FOSSIL WOOD-ASSOCIATED DOLOMITE RECORDS AN UNUSUAL RIFT RELATED FLUID

In the Lothidok Hills of the Turkana Basin, Kenya, a unique dolomite (MgCa(CO₃)₂) has been discovered in association with fossilized wood, from middle Miocene fluvial sediments that contain important early primate and afrotherian fossils. This dolomite has elevated U, almost no initial Pb, and yields a near-concordant U-Pb age of 13.68 ± 0.01 Ma from LA-ICP-MS analysis. Dolomite carbonate clumped isotope analysis yield an average formation temperature of 57 ± 6 °C and a calculated fluid δ¹⁸O that suggests a meteoric source for the warm fluid. Lower dolomite δ¹³C values, sampled along the axis of growth, reflect the incorporation of wood-derived carbon during initial mineralization, which get heavier toward the outside of the sample. Sr isotopes are consistent with a mantle-like source and suggest fluid interaction with volcanic rocks that are abundant across the region. The preservation of cellular wood textures in the dolomite indicates that mineralization occurred soon after burial of the wood. This dolomite also occurs as pore filling cements in the conglomerate that hosts the fossil wood, and this conglomerate occurs immediately above a regional unconformity that marks a change in fluvial transport direction. The geochemically unique dolomite has elevated Zr and a high Zr/Hf ratio, which is also seen in carbonates along a ~150 km transect on the western side of Lake Turkana. The age of the dolomite is coincident with a period of increased rifting across the basin, suggesting a regional process for moving the hydrothermal fluids with this unique chemistry. Because the cements must have occurred early to preserve wood textures and because they occur in a basal conglomerate overlying a change in fluvial transport directions, the age of the dolomite provides an important constraint on the tectonics that led to this disruption in stratigraphy.