Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia: Implications for volatile budgets of recycled oceanic crust

Ocean island basalts (OIB) with extremely radiogenic Pb-isotopic signatures are melts of a mantle component called HIMU (high µ, high ²³⁸U/²⁰⁴Pb). Until now, deeply dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6 ± 3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in end-member HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11-0.12), Br/Cl (0.0024), and I/Cl (5-6 × 10^-5) ratios that preclude significant assimilation of seawater-derived Cl. The new HIMU glasses that are least degassed for H₂O have low H₂O/Ce ratios (75-84), similar to ratios identified in end-member OIB glasses with EM1 and EM2 signatures, but significantly lower than H₂O/Ce ratios (119-245) previously measured in melt inclusions from Mangaia. CO₂-H₂O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2500-3600 m water depth) have eruption pressures of 295-400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H₂O. Thus, the lower H₂O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H₂O/Ce (∼50-80) in the EM1, EM2, and HIMU mantle domains.