OsHf isotopic insight into mantle plume dynamics beneath the East African Rift System

摘要

We report new Os and Hf isotopic data on mafic lavas from several key portions of the East African Rift System (EARS) with the goal of determining how contributions from various source domains influence volcanism in the evolving rift system. Our study uses picrites and basalts associated with the Afar plume in NW Ethiopia and with prolonged extension in Turkana, N Kenya, as well as mafic lavas from Kivu and Rungwe in the Western Branch of the EARS. Basalts from NW Ethiopia and Turkana have low Os concentrations (9-22ppt) and display a range of ~(187)Os/ ~(188)Os (0.1239-0.4366). The 30Ma high-TiO _2 picrites from NW Ethiopia and 20-23Ma picrites from Turkana have higher Os concentrations (579-1120ppt) than associated basalts. Picrites from NW Ethiopia have initial ~(187)Os/ ~(188)Os=0.1239-0.1311 and ε _(Hf)=12.0-13.4, consistent with derivation from a mantle source common to global OIB (i.e. "C"). In contrast, 20-23Ma Turkana picrites have more radiogenic initial ~(187)Os/ ~(188)Os (0.1450-0.1483). None of the picrites display convincing evidence for crustal or subcontinental lithospheric mantle input. Instead, the data are consistent with geochemical and geophysical models that demonstrate early evolution of the EARS was supported dynamically by geochemically distinct regions of mantle upwelling. Specifically, NW Ethiopian lavas are chemically analogous to the "C"-like Afar plume while Miocene Turkana lavas display HIMU-like geochemical features. The HIMU component in Turkana lavas can be generated by mixing ~30% ancient (1.7-2Ga) hydrothermally altered subducted oceanic crust with ~70% "C"-like mantle material (i.e. < 1Ga recycled hydrothermally altered oceanic crust). In contrast, Kivu and Rungwe lavas have low Os concentrations (3-87ppt) and more radiogenic ~(187)Os/ ~(188)Os (0.1615-0.3610) that appear to be dominated by contributions from metasomatized lithospheric mantle. Seismological observations indicate that there are thermochemical heterogeneities within the deep-seated African superplume; these heterogeneities are a plausible source for the ancient recycled oceanic crust contributing to Miocene volcanism in Turkana. We propose that mafic magmatism in both the Afar region and northern Kenya are derived from different portions of this long-lived thermochemical feature.