The role of slab serpentinite and lower crust in deep water cycles: Insights from B-Sr isotopes

The water cycle between Earth’s surface and interior plays a critical role in maintaining long-term sea-level stability. The discovery of ~24-km-thick serpentinized mantle in the Pacific plate near the Mariana Trench suggests that water influx could be up to three times greater than previous estimates. However, the contributions of thick slab serpentinite to the magma genesis in the subduction zones remain uncertain.

 

We integrate new boron isotopic data from Mariana arc volcanics and Lau Basin basalts with Sr isotopic, halogen, and trace element data. One rear arc volcano shows evidence of contributions from slab serpentinite, which otherwise has a limited role in the overall chemical variations of arc-basin magmatism. Instead, water-rich fluxes from lower crustal gabbro contribute to back-arc magmatism in both the Mariana and Tonga subduction zones. The geochemical signature of lower crustal gabbro-derived fluxes is also evident in volcanoes from the Izu rear-arc, the Lesser Antilles Arc, and the Cascades arc.

 

Water flux calculations demonstrate that the slab crust alone provides sufficient water influx to balance outflux through Mariana magmatism, raising the question of the fate of slab serpentinite. It appears that most of the water from serpentinite does not contribute to the arc, and hence would lead to a large deep water return flux to the mantle. If applied globally, however, such subduction would imply a sea-level drop of ~500 m per 100 million years, contradicting geological evidence of long-term sea-level stability. To reconcile this discrepancy, we propose that the water budget of slab serpentinite is either overestimated or not representative of cold slabs. A globally averaged thickness of ~2 km of slab serpentinite with 15 vol% serpentinization (total water budget within 3.4 × 10⁷ Tg/Myr) is permitted to maintain the sea-level stability during the Phanerozoic.