[S-GC38] Solid Earth Geochemistry, Cosmochemistry

[SGC38-04] Origin of highly depleted peridotites in supra- subduction zones: whole-rock geochemistry and Os isotope of ultramafic rocks in the Kamuikotan Belt

Hidetoshi Mori¹, *Akira Ishikawa¹, Tetsuya Yokoyama¹, Tetsu Kogiso², Norikatsu Akizawa³ (1.Department of Earth and Planetary Sciences, Institute of Science Tokyo, 2.Graduate School of Human and Environmental Studies, Kyoto University, 3.Graduate School of Human and Environmental Studies, Kyoto UniversityEarth and Planetary System Science, Hiroshima University)

Keywords:highly siderophile element, Os isotope, mantle, peritodite

Studies of peridotite xenoliths from kimberlites has shown that the cratonic lithosphere underlying the Archean crust is particularly depleted in melt components. Elucidating the origin of these peridotites is important for understanding the tectonics dominated in Archean Earth. Highly depleted peridotites formed by the similar degree of depletion as cratonic peridotites have been reported in circum-Pacific regions. Such a highly depleted mantle, which is incorporated into the modern mantle, may serve as analog material for considering the depletion of Archean lithospheric mantle. One of these peridotites is a series of ultramafic rocks from the Kamuikotan Belt, Hokkaido. It has been reported that serpentinite, which originates from peridotite, is discontinuously exposed in a narrow, elongated geological body extending from north to south in this region. Attempts have been made to consider the origin of these rocks based on their mineral chemistry. However, because the entire region is significantly serpentinized, a comprehensive survey of the whole-rock chemistry has not yet been conducted. In this study, we performed whole-rock chemical analyses of ultramafic rocks in an area spanning 300 km from north to south in Pinneshiri, Horokanai, Takadomari, and Iwanai-dake, and reported a dataset of major and minor elements, trace and transition metal elements, highly siderophile elements (HSEs), and Os isotope ratios. Using these data, we also attempted to constrain the melting conditions and ages of the rocks.
Although the ultramafic rocks of the Kamuikotan Belt have been strongly serpentinized, no effect was observed on the major elements, trace elements, HSEs, and Os isotope ratios with the exception of some fluid-mobile elements. The trace element composition of the Kamuikotan Belt shows a special characteristic, with heavy rare earth elements (HREE) being depleted in the northern region, while some elements, such as Zr, Sr, and light rare earth elements (LREE), are enriched. To explain such compositions, it is necessary to consider melting in an open system, where the inflow of melt into the system and extraction of melt components occur simultaneously. In addition, the HSEs abundance pattern of the Kamuikotan ultramafic rocks showed a unique pattern that was strongly depleted in Os-Ir. This suggests the involvement of a unique melt that disolves Os-Ir, which is usually concentrated in the solid phase, is distributed in the liquid phase, which is considered to be a boninitic magma depleted in S. The Os isotope ratio showed a small value of approximately 0.113 and was not correlated with the disturbance in Os abundance. It has acquired a highly depleted composition and unique trace element and HSEs compositions due to special melting involving boninitic magma in the subduction zone. In addition, the Os isotope ratio was thought to be unaffected by melting in the subduction zone, and the Re depletion model age was maintained at 0.8-2.2 Ga. This suggests that the mantle domain that underwent the mantle paleo-melting event existed before melting in the subduction zone.