HYDROTHERMAL CARBONATES FROM TURKANA, KENYA, PRESERVE EVIDENCE OF AN UNUSUAL MIDDLE MIOCENE THERMAL FLUID PULSE (Invited Presentation)

The Turkana Depression is located within the East African Rift system (EARs) situated between the Ethiopian highlands and the Kenya Dome. The presence of more extensive basin fill and evidence from thermochronology (Boone et al. 2019) suggest that rapid extension in the Turkana Depression began in the Middle Miocene. Carbonates preserve information about their formation based on mineralogy of the carbonates and through geochronology, elemental and stable isotope analyses as a proxy for conditions. Peculiar Miocene carbonates (i.e., non-pedogenic or lacustrine) are localized but widespread across the region, with evidence for major spring deposits recognized at Ngakoringora Ridge, a 300m long linear mound on the southwestern edge of the Lothidok Hills. Meter-wide walls of carbonate occur along fault traces along the western boundary faults of the Kalodirr graben (Boschetto 1988), and conglomeratic units with substantial carbonate cementation and petrified wood are regionally associated with the ridge. Carbonates associated with petrified wood have been found at Nakwai and Napudet. In this study, we focus on carbonates associated with petrified wood, particularly dolomites, many of which have elevated U concentrations and high U/Pb ratios making them amenable to U-Pb dating. U-Pb ages vary but give a range from 13.68 ± .01Ma (Kalodirr West), 15.26 ± 1.53Ma (Nakwai), and 13.5 ± .8Ma (Napudet). Dolomite and some calcite samples have high Zr concentrations and extremely elevated Zr/Hf ratios (<250) and ⁸⁷Sr/⁸⁶Sr of 0.703 to 0.704. These unusual characteristics have only been reported from carbonatites. While carbonatites are not recognized in the area, widespread phonolite deposits are time equivalent to carbonatites that formed south of the study area and perhaps tapped a similar mantle source. Clumped isotope measurements yield a carbonate formation temperature of ~50°C and fluid δ¹⁸O values of -4.2‰ to -5.0‰ VPDB. Despite these samples spanning >100km, these values suggest the influence of a basin wide fluid that mixed with meteoric waters and precipitated carbonates at or near the surface. Better understanding this fluid could provide insight into the continental rifting process and the landscape following the Middle Miocene Climate Optimum in Turkana.