Geochemical Study Of Volcanism Associated With The Early Stages Of Continental Rifting In Northern Tanzania PDF Download

A close relationship between lithospheric extension and mantle plumes is commonly assumed as the driving force for evolving divergent plate boundaries that manifest as a rift in either oceanic or continental settings. While lithospheric extension is robustly supported by geophysical and geochemical evidence, the role of mantle plumes is often less clear. In fact, in many cases, ongoing integrated geochemical and geophysical work suggest that heterogenous asthenospheric and sub-continental lithospheric mantle components or complex interactions between those two reservoirs (i.e., variable asthenospheric erosion or delamination of lithospheric mantle, especially the SCLM associated with continental rifts) may more robustly account for geological, geochemical, and geophysical observations at many of the prominent rifts, including each of the three rifts explored as part of this thesis. Herein, we integrate trace element and gas geochemistry from three prominent rift zones, including: 1) the Equatorial Mid-Atlantic Ridge (MAR) (5°N to 7°S), 2) the West Antarctic Rift System (WARS), and 3) the East African Rift System (EARS), to evaluate magmatic sources and processes that influence their evolution. Basalts from the Equatorial MAR provide an ideal setting to study the compositional variations in the upper mantle related to mixing and investigate interactions between mantle plumes and ridges without the risks of crustal contamination associated with working in continental settings. MORBs from the northern Equatorial MAR (5°N to 3°S) display trace element, radiogenic isotope, and helium isotopic characteristics indicative of mixing between the depleted mantle (DM) and a HIMU plume while the southern Equatorial MAR (3°S to 7°S) exhibit extremely depleted MORB compositions. The DM basalts have significantly lower (factor of 2) volatile (H2O and Cl)/Pb ratios in samples with the extremely non-radiogenic Pb isotope ratios, but vary by less than ~50% when volatile concentrations are normalized to 204Pb to control for partial melting/fractional crystallization, suggesting the volatile contents fluctuate with the degree of partial melting and source variation. In contrast to the MAR, the WARS and EARS are both continental rifts with ample opportunity for influence by crustal contamination. WARS volcanics in Victoria Land exhibit OIB-like trace element compositions with dominantly MORB-like gas signatures (e.g., La/Lu up to 155; 3He/4He ~7RA). Elevated CO2/3He cannot be accounted for strictly by the influences of magmatic degassing, indicating an addition of CO2 from the more alkaline source in the magmatic mixture. Based on the positive correlations between CO2 and several trace element ratios (e.g., Zr/Hf, Rb/Sr, Th/U), we suggest that subduction-related alteration of the mantle induced metasomatism of the SCLM across the region and eventually led to the formation of alkaline-rich volcanics, contrary to previous suggestions of a prolonged HIMU (high 238U/204Pb) plume in the region. Mantle upwelling in this region entrained magma bodies with noble gases derived from the uprising asthenosphere, which can be characterized by higher 3He/4He than typical subduction, SCLM, or HIMU sources. Volcanics in the Virunga Volcanic Province in the EARS (specifically at Mount Nyiragongo and Mount Nyamuragira) exhibit highly alkaline, silica-undersaturated lavas with even more extreme enrichments of incompatible elements (e.g., high field strength elements) than the WARS. Although VVP volcanics consistently display these atypical, but starkly similar compositions, we note significant and corresponding changes in both helium isotopes and trace elements even over the course of a few years in the region. To a first order, volcanics from the VVP region are consistent with WARS volcanism, and we envision a similar conceptual model for the evolution of the SCLM and associated volcanics in this region. We suggest that subduction-related alteration of the mantle during the Proterozoic accretion of the Tanzania Craton induced metasomatic enrichment of incompatible elements into the SCLM across the region and formed the basis for the formation of alkaline-rich volcanics. Next, we hypothesize that following the cessation of subduction during the Proterozoic, asthenospheric upwelling, and lithospheric delamination led to partial melting of highly devolatilized and incompatible-rich SCLM to generate the volcanic products we observe today. With this conceptual model in mind, we envision that the helium isotopic values observed in the current study area result from the influence of dominantly asthenospheric mantle interacting with and generating variable degrees of partial melts of the SCLM instead of a HIMU (high 238U/204Pb) plume in the region.