Steps toward dating early diagenetic K-feldspar by the 40Ar– 39Ar method

Abstract

Authigenic sedimentary low-temperature K-feldspar separated from Albian–Turonian carbonates in Israel was dated by the step-heating 40Ar– 39Ar method. In contrast to high-temperature K-feldspars, ca. 90% of the radiogenic Ar was released at temperatures lower than 600 °C and fusion mostly occurred below 750 °C. Though formed under low temperature, Ar loss in all but one sample is estimated to be less than 2%. Nevertheless, the effect of 39Ar recoil is evident in some separates, probably due to their very fine size (4–10 μm). The plateau age of one sample with the highest content of authigenic K-feldspar (93%, 96 Ma) is slightly younger than the assumed stratigraphic age and thus defines the timing of an early diagenetic event within a few million years after deposition. Other samples where a plateau age could be determined yield ages slightly older. Strong acid etching (7 N HNO 3, 85 °C) of the K-feldspar-enriched fraction may have improved the 39Ar– 40Ar spectra but did not eliminate the presence of minute quantities of detrital K-bearing minerals. Though the present set of data is insufficient for a clear-cut conclusion, the activation energy of the authigenic K-feldspar is in the range of 14–26 kcal mol − 1 s − 1 , much lower than that of magmatic and hydrothermal K-feldspars and can probably be attributed to the heating schedule and the small crystal size. It is not clear whether the formation at low temperatures (< 50 °C) has any effect on the activation energy.