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Volcanic biotite-sanidine (super 40) Ar/ (super 39) Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite

John M. Hora, Brad S. Singer, Brian R. Jicha, Brian L. Beard, Clark M. Johnson, Shan de Silva and Morgan Salisbury
Volcanic biotite-sanidine (super 40) Ar/ (super 39) Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite
Geology (Boulder) (October 2010) 38 (10): 923-926

Abstract

The (super 40) Ar/ (super 39) Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between (super 14) C and longer-lived isotopic systems that are too imprecise for dating young samples. However, (super 40) Ar/ (super 39) Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, (super 238) U- (super 230) Th disequilibrium, to demonstrate that (super 40) Ar/ (super 39) Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous (super 40) Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in (super 40) Ar/ (super 39) Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of (super 40) Ar due to rapid loss to an "infinite reservoir" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 38
Serial Issue: 10
Title: Volcanic biotite-sanidine (super 40) Ar/ (super 39) Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite
Affiliation: University of Wisconsin, Department of Geoscience, Madison, WI, United States
Pages: 923-926
Published: 201010
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 29
Accession Number: 2010-090909
Categories: Geochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2010256
Illustration Description: illus.
Secondary Affiliation: Oregon State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201048
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