University of Oregon, United States

Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H (sub 2) O, CO (sub 2) , 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 H (sub 2) O contents (average 0.22 wt% H (sub 2) O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H (sub 2) O (average 0.16 wt%). Average S concentrations are 910 + or - 60 ppm for the high-MgO glasses v. 1030 + or - 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeO (sub T) . This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K (sub 2) O and S/TiO (sub 2) with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S Kalpha 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 H (sub 2) O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H (sub 2) O/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% H (sub 2) O, corresponding to H (sub 2) O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H (sub 2) O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H (sub 2) O content of the OJP mantle source region (170 + or - 30 ppm H (sub 2) O) is similar to that of the depleted MORB source (140 + or - 40 ppm H (sub 2) O). 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.