Abstract
Many geological contexts lack suitable material for commonly applied absolute dating methods. Even if some regions contain potentially datable materials, they may be inappropriate for dating by existing geochronological techniques. Paleoanthropological localities on the Homa Peninsula, southwestern Kenya, have yielded important paleoanthropological material. These localities are, however, poorly constrained in time, and few radioisotopic ages have been published, because sediments derived from the Homa Mountain carbonatite complex commonly do not contain zircons for UPb dating or feldspars for 40Ar/39Ar dating. Additionally, apatite grains that are present have failed to produce precise isotope dilution-thermal ionization mass spectrometer (ID-TIMS) results due to their high common lead content. Apatite (UTh)/He dating using single-crystal laser fusion He determinations and the UTh isotope dilution method was applied to three formations with well-constrained ages to test the suitability of this technique for dating sediments on the peninsula. Two of the three Kanam Formation samples provide dates close to the approximately 6 million year age (Ma) provided by 40Ar/39Ar dates. The Abundu Formation was dated to 2.39 ± 0.29 Ma overlapping with the age of the Rawi Formation (C2An.1n Subchron, 3.05–2.59 Ma) to which it has been correlated. The age for the Kanjera South Member 1.54 ± 0.85 Ma overlaps with the expected 2 Ma age, though it has a large uncertainty. The apatite raw (UTh)/He ages are somewhat younger than the expected dates, but the technique shows promise.
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