Dynamical Geochemistry

Dynamical and chemical interpretations of the mantle have hitherto remained incompatible, despite substantial progress over recent years. It is argued that both the refractory incompatible elements and the noble gases can be reconciled with the dynamical mantle when mantle heterogeneity is more fully accounted for. It is argued that the incompatible-element content of the MORB source is about double recent estimates (U~10 ng/g) because enriched components have been systematically overlooked, for three main reasons. (1) in a heterogeneous MORB source, melts from enriched pods are not expected to equilibrate fully with the peridotite matrix, but recent estimates of MORB-source composition have been tied to residual (relatively infertile) peridotite composition. (2) about 25% of the MORB source comes from plumes, but plume-like components have tended to be excluded. (3) a focus on the most common “normal” MORBs, allegedly representing a “depleted” MORB source, has overlooked the less-common but significant enriched components of MORBs, of various possible origins. Geophysical constraints (seismological and topographic) exclude mantle layering except for the thin D” layer and the “superpiles” under Africa and the Pacific. Numerical models then indicate the MORB source comprises the rest of the mantle. Refractory-element mass balances can then be accommodated by a MORB source depleted by only a factor of 2 from chondritic abundances, rather than a factor of 4-7. A source for the hitherto-enigmatic unradiogenic helium in OIBs also emerges from this picture. Melt from subducted oceanic crust melting under MORs will react with surrounding peridotite to form intemediate compositions here termed hybrid pyroxenite. Only about half of the hybrid pyroxenite will be remelted, extracted and degassed at MORs, and the rest will recirculate within the mantle. Over successive generations starting early in Earth history, volatiles will come to reside mainly in the hybrid pyroxenite. This will be denser than average mantle and will tend to accumulate in D”, like subducted oceanic crust. Because residence times in D” are longer, it will degas more slowly. Thus plumes will tap a mixture of older, less-degassed hybrid pyroxenite, containing less-radiogenic noble gases, and degassed former oceanic crust. Calculations of degassing history confirm that this picture can quantitatively account for He, Ne and Ar in MORBs and OIBs. Geophysically-based dynamical models have been shown over recent years to account quantitatively for the isotopes of refractory incompatible elements. This can now be extended to noble gas isotopes. The remaining significant issue is that thermal evolution calculations require more radiogenic heating than implied by cosmochemical estimates of radioactive heat sources. This may imply that tectonic and thermal evolution have been more episodic in the Phanerozoic than has been generally recognised.