New genesis of felsic magma: the unmixing of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas with or without crustal assimilation (DePaolo, 1981), partial melting of crustal materials (Huppert and Sparks, 1988), and partial melting of subducting slabs (Defant and Drummond, 1990) have been proposed. Here, we study calc-alkaline andesitic and dacitic lavas from Rishiri volcano, to elucidate the origin of felsic magmas.

Rishiri volcano is located at the Kuril arc, and is 300 km above the Wadati-Benioff Zone. The calc-alkaline andesitic products and the following dacitic lavas, investigated in this study, comprise Middle stage of the volcanic activity (Ishizuka, 1999). Our new ⁴⁰Ar/³⁹Ar age data for the dacitic and andesitic lavas are 35.5 ± 1.4 ka and 34.6 ± 3.0 ka, respectively. The andesitic lavas have disequilibrium petrographic features, which can be explained primarily by mixing of felsic magma and primitive mafic magma which generated by fluid-fluxed melting of mantle peridotite. On the other hand, the dacitic lavas are characterized by high Sr/Y and La/Yb ratios, no Eu anomaly, and no evidence of magma mixing. The absence of Eu anomaly negates a primary role of fractional crystallization in the magma genesis. In addition, the adakitic signature cannot be produced by crustal melting, because the underlying crust is too thin (Niu et al., 2016) for garnet to be stable (Rapp and Watson, 1995). Therefore, it is suggested that the dacitic magma was derived from the subducting slab.

One possible origin of the dacitic magma would be direct partial melting of the subducted slab. However, extremely high temperatures with >1300ºC (Mibe et al., 2011) is required to melt the slab at ~300 km depth, which is highly unlikely. The petrological and geochronological observations suggest that the slab-derived dacitic melt coexisted with slab-derived aqueous fluid in the upper mantle beneath Rishiri at ~35 ka. Therefore, the dacitic magma might represent hydrous melt, which was separated from the ascending slab-derived supercritical liquid above the depth corresponding to the critical end-point (e.g. Kawamoto et al., 2012), leaving behind fluids that induced the generation of the primitive basaltic magma.