HR: 08:20h
AN: V51F-02    [Abstracts]
TI: Carbonate Introduction to, Storage in, and Release from, Archean Subcontinental Mantle and its Environmental Impact
AU: * Bell, D R
EM: David.R.Bell@asu.edu
AF: Dept of Geological Sciences and Dept. of Chemistry and Biochemistry, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Buseck, P R
EM: pbuseck@asu.edu
AF: Dept of Geological Sciences and Dept. of Chemistry and Biochemistry, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Spinsby, J
EM: jspinsby@asu.edu
AF: Dept of Geological Sciences and Dept. of Chemistry and Biochemistry, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Orstrom, P
V51F-02 AF: Dept of Geological Sciences and Dept. of Chemistry and Biochemistry, Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AB: Carbon storage mechanisms in the upper mantle are understood in theory but C-bearing minerals are rarely observed in mantle rocks. Our observations on Archean low temperature harzburgites from Kimberley, South Africa, suggest that carbonate is more common in Archean subcontinental lithospheric mantle (SCLM) than previously recognized. Catastrophic release from this reservoir during flood basalt volcanism could potentially affect surface environments. Patches of brucite + serpentine with the outline of primary metamorphic mineral grains up to 5 mm in diameter are interpreted as the secondary hydration products of primary magnesite and dolomite (Berg 1986). Modal analysis of 72 peridotite xenoliths [125 slabs; ~1m2] reveals that carbonate pseudomorphs occur in 75% of samples, with a mean modal abundance of 0.07 vol. %. A textural association with metasomatism is apparent, including Mesozoic kimberlite-related metasomatism, and ancient (probably Archean) subduction-related orthopyroxene introduction. Graphite and sulfide are occasionally associated. Diamond occurs in refractory olivine-rich harzburgites that represent a low fluid/rock environment. Carbonate:diamond in Kimberley SCLM is ~1800:1. The average CO2 content (~350 ppm) accounts for the CO2 content of orangeites thought to be lithospheric melts of the Kaapval craton SCLM. Incorporation of lithospheric melt (orangeite) accounts for the enriched trace element chemistry of Karoo flood basalt province picrites (Ellam and Cox 1992). A calculation based on the CO2 and K2O content of orangeite suggests that these picrites could contain 2 to 4 wt. % CO2 and may be vapor-saturated at mantle depths. The picrites are thus capable of delivering a high CO2 flux to the atmosphere during certain phases of flood basalt volcanism. Most Phanerozoic flood basalt provinces with contemporaneous mass extinctions erupted proximal to Archean cratons with major diamond deposits that indicate C-rich SCLM. Flood basalts not contemporaneous with mass extinctions did not. This pattern suggests that the CO2 content of the mantle source region of flood basalts influences their environmental impact.
DE: 1025 Composition of the mantle
DE: 3621 Mantle processes (1038)
DE: 3625 Petrography, microstructures, and textures
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8408 Volcano/climate interactions (1605, 3309)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005