HR: 1340h
AN: V53A-0610    [Abstracts]
TI: Using temporal-compositional trends in single eruption sequences of primitive basalts to observe systematic time-dependent mantle source variation: an example from the Big Pine Volcanic Field, California
AU: * Blondes, M S
EM: madalyn.blondes@yale.edu
AF: Yale University Dept of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511
AU: Reiners, P W
EM: peter.reiners@yale.edu
AF: Yale University Dept of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511
AU: Kayzar, T M
EM: tkayzar@email.arizona.edu
AF: University of Arizona Dept of Geological Sciences, 1040 E. 4th St., Tucson, AZ 85721
AU: Ducea, M N
EM: ducea@geo.arizona.edu
AF: University of Arizona Dept of Geological Sciences, 1040 E. 4th St., Tucson, AZ 85721
AU: Chesley, J
EM: jchesley@geo.arizona.edu
AF: University of Arizona Dept of Geological Sciences, 1040 E. 4th St., Tucson, AZ 85721
AB: Few studies in basalt petrology and geochemistry have examined geochemical variation on the scale of single eruptions, despite the fact that in cases where conditions permit such observation, large and systematic chemical variations are observed that correlate with eruption sequence. Because these trends are observed in near-primary mantle-derived melts, they may bear important signals related to the dynamics of melting, melt extraction, and mantle source changes that would not be evident at other sampling scales. Here we present data from 25 samples of the Papoose Canyon primitive, mantle xenolith-bearing single eruption sequence of the Big Pine Volcanic Field (BPVF), CA. Xenoliths are present in approximately the first half of the eruption, but are absent thereafter, when flows become markedly thinner. Temporal geochemical variations are large and systematic, and, in most cases, similar to previous primitive single eruption observations. Despite essentially constant MgO throughout the sequence (9.8 $\pm$ 0.3 (1$\sigma$) wt.%; Mg\#=70), incompatible elements systematically decrease by as much as a factor of two (e.g., La from 68 to 34 ppm). Both SiO$_{2}$ and some compatible elements such as Ni show systematic increases with time (45-48 wt.%, and 150-210 ppm, respectively). As in previous examples, however, some typically incompatible elements show puzzling behavior: Na$_{2}$O shows no change (3.6 $\pm$ 0.4 (1$\sigma$) wt.%) with time, and Nb and Ta do not decrease until the middle stages of the sequence, when HREEs reach maximum concentrations. $^{87}$Sr/$^{86}$Sr systematically decreases from 0.7063 to 0.7055 with time, and $\epsilon$$_{Nd}$ increases from -3.4 to -1.1. This single-eruption isotopic variation spans nearly that of all the BPVF basalts in the region. These temporal-compositional trends cannot be due to crystal fractionation, as the high MgO and Ni limit olivine crystallization to less than 5-10%, while Sr concentrations would require 40-50%. Crustal contamination also is considered unlikely not only because the basalts are primitive and contain mantle xenoliths, but also because SiO$_{2}$ and incompatible elements are inversely correlated. The isotopic data require at least two source components for these melts (at least one of which had residual garnet), and therefore rule out increasing degrees of partial melting of a single source. These temporal-compositional trends, which seem to be common to most primitive basaltic eruptions, appear to require mixing of at least two components. In any mixing scenario, the early-erupting, relatively alkalic melt batches contain larger proportions of the enriched component (higher incompatible elements, and in this case, high $^{87}$Sr/$^{86}$Sr and low $\epsilon$$_{Nd}$), and later-erupting, more silicic melts have increasing proportions of the more depleted component. Melt-rock reaction in the mantle, involving mixing of a relatively large-degree primary melt and a small-degree reaction-product melt, is a likely mechanism. This may involve partial melting of mantle wallrock or entrained xenoliths, alkali-diffusion-induced melting of surrounding peridotite, or systematic tapping from the center to the outside of melt channels within the mantle.
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting