Major and Trace Element Composition of the Depleted MORB Mantle (DMM)

We derive an estimate for the chemical composition of the depleted MORB mantle (DMM), the source reservoir to mid-ocean ridge basalts (MORBs), which represents at least 30% the mass of the whole silicate Earth. A database for the chemical and physical properties of abyssal peridotites has become robust and complete enough to truly access a reference DMM. Using trace element depletion trends from the abyssal peridotites, it is possible to construct a large part of DMM’s trace element pattern. Splicing this information with isotopic constraints (Sr-Nd-Pb-Hf) and canonical ratios (Ce/Pb, Nb/Ta, Nb/U, Ba/Rb and H2O/Ce), we can extend abundance estimates to all the incompatible elements including water content. The resulting trace element pattern for average DMM constrains parental MORB to be generated by 6% aggregated fractional melting – consistent with recent models for hydrous melting of the mantle (Asimow et al., 2004). We show that DMM is roughly balanced by the continental crust and better balanced upon inclusion of ocean island basalt source and oceanic crust components. Compared to the primitive mantle, DMM has been depleted by 2-3% melt extraction and has only 15% the radiogenic heat production. * To whom correspondence should be addressed.