Extensive H2O degassing in deeply erupted submarine glasses inferred from Samoan melt inclusions: The EM2 mantle source is damp, not dry

Abstract

Submarine glasses erupted at intraplate volcanic hotspot settings sampling enriched mantle (EM)—characterized by high 87Sr/86Sr—exhibit lower H2O/Ce than glasses representing less enriched mantle domains, leading to the interpretation that the EM mantle is H2O-poor (“dry”). We test whether low H2O/Ce observed in pillow glasses of EM lavas resulted from degassing of higher H2O/Ce primary melts by measuring H2O/Ce and 87Sr/86Sr in olivine-hosted melt inclusions from two deeply-erupted (3950 and 2190 m below sea level (mbsl)) Samoan submarine lavas from Vailuluʻu and Malumalu seamounts. Vailuluʻu (H2O/Ce = 161–275) and Malumalu (180–215) melt inclusions with 87Sr/86Sr data have an average H2O/Ce (197 ± 58 2SD, N = 15) that is nearly twice as high as H2O/Ce in pillow glasses from these two seamounts (average H2O/Ce = 106 ± 51, N = 65), and comparable to pillow glasses from non-EM hotspots. We show that lower H2O/Ce in submarine Samoan glasses compared to melt inclusions results from greater closed-system degassing, and concomitant loss of H2O, because EM melts have higher initial concentrations of CO2. We show that the lower H2O/Ce in global EM pillow glasses compared to non-EM pillow glasses can be modeled to be the result of more extensive degassing of H2O in EM melts, which owes to higher CO2 in primary melts (20,000–90,000 ppm) of EM sources compared to non-EM melts (300–50,000 ppm CO2). Instead of originating from a dry mantle, we conclude that EM lavas derive from a damp mantle, but EM melts lose more H2O by degassing than non-EM melts.