Ar Systematics of Illite

H. Dong, C. M. Hall, W. C. Elliott, D. R. Peacor, and A. N. Halliday

The ⁴⁰Ar^-39Ar vacuum encapsulation dating technique can be used to determinc the ages of events affecting clay minerals in pelitic rocks which have undergone diagenesis and low-grade metamorphism. For the samples from the Welsh sedimentary basin with grades equal to or greater than that of upper anchizone, the ⁴⁰Ar^-39Ar total gas ages are identical to those determined by the conventional K-Ar method, reflecting transformation of illite to muscovite. However, more information can be obtained from analysis of age spectra obtained by vacuum-encapsulated ⁴⁰Ar^-39Ar analysis. For diagenetic samples from the Welsh basin and eastern U.S., the ⁴⁰Ar^-39Ar total gas ages are in general concordant ith K-Ar ages. Although the concordance between ⁴⁰Ar^-39Ar total gas ages and K-Ar ages yields geologically meaningful ages, it is not necessarily the case, based on data for the samples from the Welsh basin. The concordant age may be younger than the independently determined depositional/diagenetic age. However, the "retention age", obtained from ⁴⁰Ar and ³⁹Ar atoms retained in the mineral at room temperature after irradiation, are generally consistent with the depositional age. We inferred that ³⁹Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that havc lost radiogenic ⁴⁰Ar in nature. lllite crystallinity appears to be a dominant controlling factor in determining the degree of ³⁹Ar loss. Our most recent data demonstrated that the "retention age" so determined is reproducible for sub-microgram sample sized without loss of precision and accuracy.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California