HR: 1330h
AN: V32A-1004    [PDF]
TI: Preliminary Experimental Results on a Volcanic Meimechite Composition From Meymecha, Siberia
AU: * Elkins-Tanton, L T
EM: Linda_Elkins_Tanton@brown.edu
AF: Brown University, Dept. Geological Sciences 324 Brook St., Providence, RI 02912 United States
AU: Jewell, J
EM: Jessica_Jewell@brown.edu
AF: Brown University, Dept. Geological Sciences 324 Brook St., Providence, RI 02912 United States
AU: Hess, P C
EM: Paul_Hess@brown.edu
AF: Brown University, Dept. Geological Sciences 324 Brook St., Providence, RI 02912 United States
AB: Ultramafic lavas, predominantly meimechites, form an approximately 1400 m stack at the top of the Siberian flood basalt section in the Maymecha-Kotuy region (see in particular Fedorenko and Czamanske, 1997). The meimechite stratigraphy correlates with an area above the top of the Siberian flood basalt stratigraphic sections in Noril'sk, Tunguska, and Putorana. Meimechites are exceptionally high in magnesium and iron (up to 38 wt% and 17 wt% respectively), low in silica (40 to 42 wt%). Although the major element compositions of the meimechite melt inclusions measured by Sobolev et al. (1991) plot on the same trends as the bulk rocks, the alkali contents of the melt inclusions are systematically higher than the lava compositions, suggesting that the lavas have lost alkalis. The majority of whole rock compositions have sodium to potassium ratios below one, while the majority of melt inclusions have ratios above one, indicating loss of sodium. With the higher alkali content, up to 5 wt% sodium plus potassium in the melt inclusions, the melt inclusion liquids no longer qualify as meimechites under the new IUGS definition, but rather as foidites. Further analysis of major element trends indicates that, in agreement with Arndt et al. (1995) and others, compositions with MgO contents of approximately 25% represent liquid compositions, and we conclude that lower magnesium compositions are the results of olivine fractionation, while higher magnesium composition have accumulated olivine. Preliminary one-atmosphere and piston-cylinder experiments have been performed on a synthetic analog of a meimechite composition obtained from an olivine melt inclusion, with 26.9 wt% MgO, 8.2 wt% CaO, and 40.2 wt% silica. The experimental composition, with an Mg\# of 78.3, has a nearly identical major element composition as whole rock compositions, except that the melt inclusion has higher alkali content, at about 3 wt% sodium oxide plus potassium oxide. The preliminary experiments show that the composition's liquidus at one atmosphere is at about 1450 degrees C, and at 1 GPa the liquidus lies at about 1550 degrees C. Olivine and spinel are stable at the liquidus from one atmosphere to 1 GPa, and pyroxene appears at one atmosphere at about 1250 degrees C, after approximately 200 degrees of olivine crystallization. While the roles of hydrous fluids and magma mixing have yet to be determined, it can be shown that a number of the lavas erupted within their olivine stability fields, with pyroxene only in the groundmass, and from our preliminary experimental results this constrains the eruption temperature to between 1250 and 1450 degrees C. The high magnesium content of these lavas have lead some workers to conclude that they are the definitive plume signature, arguing that they could only have been formed by very deep and hot melting. Others have suggested that they are olivine cumulates, supported by the large, abundant olivine grains in the lavas, because the linear compositional trends in major elements of all the meimechite bulk compositions are consistent with an accumulation or loss of olivine. The high concentration of incompatible elements of the lavas also may suggest either a lithospheric component, a very small degree of mantle melting, or possibly magma mixing. The very high liquidus temperature for this composition is a strong contraint on processes of formation for the Siberian flood basalts, and as experiments are performed at higher temperatures, suggestions for the mantle source lithology will be discovered.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting