A.E. Fallick | Search USGS

USGS1984

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

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.

Earth and Planetary Science Letters↗

USGS1983

The isotopic and chemical evolution of Mount St. Helens

Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with ε Nd = +5 , ε Sr = −10 , variable δ 18 O , 206 Pb/ 204 Pb ∼ 18.76, Ca/Sr ∼ 60, Rb/Ba ∼ 0.1, La/Yb ∼ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ε Nd = +4 to +8, ε Sr = −7 to −22 , variable δ 18 O (thought to represent melting of differing mantle-crust reservoirs), 206 Pb/ 204 Pb= 18.81−18.87 , variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with ε Nd = +6, ε Sr = −13, δ 18 O∼6‰ , variable 206 Pb/ 204 Pb, Ca/Sr ∼ 77, Rb/Ba= 0.1 , La/Yb ∼ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between 87 Sr/ 86 Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.

Earth and Planetary Science Letters↗