HR: 0800h
AN: V31A-1418    [Abstracts]
TI: Evidence for Extremely Large Lava Flows on Ontong Java Plateau from High Precision Measurements of Volatiles and Major Elements in Natural Glasses
AU: * Michael, P J
EM: pjm@utulsa.edu
AF: The University of Tulsa, 600 S. College Ave., Tulsa, OK 74104 United States
AB: 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). In this study, we use high precision microprobe measurements of Cl, K, S, H2O, CO2 and major elements in glasses to show that magmas from widespread locations on OJP are identical in composition and are probably from the same eruption and quite possibly from the same series of lava flows. By same eruption, we mean the quasi-continuous issuance of magma from a continuous chamber over a time period that is insufficient for further differentiation or assimilation. By same lava flow, we mean lavas that have issued from the same or nearby vents and were part of a sequence that that was continuously molten at the surface or beneath a crust. Cl concentrations are controlled by assimilation that takes place fairly 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 we feel are multiple surges of lava from a quasi-continuous eruption. Glass compositions (major elements and volatiles) do not vary more than analytical uncertainties within each hole. Differences between the two holes are also less than analytical uncertainties. 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. Counting precision is ñ15 ppm (2 sigma) for each element in each single glass analysis. Precision is better than accuracy. Similar CO2 contents in the glasses of the separate holes (Roberge et al., 2004), despite several hundred meters difference in their current reconstructed basement depth suggests that the lavas were erupted from the same vents and flowed for long distances: that they are not merely separate flows of a large eruptive episode that issued from a single magma chamber that had many vents. This observation does not mean that dissolved CO2 contents cannot be used to estimate paleoeruption depths, but that caution is required. 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 virtually no cooling over 10s of km of flow. This requires an insulating layer above a rapidly flowing magma (Gregg and Fornari, 1998). We are currently evaluating whether other widely separated recoveries on OJP are also from single lava flows. Possible matches include 1185B (lower) and 1186A, which are about 150-200 km apart. They would indicate even larger eruptive volumes. It is possible that certain glasses from 807C are the same as a thin layer in 1185B. References: Gregg and Fornari, JGR 103, 27517. Mahoney et al. Proc. ODP: Sci Results 130, 3-22, 1993. Roberge et al. Geol. Soc. Lond. Spec. Pub 229, 239-257, 2004.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
DE: 5480 Volcanism (8450)
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting