ISOTOPIC FINGERPRINTING OF THE PLIO-PLEISTOCENE TURKANA TUFFS

The Turkana Basin region is home to extensive Plio-Pleistocene volcanic tuff deposits which are notable for their vast spatial extent within a stratigraphic sequence over 700 metres thick. These spatiotemporally widespread tuff deposits have immense paleoanthropological importance, hosting an extraordinary fossil record of human evolution (>500 fossils). However, despite their broad scientific significance, little is known at present about the silicic magma genesis that gave rise to the Turkana Basin tuffs, nor the locations of the source volcanic centre(s). Indeed, much of the detailed geochemical studies in this region have focused on the older, basaltic volcanism. Here, we present the first Sr-Nd-Hf-Pb radiogenic isotope investigation of these extensive volcanic sequences. K-feldspar crystals from pumice clasts, pumice glass shards, and tuff glass shards were analysed to identify phases preserving the true magmatic signature of each eruption.

Our results reveal that unique volcanic eruptions can be readily distinguished on the basis of their radiogenic isotope compositions. For instance, Upper and Lower Nariokotome Tuffs, separated by ~50,000 years, exhibit distinct source signatures: εNd_(i) of 3.8 ± 0.1 and 3.1 ± 0.1 (1SD), ⁸⁷Sr/⁸⁶Sr_(i) at 0.71452 ± 0.0003 and 0.70774 ± 0.0006, and ²⁰⁶Pb/²⁰⁴Pb ratios of 18.2 ± 0.01 and 18.0 ± 0.02. The implications of these findings are two-fold. First, major and trace element geochemistry are often used to correlate tuffs both across Turkana and into Ethiopia, where such correlations are crucial in transferring ages to localities where datable material is absent. However, tephra from the same eruption may yield a wide array of composition in terms of major and trace elements. Radiogenic isotopes, therefore, may serve as an additional tool for geochemically fingerprinting distinct horizons. Second, these data can provide insights into the genesis of each eruption, such as the contribution of mantle and crustal components in their source region and the possibility of any genetic links to the basaltic volcanism which may have preceded them. For example, the εNd_(i) and ²⁰⁶Pb/²⁰⁴Pb values described here are like those of the Turkana Pliocene ‘Stratoid’ lavas but distinct from those of the Pliocene ‘Shield’ lavas, while the elevated ⁸⁷Sr/⁸⁶Sr_(i) signatures indicate a significant crustal input in the source of the Nariokotome tuffs.