ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Diffusion of argon from two bronzite pyroxenes from the Ultramafic Zone of the Stillwater complex, Montana, has been measured by both a stepwise heating Ar release process and long term isothermal heating. Both pyroxenes contained excess Ar40. This amounted to at least 88% of the total radiogenic Ar40 in the minerals. The Ar release occurred by at least two mechanisms, and possibly a third. One of the most important, quantitatively, occurs between 600° and 1000° C and has an activation energy in excess of 72 kcal/gm-atom Ar. A second occurs above 1000° C. A small, but possibly significant, loss occurs rapidly below 600° C. Both of the major Ar release processes yield some excess Ar, and there is no single temperature range or mechanism which can be assigned unambiguously to the in-situ produced radiogenic Ar. Consequently, no K-Ar age can be obtained which gives the true age unambiguously. Ar36 tracer was used to conduct absorption experiments using the same bronzites. Runs were made at 800° and 1000° C at Ar pressures equal to total pressure of 1 atm. The absorption was found to be the same at the two temperatures and was 6 × 10−5 sccAr/Atmos.-gm px. It is suggested that the excess Ar in the bronzites was introduced at the time of their crystallization. The ambient Ar partial pressure in the melt at that time would have been 0.1 atm. Estimates of the Ar in the melt yield values of (3 to 80) × 10−6 sccAr/gm melt.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00372279
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