Separation, K / Ar dating and 18 O / 16 O ratio measurements of diagenetic K-feldspar overgrowths: An example from the Lower Cretaceous arkoses of the Angola Margin *

Details Separation, K / Ar dating and 18 O / 16 O ratio measurements of diagenetic K-feldspar overgrowths: An example from the Lower Cretaceous arkoses of the Angola Margin * Separation, K / Ar dating and 18 O / 16 O ratio measurements of diagenetic K-feldspar overgrowths: An example from the Lower Cretaceous arkoses of the Angola Margin *

Sep 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2207g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 9, pp.2207-2214

Physics

6

Scientific paper

K-feldspar overgrowths (adularia) on detrital K-feldspar were removed from the Lower Cretaceous (130-115 Ma) arkoses of Angola and concentrated for oxygen isotopic analysis and K / Ar dating. The concentration technique consisted of etching with dilute H 2 SiF 6 followed by ultrasonic agitation and particle size separation. The diagenetic K-feldspar content of each of fourteen subfractions, as estimated by cathodoluminescence was between 45 and 92 percent, the rest being mostly detrital microcline, and in some cases small amounts of quartz and/or albite. Extrapolation of 40 Ar / 40 K and 18 O / 16 O ratios indicated a K / Ar age of the adularia of 98 ± 16 Ma and a 18 O of 20.2 ± 0.8 per mil (SMOW). The age is consistent with petrographic observations which suggest that precipitation of adularia occurred relatively early in the diagenetic history. The K / Ar age, together with the 18 O value, burial history and fluid inclusion data indicate that adularia formed at relatively shallow depth (<1200 m) between 114 and 82 Ma ago from high temperature (150°C), high salinity (24 wt. pet. eq. NaCl) and 18 O-rich (6 to 9 per mil) fluids. These fluids were probably residual brines derived from the Aptian evaporites overlying the sediments studied.

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