Abstract Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H 2 O, CO 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 2 O contents (average 0.22 wt% H 2 O) than high–MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low–MgO glasses have lower H 2 O (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/K 2 O and S/TiO 2 with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S K α 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 2 O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H 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 2 O, corresponding to H 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 2 O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H 2 O content of the OJP mantle source region (170 ± 30 ppm H 2 O) is similar to that of the depleted MORB source (140 ± 40 ppm H 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.

Abstract The compositions of glass clasts in volcaniclastic rocks recovered from drilling at Site 1184 on the eastern salient of the Ontong Java Plateau (OJP) are investigated using microbeam analytical methods for major, minor and trace elements. These data are compared with whole-rock elemental and isotopic data for bulk tuff samples, and with data from basalts on the high plateau of the OJP. Three subunits of Hole 1184A contain blocky glass clasts, thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks. The glass clasts have unaltered centres, and are all basaltic low-K tholeiites, with flat chondrite-normalized rare earth element (REE) patterns. Their elemental compositions are very similar to the Kwaimbaita-type and Kroenke-type basalts sampled on the high plateau. Each subunit has a distinct glass composition and there is no intermixing of glass compositions between subunits, indicating that each subunit is the result of one eruptive phase, and that the volcaniclastic sequence has not experienced reworking. The relative heterogeneity preserved at Site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau, and suggests that the eastern salient of the OJP had a different type of magma plumbing system. Our data support the hypothesis that the voluminous subaerially erupted volcaniclastic rocks at Site 1184 belong to the same magmatic event as the construction of the main Ontong Java Plateau. Thus, the OJP would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans, and these factors may have influenced the contemporaneous oceanic anoxic event.

Abstract Oceanic plateaus are areas of elevated and anomalously thick oceanic crust that are believed to form by enhanced partial melting in a mantle plume that is hotter than ambient upper asthenosphere. They are regarded as the oceanic equivalent of continental flood-basalt provinces. Because of the continual subduction of oceanic crust, the oldest known oceanic plateaus occurring in situ are Cretaceous in age. In order for oceanic plateaus to be preserved in the geologic record, they must be accreted onto continental margins. This process, involving their preservation as tectonic slices, depends on the fact that oceanic plateaus are more buoyant than normal ocean floor; thus, they are not easily subducted. If these plateaus encounter an oceanic arc, subduction polarity reversal may occur, and/or the locus of subduction may step back behind the trailing edge of the advancing plateau. At a continental subduction zone, only subduction back-step occurs. Geochemical evidence shows that basaltic and picritic rocks exposed in the thickened part of the Caribbean plate and around its margins (including northern South America) are parts of an accreted oceanic plateau that originated in the Pacific Ocean during the middle-to-late Cretaceous. Cretaceous subduction-related rocks also occur around the Caribbean margins and possess geochemical signatures (e.g., lower Nb and Ti) that are distinct from those of the oceanic plateau rocks. This arc material represents the remnants of the subduction-generated rocks with which the plateau collided at 80–90 Ma. Both island arc tholeiite and calc-alkaline magmatism occurred in these Cretaceous arcs, but the changeover between the two types appears to be gradual and cannot be used to determine the timing of subduction polarity reversal. Many Cretaceous tonalitic batholiths around the Caribbean margins appear to have formed during or shortly after accretion of the plateau rocks. In addition to the arc and oceanic plateau assemblages, Jurassic to Early Cretaceous fragments of the preexisting oceanic crust also occur around the region. The environmental impact of oceanic plateau volcanism around the Cenomanian-Turonian boundary and its link to the formation of organic-rich black shales is discussed in this paper.