Vesiculation and enclave formation at Chaos Crags
Abstract
Observations from Chaos Crags, Lassen Peak national park indicate that enclave formation is an important stage in the magma mingling process. There is general agreement that enclave formation is related to vesiculation and the generation of density instabilities as a more mafic magma comes into contact with a cooler, more differentiated magma in a magma reservoir. In addition the evidence for exchange of crystals during mingling in these systems is overwhelming. At Chaos Crags, mafic enclaves in a dacite host contain crystals of biotite and sodic plagioclase which clearly originated in the dacite. In addition microlites which originated in both the host and the inclusion and which crystallized in response to quenching and volatile exsolution have been exchanged between the magmas prior to ascent and eruption. Less clear is how the crystals are transferred before and/or during the enclave formation process and how this relates to the vesiculation process. We have quantified major element, trace element and H2O contents of the magmas involved during magma mingling using EMP and SIMS analysis of olivine, feldspar, amphibole and quartz hosted melt inclusions and matrix glass in both the host lava and enclave. H2O contents range up to 5.06 wt% with the highest H2O contents being found in quartz hosted melt inclusions with ~74wt% SiO2. Only olivine hosted melt inclusions have more primitive melts at 66wt% SiO2 but these contain significantly less H2O. Quenched matrix glass from the enclave shows higher and more variable H2O contents than quenched matrix glass in the dacite host. By combining the textural information of the crystals with compositional data we have been able to determine the relative timing of vesiculation and crystal transfer at Chaos Crags.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V41F..02C
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 1043 GEOCHEMISTRY / Fluid and melt inclusion geochemistry;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies