HR: 1340h
AN: V33A-1166    [Abstracts]
TI: The Afar Plume and Ethiopian Lithosphere: A Hf and He Isotope Study of Primitive Quaternary Rift Lavas.
AU: * Rooney, T
EM: rooneyt@msu.edu
AF: Dept. of Geological Sciences, Michigan State University, East Lansing, MI 48824, United States
AU: Hanan, B
EM: bhanan@mail.sdsu.edu
AF: Dept. of Geological Sciences, San Diego State University, San Diego, CA 92116, United States
AU: Graham, D
EM: dgraham@coas.oregonstate.edu
AF: College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331,
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802, United States
AB: Quaternary magmatism in the Ethiopian rift records the interaction of a deeply sourced mantle plume with the ambient upper mantle and the continental lithosphere. We present the first hafnium isotope values for the region and utilize new helium data from these well-characterized samples to gain insight into the distribution of Afar plume material in the Ethiopian mantle. Modern volcanism in the region is focused in two distinct NE-SW trending linear zones of extension within the rift. Chains of cinder cones erupting between large silicic centers typify both zones. The Wonjii Fault Belt (WFB) is the larger of these zones both in length and eruptive volume, extending into the Afar Depression. The Silti-Debre Zeyit Fault Zone (SDFZ) is the smaller, extending only to 8.5 degrees north, and ending against a protrusion of the rift wall. We present 12 new ³He/⁴He and 18 new ¹⁷⁶Hf/¹⁷⁷Hf results on samples that have been previously analyzed for Sr, Nd and Pb isotopic ratios, making these samples among the best geochemically characterized in the region. We observe that samples within the WFB consistently have higher ³He/⁴He (R/Ra = 12.2 to 15.1) than those of the SDFZ (R/Ra = 5.6 to 8.8) and extend to more radiogenic Hf isotope values. Hf isotope values correlate positively with ³He/⁴He and ²⁰⁶Pb/²⁰⁴Pb. Distinct linear Pb isotope arrays for Butajira, WFB, and Debre Zeyit, suggest 3-component mixing between the "C"-like Afar Plume, asthenosphere and lithosphere. In this scenario the proportions of Afar plume and ambient asthenospheric mantle are relatively fixed, suggesting that they interacted prior to mixing with heterogeneous lithosphere. The WFB, Debre Zeyit, and Butajira lavas define Pb-Pb trends that radiate away from the Afar plume-ambient mantle mix towards continental lithosphere-like compositions. The WFB has a greater proportion of Plume plus asthenosphere source material while lavas along the SDFZ have an increased lithospheric source contribution. We suggest that plume material may be channeled along the rift's thinned lithosphere southward from its hypothesized current location in the Afar Depression, mix with the ambient upper mantle, and subsequently interact with the continental lithosphere to form the rift lavas.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting