Collaborative Research: Origins of Phenocrysts and Xenocrysts in the Bishop Tuff Rhyolitic Magma

Texas A&M Research Foundation, College Station TX

Investigators

Abstract

Anderson/Wallace EAR-0001154 EAR-0001182 The objectives of this study are to document the sources of crystals in the Bishop Tuff, a large volume, fragmental deposit of silicic volcanic rock that formed as a result of a huge explosive eruption, about 500 times larger than the 1980 Mt. St. Helens eruption. The crystals are important because they contain a record of processes related to how the magma formed and how and why it erupted. The traditional interpretation for the origins of such crystals is that they grow from the melt in which they erupt (such crystals are termed 'phenocrysts'). Recently, this traditional view has been challenged by those working with radiometric isotopic systems capable of providing independent measures of time. New interpretations are that most of the crystals formed a long time before eruption and were recycled from older, previously crystallized magma bodies (such crystals are termed 'xenocrysts'). Reheating of these older, largely crystalline bodies is hypothesized to have liberated crystals that became entrained in the erupting magma. In this interpretation, crystals contain little or no information about either how the magma formed or what made it erupt. Large bodies of silicic magma are an important part of planetary differentiation, and it is vital to test concepts that are related to their origin. A record of apparent formation of phenocrysts is contained in successively grown zones on crystals and in melt inclusions (now glass) that were trapped during crystal growth. We will use this record to test ideas regarding the sources of crystals in the Bishop Tuff magma. This study involves field relations of volcanic deposits, textural information, and geochemistry, and will provide an opportunity for students to learn and appreciate the need for field tests of hypothetical ideas.