EXPLORING THE LINK BETWEEN METASOMATIZED LITHOSPHERE AND CONTINENTAL RIFTING: A CASE STUDY OF THE EAST AFRICAN RIFT SYSTEM

The subcontinental lithospheric mantle (SCLM) is foundational to understanding the construction, destruction, and division of tectonic plates. Tectonic processes, in turn, both directly and indirectly influence the lithosphere’s thermal, physical and mineralogical properties. Mantle melting and melt/fluid percolation cause fundamental changes to the lithosphere that affect its composition and stability. Specifically, metasomatism by silicate melts and hydrous/carbonated fluids can create lithologies (i.e. pyroxenites) that are denser, more fusible, and less viscous than adjacent peridotite. The resulting density instabilities may lead to lithospheric erosion, topographic uplift and even continental rifting. We explore the link between metasomatized SCLM, mafic volcanism and associated continental rifting in the Ugandan portion of the Western Rift of the East African Rift System using Re-Os isotopes from both alkaline mafic lavas and pyroxenite mantle xenoliths. The lavas record age-corrected ¹⁸⁷Os/¹⁸⁸Os that range from 0.1421 to 0.2105, which is more radiogenic than primitive mantle (0.1296; Meisel et al., 2001). These data demonstrate that many of the lavas were derived from a metasomatized mantle source though a few have experienced crustal contamination. The mantle xenoliths also record a wide range of ¹⁸⁷Os abundances. One peridotite xenolith has a mildly radiogenic signature (¹⁸⁷Os/¹⁸⁸Os = 0.1342) whereas the pyroxenites span a wide range of ¹⁸⁷Os/¹⁸⁸Os ratios (0.1401-0.5052). One pyroxenite recorded a mildly unradiogenic ¹⁸⁷Os/¹⁸⁸Os value (0.127) and has 0.96 ppb of Os. Based on these data, we conclude that the lavas were derived from metasomatized SCLM. Some of the SCLM was sampled by mantle xenoliths but, as a whole, the SCLM is more heterogeneous than the lavas suggest. The widespread, metasomatized SCLM readily contributed to melt generation both in situ as well as during foundering via lithospheric drip (Furman et al., in review). The SCLM-derived volcanism occurred prior to and during Western Rift extension, suggesting that the metasomatized SCLM played a vital role in rift development.