Fig. 8: Bulk MgO/SiO2 versus Ti in cratonic mantle peridotites and melting residues predicted from thermodynamic models.
From: The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots
Curved lines show models for isobaric melting of mantle peridotite (KR4003) at 3 and 6 GPa; and reaction of komatiite melts with peridotite (KR4003) at 6 GPa18. The 2 and 3 GPa melting trends are similar18. The amount of melting associated with the residues is given as % values. All symbols are coloured according to the modal % of orthopyroxene. Only melting at 6 GPa generates the high Mg# of olivine and orthopyroxene observed in the orthopyroxene-rich harzburgites, but the Kaapvaal orthopyroxene-rich harzburgites have much lower bulk Ti contents than the residues predicted from 6 GPa melt models. An alternative mechanism that causes depletion in Ti and enrichment in Si is therefore required. Bulk MgO/SiO2 data for peridotite xenoliths are reconstructed from EPMA analyses of minerals based on this work (Supplementary Data 1); Gibson et al.5 and whole-rock XRF analyses (Simon et al.9). Bulk xenolith Ti (µg/g) contents are also reconstructed, but from LA-ICP-MS analyses, which are at a higher precision than those from EPMA. The highest MgO/SiO2 (1.18) in our sample suite from the Kaapvaal craton occurs in a metasomatised dunite (BD2153).
