Petrogenesis of Eocene Flood Basalts in the Turkana Region of the East African Rift System: Re-Os Constraints

The earliest volcanism in the East African Rift System manifests as flood basalts erupted prior to the onset of rifting. Two temporally and spatially distinct eruptive episodes are recognized: Oligocene flood basalts blanketing the northwest Ethiopian Plateau and Eocene Turkana flood basalts located in northern Kenya and southern Ethiopia. While numerous studies have focused on the petrogenesis of Oligocene NW Ethiopian lavas, the paucity of studies on the Eocene Turkana flood basalts limits our understanding of the melting dynamics and the mantle sources contributing to the onset of Cenozoic volcanism in East Africa. In this study, we examine the petrogenesis of Eocene flood basalts using Re-Os isotopic compositions of 30 Eocene flood basalts spanning six volcanic centers across northern Kenya and southern Ethiopia. The lavas experienced variable degrees of fractional crystallization and olivine accumulation with MgO= 6.9-15.1wt.%. The Os abundances in the lavas range from 2 297ppt, with most samples between 2 100ppt. Abundances of Re range from ~200ppt to >1200ppt. Most age-corrected 187Os/188Os ratios are moderately to highly radiogenic (0.1409-1.040), with only one sample recording a mildly depleted isotopic signature (0.1276). Five samples have low (<0.1200) or negative age-corrected 187Os/188Os values, indicating an excess of Re, potentially due to crustal assimilation. Age-corrected 187Os/188Os ratios are generally more radiogenic than NW Ethiopian flood basalts (0.1239-0.1329), demonstrating notable differences in the isotopic signature of the source materials between Eocene and Oligocene flood basalts. Of the volcanic centers measured, only one records a pseudo-isochron (~42Ma), suggesting the radiogenic values recorded by these lavas were inherent to the magma source. Lavas from other volcanic centers show poor correlations between Re and Os abundances and isotopic ratios, which may be due to crustal assimilation.