Tholeiitic vs Calc-alkalic Differentiation and Evolution of Arc Crust: Constraints from Melting Experiments on a Basalt from the Izu^Bonin^Mariana Arc

The liquid line of descent (LLD) for a representative basalt from the Izu^Bonin^Mariana (IBM) arc was investigated at 03 GPa in the presence of 049^283 wt % H2O to constrain the differentia-tion of arc magmas and the evolution of the arc’s crust. This is of interest as differentiated continental crust may form as the middle crust of the intra-oceanic IBM system.The tholeiitic differentiation trend, which is often documented in the volcanic sequences of intra-oceanic arcs, is best reproduced by the LLD of an H2O-poor (05 wt %) basalt, whereas the calc-alkalic trend in the IBM rocks, which are likely to form middle crust of intermediate composi-tion, mimics the LLD of a hydrous basalt with 25^30 wt % H2O. Magmatic temperatures estimated for IBM calc-alkalic rocks, however, tend to be higher than those for the hydrous LLD. It may thus be suggested that an alternative mechanism, mixing of basaltic and felsic magmas, could play a major role in calc-alkalic middle crust formation in the IBM. Seismic velocities of the inferred middle crustal rocks, obtained based on both the experimental results and theoretical calculations, agree well with the observed seismic crustal structure. KEY WORDS: andesite; basalt; calc-alkalic: crust; tholeiitic I NTRODUCTION Basaltic magmas generated by mantle melting transport both mass and heat to the crust. When basalt magmas reach crustal depths they may produce more evolved magmas via closed-system fractionation (Sisson & Grove