Abyssal Peridotite as a Component of Forearc Mantle: Inference from a New Mantle Xenolith Suite of Bankawa in the Southwest Japan Arc

Abstract

Lithology and petrologic nature of the forearc mantle have been left unclear due to the very limited sampling to date. Here, we present petrological data on a forearc peridotite suite obtained as xenoliths in an alkali basalt dike (7.5 Ma) from the Bankawa area in the Southwest Japan arc for our better understanding of the forearc mantle. The host alkali basalt is of asthenosphere origin, and passed through a slab window with slight chemical modification by the slab-derived component. The Bankawa peridotite suite is comprised of lherzolites, which contain various amounts of secondary phlogopite and were metasomatized to various degrees. The least metasomatized lherzolite exhibits Fo91 of olivine, Cr/(Cr + Al) = 0.3 of chromian spinel, and depletion of middle to light rare-earth elements in clinopyroxene, and is overall similar to an abyssal lherzolite. It had originally formed at the proto-Pacific Ocean and then was trapped at a eastern margin of Eurasian continent by initiation of subduction. The forearc mantle peridotite formed as a residue of proto-arc magma formation is depleted harzburgite as represented by the peridotites obtained from the forearc seafloor, but can be less depleted abyssal peridotite if being devoid of partial melting or reaction with magmas after entrapment.

Editorial for Special Issue “Petrology, Geochemistry and Mineralogy of the Mantle as Tools to Read Messages from the Earth’s Interior”

“…This possibly means that part of the mantle wedge beneath Luzon Island is composed of abyssal peridotites. This result is consistent with that of Arai et al [1] and also contributes to our better understanding of mantle wedges and island-arc systems.…”

“…This Special Issue contains nine articles [1][2][3][4][5][6][7][8][9]. Seven of them [1][2][3][4][5][6]8] deal with mantle-derived rocks, which have played an essential and important role in providing us with direct information about the chemistry and processes that take place in the deep part of Earth [10,11]. Their geochemistry and mineralogy give us excellent tools to unravel various mantle processes, i.e., partial melting, melt extraction, melt-rock interaction, metallogeny, metasomatism and metamorphism.…”

“…Their geochemistry and mineralogy give us excellent tools to unravel various mantle processes, i.e., partial melting, melt extraction, melt-rock interaction, metallogeny, metasomatism and metamorphism. Five papers [1,3,4,6,8] describe and discuss mantle-derived peridotites from Asia. Interesting new data are presented regarding the origin of mantle wedges and the nature of ophiolites [1,3,4,6].…”

“…Five papers [1,3,4,6,8] describe and discuss mantle-derived peridotites from Asia. Interesting new data are presented regarding the origin of mantle wedges and the nature of ophiolites [1,3,4,6]. Readers will be interested in the relationship between abyssal peridotites and arc peridotites.…”

“…Arai et al [1] describe peridotite xenoliths from a new locality (Bankawa), which is located in forearc area of the Southwest Japan arc, and discuss the petrologic nature of the mantle wedge. The Bankawa peridotite xenolith suite is composed of almost uniform lherzolite, which is similar to some abyssal lherzolites.…”