Uranium and thorium decay series disequilibria in young volcanic rocks
Two of the central questions in igneous geochemistry that study of radioactive disequilibria can help to answer are: what are the rates of magma genesis; and what are the timescales of magma separation and transport. In addition to the temporal information that may be extracted from disequilibria data, the {sup 230}Th/{sup 232}Th of a young rock may be used as a tracer of the Th/U ratio of its source region. Measurements were made by isotope dilution alpha-spectrometry of {sup 238}U, {sup 234}U, {sup 230}Th, and {sup 232}Th in 20 subduction related, 3 oceanic intraplate, and 10 continental intraplate volcanics. {sup 210}Pb was measured in all, {sup 226}Ra was measured in about half, and {sup 228}Th was measured in 10 of the most recent samples. Disequilibrium between {sup 228}Th and {sup 232}Th was found only in the Nacarbonatite samples from Oldoinyo Lengai volcano in Tanzania, which is attributable to {sup 228}Ra/{sup 232}Th {approximately} 27 at the time of eruption. These rocks also have {sup 226}Ra/{sup 230}Th > 60. Three Ra-enrichment models are developed which constrain carbonatite magma formation at less than 20 years before eruption. The effects of different partial melting processes on the {sup 238}U decay series are investigated. If mid-ocean ridge basalts are formed by a dynamic melting process, the {sup 230}Th/{sup 232}Th of the basalts provides a minimum estimate of the Th/U ratio of the source region. The {sup 238}U enrichment in arc volcanics is probably the results of metasomatism of the source by fluids derived from the subducting slab, and the {sup 230}Th enrichment observed for other volcanics is probably due to the partial melting process in the absence of U-bearing fluids.
- Research Organization:
- California Univ., Santa Cruz, CA (USA)
- OSTI ID:
- 5470609
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580202* -- Geophysics-- Volcanology-- (1980-1989)
580400 -- Geochemistry-- (-1989)
ACTINIDE ISOTOPES
ACTINIDE NUCLEI
AFRICA
ALKALINE EARTH ISOTOPES
ALPHA DECAY RADIOISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CARBON 14 DECAY RADIOISOTOPES
CHEMISTRY
DEVELOPING COUNTRIES
ENVIRONMENTAL TRANSPORT
EQUILIBRIUM
EVEN-EVEN NUCLEI
GEOCHEMISTRY
GEOLOGIC HISTORY
HEAVY ION DECAY RADIOISOTOPES
HEAVY NUCLEI
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
LEAD 210
LEAD ISOTOPES
MAGMA
MAGNESIUM 28 DECAY RADIOISOTOPES
MASS TRANSFER
MELTING
NEON 24 DECAY RADIOISOTOPES
NUCLEI
PHASE TRANSFORMATIONS
PLATE TECTONICS
RADIOISOTOPES
RADIUM 226
RADIUM 228
RADIUM ISOTOPES
ROCKS
SPECTROSCOPY
SUBDUCTION ZONES
TANZANIA
TECTONICS
THORIUM 228
THORIUM 230
THORIUM 232
THORIUM ISOTOPES
URANIUM 234
URANIUM 238
URANIUM ISOTOPES
VOLCANIC REGIONS
VOLCANIC ROCKS
YEARS LIVING RADIOISOTOPES