Dehydration of subducted slab gabbro in Lau Basin and its implications

Abstract The role of subducted lower oceanic crust gabbro in the water cycle of subduction zones has been underappreciated. This study conducted boron and strontium isotope analyses for basaltic glasses from the Eastern Lau Spreading Center (ELSC) at the Tonga subduction zone. Our results indicate that ELSC1 magmas, with heavy B isotopes and elevated H2O/Ce ratios, exhibit the lowest 87Sr/86Sr ratios among Lau Basin basalts, suggesting contribution of water released from lower crust gabbro. Globally, the lower crust gabbro, hosting at least 20% of total water budget of the subducted ocean plate, would release water continuously up to depths of ~250 km, implying a globally important role in subduction zone water cycle. Similar signals identified in the Izu, Lesser Antilles, and Cascades arcs support the widespread water contribution from gabbro. Our findings call for a reassessment of the lower crust’s role in the global water budget of subduction zones.