The Genesis of the Abukuma Adakites, Northeast Japan Resulting From the Interaction Between Hot Asthenospheric Mantle and Subducting Slab: A Numerical Model Study

The geochemistry of the transient Miocene adakites (~16 Ma) in the Abukuma Mountains, Northeast Japan shows that the adakites were generated by the partial melting of the subducted oceanic crust. However, the very old age of the converging oceanic plate which cannot yield high slab temperatures enough for the partial melting poses a problem for the genesis of the adakites. Other possible geneses such as the partial melting of the lower crust, flat subduction and/or transient cold plume are not relevant to the genesis of the adakites. Instead, it is thought that the injection of the upwelling hot asthenospheric mantle to the mantle wedge caused by the East Sea (Japan Sea) opening heats the cold subducting slab hotter enough for the partial melting of the oceanic crust. Although the hypothesis is promising, quantitative evaluation of the interaction between the cold Pacific slab and hot asthenospheric mantle has not been carried out. Thus, we conducted a series of 2-dimensional kinematic-dynamic subduction model experiments to evaluate the thermal structures of the subducting slab, essential for the partial melting of the oceanic crust. Since time-dependence is crucial for the transient adakites, the time-evolving convergence rate and slab age of the incoming Pacific plate for the last 65 Ma constrained from a recent plate reconstruction model are implemented in the numerical models with the transient hot asthenospheric mantle. The convergence rate and slab age are implemented along the oceanward wall boundary and updated each time step. The mantle potential temperature of 1350 °C and the mantle adiabat of 0.35 °C/km are used. The transient injection of the hot asthenospheric mantle to the mantle wedge is implemented as a function of depth- and time-dependent normal temperature distribution along the arcward wall boundary and updated each time step. The peak temperature of the hot asthenospheric mantle is assumed as 1550 °C at 100 km depth and the standard temperature deviation corresponding to the thickness of the hot asthenospheric mantle is varied as 20, 30 and 40 km. The peak time of the hot asthenospheric mantle is assumed as 22.5 Ma, corresponding to the initiation of the East Sea opening, and the standard time deviation is varied as 2.5 Ma, 5 Ma and 10 Ma. Results show that the transient injection of the hot asthenospheric mantle is required for the partial melting of the oceanic crust. Larger thickness of the hot asthenospheric mantle expressed as larger standard temperature deviation results in larger slab temperatures. Longer injection of the hot asthenospheric mantle results in longer and larger slab temperatures. All the experiments show that the peak slab temperatures over the wet solidus of the oceanic crust are attained on ~17-18 Ma, consistent with the ~16 Ma adakites in the Abukuma Mountains. However, a longer span of the injection of the hot asthenospheric mantle except for the standard time deviation of 2.5 Ma results in a longer span of the slab temperatures over the wet solidus from ~12.5 to ~22.5 Ma, inconsistent with the short eruption span of the Miocene adakites on ~16 Ma. This indicates that the injection of the upwelling hot asthenospheric mantle occurred very shortly (< 5 Ma) when the East Sea opened.