Combined trace element and oxygen isotope measurements in Mount Hood andesites: a new approach for studying mixed crystal populations in intermediate magmas
Abstract
Many intermediate magmas, particularly in arc settings, contain complex mineral populations as a result of magma mixing. Traditional studies of whole rock chemical and isotopic compositions in such magmas are often difficult to interpret, and a focus on single-mineral studies provides valuable constraints on magma sources. Studies of trace element, radiogenic isotope and stable isotope compositions based on individual minerals (or domains within minerals) have all proven to be valuable approaches for studying intermediate magmas, but these approaches are less commonly combined. We present the results of a coupled trace element and stable isotope (O, H) study of plagioclase and amphibole from compositionally restricted andesites and dacites from Mount Hood, Oregon. Previous studies show that these lavas contain mixtures of crystals derived from both mafic and felsic source magmas, and minerals sourced from these parental magmas are clearly distinguishable on the basis of trace element contents. For this study we have analyzed individual crystals for trace element abundances using laser ablation ICP-MS prior to analysis for O and H isotopic composition. Oxygen isotopes show significant variations: amphiboles have δ18O values that range between 4.8-5.4 ‰ and plagioclase between 6.0-6.3 ‰. Amphiboles derived from a mafic parent have δ18O of ~5.3 ‰, those from felsic parent magma have δ18O from 4.8-5.2 ‰. Comparison of amphibole δ18O with trace element indices that are sensitive to crystal fractionation (e.g. Eu/Eu*) show that lighter δ18O values correlate with greater extents of fractionation (lower Ευ/Ευ*). This correlation is inconsistent with oxygen isotope fractionation solely due to crystallization, which would increase δ18O. These amphiboles also have lower Al2O3 contents, and formed at relatively shallow crustal levels (~5-7 km), suggesting that lower δ18O reflect assimilation or melting of wallrocks that have interacted with meteoric water as recognized elsewhere in the Cascades (e.g. Crater Lake, Mount Jefferson). This data show the importance of shallow crustal processes in the formation of felsic magmatic components that are involved in intermediate magma generation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V52B..05K
- Keywords:
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- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 1036 GEOCHEMISTRY Magma chamber processes