HR: 1340h
AN: V13E-0601    [Abstracts]
TI: Evolution of the Craters of the Moon Lavas from primitive Snake River Plain basalts: inferences from plagioclase-melt thermobarometers and whole rock compositions
AU: * Vaid, N
EM: nitin_csu@yahoo.com
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2576 E. San Ramon Ave., MS/ST24, Fresno, CA 93710 United States
AU: Putirka, K
EM: kputirka@csufresno.edu
AF: California State University, Fresno, Department of Earth and Environmental Sciences, 2576 E. San Ramon Ave., MS/ST24, Fresno, CA 93710 United States
AU: Kuntz, M
EM: mkuntz@usgs.gov
AF: U. S. Geological Survey, Earth Surface Processes Team MS 980, Box 25046, Denver, CO 80225 United States
AB: The volcanic rocks of the Craters of the Mon Lava field provide an ideal laboratory for testing models of magma transport and evolution. Their compositions, relative ages and volumes are well known, as are the fractionation processes leading to their evolution (Leeman, 1976). The COM is somewhat distinctive in the Snake River Plain (SRP) region, due to its evolved character, and an apparent compositional segregation from associated SRP basalts. Some have suggested that the high Fe liquids of the COM demand an origin separate from that of SRP basalts, possibly involving an Fe-enriched mantle, while others have suggested that the COM lavas may be derived by fractionation at moderate depths (30 km). In either case, there are important implications in regard to mantle composition and the nature and distribution of thermal energy. We use plagioclase-melt pairs and an analysis of whole rock compositions in attempt to test models of COM magmatic evolution. Plagioclase-melt thermobarometers provide rough estimates of crystallization depths, and show that COM and SRP lavas partially crystallized at similar depths of 14 +/- 6 km. However, plagioclase crystallization temperatures for SRP basalts (1400 +/- 25 K; Kings Bowl, Cerro Grande, North and South Robbers) exceed temperatures for COM lavas (1358 +/- 45 K) by 40 K. Our data also show that fractional crystallization (ol + plag) can explain the evolution of surrounding SRP basalt flows, and that the most evolved SRP basalts approach primitive COM lava compositions. The most primitive of COM magmas appear to be characterized by the appearance of apatite + magnetite as fractionating phases. Our results thus confirm the geochemical model of Leeman (1976) and the physical model of Kuntz (1992), with the added insight that SRP basalts are parental to the more evolved COM lavas, through low-pressure fractional crystallization in the upper crust. The principal differences between SRP and COM magmas appear to relate more to the presence or absence of density contrasts in the crust than differences in composition or temperature of mantle source materials. SRP basalts lie near the axis of the SRP where the granitic upper crust may have been obliterated by earlier volcanic episodes. In contrast, COM lavas, whose vents lie off axis, appear to have been trapped within the upper crust for longer periods, sufficient for further differentiation. Finally, SRP rhyolite compositions lie on the same fractionation trend as COM and SRP lavas, at very low values of MgO. We propose that highly evolved lavas throughout the SRP may form by fractional crystallization mechanisms alone, rather than through the partial melting and remobilization of preexisting felsic crustal materials.
DE: 1020 Composition of the continental crust
DE: 1036 Magma chamber processes (3618)
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005