| Abstract: | 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 ??{lunate}Nd = +5, ??{lunate}Sr = -10, variable ??18O, 206Pb/204Pb ??? 18.76, Ca/Sr ??? 60, Rb/Ba ??? 0.1, La/Yb ??? 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ??{lunate}Nd = +4 to +8, ??{lunate}Sr = -7 to -22, variable ??18O (thought to represent melting of differing mantle-crust reservoirs), 206Pb/204Pb = 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 ??{lunate}Nd = +6, ??{lunate}Sr = -13, ??18O ???6???, variable 206Pb/204Pb, 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 87Sr/86Sr 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. ?? 1983. |
| Genre: | Article |
| ProdID: | 70011362 |
| Citation Author: | Halliday, A. N.; Fallick, A. E.; Dickin, A. P.; Mackenzie, A. B.; Stephens, W. E.; Hildreth, W. |
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| Citation End Page: | 256 |
| Citation Issue: | 2 |
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| Citation Language: | English |
| Citation Larger Work Title: | Earth and Planetary Science Letters |
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| Citation Number Of Pages: | 16 |
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| Citation Search Results Text: | The isotopic and chemical evolution of Mount St. Helens; 1983; Article; Journal; Earth and Planetary Science Letters; Halliday, A. N.; Fallick, A. E.; Dickin, A. P.; Mackenzie, A. B.; Stephens, W. E.; Hildreth, W. |
| Citation Start Page: | 241 |
| Citation Volume: | 63 |
| Citation Year: | 1983 |
| Type: | citation/reference |
| Text: | The isotopic and chemical evolution of Mount St. Helens; 1983; Article; Journal; Earth and Planetary Science Letters; Halliday, A. N.; Fallick, A. E.; Dickin, A. P.; Mackenzie, A. B.; Stephens, W. E.; Hildreth, W. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Sat, 1 Jan 1983 00:00 -0600 |
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