The Fractionation of Sulfur Isotopes during Arc Initiation – Preliminary Data

Abstract:

The “Stable Isotope Sensitive High-Resolution Ion Microprobe” (SHRIMP-SI) at the ANU in Canberra provides a powerful tool for in-situ analyses of light isotopes. Recently, we developed a technique to analyze the sulfur isotope composition (δ³⁴S) of volcanic glasses. Since no interlaboratory reference material is available for this purpose, we carried out piston cylinder experiments to create artificial glasses of known sulfur isotope composition. We used natural sulfides mixed with CaO-Al₂O₃-SiO₂ (with PtO₂ in 5 mm Pt capsule) and high-Ti Mare basalt powder (graphite capsule inside the Pt capsule). Experimental conditions were set to 1400°C, 5 kbar and 4 hours. We used these artificial glasses to survey sulfur isotope fractionation during our experimental runs and for use as standard material with calibrated δ³⁴S. Additionally, natural standards Juan de Fuca basalt (VG-2), Makaopuhi basalt (VG-A99; Hawai’i), SEIR MORB (NMNH 113716), and one rhyolite (VG-568) were analyzed repeatedly to provide sulfur isotope data of commonly used natural glass standards.

IODP expedition 351 to the Amami-Sankaku Basin adjacent to the Kyushu-Palau Ridge in June-July 2014 recovered a variety of rocks related to the history of the Izu-Bonin Mariana (IBM) Arc. Basalts of the oceanic igneous crust underlying the IBM arc give us information of the basement prior to arc initiation and volcaniclastic rocks eroded from active volcanoes of the IBM arc record its evolution from inception in the Eocene through Oligocene and Miocene. Ash layers intercalated in hemipelagic sediments record the evolution of the IBM and Ryukyu arcs during the Neogene. The aim of this study is to track changes in the oxidation state of island arc magmas and the mantle wedge during the time of arc initiation. Experimental results as well as preliminary data from IODP Exp. 351 will be presented by the time of the conference.