Refractory magmas in back-arc basin settings - Experimental constraints onthe petrogenesis of a Lau Basin example

We present the results of an experimental study on a refractory back-are ba sin glass composition 123 95-1 recovered close to the intersection of the,N orth Western Lau Spreading Centre and the Peggy Ridge in the Central Lau Ba sin. We used both the inverse and forward experimental approaches to determ ine that a picrite composition of similar to 16 wt % MgO was parental to th e refractory back-art basalt composition and that this picrite was in equil ibrium with a residual lherzolite assemblage at similar to 2.1 GPa; similar to 1460 degrees C. Our, experimental results suggest that this primary pic rite represents neither an aggregate of small melt fractions collected over a depth interval in a melting column nor a small melt fraction. Instead, t he primary picrite composition represents a significant melt fraction (15% melting) of a lherzolite source, and its composition is closely modelled by equilibrium batch melting. Our preferred model of magma genesis involves t hree-dimensional diapiric flow during which significant solid-melt re-equil ibration occurs. Melts als trapped within an ascending diapir until melting reaches a moderate to high fraction (15-25%), at which point rite primary picrite magmas segregate from the diapir.