Delineating contamination of basaltic magma by mafic crust VIA U/Th disequilibrium determined by TIMS, Pisgah Craters, Mojave Desert, California

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5019788

Ramos, F C; Reid, M R ^[1]

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1. Univ. of California, Los Angeles, CA (United States). Dept. of Earth and Space Sciences

Compositional changes in basaltic lavas at Pisgah Craters, CA, are hypothesized to reflect contamination by variable amounts of mafic crust (Glazner et al, 1991). Data determined by Glazner and coworker indicate that the lavas range in isotopic composition from [sup 143]Nd/[sup 144]Nd = 0.51293--0.51276 and [sup 87]Sr/[sup 86]Sr = 0.7038--0.7050, and have decreasing concentrations of incompatible elements with decreasing MgO. The lavas are postulated to range from nearly pure primitive mantle-derived magmas (>8 wt % MgO) early in the eruptive history to nearly pure melts of mafic crust (<4.5 wt % MgO) late in the eruptive history. Preliminary U-series isotopic results obtained by thermal ionization mass spectrometry suggest that the crustal contaminant contains minor concentrations of Th and U. Th and U concentrations in the lavas correlate positively with MgO contents, with ([sup 238]U)/([sup 232]Th) increasing by almost 30% over a variation from 8 to 4.5 wt % MgO. In contrast, [sup 232]Th/[sup 230]Th varies little with MgO concentration and averages 1.8 [times] 10[sup 5] (([sup 230]Th)/([sup 232]Th) [approximately]1.03; 5 samples). The [sup 232]Th/[sup 230]Th ratios of the Pisgah lavas are those expected for mantle-derived magmas based on their Sr and Nd isotopic compositions. The lack of Th isotopic variation coupled with decreasing Th concentrations suggests that the contaminant contained very little Th, consistent with a mafic and/or deep crustal source. U/Th disequilibrium in these lavas indicates that contamination and eruption occurred within the past 50,000 years from the present.

• OSTI ID: 5019788
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

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