Reply to discussion of “Extracting K-Ar ages from shales: a theoretical test”

As plots of K-Ar ages vs. percentage of the detrital component of mineral mixtures containing a detrital and a diagenetic illite (or illite-smectite) have different curved shapes, the linear extrapolation method of extracting the end-member ages is not justified. The curvature depends on the difference in K2O contents; thus it can be minimized if the percentage of detrital illite is normalized to the sum of detrital illite plus illitic fraction of diagenetic illite-smectite.

U-Series Dating by the TIMS Technique of Land Snails from Paleosols in the Canary Islands

A sequence of seven superimposed paleosols developed on eolian calcarenites and alluvium was sampled on the island of Lanzarote in order to examine the possibility of dating land snail shells by the U-series method, using a TIMS technique allowing measurement of U and Th isotopes in very small samples. In the lower six units, the fossil shells yielded D-allo/L-isoleucine (A/I) ratios of about 0.5 and apparent AMS 14C ages ranging from 41,000 to 34,000 yr B.P., indicating that most paleosols formed during a relatively short mid-Würm humid episode. The upper unit (paleosol 7) yielded more variable A/I ratios (ranging from 0.6 to 0.2) and a younger 14C age ∼27,000 yr B.P. Most samples contained enough U to allow the calculation of U-series ages, after correction for the presence of a detrital component. In samples containing a few tens of ppb of U (paleosols 1, 2, 3, 6, and 7), the ages are strongly dependent upon the model used for the correction. In samples containing more than 300 ppb of U (paleosols 4 and 5), concordant ages of ∼31,000 ± 1000 yr were obtained regardless of the correction model used. U uptake in these shells occurred during one single early diagenetic phase, soon after burial, since shells of modern snails do not contain any significant amount of U. The arid conditions subsequent to the mid-Würm humid episode have likely ensured since then a fair closure of the radioactive system.

14C and Th/U Dating of Pleistocene and Holocene Stromatolites from East African Paleolakes

During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ13C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO2, thereby lending credence to the reliability of the 14C. An initial 230Th/232Th ratio in the detrital component was determined by Th/U measurements on the 14C dated stromatolites. Using this value a 230Th/234U chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18O and 13C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.

Crandallite group minerals in the Helikian Athabasca Group in Alberta, Canada

Members of the crandallite group of aluminous hydroxy phosphates are present in trace amounts in every formation of the Athabasca Group in Alberta. The minerals of the group present in Alberta form a solid-solution series with end members goyazite (SrAl3(PO4)OH5), crandallite (CaAl3(PO4)OH5), and gorceixite (BaAl3(PO4)OH5). These minerals are present as cubes and subhedral grains, 2–20 μm across, in isolation or in clusters interstitially in the Athabasca Group sandstones, siltstones, and tuffs. Petrographic study indicates an authigenic origin for the crandallite-group minerals in the Athabasca Group. Their presence, locally, beneath quartz overgrowths and early diagenetic fluorapatite suggests formation very early in the post-depositional history of the rock. The presence of the crandallites within the regolith beneath the Athabasca Group and within a metamorphic rock fragment incorporated into the sandstone suggests more than one origin for the minerals and possibly a detrital component.