The feasibility of determining the age of diagenetic K-feldspar overgrowths utilizing 40Ar 39Ar step-heating and laser-probe dating techniques was tested on samples from the Lower Cretaceous arkoses of the Angola offshore basin. A previous study of these samples combining cathodoluminescence and K Ar dating indicated that the K-feldspar diagenetic overgrowths were 98 ±16 Ma and the detrital cores were 516 ± 26 Ma. Conventional 40Ar 39Ar step-heating of overgrown grains from the bulk K-feldspar fraction failed to unambiguously distinguish detrital from diagenetic ages. Similarly, step-heating analysis of overgrowth concentrates (>84% diagenetic) did not yield any distinct plateau age attributable to the diagenetic K-feldspar. Detrital and diagenetic K-feldspars from the arkoses studied appear to release their Ar over similar ranges of temperature. Partial laser fusion of individual overgrown grains, using lowenergy, short-duration laser pulses carefully focussed on the overgrowth, yielded mixed ages, with Ar contributed from both the overgrowth and the core. Heat conduction calculations indicate that conductive heating through the core-overgrowth boundary is not responsible for partial degassing of the core. Rather, beam scattering along intragrain structural defects and incipient melting of the core must occur. A more successful approach involved total laser fusion of individual grains from the overgrowth concentrates, which were physically separated from their detrital core. One limitation of this approach was that because of the young age and the small size of the diagenetic feldspar, simultaneous fusion of several grains was required for accurate age determination. Our results suggest that future investigations will greatly benefit from the use of on-line cathodoluminescence capability to reduce the possibility of contamination by detrital material and the use of a laser beam whose wavelength is strongly absorbed by diagenetic K-feldspar.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology