Volatile (H, C, Cl, S) Concentrations in Ocean Island Basalt Glasses From Pitcairn and the Society Islands

Submarine lavas from Pitcairn and the Society Islands offer a unique opportunity to investigate volatile contents in melts derived from mantle OIB sources, and in particular to constrain the composition and origin of the EM-1 and EM-2 mantle endmembers. We present here the results of a study on volatile abundances in dredged glasses from Pitcairn and the Society Islands, two well-documented OIB localities with EM-1 and EM-2 isotopic compositions. Samples for this study consist of submarine glasses, dredged from 485-2829 m depth and representing a range of melt compositions (42.7-59.7 wt% SiO2, 1.08-11.69 wt% MgO). The EM-2 lavas of the Society Islands have H2O concentrations between 0.9-2.6 wt%, whereas Pitcairn's EM-1 lavas typically contain 0.8-1.8 wt% H2O. Both locations have CO2 abundances that range from low (<190 ppm for Society, <115 ppm for Pitcairn) to below detection, suggesting significant degassing of CO2 ± H2O. H2O/Ce ranges from 55-170 for Pitcairn lavas, and the highest H2O/Ce ratios are observed in samples with the highest CO2 concentrations (least degassed). Samples with H2O/Ce lower than typical MORB or EM sources (<100) may have volatile contents modified by magmatic degassing. Sulfur concentrations (200-1000 ppm for Society, 400-1450 ppm for Pitcairn) may be largely controlled by sulfide saturation. Concentrations of Cl in Society (650-1400 ppm) and Pitcairn (400-1100 ppm) basalts correlate inversely with 87Sr/86Sr, and do not show evidence of Cl assimilation. Cl/K ratios vary from 0.03 to 0.07 for both sample suites, ranging from only slightly higher than enriched MORB (0.01-0.02) to within the HIMU field (0.04-0.08). Pitcairn lavas with the lowest Cl/K have the strongest EM-1 signature (lowest 206Pb/204Pb). Similarly, Society lavas with the lowest Cl/K have the strongest EM-2 signature (highest 87Sr/86Sr). We suggest that these represent primary mantle signatures.