Volatiles in Submarine HIMU Basalts from the Austral Islands, South Pacific

Abstract:

Submarine basalts have been collected from the slopes of Rurutu and Tubuai in the Austral Islands, South Pacific with the manned submersible Shinkai 6500. Previous work on the bulk radiogenic isotope and trace element chemistry of these samples suggests that the basalts were generated from a HIMU reservoir derived from an ancient subducted slab that was entrained and mixed with the depleted asthenospheric mantle. Olivines and glasses from the submarine basalts show lower ³He/⁴He than MORB, similar to subaerial basalts from these islands. Sixteen glass chips from the same submarine samples have now undergone in-situ analysis for major elements (including S and Cl) by EPMA, trace elements by LA-ICP-MS, H₂O and CO₂ by FTIR, and bulk volatile analysis (S, Cl, F) by ion chromatography combined with pyrohydrolysis. H₂O ranges from 0.62-2.44 wt%, while CO₂ is below detection (<20 ppm). S measured by EPMA ranges from 612-1889 ppm and by bulk analysis from 582-1301 ppm and, with the exception of one sample, concentrations agree well. Cl measured by EPMA ranges from 151-538 ppm, and by bulk analysis from 188-980 ppm. The higher values suggest that the bulk samples may be contaminated by seawater; otherwise Cl correlates strongly with incompatible elements. F measured in the bulk samples ranges from 221-1243 ppm. S correlates positively with FeO and Cu, but not with incompatible elements, suggesting sulfide saturation. While the highest H₂O contents may reflect late-stage hydration and are oversaturated at the depth of collection, the low H₂O contents (11 samples with 0.62-0.96 wt%) are undersaturated, and there is a positive correlation between the H₂O contents of all chips and their incompatible element concentrations. This suggests that H₂O/Ce and Cl/Ce filtered for shallow level processes may reflect source compositions, providing constraints on volatiles in the sources of Rurutu and Tubuai, and indications about the efficiency of subduction-related volatile-loss in the HIMU component.