of ancient subducted sediments

erupted from Debre Zeyit, Butajira, and the Wonji Fault Belt of the Main Ethiopian Rift, show systematic mixing relationships involving three distinct mantle sources.The Pb, Sr, Nd, and Hf isotopic arrays converge in a specific region of isotopic multi-space where they define the composition of the Afar mantle plume (centered

www.elsevier.com/locate/chemgeo Nd–Hf–Sr–Pb isotopes and trace element geochemistry of Proterozoic lamproites from southern India: Subducted komatiite in the source

Concentration ratio An assemblage of MORB analyses (n=792 samples), including a suite of new, high-precision LA-ICP-MS measurements (n=79), has been critically compiled in order to provide a window into the chemical composition of these mantle-derived materials and their respective source region(s), commonly referred to as the depleted MORB mantle (DMM). This comprehensive MORB data set, which includes both “normal-type ” (N-MORB, defined by (La/Sm)Nb1.00) and “enriched-type ” samples (E-MORB, (La/Sm)N≥1.00), defines a global MORB composition that is more enriched in incompatible elements than previous models. A statistical evaluation of the true constancy of “canonical ” trace element ratios using this data set reveals that during MORB genesis Ti/Eu, Y/Ho and Ce/Pb remain constant at the 95 % confidence-level; thus, the ratios recorded in MORB (Ti/Eu=7060±1270, 2σ; Y/Ho=28.4±3.6, 2σ; Ce/Pb=22.2±9.7, 2σ) may reflect the composition of the DMM, presuming the degree of source heterogeneity, component mixing and conditions of melting/crystallization of the DMM are adequately recorded by global MORB. Conversely, Ba/Th, Nb/U, Zr/Hf, Nb/Ta, Sr/Nd, and Th/U are shown to fractionate as a function of MORB genesis, and thus these ratios do not faithfully record the composition of the DMM. Compared to samples from the Pacific and Indian Oceans, MORB derived from Atlantic ridge segments are

n nt i b hemie, P

n nt i b hemie, P

The 4000 km2 Androy massif in southeastern Madagascar is a 42000 m thick sequence of interbedded basalt and rhyolite erupted during a widespread Cretaceous episode of predominantly basaltic volcanism. Two geochemically different groups of basalt, tholeiitic group B1 and mildly alkalic B2, are present, as are two different groups of rhyolite, R1 and R2. Both the basalts and rhyolites appear to have issued from relatively nearby feeders, as compositionally equivalent intrusions are exposed in the vicinity. The R2 rhyolites define a whole-rock Rb^Sr isochron of 840 24Ma (2), the same, within error, as an 40Ar^39Ar sanidine age reported by earlier workers. Plate reconstructions suggest that the area was near the Marion hotspot at this time. Some involvement of hotspot mantle is allowed, but not required, by Nd^Pb^Sr isotope data for the basalts. The two types of basalt may have formed by different amounts of melting of the same mantle source, which remains rather poorly