Cratonic mantle eclogites as palaeo-dynamic archives
Final Report Abstract
Eclogite xenoliths derived from the mantle lithosphere of ancient continental cores have lowpressure protoliths that were emplaced in palaeo-ocean floors after decompression melting of the uppermost convecting mantle beneath spreading ridges. Like modern mid-ocean ridge basalts, these “oceanic” eclogites are therefore probes of the chemical, thermal and redox state of the ambient mantle from which their protoliths formed, extending the record back to ca. 3 Ga. Constraints on these parameters are critical to understanding crust-mantle evolution and geodynamics through time. This project focussed on (1) unravelling the origin and evolution of mantle eclogite from different cratons and tectonic settings, using comprehensive geochemical and isotopic information, (2) using the least differentiated and modified samples to find reliable proxies for the chemical, oxidation and thermal state of the ancient ambient mantle, (3) test the link between mantle eclogites and regional continental crust formation from a compositional and chronological perspective. Combining new data acquired during the project with published data from cratonic eclogites, it was demonstrated that two redox proxies, V/Sc and Fe3+/ΣFe, behave sympathetically, despite different responses of these ratios to differentiation and post-formation processes, which therefore did not obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/ΣFe in two Archaean eclogite suites are significantly lower than those of modern midocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Because mantle oxygen fugacity (ƒO2) links to the redox state of volcanic gases, these data affirm a role of the convecting mantle in the oxygenation of the atmosphere, known as the Great Oxidation Event. Oxybarometry also shows that after metamorphism, the vast majority of eclogites, including those showing evidence of seafloor weathering (non-mantle δ18O), record ƒO2 lower than both modern MORB and its Archaean equivalent. Low ƒO2 stabilises diamond relative to oxidised carbon species during interaction of fluids or melts with eclogite, presenting a pathway for deep carbon recycling in the Archaean, and indicating little oxidising power for deeply subducted oceanic crust. Furthermore, when filtered to exclude specimens with cumulate protoliths or showing evidence for later metasomatism, cratonic eclogites have elemental relationships indicating formation of their protoliths by melt segregation predominantly from a garnet-free peridotite source, qualitatively indicating low pressures of solidus intersection and mantle potential temperature (TP). The moderately incompatible elements Ti and Sm were shown to be relatively insensitive to melt loss from eclogite, while extremely low measured or modelled 87Sr/86Sr (≤0.7007) in some eclogites suggests depletion of the ambient mantle by 3 Ga. Their abundances were therefore used to calculate the melt fraction (F) by which their protolith formed, assuming a Depleted Mantle source and using the batch melting equation. As Ti and Sm abundances increase through time, F and therefore TP decrease, consistent with ambient mantle cooling. Modelling, which quantitatively relates TP to F shows that TP at 3 Ga was only some 100°C higher than today, supporting early plate strengthening and transition to plate tectonics. Application of 87Sr/86Sr and Sm-Nd-U-Pb isotopes reveals that accurate and precise age dating of mantle eclogite remains a difficult task. Within the available chronological and compositional constraints, a link of cratonic eclogite to continental crust formation via loss of a silicic partial melt is not strongly supported, although more data are needed for firm conclusions on this issue.
Publications
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(2019) Ages and sources of mantle eclogites: ID-TIMS and in situ MC-ICPMS Pb-Sr isotope systematics of clinopyroxene. Chemical Geology 503: 15-28
Aulbach S, Heaman LM, Jacob DE, Viljoen KS
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(2019) Eclogites as Palaeodynamic Archives: Evidence for Warm (not hot) and Depleted (but heterogeneous and evolving) Archaean Ambient Mantle. Earth and Planetary Science Letters 505: 162-172
Aulbach S, Arndt NT
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(2019) Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust. Scientific Reports 9: 20190
Aulbach S, Woodland AB, Stern RA, Vasilyev P, Heaman LM, Viljoen KS
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(2020) Diamondiferous and barren eclogites from the western Kaapvaal craton record subduction metasomatism and mantle metasomatism, respectively. Lithos 368-369: 105588
Aulbach S, Viljoen KS, Gerdes A
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(2020) Temperature-dependent rutile solubility in garnet and clinopyroxene from mantle eclogite: Implications for continental crust formation and V-based oxybarometry. Journal of Petrology 61
Aulbach S
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(2020) Ultramafic carbonated melt- and auto-metasomatism in mantle eclogites: Compositional effects and geophysical consequences. Geochemistry Geophysics Geosystems 21(5): e2019GC008774
Aulbach S, Massuyeau M, Garber J, Gerdes A, Heaman LM, Viljoen KS
