EXTREMELY LARGE ERUPTIONS FROM ONTONG JAVA PLATEAU

Magmas of Ontong Java Plateau (OJP) display little geochemical variation, having only a few widely dispersed magma types (Mahoney et al. , 1993). Here we define magma type as all lavas that have evolved by similar extents of melting of a similar mantle source, and have undergone similar polybaric fractionation histories. It does not imply they are from the same eruption. In this study, precise microprobe measurements of Cl, K, S, H2O, CO2 and major elements in glasses are used to show that magmas from widespread locations on OJP are nearly identical in composition and are from the same eruption and quite possibly from the same series of lava flows.

Cl concentrations are controlled by assimilation that takes place late at shallow levels in the magma chamber. The amount of assimilation and Cl content of assimilated material control Cl contents of magmas, and are expected to be highly variable in this stochastic process. It is inconceivable that magmas erupted at different times would have precisely the same Cl content, even if they have the same major element chemistry from identical cotectic evolution. The clearest case of distant lavas being from the same eruption is the Kroenke-type lavas from ODP holes 1187A and 1185B, about 140 km apart. The lavas form roughly 150 flow units of about 1 meter average thickness, which are most likely multiple surges of lava from a quasi-continuous eruption. Glass compositions (major elements and volatiles) follow an olivine control line that suggests a maximum of 17° C cooling throughout the large eruption which resulted in only 1.8% crystallization. Averages of 4 samples from each of the two holes are: Cl 750 vs 732 ppm; s.d.=15 ppm. S 988 vs 969 ppm; s.d.=5 ppm. K 616 vs 608 ppm; s.d.=22 ppm.

Differences in glasses' dissolved CO2 contents between different ODP holes suggests that lavas erupted from multiple vents and that flow lengths were at least 40 km (based on plateau slopes) and probably much longer. The volume of the eruption exceeds that of any known submarine eruption by a great deal. If its lateral dimensions are similar to the length between the holes, then its area would be >200 km2 and its volume would be >3000 km3. That such widely separated lavas have identical chemistry means that there was very little cooling over 10s of km of flow. References: Mahoney et al. Proc. ODP: Sci Results 130, 3-22, 1993.