"Excess Ar" by laboratory alteration of biotite

Villa, Igor M.; Bosio, Giulia (2023). "Excess Ar" by laboratory alteration of biotite. Geology, 51(1), pp. 121-125. Geological Society of America 10.1130/G50503.1

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Many biotite phenocrysts from marine tephra layers have substoichiometric potassium concentrations and alkali occupation << 2.0 atoms per formula unit. Diagenetic alteration is an expected effect of exposure of fresh magmatic minerals to interstitial water and brine intrusions after the deposition and burial of sediments. To test the effect of diagenetic alteration on potassium-argon ages, we irradiated and step heated untreated Fish Canyon biotite (t = 28.2 Ma) and several aliquots leached to various extents in strong and weak acids. Laboratory alteration caused loss of K, age spectrum discordance, high step ages and total gas ages, Ar release at lower furnace temperature, higher Cl/K and Ca/K, and a slight decrease in 36Ar concentration. Potassium loss was always higher than 40Ar* loss. Electron microprobe element maps document that acids preferentially penetrate in phyllosilicate interlayers, removing K (and Na). Because Ar* is removed to a lesser extent than K, we propose that natural 40K decay partly implants radiogenic Ar* into the tetrahedral-octahedral-tetrahedral (T-O-T) phyllosilicate layer, where Ar is shielded from interlayer leaching. The recoiled 39Ar, which was produced by irradiation after the leaching, also partitioned between T-O-T and the interlayer; age spectrum discordance was probably enhanced by the heterogeneous partition of 39Ar and 40Ar* in leached samples.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Other Institutions > Emeriti, Faculty of Science
08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology

UniBE Contributor:

Villa, Igor Maria

Subjects:

000 Computer science, knowledge & systems
500 Science > 550 Earth sciences & geology
500 Science

ISSN:

0091-7613

Publisher:

Geological Society of America

Language:

English

Submitter:

Igor Maria Villa-Toscani

Date Deposited:

20 Mar 2023 06:56

Last Modified:

20 Mar 2023 23:27

Publisher DOI:

10.1130/G50503.1

Uncontrolled Keywords:

alteration, biotite, 39Ar-40Ar dating, 38Ar, element maps

BORIS DOI:

10.48350/180319

URI:

https://boris.unibe.ch/id/eprint/180319

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