Heat producing elements in the modern MORB mantle

Primordial and radiogenic heat together account for the Earth's surface flux of 46 ± 2 TW, with competing compositional models for the bulk silicate Earth (BSE) between 10 and 36 TW of this power comes from radiogenic sources, mainly K, U, and Th. The abundances of these radiogenic sources vary by a factor of over 100 between the crust and mantle, with the continental crust emitting 7 ± 1 TW of radiogenic heat. Interestingly, several recent studies have suggested that K/U ratio of the MORB source may in fact be higher than the continental crust despite the similar geochemical behavior exhibited by K and U during mantle anataxis; in contrast, the κ_Pb values of MORB and continental crust are statistically indistinguishable, indicating negligible fractionation of Th from U.

Here, we re-examine multiple global MORB data sets to try to isolate potential analytical and/or geographical biases and reconcile different model K/U ratios of the MORB source. A statistical evaluation of the correlation between K and U documents a higher incompatibility of U relative to K during partial melting and/or fractional crystallization, requiring that the mantle sources of oceanic basalts (including MORB and ocean island basalts, OIB) be characterized by higher K/U ratios than their derivatives. For example, assuming a global MORB average of 14,000, in line with several recent and historical compositional models, the MORB source must have a K/U value on the order of 18,000 given observed differences in the partitioning behavior of K and U.

Estimates of MORB U concentration range from 3-10 ppb, with mass fraction estimates of the MORB source region ranging from 30-95% of the mantle. We calculated the MORB source U concentration, assuming an OIB source region (OSR) concentration of 40 ppb and mass fractions from 1% to 25%, and a BSE model with 20 ppb U. This yields complementary DMM compositions with 13 to 5 ppb U following the assumed range of OSR mass fractions.