Refractory Magmas in Back-Arc Basin Settings—Experimental Constraints on the Petrogenesis of a Lau Basin Example

We present the results of an experimental study on a refractory back-arc basin glass composition 123 95-1 recovered close to the intersection of the North Western Lau Spreading Centre and the Peggy Ridge in the Central Lau Basin. We used both the inverse and forward experimental approaches to determine that a picrite composition of ~16 wt %MgO was parental to the refractory back-arc basalt composition and that this picrite was in equilibrium with a residual lherzolite assemblage at ~2.1 GPa, ~1460°C. Our experimental results suggest that this primary picrite represents neither an aggregate of small melt fractions collected over a depth interval in a melting column nor a small melt fraction. Instead, the 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 three-dimensional diapiric flow during which significant solid-melt re-equilibrium occurs. Melts are trapped within an ascending diapir until melting reaches a moderate to high fraction (15-25%); at which point the primary picrite magmas segregate from the diapir.