Dynamical geochemistry of the mantle Research School of Earth Science, Australian National University, Canberra, ACT 0200, Australia
13 Sep 2011
Received: 02 Mar 2011 – Published in Solid Earth Discuss.: 24 Mar 2011 Abstract. The reconciliation of mantle chemistry with the structure of the mantle
inferred from geophysics and dynamical modelling has been a long-standing
problem. This paper reviews three main aspects. First, extensions and
refinements of dynamical modelling and theory of mantle processing over the
past decade. Second, a recent reconsideration of the implications of mantle
heterogeneity for melting, melt migration, mantle differentiation and mantle
segregation. Third, a recent proposed shift in the primitive chemical
baseline of the mantle inferred from observations of non-chondritic
142Nd in the Earth. It seems most issues can now be resolved, except
the level of heating required to maintain the mantle's thermal evolution.
Revised: 30 Jul 2011 – Accepted: 10 Aug 2011 – Published: 13 Sep 2011
A reconciliation of refractory trace elements and their isotopes with the
dynamical mantle, proposed and given preliminary quantification by Hofmann,
White and Christensen, has been strengthened by work over the past decade.
The apparent age of lead isotopes and the broad refractory-element
differences among and between ocean island basalts (OIBs) and mid-ocean
ridge basalts (MORBs) can now be quantitatively accounted for with some
The association of the least radiogenic helium with relatively depleted
sources and their location in the mantle have been enigmatic. The least
radiogenic helium samples have recently been recognised as matching the
proposed non-chondritic primitive mantle. It has also been proposed recently
that noble gases reside in a so-called hybrid pyroxenite assemblage that is
the result of melt from fusible pods reacting with surrounding refractory
peridotite and refreezing. Hybrid pyroxenite that is off-axis may not remelt
and erupt at MORs, so its volatile constituents would recirculate within the
mantle. Hybrid pyroxenite is likely to be denser than average mantle, and
thus some would tend to settle in the D" zone at the base of the mantle,
along with some old subducted oceanic crust. Residence times in D" are
longer, so the hybrid pyroxenite there would be less degassed. Plumes would
sample both the degassed, enriched old oceanic crust and the gassy, less
enriched hybrid pyroxenite and deliver them to OIBs. These findings can
account quantitatively for the main He, Ne and Ar isotopic observations.
It has been commonly inferred that the MORB source is strongly depleted of
incompatible elements. However it has recently been argued that conventional
estimates of the MORB source composition fail to take full account of mantle
heterogeneity, and in particular focus on an ill-defined "depleted" mantle
component while neglecting less common enriched components. Previous
estimates have also been tied to the composition of peridotites, but these
probably do not reflect the full complement of incompatible elements in the
heterogeneous mantle. New estimates that account for enriched mantle
components suggest the MORB source complement of incompatibles could be as
much as 50–100 % larger than previous estimates.
A major difficulty has been the inference that mass balances of incompatible
trace elements could only be satisfied if there is a deep enriched layer in
the mantle, but the Earth's topography precludes such a layer. The
difficulty might be resolved if either the Earth is depleted relative to
chondritic or the MORB source is less depleted than previous estimates.
Together these factors can certainly resolve the mass balance difficulties.
Citation: Davies, G. F.: Dynamical geochemistry of the mantle, Solid Earth, 2, 159-189, doi:10.5194/se-2-159-2011, 2011.