The renaissance of redox melting

The concept of redox melting was introduced by Taylor
[1] to designate melting caused by the increase in water
activity owing to the oxidation of CH4-rich fluids, thus
explaining melting of the mantle without a change in
temperature. This is thought to be especially relevant to lowdegree
melting where volatile components are enriched, and
also to melting in the lower parts of cratonic lithosphere
because of the lack of heat sources and the association with
diamond, which may also result from the oxidation reaction
CH4 + O2 = 2H2O + C. A possible role for reduced fluids in
the origin of kimberlites in the lower had previously been
postulated by Wyllie [2], although this has not generally found
favour due to the carbonate-rich nature of kimberlites.
Recent experiments at pressures relevant to the lower
cratonic lithosphere have shown that small quantities of H2O
and CO2 will cause mantle peridotite to melt at lower
temperatures than with either volatile component alone, so that
oxidation of C to CO2 may provide an additional redox
melting mechanism [3]. In the first "wet" redox melting
mechanism, melting is caused by an increase in H2O-activity,
whereas in the second "dry" mechanism, melting results from
the oxidation of C to CO2 without a major change in H2O
activity. Both mechanisms may occur during the erosion of
continental lithosphere from below in the early stages of
development of continental rifts [3], and are documented in
the rock types produced during development of the Labrador
Sea rift [4]. Lamproites, which are typical of melting in
reduced environments [5], may be produced in the early
stages, whereas ultramafic lamprophyres are produced later by
melting at shallower levels in more oxidizing conditions, and
may result from the second redox melting mechanism. This
sequence of appearance of the two mechanisms should be
typical of lithosphere erosion, and the second may explain the
abundance of CO2-rich magmatism in continental rift zones.

[1] Taylor, W.R. (1985) Ph.D. thesis, University of Tasmania.
[2] Wyllie, P.J. (1980) J. Geophys. Res. 85, 6902-6910.
[3] Foley, S.F. (2008) Nature Geosci. 1, 503-510. [4] Tappe,
S. et al. (2008) Geochim. Cosmochim. Acta 72, 3258-3286. [5]
Foley, S.F. (1989) Eur. J. Mineral. 1, 411-427.