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Bulk-rock Lu-Hf and Sm-Nd isotope compositions, as well as major and trace element data are presented for metavolcanic rocks from the Mesoarchaean (ca. 3075 Ma) Ivisaartoq Supracrustal Belt in the Nuuk region of southern West Greenland. The εHf t calculated at 3075 Ma range from +0.8 to +3.1 and the corresponding εNd t values range from +0.7 to +3.6, which forms an array that is displaced off the mantle array for these two isotopic systems. Primitive mantle normalized trace element plots of the metabasalts display negative Nb- and Ti-anomalies in combination with the elevated Th abundances, which is consistent with a subduction zone affinity as proposed by previous studies of this metavolcanic belt. No significant correlations are observed between the isotope compositions and proxies of shallow crustal contamination in the Ivisaartoq rocks, despite clear evidence for inherited Eoarchaean zircon [ Polat et al. (2009a) Chemical Geology 268, 248-271 ], which would have dominated the bulk-rock Hf-isotope budget. Furthermore, the measured samples are less radiogenic than the estimate for the depleted mantle composition at 3075 Ma. The lack of isotope and trace element correlation suggests incomplete equilibration between the crustal contaminant and the parental Ivisaartoq melts. We prefer a petrogenetic model with some combination of slab-derived metasomatism of the mantle source region for the Ivisaartoq magmas, which homogenized their trace element contents, in combination with the incorporation of granitoid residue with unradiogenic Hf-isotope composition at higher degrees of partial melting and finally the eruption of mechanically entrained Eoarchaean crust without significant chemical equilibration. The geochemical arc-affinity and lower than depleted mantle (DM) isotope compositions of these metavolcanic rocks support the notion that crustal recycling and plate tectonics has been operating on Earth since at least the Mesoarchaean Eon. [ABSTRACT FROM AUTHOR]

The first 2.5 billion years of the terrestrial impact history are not documented by any impact structures. Only a few spherule layers of impact origin are known, most of them of late Archean to early Proterozoic age. In the Barberton Greenstone Belt (South Africa), several spherule horizons (layers S1 to S4, possibly up to S8, with ages between ∼3.5 and ∼3.2 Ga) are amongst the oldest deposits from large bolide impacts onto Earth. Impact evidence is limited to (highly) elevated siderophile element contents and Cr isotopic compositions. Other isotope tools, such as the 187 Re- 188 Os radionuclide system in combination with high-precision concentration data for siderophile elements, might be useful to confirm the propositions regarding the presence of meteoritic components made so far. Two recently recovered drill cores from the central and northern Barberton area (CT3 and BARB5) with as many as 18 spherule layer intersections of Paleoarchean age (some of which may be due to tectonic duplication, some might correlate with the S2 to S4 layers) provide an outstanding opportunity to gain new insight into the early impact bombardment of Earth. We present new mineralogical, chemical, and 187 Re- 188 Os isotope data on CT3 and BARB5 drill core samples. Spherules in most layers exhibit undeformed shapes and include vesicles. Sulfides frequently are present in both matrix and spherules. Osmium data reveal a trend between the spherule-free horizons (intercalating the spherule layers) and spherule-matrix aggregates. Whereas the former typically exhibit elevated 187 Os/ 188 Os ratios of up to ∼1.2 and low Os and Ir concentrations below several hundred ppt, spherule-matrix aggregates tend to be less radiogenic (down to subchondritic present day 187 Os/ 188 Os ratios) with Os and Ir concentrations as high as in chondrites. Chromium-Ir correlations for CT3 and BARB5 samples mirror earlier results on S1 to S4 layers and can be interpreted in favor of an impact origin of the here investigated spherule horizons. [ABSTRACT FROM AUTHOR]