Oxidation State of Iron in the Izu-Bonin Arc Initial Magma and Its Influence Factors

Friday, 19 December 2014
He Li1, Richard J Arculus2, Philipp A Brandl3, Morihisa Hamada4, Ivan P Savov5, Sanyuan Zhu1, Rosemary Hickey-Vargas6, Frank J Tepley III7, Sebastien Meffre8, Gene M Yogodzinski9, Anders McCarthy10, Andrew P Barth11, Kyoko Kanayama12, Yuki Kusano12 and Weidong Sun1, (1)GIG Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China, (2)Australian National University, Canberra, Australia, (3)Australian National University, Research School of Earth Sciences, Canberra, ACT, Australia, (4)Japan Agency for Marine-Earth Science & Technology, Yokosuka, Japan, (5)University of Leeds, Leeds, United Kingdom, (6)FIU, Miami, FL, United States, (7)Oregon State University, Corvallis, OR, United States, (8)University of Tasmania, Hobart, Australia, (9)University of South Carolina, Columbia, SC, United States, (10)University of Lausanne, Lausanne, Switzerland, (11)Indiana Univ, Indianapolis, IN, United States, (12)Kanazawa University, Kanagawa, Japan
The redox state of mantle-derived magmas is a controversial issue, especially whether island arc basalts are more oxidized than those from mid-ocean ridges. Usually, arc magmas have higher Fe3+/Fe2+ and calculated oxygen fugacity (fO2) than mid-ocean ridge basalts (MORB). It is the high fO2 of arc magma that apparently delays onset of sulfide fractionation and sequestration of precious/base metals thereby facilitating the formation of many giant gold-copper deposits typically associated with subduction zones. But due to a paucity of Fe3+/Fe2+ data for primary mantle-derived arc magmas, the cause for high fO2 of these magma types is still controversial; causes may include inter alia subduction-released oxidized material addition to the mantle wedge source of arc magma, partial melting of subducted slab, and redox changes occurring during ascent of the magma. Fortunately, IODP expedition 351 drilling at IODP Site U1438 in the Amami-Sankaku Basin of the northwestern Philipine Sea, adjacent to the proto-Izu-Bonin Arc at the Kyushu-Palau Ridge (KPR), recovered not only volcaniclastics derived from the inception of Izu-Bonin Mariana (IBM) arc in the Eocene, but also similar materials for the Arc’s subsequent evolution through to the Late Oligocene and abandonment of the KPR as a remnant arc. Samples of the pre-Arc oceanic crustal basement were also recovered enabling us to determine the fO2of the mantle preceding arc inception.

As the oxidation state of iron in basaltic glass directly relates to the fO2 , the Fe3+/∑Fe ratio [Fe3+/(Fe3++ Fe2+)] of basaltic glass are quantified by synchrotron-facilitated micro X-ray Absorption Near Edge Structure (XANES) spectroscopy to reflect its fO2. Fe K-edge µ-XANES spectra were recorded in fluorescence mode at Beamline 15U1, Shanghai Synchrotron Radiation Facility (SSRF). Synthetic silicate glass with known Fe3+/∑Fe ratio was used in data handling. The experimental results as well as preliminary data from IODP Expedition 351 will be presented