Volatiles in Submarine Basaltic Glasses from the Ontong Java Plateau (ODP Leg 192): Implications for Magmatic Processes and Source Region Compositions

The Ontong Java Plateau (OJP) in the western Pacific is the largest volcanic oceanic plateau and may represent the largest magmatic event on Earth in the last 200 m.y. During ODP Leg 192, unaltered basaltic glass was recovered from igneous basement at five widely separated sites in previously unsampled areas of the plateau. At four of the Sites (1183, 1185, 1186, 1187) the glass is derived from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. We have analyzed the glasses for major and volatile elements (H₂O, CO₂, S, Cl) using FTIR spectroscopy and electron probe. Our results complement previously published data for glasses recovered from Sites 803 and 807 on the OJP (Michael, 1999). An exciting discovery of Leg 192 was that basement at Site 1187 and the upper group of flows at Site 1185 are composed of high-MgO, incompatible-element-poor basalt that is unlike basalts found elsewhere on the OJP. Glassy pillow rims from these basalts have 8.3 to 9.3 wt% MgO compared with values of 7.2 to 8.0 wt% MgO for glasses from Sites 1183, 1184, 1186, and the lower group of flows at 1185. Relatively low K₂O, Na₂O, and P₂O₅ in all glasses suggest that OJP basaltic magmas formed by large extents of melting. H₂O concentrations are similar in the two basalt types (high-MgO average 0.19 +/- 0.01 wt% H₂O; low-MgO average 0.22 +/- 0.02 wt%) despite the lower K₂O and TiO₂ of the high-MgO glasses. Average S concentrations are 910 +/- 60 ppm for the high-MgO glasses vs. 1030 +/- 60 ppm for the low-MgO glasses. When compared with MORB, the OJP glasses have lower S at comparable FeO, indicating that OJP basaltic magmas were not saturated with immiscible sulfide during crystallization. Cl contents of the glasses are very high compared with MORB, as was found previously for glasses from Sites 803 and 807. Cl/K ratios for all glasses are relatively high (> 0.7). This ratio is sensitive to assimilation of hydrothermally altered material (Michael and Cornell, 1998), so the high values indicate extensive assimilation during shallow level crystallization of OJP magmas. Ratios of H₂O to Ce, which have similar incompatibility, are 355-370 for high-MgO glasses and ~270 for low-MgO glasses. These values are slightly higher than most depleted and enriched MORB (Michael, 1995). However, the high H₂O/Ce values of the high-MgO glasses may be caused in part by assimilation. As with the previous results for Sites 803 and 807, there is no evidence for high H₂O contents in OJP basalts that might have significantly increased extents of mantle melting beneath the OJP. Instead, large extents of melting must have been caused by a relatively high mantle potential temperature.