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A Nd, Sr and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff, California

A. N. Halliday·A.E. Fallick·J. Hutchinson·W. Hildreth
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Abstract

The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial 87 Sr/ 86 Sr of 0.7060–0.7092 and δ 18 O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial 87 Sr/ 86 Sr of 0.7061–0.7069 and δ 18 O of 6.7–7.9. In contrast 143 Nd/ 144 Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of 0.51258 ± 1 . These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had ɛNd = −0.9, ɛSr = +23 and δ 18 O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith.

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A. N. Halliday, A.E. Fallick, J. Hutchinson, W. Hildreth. 1984. A Nd, Sr and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff, California. https://doi.org/10.1016/0012-821x(84)90123-7

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