Sr Isotopic Variation of Plagioclase Crystals in Mount St. Helens, Southern Washington
Abstract
Deciphering magma chamber dynamics through the use of petrologic methods on the scale of single crystals has been of considerable interest recently. In particular, the textural and compositional records preserved in zoned plagioclase offers a window into the history of magmas. Such information is most valuable when the products of recent or current eruptions can be compared with eruption chronology. The recent (2004-2006) volcanic activity at Mount St. Helens (MSH) and growth of the new lava dome provide an excellent opportunity to explore these dynamics. In this study, we present new Sr-isotope and trace element data on plagioclase crystals from samples collected from the growing lava dome. Sr-isotope variations within single plagioclase feldspar grains were obtained using laser ablation micro-sampling techniques. Isotopic ratios were corrected for potential Rb interference and adjusted for mass fractionation. Crystals vary from clear euhedral grains and rims, to resorbed and inclusion-rich grains in apparent disequilibrium with the glass. Plagioclase grains within a single thin section of the new MSH dome dacite have a range of Sr isotope ratios greater than that of an array of whole rocks from the entire volcano. Most plagioclase crystals have Sr-isotopic ratios that show a near-normal distribution between 0.7032 and 0.7040, about an average of 0.7036. Some rare crystals have Sr- isotopic values as high as 0.7049 and have elevated incompatible trace element concentrations. LREE, Ba, Pb, Ba/La and Ba/Sr, decrease over the Sr isotopic range of 0.7032 - 0.7035, then increase sharply between 0.7035 and 0.7038. These systematic variations in plagioclase crystals suggest that up to four different components are present in the 2004-2006 Mount St. Helens dacite. These four components are 1) a slightly enriched component with a Sr-isotope composition of ~0.7032, 2) a more depleted component with an isotope composition of ~0.7035, 3) an enriched component with an isotope composition of ~0.7038, and 4) a minor component with Sr-isotope compositions >0.7040. These observations reinforce the impression of a complex and dynamic magmatic history involving recharge events that is recorded in plagioclase textures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V54B..04H
- Keywords:
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- 1036 Magma chamber processes (3618);
- 1042 Mineral and crystal chemistry (3620);
- 3625 Petrography;
- microstructures;
- and textures;
- 8178 Tectonics and magmatism;
- 8439 Physics and chemistry of magma bodies