Volatile abundances in OIBs hosting a recycled sediment signature: Evidence from Samoa

During subduction, sediment suffers dehydration and volatile loss in stages, and eventually melts and becomes incorporated into the overlying peridotite mantle wedge. While much of the subducted sediment is short-cycled into arc volcanoes, a small portion of sediment is thought to enter the convecting mantle. Little is known about the volatile budget of sediment that becomes incorporated into the convecting mantle, and knowledge of the volatile content of deeply recycled sediment has potential to further reveal processes that recycle volatiles in subduction zones. Lavas dredged off the southern flanks of the Samoan island of Savai’i exhibit trace element and isotopic fingerprints associated with sediment, suggesting a component of deeply-cycled sediment in their mantle source. The suite of lavas from one dredge (ALIA-DR115) hosts whole rock 87Sr/86Sr ratios ranging from 0.7082 to 0.7205. Sample ALIA-115-18 was selected for this study, and hosts whole rock 87Sr/86Sr of 0.7186. Cpx (clinopyroxene) separates host even higher 87Sr/86Sr (0.7202 to 0.7208; Jackson et al., 2007). Volatiles were measured in 9 cpx-hosted melt inclusions from sample ALIA-115-18. Major element analyses from a subset of the inclusions reveal highly evolved compositions; SiO2 (64.5-68.5 wt.%), MgO (0.04-1.08 wt.%) and K2O (5.8-8.3 wt.%) concentrations are extreme, and are similar in composition to phonolitic glasses hosted in ultra-enriched, metasomatized peridotite xenoliths from Savai’i (69.2 wt.% SiO2; Hauri et al., 1994). CO2 abundances in the melt inclusions correlate with proxies of magma evolution, including MgO (R2=0.92) and FeO (R2=0.75). The least evolved melt inclusions have the highest CO2 concentrations (up to 74 ppm CO2). The highest CO2 concentrations are similar in magnitude to the CO2 concentrations previously reported in Malumalu seamount (33-84 ppm; Workman et al., 2006), the volcano with the highest 87Sr/86Sr (0.7089) in the eastern Samoan volcanic province. Unlike Malumalu, which has 0.63-1.01 wt.% H2O, the ALIA melt inclusions have H2O abundances of 0.17-0.72 wt.%. An H2O-CO2 solubility model (Dixon, 1997) indicates that the least degassed ALIA melt inclusion has a vapor saturation pressure of ~140 bar, suggesting that this inclusion did not lose significant water before entrapment. The same inclusion has high F (1375 ppm) and Cl (1606 ppm), but low S (295 ppm) abundances, and along with the other ALIA melt inclusions, the new data present an extreme in the Samoan suite. Assuming that the cpx-hosted melt inclusions have Sr and Nd isotopes similar to cpx separates from the same lava, we compare the volatile abundances of the extreme EM2 melt inclusions with previously reported volatiles measured on Samoan pillow-rim glasses. Raw H2O, S and F concentrations exhibit correlations with Sr and Nd isotopes, and volatile normalization to K or Ti improves the correlations. The extreme enriched inclusions anchor the isotopically-enriched portion of the arrays, and host the lowest S and H2O, but the highest F and Cl in Samoa (low Cl/K in the inclusions—0.024 to 0.070—rules out seawater assimilation). These volatile abundances constrain the volatile budget of recycled sedimentary material that contributed to Samoan magmatism.