HR: 14:40h
AN: V53C-05    [Abstracts]
TI: High Volatile Content and Shallow Melting at the end of the Siberian Flood Basalts: Experimental Results
AU: Draper, D S
EM: dave@draper.name
AF: Institute of Meteoritics, 1 University of New Mexico, Albuquerque, NM 87131 United States
AU: * Elkins-Tanton, L T
EM: Lindy@brown.edu
AF: Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
AU: Jewell, J D
EM: jessica.jewell@gmail.com
AF: Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
AU: Thorpe, A
EM: akthorpe@hotmail.com
AF: Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
AU: Agee, C B
EM: agee@unm.edu
AF: Institute of Meteoritics, 1 University of New Mexico, Albuquerque, NM 87131 United States
AB: Constraints on the depth and temperature of melting for flood basalt lavas are critical for evaluating melting models. Obtaining primary melting conditions through straightforward experimental petrology is unfeasible for most flood basalts because of significant secondary processing. The final lavas in the Siberian flood basalts (SFB), however, are candidates to be nearly primary magmas, having experienced only olivine addition or subtraction following mantle melting. These ultramafic lavas, predominantly meimechites, form a 1400 m stack at the top of the SFB section in the Maymecha region. One-atmosphere, piston-cylinder, and multi-anvil experiments have been performed on a synthetic analog of a meimechite olivine melt inclusion to determine its pressure and temperature of original mantle melting. Although the major element compositions of meimechite melt inclusions have the same trends as do the bulk rocks, the alkali contents of the melt inclusions are systematically higher, suggesting that the lavas have lost alkalis in a post-eruption serpentinization event. Meimechites are enriched in incompatible elements, particularly the LREE (Basu et al., 1995; Arndt, 2003), and are hydrous, evidenced by groundmass phlogopite. Analysis of meimechite major element trends indicates that compositions with approximately 25 wt% MgO represent liquid compositions. The experimental composition has 25.5 wt% MgO, 8.3 wt% CaO, and Mg\# 77. The experimental composition with 2% water is multiply saturated on its liquidus at 3.0 GPa and about 1600\deg C with olivine, sub-calcic augite, garnet, and Ti spinel. This multiple saturation is at an anomalously high temperature, interpreted to imply an even larger volatile concentration in the source region, which would likely lower the temperature of melting by as much as 100 to 150\deg without largely changing pressure. Pre-eruptive water and carbon dioxide content estimates are being obtained in current research analyzing melt inclusions. The multiple saturation point is interpreted as the conditions of batch melting in the mantle, or the mid-point of melting during an adiabatic ascent. The experimental meimechite therefore is inferred to have originated from mantle melting at about 100 km depth, surprisingly shallow for melting in a plume under an ancient continental lithosphere. Alternative models for melting are also required to explain the strong geologic evidence for subsidence during the first kilometer of eruption (Federenko and Czamanske, 1997). We suggest that the lower lithosphere delaminated, pulling topography down and allowing shallow melting. As it sank and heated, the lithosphere would have dewatered, providing volatiles for the meimechite source. The volatile input from the lithosphere can also explain the unusual meimechite trace element compositions. We also note that orthopyroxene instability in the source region, implied by its absence at multiple saturation, is further evidence for hydrous metasomatism of the mantle. This model is not contradictory to a plume model, but requires an upwelling weak enough (with relatively little buoyancy) to allow the delaminating material to cause subsidence in the lithosphere.
DE: 8400 VOLCANOLOGY
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting