Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v34a..07g&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V34A-07
Other
1038 Mantle Processes (3621), 1040 Radiogenic Isotope Geochemistry, 1065 Major And Trace Element Geochemistry, 8415 Intra-Plate Processes (1033, 3615)
Scientific paper
The geochemistry of basaltic magmas erupted in the Basin and Range province of the western USA has demonstrated that at least two mantle sources exist, one with a subduction signature and another with an "ocean island basalt" (OIB) signature. Here we investigate the distribution of these two sources during the Pleistocene and Holocene in a 250 km-long transect from the eastern Sierra Nevada near Reno, NV, into central Nevada. Samples were collected from young, dated mafic lava flows from the Carson Range (2.5 to 1.4 Ma), Steamboat Hills (2.6 Ma), Virginia City and Chalk Hills (1.5 to 1.44 Ma), east of Carson City (1.36 Ma), Rattlesnake Hill (1.2 to 0.9 Ma), Buffalo Valley (1.1 to 0.95 Ma), Upsal Hogback (0.6 Ma), and Soda Lake (Holocene). With the exception of Carson Range andesites, all of the lavas are alkaline basalts and basaltic trachyandesites with K2O/Na2O > 0.4. Incompatible element abundances, incompatible element ratios, and radiogenic isotope ratios vary widely between locations. Many key incompatible element ratios, such as Ce/Pb, Sr/P, Ba/Nb, and Nb/La, and isotopic ratios vary as a function of age and longitude. Lavas less than 1 Ma in age have low Ba/Nb, Sr/P, 87Sr/86Sr, 206Pb/204Pb, and high Ce/Pb and Nd/La compared to lavas greater than 1Ma in age. These ratios vary more strongly as a function of longitude, from high Ba/Nb, Sr/P, 206Pb/204Pb, 87Sr/86Sr and low Ce/Pb and Nb/La (subduction signature mantle) lavas in the Sierra Nevada margin to lavas with the opposite characteristics (OIB signature mantle) in central Nevada. La/Sm does not vary with either age or longitude. The relationship with longitude indicates that two mantle sources currently exist beneath western Nevada, subduction-modified mantle to the west and OIB-type mantle to the east, and that these two mantle types probably taper in thickness towards one another. The termination of subduction beneath the Reno area at 5-3 Ma, in conjunction with 87Sr/86Sr greater than modern Cascade arc lavas, suggests that the subduction-modified source resides primarily in the lithospheric mantle and that melting occurs due to extension of the lithosphere. Either an asthenospheric or lithospheric source of OIB-type lavas is possible. The results for this cross-section mirror those reported for a transect across southern and central Nevada, with the exceptions that a) the age range considered here is shorter and closer to the present day, and b) isotopic compositions from Buffalo Valley at the east end of the section do not as closely approach depleted upper mantle values such as those reported for the Lunar Crater volcanic field to the south.
Cousens Brian L.
Gupta Varsha
Henry D. C.
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