cycles in the Earth - from Surface to Deep Interior

Mon. May 22, 2017 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall HALL7)

convener:Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry), David R Hilton(University of California San Diego), Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

[SGC52-P03] Systematics of volatile elements in melt inclusions from the proto-Izu-Bonin-Mariana arc (30-40 Ma)

*Morihisa Hamada1, Philipp A. Brandl2, Takayuki Ushikubo3, Kenji Shimizu3, Motoo Ito3, He Li4, Ivan P. Savov5 (1.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, 2.GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany, 3.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 4.Guangzhou Institute of Geochemistry, Chinise Academy of Science, China, 5.School of Earth and Environment, The University of Leeds, UK)

Keywords:IODP, Izu-Bonin-Mariana arc, Amami Sankaku Basin, Melt inclusion, SIMS

IODP Expedition 351 “Izu-Bonin-Mariana (IBM) Arc Origins” drilled at Site U1438 in the Amami Sankaku Basin, about 100 km west of the Kyushu-Palau Ridge, a remnant of the IBM arc. We recovered 1611-m-long cores composed of 150-m of igneous basement (Unit 1), which were formed when subduction initiated at ~52 Ma, and 1461-m of sediments (Units I, II, III and IV), which were deposited immediately after subduction initiation (Fig. a). We focused on Unit III (30-40 Ma) and have already analyzed the major elements and volatile elements (S and Cl) of more than 300 melt inclusions from Unit III with electron probe microanalyzers and have discussed the causes for temporal changes in volcanism of the proto-IBM arc (Brandl et al., 2017; Hamada et al., under review).

In order to extend our previous studies, we analyzed the concentrations of four volatile elements (H2O, S, Cl, F) and P2O5 of a sub-set of 56 representative melt inclusions by Secondary Ion Mass Spectrometry (SIMS) at the Kochi Institute for Core Sample Research of JAMSTEC. Quantification of volatile elements was based on callibration lines and volatile standards by Shimizu et al. (2017). Generally, abundances of volatile elements increase from 40 Ma to 30 Ma (Figs. b-e), along with incompatible elements such as K2O and P2O5 (Fig. f). As a result, the ratios of volatile elements to incompatible elements, such as F/K2O and Cl/K2O, are almost constant from 40 Ma to 30 Ma, irrespective of the rock series (low-K series or medium-K series) and chemical groups (clusters) of melt inclusions assigned by Hamada et al. (under review) (Figs. g and h). Because frontal-arc volcanoes and rear-arc volcanoes of the IBM arc are characterized by low-K series rocks and medium-K series rocks, respectively, these results suggest that (i) the volcaniclastics that accumulated at Site U1438 originate from both frontal-arc volcanism and rear-arc volcanism, and that (ii) volcanism around Site U1438 shifted from frontal-arc to rear-arc volcanism over time.

Volatiles in silicic (dacitic~rhyolitic) melt inclusions (Cluster 6 melt in Figs. g and h) seem to behave differently from those dissolved in mafic melts. Fluorine concentration of silicic melt inclusions (600-800 ppm) does not increase with increasing K2O (Fig. g). Silicic melt inclusions mainly occur at ~30 Ma, the upper level of Unit III, which corresponds to the timing just before the arc rifting and back-arc opening of the IBM arc from ~25 Ma. These silicic melts may be a products of fractional crystallization or may be crustal anatexis (Ikeda and Yuasa, 1989). With respect to the occurences of extremely Cl-rich melt inclusions (Cluster 2 melt inclusions in Figs. g and h), we discuss two possibilities for their origin: one is that they are the “halogen-rich andesite melts” as proposed by Straub and Layne (2003) for the IBM arc; the other is brine assimilation in a submarine hydrothermal system. No F enrichment is observed for extremely Cl-rich (Cluster 2) melt inclusions, and they cannot be “halogen-rich andesite melts” (Straub and Layne, 2003). Therefore, brine assimilation likely explains the origin of extremely Cl-rich melt inclusions.

Brandl, P.A., Hamada, M., Arculus, R.J., Johnson, K., Marsaglia, K.M., Savov, I.P., Ishizuka, O., Li, H. (2017) The arc arises: The links between volcanic output, arc evolution and melt composition. Earth Planet. Sci. Lett. 461, 73-84.

Hamada, M., Iwamori, H., Brandl, P.A., Li, H., Savov, I.P. Evolution of the proto-Izu-Bonin-Mariana arc volcanism: Constraints from statistical analysis on geochemical data of melt inclusions (under review).

Shimizu, K., Ushikubo, T., Hamada, M., Itoh, S., Higashi, Y., Takahashi, E., Ito, M. (2017) H2O, CO2, F, S, Cl, and P2O5 analyses of silicate glasses using SIMS: Report of volatile standard glasses. Geochem. J. 51 (in press).

Ikeda, Y., Yuasa, M. (1989) Volcanism in nascent back-arc basins behind the Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, northwest Pacific: evidence for mixing between E-type MORB and island arc magmas at the initiation of back-arc rifting. Contrib. Mineral. Petrol. 101, 377-393.

Straub, S.M., Layne, G.D. (2003) Decoupling of fluids and fluid-mobile elements during shallow subduction: Evidence from halogen-rich andesite melt inclusions from the Izu arc volcanic front. Geochem Geophys Geosyst 4(7):9003