Ultramafic rocks found within the ~ 3.81 Ga Itsaq Gneiss Complex (IGC) have some mantle-like geochemical characteristics that have led to them being used to directly constrain the nature of the Eoarchean mantle. The discrimination of mantle peridotites that are the residues of partial melting, from cumulate peridotites generated by crystal accumulation from mantle-derived magmas can be difficult in ancient, altered ultramafic rocks whose field relations have been obscured by multiple tectonic episodes. Hence it is important to scrutinize significant individual occurrences of Eoarchean ultramafic rocks in some detail prior to using them to constrain the nature of Earth's early mantle. Here we present mineral chemistry, whole rock major-, trace-, and platinum-group-element abundances, and Re-Os isotope compositions of a previously unstudied large ultramafic enclave in the IGC-the Tussaap Ultramafic Complex (TUC)-with the aim of documenting its origin. High FeO contents of up to 15.5 wt% and correlations between MgO and Os provide strong evidence that the TUC evolved through fractional crystallization rather than partial melt extraction. In addition, co-variations of major elements in the TUC lithologies can be modeled via fractional crystallization of picritic basalts using MELTS. Later alteration and metasomatism of these ultramafic rocks has largely overprinted primary mineral chemistry and resulted in a redistribution of light rare earth elements, rendering these tools ineffective for ascertaining the origin of the TUC or quantifying some of the petrogenetic processes that formed the body. In addition, it is clear that many geochemical features used to identify residual mantle peridotites can also be produced by cumulate or alteration processes, such as some variations in olivine and chromite chemistry, whole rock Al/Si vs Mg/Si systematics, and trace and platinum group element patterns. Finally, combined discrimination diagrams for high field strength elements and moderately high 187 Os/ 188 Os ratios suggest the parental melt of the TUC partially assimilated basaltic crust prior to precipitating the TUC cumulates. As such, these rocks represent a variably obscured record of Eoarchean crystal fractionation from mantle-derived melts. Despite not being prima facie mantle rocks, it is possible that such early formed ultramafic cumulates in nascent continents found their way into the later-stabilized roots of Archean cratons, helping to explain the high compositional variability of cratonic peridotites. Keywords Eoarchean • Itsaq gneiss complex • Ultramafic cumulates • Platinum group elements • Re-Os isotopes Communicated by Timothy L. Grove.
Modeled geothermal gradients, built from estimated thermal properties of the cratonic lithosphere, provide important insight into the thermal structure of Archean continental lithosphere. An important and yet poorly determined parameter affecting estimates of geothermal gradients within cratonic lithosphere, is the heat production in the cratonic lithospheric mantle (CLM) due to the decay of heat producing elements (HPE: K, U, and Th). Heat production in the CLM has been estimated by direct measurements of HPEs in mantle material (Goes et al., 2020;Rudnick et al., 1998;Russell et al., 2001) or by projecting geothermal models through pressure and temperature estimates from cratonic mantle xenoliths and varying heat production and other lithospheric properties to estimate the best-fits to the data (
Chemical sedimentary rocks in the Red Lake-Wallace Lake area of the Canadian Shield form the Earth's oldest known carbonate platform and, as such, provide a unique opportunity to explore the floor of a 2930 Myr old warm, shallow sea. Peritidal depositional features dominate the platform top, ranging from colloform crusts, teepee structures, and evaporate pseudomorphs in the supratidal; pseudomorph crystal fans and laterally linked domal stromatolites, associated with stromatactis-like structures, sheet cracks and liminoid fenestral fabrics extending from the lower supratidal through the intertidal; and herringbone cross-stratification, isolated domal stromatolites and herring-bone calcite cement in tidal channels. These lithofacies indicate a low energy, restricted evaporitic environment. Limited subtidal platform top deposits are characterized by laterally linked domal stromatolites and pseudomorph crystal fans on a larger scale than those found in the intertidal areas. A transitional lithofacies to deeper water were deposited further offshore. It consists of ribbon rock (mixed laminae and beds of carbonate, slate and iron oxide sediments), slump structures composed of intraclastic carbonate lithoclasts in a marl matrix and carbonate-associated iron formation. Basinal deposits consist of chert and chert-oxide facies iron formation. Peritidal lithofacies are composed of ferroan dolomite, whereas deep subtidal to upper slope lithofacies are composed of calcite. The dolomites have O and Sr isotopic ratios which were probably reset during dolomitization, whereas only O is altered in the limestones. The d 13 C values for all the carbonate samples were 0 AE 1Á1& V-PDB , with samples formed in deeper water having lighter d 13 C values, suggesting the deeper open ocean had a lighter d 13 C budget than water in the platform interior. Post Archean Australian Shale normalized rare earth element patterns for the carbonate samples have positive La and Eu anomalies, suprachondritic Y/Ho ratios and slight heavy rare earth element enrichment in most samples. Basin lithofacies are characterized by heavy rare earth element enrichment and positive Eu anomalies. One sample had a significant negative Ce anomaly. These data probably indicate restricted circulation in the supra and intertidal areas and possibly the development of spatially limited areas where oxygen production could move the redox boundary out into the water column.
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