Rutile-bearing refractory eclogites: Missing link between continents and depleted mantle

A mass imbalance exists in Earth for Nb, Ta, and possibly Ti: continental c rust and depleted mantle both have subchondritic Nb/Ta, Nb/La, and Ti/Zr, w hich requires the existence of an additional reservoir with superchondritic ratios, such as refractory eclogite produced by slab melting. Trace elemen t compositions of minerals in xenolithic eclogites derived from cratonic li thospheric mantle show that rutile dominates the budget of Nb and Ta in the eclogites and imparts a superchondritic Nb/Ta, Nb/La, and Ti/Zr to the who le rocks. About 1 to 6 percent by weight of eclogite is required to solve t he mass imbalance in the silicate Earth, and this reservoir must have an Nb concentration greater than or equal to 2 parts per million, Nb/La greater than or equal to 1.2, and Nb/Ta between 19 and 37-values that overlap those of the xenolithic eclogites. As the mass of eclogite in the continental li thosphere is significantly lower than this, much of this material may resid e in the lower mantle, perhaps as deep as the core-mantle boundary.