Paper No. 87-2
Presentation Time: 1:15 PM
TIME SCALES AND THERMAL CONDITIONS OF MAGMA STORAGE IN THE CRUST: INSIGHTS FROM CRYSTAL RECORDS
The thermal conditions of magma storage in sub-volcanic crustal reservoirs largely control the physical state of magmas (e.g., magma viscosity is directly related to temperature and indirectly related through crystallinity). Understanding what proportion of time magma bodies spend largely-liquid vs. a locked mush is critical to understanding processes such as magma mixing and crystal-liquid segregation, which contribute to compositional diversity of magmas and can control development of an eruptible body of magma. Crystal ages combined with sub-crystal-scale compositional information provide information about the conditions of storage. For example, rhyolitic erupted products from South Sister, OR, Okataina Volcanic Center, New Zealand, and Yellowstone Caldera, WY, show surprisingly similar patterns when comparing ages and chemical compositions of major vs. accessory phases, despite orders of magnitude variation in eruption volumes. Commonalities are that: 1) eruptions have diverse zircon populations documenting that eruptions gather crystals that originated in coeval yet chemically distinct magma bodies that existed for tens of thousands of years, and 2) these same eruptions contain feldspar with significantly shorter pre-eruptive histories and less chemical diversity. These data suggest that zircon were physically separated from coeval feldspar, likely due to melt+zircon extraction from a locked crystal network containing the major phases, and the resulting magma bodies subsequently crystallized new feldspar during relatively brief periods of storage prior to eruption. Similar conditions of storage are implied using a novel approach combining time scales of crystal residence derived from time scales derived from U-series disequilibria, textural information, and trace-element zoning in crystals. For crystals derived from the silicic end-member at Mt. Hood, OR, we show that only a small fraction (likely < 1%) of the >21 kyr total storage duration has been spent at temperatures above the critical crystal fraction of 40-50%, where the magma body is easily mobilized. Thus, our results collectively imply that silicic magma bodies of diverse sizes and settings reside dominantly as locked mushes, with largely-liquid bodies present only for a short time prior to eruption.