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

Abstract

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 FeOT. 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.

Origin and evolution of magmas on the Ontong Java Plateau

“…The only evidence for subaerial or shallow-water volcanism during emplacement of the OJP is provided by the thick volcaniclastic succession at Site 1184 (Thordarson 2004;White et al 2004), two thin intervals of vitric tuff in the Aptian limestone immediately above basement at Site 1183 (Mahoney et al 2001) and a vitric tuff just above basement at Site 289. There is no evidence for subaerial eruption of lava flows, and the CO2 contents of basaltic glass (Michael 1999;Roberge et al 2004) provide convincing evidence for relatively deep submarine eruption at all the drill sites on and around the OJP. For example, Roberge et al (2004) estimate eruption depths of approximately 1000 m at Site 1183 on the crest of the plateau and approximately 2500 m at Site 1187 on its eastern margin (Fig.…”

“…There is no evidence for subaerial eruption of lava flows, and the CO2 contents of basaltic glass (Michael 1999;Roberge et al 2004) provide convincing evidence for relatively deep submarine eruption at all the drill sites on and around the OJP. For example, Roberge et al (2004) estimate eruption depths of approximately 1000 m at Site 1183 on the crest of the plateau and approximately 2500 m at Site 1187 on its eastern margin (Fig. 1).…”

Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites

“…[Norman and Garcia, 1999;GEOROC, http [Michael, 1995;Dixon and Clague, 2001;Dixon et al, 2002;Danyushevsky et al, 2000], and water depresses the freezing point of basaltic liquids slightly more efficiently than some elements with a heavier atomic weight. The effects of water have been evaluated by establishing how various amounts of dissolved magmatic water concentrations [Sobolev and Chassidon, 1996;Dixon and Clague, 2001;Michael, 1999;Danyushevsky et al, 2000;Danyushevsky, 2001;Nichols et al, 2002;Roberge et al, 2004] suppress the liquidus temperature , and results are included in Figure 5.…”

Phreatomagmatic eruptions on the Ontong Java Plateau: chemical and isotopic relationship to Ontong Java Plateau basalts

“…In spite of this problem, however, the plume-head model remains the most widely accepted hypothesis for the generation of the OJP, mainly because it is the only model amongst several contenders that adequately explains the generation of such a large magma volume derived from a relatively undepleted (i.e. non-mid-ocean ridge basalt) and volatile-poor (Roberge et al 2004a) source in a relatively short period of time.…”

“…Wignall 2001). A likely explanation for the lack of environmental catastrophe associated with the OJP is the observation that the majority of the volcanism occurred in the submarine environment, and that the magmas are relatively volatile-poor (Michael 1999;Roberge et al 2004a). This would have restricted the flux of climate-affecting volatile species (e.g.…”