Geology Reference
In-Depth Information
10.9 Assessment of mantle chemistry
This discussion of mantle chemistry has addressed the question of how it can fit into
the picture of mantle dynamics developed earlier in the topic. An understanding
of how the refractory incompatible elements fit into mantle dynamics has emerged
over the past decade or so, following the seminal work by Christensen and Hofmann
[122]. Two basic questions remained unresolved, namely how the global budgets
of refractory incompatible elements could be balanced in such a picture and how
the noble gases might fit in. Consideration of the important role of major element
heterogeneity, especially in controlling melting and melt extraction and in inhibiting
chemical equilibration, has led to plausible resolutions of both questions. These
proposals will, of course, need to be debated and tested.
The interpretations presented here have been quantified to varying degrees.
The quantification of refractory incompatible element behaviour is at the level of
numerical models of thermochemical mantle dynamics. The quantification of the
role of noble gases is quite simplified at this stage, but any quantification that
achieves plausible concordance with helium observations is an advance. It is thus
reasonable to say that quantitative dynamical treatments of mantle geochemistry
are becoming established.
Earlier interpretations invoking a primitive or undegassed reservoir were never
soundly based even on geochemical grounds, as they were contradicted by lead
isotope data from the beginning and they could only explain the diversity of OIB
data with additional
ad hoc
assumptions. However understandable the original
presumptions might have been, in the context of some geophysicists' early pre-
sumption of an inactive lower mantle or, later, a layered mantle, the subsequent
failure to critically examine basic assumptions and the reliability of key observa-
tions, and the propensity to ignore contrary observations such as the lead isotope
data, has impeded the subject for decades.
