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This depleted MORB mantle (DMM) is not well represented by OIB (see Iceland debate; Hanan et al., 2000; Fitton et al., 2003), but less-extreme depleted mantle (DM) underlies some islands (e.g., Easter).
Given the importance of subduction, it is reasonable to expect that OIB magma compositions might hold evidence for recycling of oceanic lithosphere.
Indeed, Os isotopes suggest that most OIB sources contain > 10% recycled basalt and HIMU sources are even higher (i.e., high [sup.187]Os/[sup.188][Os]; Fig.
(1999) cautioned against using Os to estimate the percentage of recycled ocean floor in OIB sources.
In a plot of [sup.143]Nd/[sup.144]Nd versus [sup.176]Hf/[sup.177]Hf, a regression line passed through depleted mantle, OIB and continental crust does not pass through bulk silicate Earth.
MORB sources show low, uniform [sup.3]He/[sup.4]He ratios whereas OIB sources are variable.
An alternative explanation for high [sup.3]He/[sup.4]He ratios in specific OIB samples is that small-scale melting leads to localized sampling of highly depleted (low U+Th) sources that produced little [sup.4]He--the opposite of the primordial mantle hypothesis (Anderson, 1999; 2001; Meibom et al., 2003)!
Thus, all normal - MORB ratios are lower than in OIB implying a highly depleted source for the former (Table 2).
The EM1 ratios involving Ba, Th, and K (in numerator) are the highest, and U, Nb and Ta are the lowest in the ocean basins, and comparing OIB only, HIMU is the reverse of EM1 (Table 2).
Variations in OIB and MORB geochemistry lead to models for chemical structure in the mantle.
Both MORB and OIB are derived from the same upper mantle source containing recycled materials.
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