Volatiles in submarine basaltic glasses from the Ontong Java Plateau (ODP Leg 192): Implications for magmatic processes and source region compositions

Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H₂O, CO₂, S, Cl). At four of the sites (1183, 1185, 1186, 1187) the glass is from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. Glassy pillow rims from Site 1187 and the upper group of flows at Site 1185 have 8.3-9.3 wt% MgO compared with values of 7.2-8.0 wt% MgO for glasses from Sites 1183, 1184 1186, and the lower group of flows at Site 1185. Low-MgO glasses have slightly higher H₂O contents (average 0.22 wt% H₂O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H₂O (average 0.16 wt%). Average S concentrations are 910 ± 60 ppm for the high-MgO glasses v. 1030 ± 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeO^T. This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K₂O and S/TiO₂ with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S Kα peak in the glasses indicate low oxygen fugacities comparable to MORB values. Chlorine contents of the glasses are very high compared with MORB, and Cl/K ratios for all glasses are relatively high (>0.7). This ratio is sensitive to assimilation of hydrothermally altered material, so the high values indicate assimilation during shallow-level crystallization of OJP magmas. Ratios of H₂O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H₂O/Ce values are probably also caused by the same assimilation process that results in high Cl. The water content of the high MgO-magmas before contamination is estimated to be approximately 0.07 wt% H₂O, corresponding to H₂O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H₂O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H₂O content of the OJP mantle source region (170 ± 30 ppm H₂O) is similar to that of the depleted MORB source (140 ± 40 ppm H₂O . Instead, large extents of melting beneath the OJP must have been caused by a relatively high mantle potential temperature, consistent with upwelling of a hot mantle plume.