Temporal evolution of 142Nd signatures in SW Greenland from high precision MC-ICP-MS measurements

Measurements of 142Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ142Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143Nd/144Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390−0.060+0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142Nd/144Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ142Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ142Nd = +9.8 ± 1.0). Even lower but homogeneous μ142Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε143Nd(i) and εHf(i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ142Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147Smsingle bond143Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules out a significant disturbance by younger alteration events. Based on isotope and trace element modelling, we argue that the temporal evolution of coupled 142,143Nd/144Nd isotope compositions in the ambient mantle beneath the Isua rocks is best explained by the progressive admixture of material to the Isua mantle source that must have had present-day-like μ142Nd compositions. In contrast, Mesoarchean mafic rocks from the ∼3.08 Ga Ivisaartoq greenstone belt and the 2.97 Ga inner Ameralik Fjord region as well as a 2.0 Ga Proterozoic dike within that region all have higher μ142Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ142Nd compositions

Characterisation of Conventional 87Sr/86Sr Isotope Ratios in Cement, Limestone and Slate Reference Materials Based on an Interlaboratory Comparison Study

An interlaboratory comparison (ILC) was organised to characterise 87Sr/86Sr isotope ratios in geological and industrial reference materials by applying the so-called conventional method for determining 87Sr/86Sr isotope ratios. Four cements (VDZ 100a, VDZ 200a, VDZ 300a, IAG OPC-1), one limestone (IAG CGL ML-3) and one slate (IAG OU-6) reference materials were selected, covering a wide range of naturally occurring Sr isotopic signatures. Thirteen laboratories received aliquots of these six reference materials together with a detailed technical protocol. The consensus values for the six reference materials and their associated measurement uncertainties were obtained by applying a Gaussian, linear mixed effects model fitted to all the measurement results. By combining the consensus values and their uncertainties with an uncertainty contribution for potential heterogeneity, reference values ranging from 0.708134 mol mol-1 to 0.729778 mol mol-1 were obtained with relative expanded uncertainties of ≤ 0.007 %. This study represents an ILC on conventional 87Sr/86Sr isotope ratios, within which metrological principles were considered and the compatibility of measurement results obtained by MC-ICP-MS and by MC-TIMS is demonstrated. The materials characterised in this study can be used as reference materials for validation and quality control purposes and to estimate measurement uncertainties in conventional 87Sr/86Sr isotope ratio measurement

The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)

Drill core samples from the 333 Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf-isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial epsilon Hf isotope values range from +6.5 to +9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial epsilon Nd values on the same geologic unit that reflect only minor depleted mantle sources of +2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high-degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites. (C) 2016 Elsevier B.V. All rights reserved

Multiple sulfur isotope signature of early Archean oceanic crust, Isua (SW-Greenland)

The Archean sulfur cycle was different from the present-day cycle, as the emission of volcanogenic sulfurous gases was the dominant process in the anoxic environment of the early Earth.This emitted sulfur exhibits mass-independently fractionated sulfur isotopes (MIF-S), resulting from photochemical reactions in the atmosphere, and it differs substantially from unfractionated sulfur in the mantle. So far, the main focus of multiple sulfur analyses (S-32, S-33, S-34 and S-36) was placed on the sedimentary part of the Archean sulfur cycle. In order to constrain the magmatic part of the sulfur cycle, we analyzed the sulfur isotopic composition of oceanic crustal rocks from the ca. 3.7-3.8 Ga Isua Supracrustal Belt (ISB). Differently altered samples were taken from two units:(1) the Undifferentiated Amphibolites (UA) and (2) the younger Amphibolites with Boninitic affinity (AB). The mean values are:delta S-34(CRS) = +0.01 +/- 0.65 parts per thousand (values range from -0.87 to 1.37%0; CRS = chromium-reducible sulfur), Delta S-33(CRS) = +0.02 +/- 0.12%0 (values range from -0.17 to 0.26 parts per thousand), Delta S-36(CRS) = -0.47 +/- 0.06 parts per thousand (values range from -0.56 to -0.38 parts per thousand). Thus, the mean isotope values support the assumption that the sulfur isotopic signature reflects the expected near-zero signature of their mantle origin. However, differences in Delta S-33(CRS) values are discernible and non-zero suggesting that different sources are contributing to the isotopic signature. An influence of alteration is excluded for all samples as different alteration-sensitive geochemical parameters do not show any correlation with the multiple sulfur isotope signatures. Further, it is unlikely that the small magnitudes in Delta S-33(CRS) are generated by microbial mass-dependent processes because of the narrow range of delta S-34(CRS) values. Possible sources contributing an atmospheric MIF-S signature include seawater sulfate (negative Delta S-33(CRS) values) through hydrothermal circulation, the assimilation of ocean floor sediments during the ascent of the melt and/or a mantle source contamination by subducted oceanic slab. (C) 2016 Elsevier B.V. All rights reserved

Alfred Kröner (1939-2019)

International audienc

Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater

It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf-Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf-Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled epsilon Nd2.60Ga-epsilon Hf2.60Ga values of -2.3 and + 9.48, respectively, and suggests that decoupled Hf-Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive epsilon Nd-2.60Ga values, but still reveal decoupled eNd-eHf values relative to an associated schist that plots slightly below the terrestrial array. This suggests that mineral sorting between a zirconbearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the zircon effect. Alternatively, the less pronounced decoupling of eHf-eNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial eNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra) mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater

Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

Pure Superior-type Banded Iron Formation (BIF) samples from the Krivoy Rog Supergroup (Ukraine) are excellent archives of ambient Early Precambrian seawater. They show low concentrations of incompatible elements such as Zr, Hf, and Th, and shale-normalized Rare Earths and Yttrium (REYSN) patterns similar to those of modern seawater, i.e. heavy REYSN enriched patterns with positive La-SN, Gd-SN and Y-SN anomalies. Lack of Ce-SN and presence of positive Eu-SN anomalies indicate REY contributions to anoxic ferruginous seawater from high-temperature hydrothermal fluids. The depositional age of the Krivoy Rog BIF is ill-defined, but a Late Archean to Paleoproterozoic age has been suggested based on U-Pb zircon ages for units stratigraphically above and below the BIF. We determined Sm-Nd isotopic compositions of pure and impure samples from the Krivoy Rog BIF, which yield an errorchron with an apparent age of 2406 +/- 350 Ma (MSWD 15), that falls within this broad age range. All pure BIF samples show chondrite-normalized (subscript (CN)) REY patterns with strong positive Eu-CN anomalies that are typical for Archean but rather rare and much less pronounced in Proterozoic BIFs. Associated schists also show Archean - rather than post-Archean-style REY distributions. The REV geochemistry of both, chemical and epiclastic sediments, therefore, is more consistent with a Late Archean rather than a post-Archean depositional age of the Krivoy Rog Supergroup. Initial epsilon Nd values of impure BIFs and of associated schist reveal variable contributions from TTGs less radiogenic in Nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying New Krivoy Rog Group. The purest Krivoy Rog BIF, representing local Krivoy Rog seawater, displays an epsilon Nd-2.60Ga value of -2.3. This value is less radiogenic than impure Krivoy Rog BIFs or other near-contemporaneous Neoarchean pure chemical sediments. To preserve this specific local isotopic fingerprint in anoxic Archean seawater, the Krivoy Rog BIF must have been deposited in an isolated sea basin with limited exchange with ferruginous deep-waters of the open ocean. A compilation of REY data for high-purity Precambrian BIFs reveals that Eu-CN/Eu*(CN) ratios of Precambrian seawater follow a general global evolution curve, that shows specific peaks which reflect times of increased high-temperature hydrothermal REY input into seawater. Following declining Eu-CN/Eu-CN ratios from the Eoarchean to the Mesoarchean, the ratios suddenly rise at 2.7 Ga and reach a maximum at 2.6 Ga, indicating an increased flux of high-temperature hydrothermal REV to Neoarchean seawater, which supports the hypothesis that times of widespread BIF deposition coincided with periods of intense submarine hydrothermal activity, probably triggered by major mantle plume events. This association is supported by a strong increase of the epsilon Nd-(t) values of pure seawater archives at 2.7-2.6 Ga, which reflects an increased flux of mantle Nd into seawater. These results suggest that Eu-REV systematics (and potentially epsilon Nd systematics) are robust tools to indentify episodes of enhanced mantle plume activity. (c) 2015 Elsevier B.V. All rights reserved

Archaean granulite-facies paragneisses from central Swaziland: inferences on Palaeoarchaean crustal reworking and a complex metamorphic history

We present a petrographic, petrological, geochemical, and geochronological study (U-Pb/Lu-Hf) on granulite-facies paragneisses of the Mahamba Gneiss Complex in central Swaziland, eastern Kaapvaal Craton. Our data suggest that prograde metamorphism occurred at c. 3.07 Ga. Dating of detrital zircons of a metapelite in combination with geochronological and ambiguous structural relationships with granitoid gneisses suggests two possible scenarios: (1) the time of deposition of the sedimentary protoliths is prior to c. 3.58 Ga; (2) c. 3.58 Ga granitoid crust was the basement for the sedimentary protoliths. Furthermore, enrichment in Ni and Cr in the Mahamba metasediments may originate from erosion of a greenstone terrane similar in composition to the dominantly mafic and ultramafic lithologies of the Barberton Greenstone Belt or Dwalile supracrustal belt. Evidence for an older crustal basement is provided by the oldest detrital zircons, which yield ages up to c. 3.7 Ga. Because the metamorphic conditions of metapelitic rocks have no similarity to P-T paths of modern subduction-collision tectonic settings we conclude that the metamorphic history of the Mahamba Gneiss Complex cannot be explained by this model. We propose that deformation and metamorphism occurred through intracratonic contraction and a thermal event possibly related to Mpuluzi batholith emplacement

Decoupled Hf-Nd isotopes in Neoarchean seawater reveal weathering of emerged continents

Marine chemical sediments from the Temagami banded iron formation (BIF) in Canada exhibit nonchondritic Zr/Hf and Y/Ho ratios and seawater-like rare earth element patterns, indicating that their Hf and Nd are not detrital, but derived from seawater. This is confirmed by Sm-Nd and Lu-Hf isochron ages of 2605 +/- 140 Ma (initial epsilon(Nd) +0.03 +/- 4.1) and 2760 +/- 120 Ma (initial epsilon(Hf) +/- 7.2 +/- 5.3), respectively, that overlap within error the 2.7 Ga U-Pb age of associated igneous rocks. The Temagami BIF is therefore an excellent archive of the Nd-Hf isotopic composition of Neoarchean seawater. Whereas epsilon(Nd2.7Ga) values cluster around +1, epsilon(Hf2.7Ga) values range from +6.7 to +24.1, substantially more radiogenic than those of ambient Neoarchean mantle and continental crust. Such an epsilon(Hf)-epsilon(Nd) distribution is typical of modern seawater, plotting above the terrestrial array as defined by igneous and clastic sedimentary rocks. The only mechanism known to produce natural waters with decoupled Nd and Hf isotope compositions is the incongruent mobilization of Hf from continental crustal material. Therefore, input of such highly radiogenic Hf into seawater requires substantial amounts of evolved Neoarchean continental crust that was exposed above sea level and available to erosion and terrestrial weathering