HR: 1340h
AN: V43B-1369    [Abstracts]
TI: Constraints on Lithospheric Enrichment and Crustal Contamination in the Central Rio Grande Rift: Evidence From Basaltic Melt Inclusions
AU: Lassiter, J C
EM: lassiter1@mail.utexas.edu
AF: University of Texas at Austin, Department of Geological Sciences, Austin, 78712,
AU: * Rowe, M C
EM: michael-rowe@uiowa.edu
AF: University of Texas at Austin, Department of Geological Sciences, Austin, 78712,
AU: * Rowe, M C
EM: michael-rowe@uiowa.edu
AF: University of Iowa, Department of Geoscience, Iowa City, 52242,
AB: Many basalts erupted during the early stages of rifting throughout the southwestern United States have an enrichment in water-soluble elements leading to high ratios of water-soluble to insoluble elements, e.g. Ba/Nb, K/Nb, and Sr/Nd, often found in volcanic arcs. Prior studies have suggested that these basalts with "arc-like" trace element signatures originate from melting of a mantle source that has experienced hydrous metasomatism as a result of shallow subduction of the Farallon Plate beneath western North America. Rio Grande Rift extension and volcanism correlates with the eastern extent of flat slab subduction of the Farallon. We have analyzed olivine- and orthopyroxene-hosed melt inclusions from Rio Grande Rift (New Mexico, U.S.A.) basalts for major-, trace-, and volatile-species concentrations. Melt inclusions from lavas with an "arc-like" signature (e.g. elevated Ba/Nb or Sr/Nd) should be also enriched in volatile species such as water and Cl. In melt inclusions from Rio Grande Rift tholeiites and alkaline basalts Cl/K and Cl/Nb ratios broadly correlate with Ba/Nb and Sr/Nd. In melt inclusions with higher Ba/Nb and Sr/Nd ratios, Cl/Nb and Cl/K ratios are also slightly elevated. Similarly, tholeiitic basalts with lower Ba/Nb and Sr/Nd possess low Cl/Nb and Cl/K ratios, more consistent with ratios observed in fresh MORB. Low Cl/Nb ratios relative to melt inclusions from modern arc and backarc basalts from the Casacade range in western North America are consistent with minor volatile enrichment occurring in an extreme backarc position. However, partial melting and assimilation of continental crust may produce qualitatively similar trends. Whole rock ²⁰⁸Pb/²⁰⁴Pb from Rio Grande Rift lavas negatively correlates with inclusion Cl/Nb, Ba/Nb and Sr/Nd ratios. Basement rocks in the study area generally have low radiogenic Pb and high Ba/Nb, consistent with a model of crustal contamination for the Rio Grand Rift basalts. However, low Sr/Nd of basement rocks precludes a model for simple assimilation of local basement. Further investigation is required to distinguish between lithospheric and crustal sources for trace element and volatile enrichment.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting