Two-plume Dynamics beneath the East African Rift System: a Geochemical Perspective

Open Access
Author:
Nelson, Wendy Rae
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 27, 2009
Committee Members:
  • Tanya Furman, Dissertation Advisor
  • Tanya Furman, Committee Chair
  • Maureen Feineman, Committee Member
  • Andrew Arnold Nyblade, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • HIMU
  • mantle plume
  • Afar
  • East African Rift
  • basalt
Abstract:
This dissertation presents new geochemical data investigating the origin of the chemically dissimilar Kenya and Afar mantle plumes and the contributions of these plumes to volcanism beneath Ethiopia. This study focuses primarily on mafic lavas from three localities on the poorly characterized eastern Ethiopian plateau but also includes lavas from the northwest Ethiopian plateau as well as Re-Os isotopic data from various key locations throughout the East Africa Rift System. The location of the samples sites provides a framework for a spatial and temporal assessment of the influence the Afar and Kenya plumes have on volcanism. We are also able to evaluate the role of the lithospheric mantle including its contribution to melt production and its influence on upwelling mantle plume material. Expansion of the data set to include northwest Ethiopian plateau lavas and key volcanic sites throughout the region allow us to consider the origin of HIMU mantle component observed in Kenya plume lavas but absent from Afar plume products. Geochemical data demonstrate the sources contributing to volcanism on the eastern Ethiopian plateau vary over space and time. Northernmost Oligocene-Miocene lavas from Asbe Tefari are chemically similar to well characterized high-TiO2 Afar plume flood basalts from northwest Ethiopia. These lavas are chemically evolved and record contributions from the continental crust. Debre Sahil and Bale Mountains lavas are respectively located 85 km and 230 km to the southwest. Unlike Asbe Tefari basalts, Miocene Debre Sahil and Bale Mountains lavas are chemically similar to Kenya plume lavas produced in Turkana (northern Kenya). Pliocene-Recent Bale Mountains lavas do not resemble their Miocene predecessors but are compositionally similar to Quaternary lavas found within the Main Ethiopian Rift indicating a change in source beneath the Bale Mountains over time. Miocene-Recent alkaline lavas from Debre Sahil and Bale Mountains were derived near the base of the subcontinental lithospheric mantle and record its variable influence. In contrast, transitional Miocene lavas were generated with the asthenosphere and did not experience lithospheric contamination, thus recording Kenya plume compositions. The Kenya and Afar mantle plumes are consistent isotopically with low 3He/4He HIMU and high 3He/4He “C” mantle components, respectively. Re-Os isotopic data demonstrate the Kenya plume is the product of mixing recycling ~ 2 Ga oceanic crust with typical enriched mantle plume material, producing 187Os/188Os = 0.1450-0.1483 similar to other global HIMU OIB. The Afar plume has typical enriched plume values (187Os/188Os = 0.1239-0.1311). Sr-Nd-Pb isotopic data from eastern plateau lava are consistent with major and trace element geochemical data. Asbe Tefari record “C” like Afar contributions. Miocene Debre Sahil and Bale Mountain lavas have a HIMU-flavor, though transitional Bale Mountains lavas display a more dilute signature due to chemical heterogeneities within the Kenya plume or mixing with Afar plume material prior to eruption. Pliocene-Recent Bale Mountains lavas record Afar contributions consistent with Main Ethiopian Rift lavas. Hf isotopic data demonstrate HIMU-Kenya plume lavas likely incorporated a recycled oceanic crust while lavas with Afar plume signature have signatures consistent with peridotite melting. Transitional Bale Mountains lavas and high-TiO2 lavas from northwest Ethiopia have high-Ni olivine, indicative of pyroxenite melting. In contrast, alkaline HIMU-flavored lavas Debre Sahil, Bale Mountains, and Turkana have low-Ni olivine. We suggest that recycled oceanic crust is present in both Afar and Kenya plumes. The thick lithosphere beneath the Ethiopian plateau promotes the reaction between eclogite melt and peridotite solid to form pyroxenite while thin lithosphere beneath Turkana allows peridotite to melt and mix with eclogite melt instead of forming pyroxenite. Pyroxenite melts will record isotopic signatures similar to the parent peridotite while eclogite-peridotite melt mixes will have a stronger recycled (HIMU) component.