Cratonic granulite, pyroxenite and eclogite xenoliths: Archives of continental crust evolution and terrestrial trace element cycling

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The processes of forming, stabilising, modifying and recycling continental crust (CC) through time are fundamental to understanding the differentiation of the Earth, global geochemical cycling and geodynamics. While the composition and origin of the upper CC are reasonably well known, those of the largely inaccessible lower CC remain enigmatic. Granulites and pyroxenites are actual samples of cratonic lower CC that are sporadically entrained as xenoliths during rare kimberlite or basaltic magmatism. Moreover, some mantle eclogites may represent residues complementary to the early upper CC, while others crystallised near the crust-mantle boundary from mafic melts during later events. Thus, all these lithologies offer important insights into processes attending the formation and modification of the CC, such as crustal differentiation and mass transfer between the crust and mantle at the Moho. I propose to comprehensively study the major-element, trace-element and isotopic composition (U-Pb, Hf-Nd-Sr, O and multiple S isotopes) and redox state of granulite, pyroxenite and eclogite xenoliths from four cratons to constrain (1) the processes, sources, conditions and timing of lower CC formation, modification and growth; (2) the thermophysical properties of granulites and pyroxenites and the resultant thermal and gravitational stability of lower CC; (3) the role of cratonic mantle eclogites in continent formation and their relationship to the lower CC; (4) the abundances, distribution and mobility of high field-strength, chalcophile and siderophile elements, with a focus on accessory rutile, ilmenite and sulphide. Taken together, these studies will have implications for Archaean geodynamics, terrestrial trace-element cycling, and geochemical processes relevant to continent formation.