FAST ASSIMILATION OF XENOLITHS IN MAGMAS

Analytical and numerical modeling indicates that melting of xenoliths in magmas may be controlled by viscous flow. The xenolith melt will be removed immediately by the host magma if flow is present and new xeno lith melt can be produced. This melting mechanism is referred to as ab lation. Ablation dominates the melting process in earlier experiments in the system diopside. We propose that complex phases (solid solution s) undergo ablation when the heat flow into a xenolith is high. Ablati on is found to be controlled by a nearly linear rate law after a short heating up period. The average computed ablation rates coincide with earlier experiments of diopside. We predict steady state ablation rate s at 20 K superheating for beta-cristobalite (3.8 X 10(-4) m/s), high- sanidine (1.6 x 10(-4) m/s), calcite (4.9 x 10(-5) m/s), clinoenstatii e (4.0 x 10(-5) m/s), and forsterite (3.2 x 10(-5) m/s). A cold xenoli th might cool down the host liquid next to it below the glass transiti on temperature and to accrete a glassy crust. Such chilled selvage acc retion will modify the total assimilation rates of granulites by less than 10% for xenolith radii <20 cm. Ablation appears to be more effect ive as compared to diffusion controlled assimilation. This could expla in the worldwide predominance of mafic and ultramafic (mantle and lowe r crust) compared to felsic lower crustal xenoliths.