Isotopic zonations in silicic magma chambers
- Univ. of Wisconsin, Madison (USA)
Many ash-flow tuffs are zoned in radiogenic isotope ratios, indicating that roofward assimilation of crust occurs in ash-flow magma chambers prior to eruption. Cases where relatively well constrained calculations may be made regarding the percentage of assimilation in the roof zone indicate that the percentage of assimilation often exceeds the percentage of phenocrysts in the tuffs. This relation, in addition to the fact that assimilation gradients are opposite to that of the percentage of phenocrysts, suggests that assimilation and crystallization in the silicic roof zones of crustal magma chambers are separated in time and space, and that these processes are best modeled as two-component mixing; true assimilation-fractional crystallization is probably restricted to the lower mafic parts. Most phenocrysts in the silicic upper parts of magma chambers crystallized after assimilation, providing minimum estimates of time between assimilation and eruption (1-100 yr). Preservation of monotonic isotopic gradients suggests that convection is minor in the upper parts of silicic magma chambers during the late stages of evolution.
- OSTI ID:
- 6918427
- Journal Information:
- Geology; (USA), Vol. 17:12; ISSN 0091-7613
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
MAGMA SYSTEMS
GEOLOGIC HISTORY
TUFF
ISOTOPE RATIO
CRYSTALLIZATION
GEOCHEMISTRY
GEOLOGIC MODELS
MIXING
NEODYMIUM ISOTOPES
STRONTIUM 86
STRONTIUM 87
ALKALINE EARTH ISOTOPES
BETA DECAY RADIOISOTOPES
CHEMISTRY
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOTHERMAL SYSTEMS
HOURS LIVING RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
NUCLEI
PHASE TRANSFORMATIONS
RADIOISOTOPES
STABLE ISOTOPES
STRONTIUM ISOTOPES
580000* - Geosciences