Compositions of primary basalt magmas were backcalculated from Miocene Mg-rich basalts in the Japan Sea back arc basin and the NE Japanese volcanic front, in order to assess the spatial variation in physicochemical conditions for magma production during opening of the back arc basin. Chemical compositions of these magmas together with available liquidus experimental data suggest that the primary basalt magmas both in the Japan Sea (BABB magma) and the NE Japanese volcanic front (VFB magma) were equilibrated with harzburgitic mantle materials at almost identical pressure and temperature conditions. The observation that the BABB magma is more depleted in Fe and some incompatible trace elements and more enriched in Al than the VFB magma then suggests that these magmas may have been derived from source mantle materials with different chemical compositions. An equilibrium melting model further demonstrates that the BABB magma was produced by a smaller degree of partial melting of a more depleted source mantle material (compositional equivalent to abyssal harzburgites) than the VFB magma. Provided that the varying degrees of depletion in the source mantle materials were manifestations of mantle upwelling efficient beneath the back arc side region, the more depleted BABB magma source may have been a plausible geochemical reservoir in the deeper portion of the mantle wedge. ¿ American Geophysical Union 1994