In vacuo crushing experiments and K-feldspar thermochronometry

Details In vacuo crushing experiments and K-feldspar thermochronometry In vacuo crushing experiments and K-feldspar thermochronometry

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.117..169m&link_type=abstract

Earth and Planetary Science Letters, Volume 117, Issue 1-2, p. 169-180.

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Scientific paper

Using in vacuo crushing methods, both Cl- and K-correlated components of trapped 40Ar have recently been identified in hypersolvus alkali feldspar and K-feldspar-bearing `chert' and stromatolite samples. If these components re-emerge during the late stages of thermal degassing, interpretation of 40Ar/39Ar age spectra could be complicated. We have extended these observations by performing similar measurements on a well-characterized low-temperature K-feldspar, MH-10. A plot of 40Ar*/K against Cl/K confirms the presence of a chlorine-related excess argon component from sites accessible by crushing MH-10, although it amounts to only 0.078% of the total potassium-derived 39Ar from this sample. No potassium-correlated component of excess argon could be clearly identified. Surprisingly, isothermal duplicate heating steps reveal a Cl-correlated component (40ArE/Cl = 2.7 +/- 0.2 × 10-4) that appears to be related to the modification of very small ( < 1 μm) inclusions. This method appears to provide a basis with which to correct for the excess argon that is commonly observed during the initial stages of step heating of K-feldspars. The product of the 40ArE/Cl and the Cl/K (determined for each step via the 38Ar/39Ar ratio) yields an ``excess age'' that is simply subtracted from the measured 40Ar*/40K to yield an age corrected for the presence of excess argon. Crushing appears to produce an artifact that seriously affects the step heating age spectrum but does shed light on possible complexities in the internal distributions of 40Ar and 39Ar. On balance, we find that any adverse consequences arising from these effects on interpreting the 40Ar/39Ar results in terms of thermochronometric models are exceedingly small.

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