Quantitative modeling of mantle heterogeneity and structure

Abstract

Mantle-derived rocks, particularly mid-ocean ridge basalts (MORB) and intraplate ocean island basalts (OIB), provide insights into the compositional heterogeneity and first-order structural make-up of the modern mantle; laser ablation (LA-) ICP-MS analysis provides the ideal analytical tool for the in situ chemical characterization of these materials. The silicate Earth, as defined by the MORB and OIB source regions plus the continental crust, is determined to have a representative W/U and K/U ratio of 0.65 ± 0.45 (2σ) and 13,800 ± 2600 (2σ), respectively, equating to 13 ± 10 ng/g W and 280 ± 120 μg/g K in the silicate Earth. Although both the isotopic composition of W and the constancy of the terrestrial W/U ratio may serve as tracers of putative core-mantle interactions, both of these proxies are sensitive to the chemical composition of the mantle source and have yet to resolve a core signal in Hawaiian picrites. The abundance of K in the silicate Earth indicates a current convective Urey ratio of ~0.34 and mantle cooling rate of 70-130 K*Gyr−1, after taking into account potential heat flux across the core-mantle boundary. The Earth's balance of radiogenic heat and budget of 40Ar necessitate a lower mantle reservoir enriched in radioactive elements. The bulk Earth Pb/U ratio, determined here to be ~85, suggests ~1200 ng/g Pb in the bulk Earth and ≥3300 ng/g Pb in the core. A compositional model of MORB, which is derived from a suite of sample measurements augmented by a critically compiled data set, shows that Atlantic, Pacific and Indian MORB can be distinguished based on both trace element abundances and ratios. The geochemical signatures associated with global MORB are not entirely complementary to the continental crust, and require an under-sampled reservoir enriched in Ti, Nb and Ta. A compositional model of OIB, which is based on the inferred chemical composition of OIB parental melts from Hawaiian shield volcanoes as well as the Austral-Cook islands, indicates that the OIB source region may only be ≥1.0x as enriched in incompatible elements as the unfractionated silicate Earth, and constitute up to ≤50% of the modern mantle mass.