Rutile-Bearing Refractory Eclogites: Missing Link Between Continents and Depleted Mantle

Abstract

A mass imbalance exists in Earth for Nb, Ta, and possibly Ti: continental crust and depleted mantle both have subchondritic Nb/Ta, Nb/La, and Ti/Zr, which requires the existence of an additional reservoir with superchondritic ratios, such as refractory eclogite produced by slab melting. Trace element compositions of minerals in xenolithic eclogites derived from cratonic lithospheric 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 whole rocks. About 1 to 6 percent by weight of eclogite is required to solve the mass imbalance in the silicate Earth, and this reservoir must have an Nb concentration >/= 2 parts per million, Nb/La >/= 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 lithosphere is significantly lower than this, much of this material may reside in the lower mantle, perhaps as deep as the core-mantle boundary.

Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

“…8a). Both andesites and dacites exhibit high Pt/Pt* (Pt anomaly), (Pt/Ir) N and Re/Os ratios higher than those of chondrites and primitive mantle (Barnes et al, 1988;McDonough and Sun, 1995), but similar to those of the ; (e) Th/La vs. Th (Plank, 2005); (f) Nb/Ta vs. Nb (Rudnick et al, 2000). Upper crust and marine sediments are from Plank (2005), Plank and Langmuir (1998), and references therein.…”

Dismantling the deep earth : geochemical constraints from hotspot lavas for the origin and lengthscales of mantle heterogeneity

“…Shallow geochemical reservoirs in the earth-continental crust and the depleted midocean ridge basalt mantle (DMM)-have a shortage of the element Ti, Ta and Nb (TITAN) Rudnick et al, 2000). Sr=0.7044), within the quoted external precision.…”

“…However, the TITAN trio of elements are prominently depleted in the continents (Rudnick and Gao, 2003), and their absence is not balanced by a corresponding enrichment in DMM (Workman and Hart, 2005). Thus, another deeper reservoir hosting the missing TITAN elements has been proposed to exist in the earth Rudnick et al, 2000;Kamber and Collerson, 2000).…”

“…A common mantle dynamics paradigm maintains that mantle plumes complete the process of recycling by transporting plate remnants from the core-mantle boundary back to the surface, where they melt and erupt as ocean island basalts (OIBs) White, 1980, 1982;Chase, 1981). However, the reservoir hosting the Earth's missing TITAN has been difficult to detect in hotspot lavas Rudnick et al, 2000).…”

“…By contrast, titanium-rich phases, such as rutile, may preferentially sequester the TITAN elements in the mafic portion of the downgoing slab, balancing the depletion observed in subduction zone lavas (Green and Pearson, 1986;Ryerson and Watson, 1987;Brennan et al, 1994;Foley et al, 2000;Schmidt et al, 2004;Kessel et al, 2005). Refractory, rutile-bearing eclogites have been subducted in large quantities over geologic time, and may form a reservoir in the mantle that hosts the Earth's missing TITAN (McDounough, 1991;Rudnick et al, 2000).…”