USING SPINEL TO TRACK MANTLE DEPLETION AND MELT-ROCK INTERACTION DURING BACK-ARC EXTENSION: A STORY FROM THE GODZILLA MEGAMULLION

The 9000 km² Godzilla Megamullion (GM) is the largest known oceanic core complex (OCC) yet identified on the seafloor. It was exposed along a low-angle detachment fault around 15 Ma during intermediate to fast spreading of the Parece Vela Rift in the back-arc region of the Izu-Bonin-Mariana subduction zone just prior to the end of spreading there. The GM exposes significant volumes of mantle peridotites that renders it an ideal location for investigating mantle evolution during back-arc basin formation and extinction. In this study, we explored variations in spinel composition from peridotite samples collected near the termination of spreading (proximal region) to understand how the “last gasp” of extension affected the mantle. Spinel major element data was collected using Cameca SX-50 electron microprobe at the University of Houston. The spinel Cr# [(Cr/Cr + Al)*100] ranges from 32 to 60 while the TiO₂ = 0.15 to 0.72 wt. %. The range in Cr# records varying degrees of peridotite melting: low Cr# is indicative of a more fertile mantle with low degrees of melting while high Cr# implies larger degrees of melt extraction. Elevated TiO₂ contents are indicative of melt-rock interaction as melts stagnated en route to the surface. Compared to spinel from the proximal region, spinel grains from the medial and distal regions of the GM are distinct compositionally (Snow et al., in review). The medial region contains an average Cr# = 17 with low (< 0.30) TiO₂ values, consistent with low degree of partial melting with little interaction with melt. The distal region (area furthest from the termination) experienced a high degree of partial melting within the mantle, generating spinel with relatively high Cr# (30-60).The composition of spinel from the proximal region demonstrates that the last stage of extension is characterized by extensive re-equilibration between the mantle and ascending melts. This is consistent with thickening and cooling of the lithosphere as spreading declined, allowing melt to stagnate and react with residual peridotite.