Cratonic granulite, pyroxenite and eclogite xenoliths: Archives of continental crust evolution and terrestrial trace element cycling
Final Report Abstract
This project focussed on the comprehensive petrographic and geochemical investigation of kimberliteborne granulite and eclogite xenoliths from various cratons, aimed at obtaining new constraints on the processes that govern the formation, stabilisation, modification and recycling of continental crust (CC) through time. A large proportion of the lower CC at Diavik in the central Slave craton formed by underplating of mafic melts leaving a garnet-bearing subduction-modified mantle source at ca. 3.3 Ga. This is evidenced by bulk-rock HFSE negative anomalies, combined with variable enrichments in Sr, Nd ± Li and Pb and depletions in HREE, consistent with Sm-Nd isotopes falling on Meso- to Neoarchaean age arrays. Quasi-concurrent plume and subduction signatures can be explained by the interaction of a thick oceanic plateau with a subduction zone just prior to plume impingement. Early differentiation of this underplate is indicated by Th, U and K2O abundances much lower than average estimates for lower CC, combined with unradiogenic Sr (down to 0.7017) consistent with early Rb loss. A long history of subsequent cooling and re-heating is constrained from petrographic, traceelement and U-Pb isotope data of rutile, ilmenite and Ti-magnetite. This cooling resulted in a large spread in Zr-in-rutile temperatures (>200°C) as well as in the concentrations of other trace elements in rutile and ilmenite, dependent also on local mineralogy, such that Ti minerals can effectively constitute HFSE “islands” in the silicate matrix. The lack of hydrous minerals in the granulites, combined with significant trace-element and radiogenic isotopic heterogeneity observed for isolated sulphide and rutile grains at the sample scale and persistence of high-energy grain boundaries, such as exsolution lamellae, attest to the long-term dryness of the lower CC beneath the craton, which, together with loss of heat-producing elements, may have been instrumental in ensuring craton longevity. Mantle eclogites have for a long time and frequently been linked to CC formation via loss of a TTG-like partial melt. Although such a link is permissive for a subset of samples at some localities from a geochronological and geochemical point of view (Koidu in the West African craton and Orapa in the Zimbabwe craton), it is precluded by available age constraints at Lace (Kaapvaal craton) and Diavik (Slave craton), in addition to the important observation that the SiO2- and LREE-depletion, often cited as evidence for loss of a silicic partial melt, in eclogites with plagioclase-rich protoliths results from accumulation rather than melt depletion. The origin and evolution of mantle eclogites was unravelled using new and published data-sets, combined with the collation of a substantial experimental data-set and new thermodynamic (MELTS) and trace-element modelling. This strongly supports the view that most mantle eclogite suites originated as recycled oceanic crust and allows the confident identification of kimberlite infiltration and metasomatism (after capture in the continental lithosphere), of fractional crystallisation and accumulation in the igneous protoliths, as well as of subduction-related processes. The latter include interaction of oceanic crust with sedimentary material in a mélange-type setting, which led to formation of kyanite- and diamondbearing eclogite from initially bimineralic eclogite at Lace. Furthermore, mantle eclogites, filtered for igneous differentiation in the protolith and later metasomatism in the cratonic mantle, were recognised as invaluable archives of the physical and chemical evolution of the ambient palaeoconvecting mantle, from which information on its temperature, redox and compositional evolution may be gleaned, analogous to modern mid-ocean ridge basalt. Thus, preliminary data using mantle eclogites investigated in this project, and other meta-basalts identified as spreading ridge-derived, indicate that the Archaean ambient convecting mantle was possibly more depleted, more reduced and only slightly (~100-150°C) warmer than today. In addition, the oxygen fugacity of this more reducing Archaean oceanic crust decreases further upon subduction, with implications for the speciation and solubility of carbon and other volatiles during fluid and melt loss from the slab. In lower crustal granulites, sulphide strongly partitions Ni, Co, As and Sb, whereas W, Sn ± Mo concentrate in rutile, and Cr, Zn, Ga favour ilmenite. Compared to upper CC, bulk-rock chalcophile and siderophile element abundances, calculated from concentrations in sulphide, rutile and ilmenite weighted by their modal abundances, show large excesses for Ni, Cr, Co, Sn, Mo, Sb and As, and it seems unlikely that sulphide- and rutile/ilmenite-saturated melting of lower crustal granulite can generate upper CC compositions. Interestingly, Diavik granulites have superchondritic Nb/Ta (median 27) compared to other silicate reservoirs (<18) and may thus be important in a solution to the so-called Nb/Ta paradox. Considering the current volume of lower CC and the proportion of lower CC recycled into the deep mantle through time (total ~2%), average calculated whole-rock Nb and Ta concentrations in granulites (7.0 and 0.29 ppm) come very close to the Nb and Ta concentrations of 7.2 and 0.35 ppm, respectively, required in the lower CC to balance the budget.
Publications
- (2015) Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back. Earth and Planetary Science Letters 431: 274-286
Aulbach S, Viljoen KS
(See online at https://doi.org/10.1016/j.epsl.2015.08.039) - (2016) Evidence for a reducing Archaean ambient mantle and its effects on the carbon cycle. Geology 44: 751-754
Aulbach S, Stagno V
(See online at https://doi.org/10.1130/G38070.1) - (2016) Formation of diamondiferous kyanite-eclogite in a subduction mélange. Geochimica Cosmochimica Acta 179: 156-176
Aulbach S, Gerdes A, Viljoen KS
(See online at https://doi.org/10.1016/j.gca.2016.01.038) - (2016) Major- and trace-elements in cratonic mantle eclogites and pyroxenites reveal heterogeneous sources and metamorphic processing of low-pressure protoliths. Lithos 262: 586-605
Aulbach S, Jacob DE
(See online at https://doi.org/10.1016/j.lithos.2016.07.026) - (2017) Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin. Geochimica et Cosmochimica Acta 213: 574-592
Aulbach S, Jacob DE, Cartigny P, Stern RA, Simonetti SS, Viljoen KS
(See online at https://doi.org/10.1016/j.gca.2017.06.038) - (2017) Effects of low-pressure igneous processes and subduction on Fe3+/∑Fe and redox state of mantle eclogites from Lace (Kaapvaal craton). Earth and Planetary Science Letters 474: 283-295
Aulbach S, Woodland AB, Vasilyev P, Galvez ME, Viljoen KS
(See online at https://doi.org/10.1016/j.epsl.2017.06.030)
