GEODYNAMIC MODELING OF SEISMIC OBSERVATIONS TO ASSESS DEGREE OF MELTING BENEATH THE HAWAIIAN ISLANDS

We explore the combined effect of thermal, chemical, and melting anomalies on shear wave velocities above the mantle transition zone. While thermal and chemical heterogeneity influence the seismic velocities at subsolidus temperatures to some extent, the velocity structures are greatly modified in the presence of partial melting. We interpret receiver function analysis of P-to-s conversions beneath an array of 28 stations surrounding the Hawaiian Islands. The seismic data detect a Low Velocity Layer (LVL) at an average of ~350 km depth and ~40 km of regional topography along the 410-km discontinuity. We calculate the local potential temperature from the elevation/depression of the 410-km discontinuity, which we then use to calculate local mantle Vs, Vp, and density. For a range of different basalt fractions, we found fifth degree polynomial fits to mineral physics models for reference values of density, Vs, and Vp as a function of temperature. We then applied the polynomial functions, calculated material properties, and microgeodynamic equations to estimate the melt fraction corresponding to the seismic observations. Temperature and subsolidus composition fail to fully explain the observed impedance contrast atop the LVL, however a range of low melt fractions describes the seismic signature. Melt fraction appears to increase from northwest to southeast along the island chain and has a positive correlation with traits of the dihedral angle.