Oceanic mantle sources

Another recent study on historic, basaltic lava flows (1848 to 1977 AD) from Karthala volcano (Comores, Indian ocean) by Claude-Ivanad et al. (1998) reveals significant variations of the ( Ra)o/Ba ratios, correlated with Sr/ Sr ratios, which are explained by mantle source heterogeneities preserved by short transfer times of the magmas (a few centuries at most). 

Grustal reservoirs are also variable in Gl-isotope compositions (Figs. 1-6) due to fractionation of the Gl-isotope compositions inherited from their mantle source through fluid-mineral reactions, incorporation of G1 derived from the oceans and fractionation within fluid reservoirs by diffusion (see below). For example, the oceanic crust is enriched in Gl (and pore fluids depleted in Gl) through reaction of seawater with basaltic crust derived from the depleted mantle (Fig. 1 Magenheim et al. 1995). Undoubtedly, future investigations of Gl-isotopes in whole rocks and mineral separates will address the Gl-isotope compositions of these reservoirs and their evolution. 

Fisher, D. E. (1985a) Noble gas data from oceanic island basalts do not require an undepleted mantle source. Nature, 316, 716-18. 

Haase K. M. (1996) The relationship between the age of the lithosphere and the composition of oceanic magmas constraints on partial melting, mantle sources and the thermal structure of the plates. Earth Planet. Sci. Lett. 144(1-2), 75-92. 

Graham D. W. (2002) Noble gas isotope geochemistry of midocean ridge and ocean island basalts characterization of mantle source reservoirs. Rev. Mineral. Geochem. 47, 247-317. 

Several lines of evidence indicate problems with strict application of the layered model. It is particularly difficult to satisfy the requirement for primitive composition of the lower mantle. For example, Zindler and Hart (1986) showed that the isotopic database for the oceanic mantle could be described by mixing between depleted MORE mantle and enriched components that have compositions quite unlike primitive mantle. Similarly, the constancy of Nb/U and Ce/Pb (Hofmann, 1986 Newsom et al., 1986) suggests that there is no current reservoir with primitive mantle ratios. Interestingly, there is a strong correlation between a depleted component, termed FOZO by Hart et al. (1992), and the high He source observed in many hot spots (Hart et al., 1992 Farley et al., 1992 Hanan and Graham, 1996). 

In recent years, we have seen the use of new isotopic systems, including those of osmium and oxygen, in the description of oceanic basalts. Osmium isotopes can trace the addition of mafic crust or melt to the mantle source (Hauri and Hart, 1993) and have been used to highlight the presence of recycled mafic crust in sources of hot spots (Shirey and Walker, 1998). In a complimentary fashion, oxygen isotope variations trace directly... 

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