Modeling Mantle Isotopic Heterogeneity:
The FOZO's in the Pudding

James B. Kellogg
October 22, 2004

ABSTRACT

Radiogenic isotope geochemistry faithfully records the history of 
differentiation, recycling and mixing in the Earth's mantle. 
While a great deal of observational effort has been directed
toward describing the isotopic heterogeneity
of the mantle, attempts to model the processes that create this heterogeneity
and link it to the dynamics of the mantle have been relatively few. We present
a new suite of "extended box models" in which we quantify the evolution not
only of the mean geochemical reservoirs that make up the Earth's interior, but
also of the chemical and isotopic heterogeneity within them.
Focusing on the Sr, Nd, and Pb systems, we recover nearly the full range
of variability observed in oceanic basalts. We show that the pseudo-isochron
in 206Pb/204Pb - 207Pb/204Pb space reflects neither a steady-state, nor a mean
differentiation age, nor a stirring time of the mantle. Rather, it likely reflects
mixing between relatively non-radiogenic residua from the extraction of conti-
nental crust and a himu-like component formed by oceanic crust that has been
stripped of ~50% of its lead during subduction.
Furthermore, by tracking the length scales of the heterogeneities in the sys-
tem and considering the statistics of sampling, we arrive at a new understanding
of putative "mantle endmembers." We argue that full mantle menagerie as ob-
served at the surface is contained within the heterogeneous depleted mantle. The
intermediate "FOZO" (FOcal ZOne) component, previously associated with the
lower mantle, may better be understood as a highly heterogeneous mixture that
constitutes the matrix in which larger plums of chemically depleted material
float. This is a reversal of the classical "plum pudding" model, in which primi-
tive or enriched plums were imagined to float in a highly depleted pudding.