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is not compatible with the physical and dynamical interpretations of the mantle,
as we have seen. The undegassed reservoir has also been claimed to explain the
presence of unradiogenic helium in OIBs [226], but it requires plumes to acquire a
small complement of unradiogenic helium from the reservoir without also acquir-
ing an unradiogenic lead signature. Given also the requirement that plumes acquire
a substantial extra complement of basaltic composition from somewhere, these
are delicate operations that are not easy to quantify. Anyway, the idea of a large
separate reservoir in the deep mantle is not dynamically tenable, so we need to
move on from it. All we have available is the D
layer and its associated large
thermochemical piles.
The consideration of a heterogeneous mantle, and of melting heterogeneous
sources, has led to an alternative interpretation that readily accounts for the helium
enigma and may also account for the argon mass balance when all relevant uncer-
tainties are taken into account. This will now be described, and some alternative
interpretations will then be briefly discussed.
10.8.1 Noble gases in the hybrid pyroxenite
Section 10.5.2 reported investigations of melting in heterogeneous sources that
indicate the formation of
hybrid pyroxenite
, produced when melts derived from
eclogitic bodies react with surrounding, more abundant, peridotite. In Section
10.5.3 it was argued that significant amounts of such hybrid pyroxenite would
not be extracted at mid-ocean ridges, and would therefore recirculate within the
mantle. Successive generations of hybrid pyroxenite would form, as one gener-
ation is carried back into a melt zone, to contribute to the formation of another
generation.
Incompatible elements, including noble gases, would be concentrated into hybrid
pyroxenites, as they are into all melts. If some of the hybrid pyroxenite recirculates
within the mantle, then it will not be degassed, so this is a way in which gas-rich
bodies could form in the mantle. If such processes have operated since early in
Earth history, before much of the noble gases had been lost from the mantle, then the
hybrid pyroxenites would have acquired significant noble gases. As old subducted
oceanic crust and hybrid pyroxenite are carried into a melting zone, both will melt
preferentially compared with the peridotite in which they are embedded. Some of
the melts will refreeze, as depicted in Figures 10.13 and 10.14.
Old oceanic crust and hybrid pyroxenite are likely to be intermingled after some
mantle stirring, so some mixing of the two kinds of melt is to be expected as the
melts migrate in the melting zone (Figure 10.24(a)). As already argued, some of
the total melt will be erupted to form oceanic crust, and this will degas. On the
other hand, some of the total melt will not be extracted, but rather will recirculate
