7 The Link between the Physical and Chemical Properties of Carbon-Bearing Melts and Their Application for Geophysical Imaging of Earth’s Mantle fabrice gaillard, nicolas sator, emmanuel garde ´s, bertrand guillot, malcolm massuyeau, david sifre ´, tahar hammouda, and guillaume richard 7.1 Introduction: Toward a Geophysical Definition of Incipient Melting and Mantle Metasomatism Geochemical observations on mantle xenoliths and experiments at pressure and tempera- ture on CO 2 - and H 2 O-bearing mantle rocks have provided the widely accepted picture that melts and fluids are flowing and reacting within the solid mantle. 1–7 Whether this must be seen as a transient and local process or a broad and planetary-scale mantle dynamic is unknown. 2,4,7 Understanding this could establish whether these melt advection processes explain some remote geophysical observations and could help with clarifying the geody- namic roles played by these melting dynamics. The question is rendered difficult since these melts may not be easily linked to the volcanic products reaching Earth’s surface; somehow, most of the mantle melting processes may produce melts that never leave the mantle and therefore remain inaccessible. The fingerprints of such deep melts have been historically characterized by geochemical means: trace element abundances and some isotopic ratios of mantle xenoliths are modified by the reactive passage of these melts. 2,4,6,8,9 Major element abundances and the modal proportions of minerals can also be significantly affected. 6,8 All of this is named mantle metasomatism, 2,8,9 and this process may explain some geophysical observations. 2,10 Notably, mantle metasomatism has been characterized on lithospheric samples only, therefore representing the shallowest part of a deeper melting dynamic that will be presented hereafter. The melt causing such modifications is usually not observable in mantle rocks and is also not generally found in most volcanic exposures (except for the enigmatic petit-spot volcanoes 11 ). Experimental petrology has therefore been used to reconstruct the chemical compositions of the parental melt coexisting at equilibrium with the solid mantle assem- blage. 3,5,7,12,13 Experiments at upper-mantle conditions have shown the key role of volatile species (i.e. H 2 O and CO 2 ) in stabilizing CO 2 -rich melts or fluid versus SiO 2 -rich melts. 1,3,5,7,12,13 The take-home message of such experimental approaches is that, in the presence of volatiles, mantle melting can occur in most of the upper mantle; 7 melting regions are commonly limited by redox process reactions favoring diamonds in the deep upper mantle and decarbonation reactions in the shallowest part of the mantle. 5,7,14 163 https://www.cambridge.org/core/terms. https://doi.org/10.1017/9781108677950 Downloaded from https://www.cambridge.org/core. IP address: 3.93.61.0, on 13 Nov 2021 at 12:12:46, subject to the Cambridge Core terms of use, available at