Melt-peridotite reactions: Roles in magma genesis beneath mid-ocean ridges, oceanic islands, and volcanic arcs

Abstract

Melt-peridotite reactions: Roles in magma genesis beneath mid-ocean ridges, oceanic islands, and volcanic arcs 木村 純一(Jun-Ichi KIMURA) Melts from asthenospheric depths, such as silica-deˆcient alkali basalts beneath mid-ocean ridges, pyroxinite melts beneath ocean islands, and felsic melts from subducted slabs beneath arcs, are usually not in equilibrium with mantle peridotite at the shallower depths. Reactions between deep melts and shallow peridotites have drawn attention because they are immediately related to the geochemical features of mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs), island arc basalts (IABs), and high-Mg andesites (HMAs). Reactions between the deep melts and the shallow peridotites are peritectic in many cases, whereby they consume and precipitate particular mineral phases as part of the reaction process. This type of reaction is highly complex because the open behavior of the reaction system contains a sig-niˆcant amount of variability. The dynamics of the melt transport alter the mass balance in the melt-peridotite reaction system by aŠecting its temperature and the chemical stoichiometry via heat transfer and chemical disequilibrium caused by melt in‰ow-out‰ow. DiŠerences in source rocks and degrees of melting cause considerable ‰uctuation in the chemistry of deep melts, which also aŠects the reaction. For example, deep alkali basalt melts dissolve orthopyroxene in the shallow mantle peridotite, whereas felsic slab melts precipitate orthopyroxene through the consumption of olivine. As such, the melt-peridotite reaction in an open system clearly aŠects the petrology and chemistry in both residual mantle peridotites and reacted melts. Moreover, evidence of these changes should be recorded in the major and trace element chemistries of residual mantle rocks and erupted melts. In this article, the author summarizes previous research and examines the role of the melt-peridotite reaction in an open system to determine the origins of geochemical diversity in residual peridotites and melts obtained from the various magmatic settings.

Origin of the Tonaru body in the Sanbagawa metamorphic belt, SW Japan

“…Nb and Ti) compared to MOR and OI magmas, because HFSE are hard to transfer from subducting slabs to slabderived fluids/melts (e.g. Kimura, 2013;J. A. Pearce, Stern, Bloomer, & Fryer, 2005;Tatsumi & Takahashi, 2006).…”