Abstract:
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.