sup 238 U- and sup 232 Th-series chronology of phonolite fractionation at Mount Erebus, Antarctica

Journal Article · Sun Mar 01 04:00:00 EST 1992 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(92)90071-P· OSTI ID:7206244

Reagan, M K ^[1]; Volpe, A M ^[2]; Cashman, K V ^[3]

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1. Univ. of Iowa, Iowa City, (United States)
2. Los Alamos National Lab., NM (United States)
3. Princeton Univ., NJ (United States)

Uranium, thorium, radium, and barium abundances and {sup 234}U/{sup 238}U and {sup 230}Th/{sup 232}Th isotopic ratios determined by thermal ionization mass spectrometry and ({sup 228}Th)/({sup 232}Th) activity ratios determined by alpha spectrometry are used to date anorthoclase growth and infer magma chamber residence times of phonolites erupted in 1984 and 1988 from Mount Erebus, Antarctica. The 1984 and 1988 glasses have slightly different ({sup 230}Th)/({sup 232}Th) ratios but both have a 10% excess of ({sup 230}Th) over ({sup 238}U) and equilibrium ({sup 228}Th) values. By comparing these data and Pb-isotopic data reported in SUN and HANSON (1975) to similar data for oceanic basalts, the duration of differentiation from basanite to phonolite is limited to less than 150,000 years. The anorthoclase separates have ({sup 230}Th)/({sup 238}U) ratios exceeding those of the associated glasses but have ({sup 230}Th)/({sup 232}Th) ratios like those of the glasses. Both glasses are depleted in {sup 226}Ra with respect to {sup 230}Th by about 25%, whereas associated anorthoclase separates have extreme excesses of {sup 226}Ra over {sup 230}Th and ({sup 228}Th)/({sup 232}Th) = 2.2. On a plot of ({sup 226}Ra)/Ba vs. ({sup 230}Th)/BA, the glass-anorthoclase pairs produce isochrons averaging 2,380 y, which represents the average age of anorthoclase growth in the shallow magma system at Erebus. The implied residence time of phonolite magmas in the shallow magma chamber system of Erebus is about 3,000 y. Final crystal growth occurred after intrusion into the convecting lava lake less than decades before eruption.

• OSTI ID: 7206244
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 56:3; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580000* -- Geosciences
ABUNDANCE
ACTINIDE ISO
ACTINIDE ISOTOPES
ACTINIDE NUCLEI
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ALKALINE EARTH ISOTOPES
ALKALINE EARTH METALS
ALPHA DECAY RADIOISOTOPES
ALPHA SPECTROSCOPY
ANTARCTIC REGIONS
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CRYSTAL GROWTH
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ELEMENTS
EVEN-EVEN NUCLEI
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GEOTHERMAL SYSTEMS
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MAGNESIUM 28 DECAY RADIOISOTOPES
MASS SPECTROSCOPY
METALS
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NUCLEI
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PHOTOGRAPHY
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POLAR REGIONS
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RADIUM
RADIUM 226
RADIUM ISOTOPES
ROCKS
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THORIUM
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THORIUM 230
THORIUM 232
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URANIUM
URANIUM 234
URANIUM 238
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VOLCANIC ROCKS
VOLCANOES
YEARS LIVING RADIOISOTOPES