Erosion of Archean continents: The Sm-Nd and Lu-Hf isotopic record of Barberton sedimentary rocks

International audienceKnowing the composition, nature and amount of crust at the surface of the early Earth is crucial to understanding the early geodynamics of our planet. Yet our knowledge of the Hadean-Archean crust is far from complete, limited by the poor preservation of Archean terranes, and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants. To address this problem and get a more comprehensive view of what an Archean continent may have looked like, we investigated the trace element and Sm-Nd, Lu-Hf isotopic records of Archean metasedimentary rocks from South Africa. We focused our study on sandstone and mudstone from drill core in the Fig Tree Group (3.23-3.26 Ga) of the Barberton granite-greenstone belt, but also analyzed the 3.4 Ga Buck Reef cherts and still older (3.5-3.6 Ga) meta-igneous rocks from the Ancient Gneiss Complex, Swaziland. Based on principal component analysis of major and trace element data, the Fig Tree metasedimentary rocks can be classified into three groups: crustal detritus-rich sediments, Sirich sediments and CaFe Fe-rich sediments. The detritus-rich sediments have preserved the Sm-Nd and Lu-Hf isotopic signatures of their continental sources, and hence can be used to constrain the composition of crust eroded in the Barberton area in the Paleoarchean period. Based on Sm/Nd ratios, we estimate that this crust was more mafic than today, with an Revised Text average SiO 2 content of 60.5 ± 2 wt.%. This composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic-ultramafic and felsic rocks. This implies that the Archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth. Neodymium and Hf crustal residence ages show that the eroded crust was, on average, ~300-400 Ma older than the deposition age of the sediments, which highlights the importance of intracrustal reworking of older crust at ~3.2 Ga in the Barberton area. The Si-rich sediments have slightly positive ε Nd (t=3.23Ga) but extremely radiogenic ε Hf (t=3.23Ga) , up to +11. Based on analyses of 3.4 Ga Buck Reef cherts, we suggest that the radiogenic Hf isotopic signature of the Si-rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks. The latter have extremely high Lu/Hf ratios such that their ε Hf values would increase dramatically, by about +100 epsilon units every 100 Ma. In the CaFe Fe-rich sediments, one sample contains carbonate that preserves the typical rare-earth element features of seawater precipitates. The initial Nd isotopic composition of this sample (ε Nd (t=3.23Ga) = +1.7) is within the range of previous estimates for Archean anoxic seawater

Sulfur-controlled iron isotope fractionation experiments of core formation in planetary bodies

A series of high pressure and temperature experiments were conducted to better constrain the Fe isotope fractionation during core-mantle differentiation in planetesimal and planetary bodies. Synthetic mixtures of oxides and metal having varying amounts of sulfur, approximating terrestrial and Martian compositions, were melted at 1-2GPa and 1650°C. Iron isotopic equilibrium between the resulting metal and glass run products was verified for all experiments using the three-isotope technique. Purified Fe from metal and glass was analyzed by multiple-collector ICP-MS in high resolution mode. Iron alloy and silicate glass show a well-resolved d57Femetal-silicate of +0.12±0.04‰ in a sulfur-free system. Isotope fractionation increases with sulfur content to +0.43±0.03‰ at 18wt.% sulfur in the metal. These results cannot be easily interpreted within the context of known Fe isotope ratios in most natural samples of planetary and asteroidal mantles and therefore suggest more complex processes affected the Fe isotope fractionation therein. However, to reconcile Martian meteorite iron isotopic signatures with geophysical models using this new experimental data requires a smaller amount of sulfur in the Martian core than previous estimates, with an upper limit of ~8wt.%. © 2014 Elsevier Ltd

Metabolismo oxidativo de neutrófilos em ovelhas naturalmente infectadas por nematódeos gastrintestinais e correlação entre nível sérico de cortisol e carga parasitária Neutrophils oxidative metabolism in sheep naturally infected by gastrointestinal nematodes and correlation between serum level of cortisol and parasitary burden

Estudaram-se as relações entre o metabolismo oxidativo dos neutrófilos, os níveis séricos de cortisol e a carga parasitária estimada pela contagem de ovos por grama de fezes (OPG) no final da gestação, durante a lactação e após o desmame de ovelhas naturalmente infectadas por nematódeos gastrintestinais. Utilizaram-se 22 ovelhas da raça Suffolk, homogêneas quanto à idade (três a quatro anos), número de parições (terceira) e época de parição. O cortisol sérico foi determinado por radioimunoensaio e o metabolismo oxidativo dos neutrófilos pelo teste de redução do tetrazólio nitroazul (NBT). Os maiores valores de OPG foram observados na quinta semana de lactação, e as maiores taxas de cortisol e de redução do NBT ocorreram na quarta semana pós-desmame. Verificou-se correlação positiva (r = 0,52; P<0,05) entre a capacidade dos neutrófilos reduzirem o NBT e a concentração sérica de cortisol no período pré-parto. Próximo ao parto o metabolismo oxidativo dos neutrófilos diminuiu. Após o desmame a correlação entre o OPG e o teste de redução do NBT foi negativa (r = -0,39; P<0,01). A diminuição do OPG observada após o desmame coincidiu com o aumento da capacidade de os neutrófilos reduzirem o NBT, indicando que animais com melhor resposta imune pós-desmame possuem neutrófilos com metabolismo oxidativo maior e menor carga parasitária.<br>The relationships between neutrophils oxidative metabolism, cortisol serum levels and worm burden, estimated by fecal egg count (EPG), were studied in sheep naturally infected by gastrointestinal parasites at the end of pregnancy, during lactation, and after weaning. Twenty-two Suffolk sheep three to four year-old, of same parity and season of parturition were used. Serum cortisol was determined by radioimmunoassay and the neutrophils oxidative metabolism by the nitroblue tetrazolium (NBT) reduction test. The highest EPG values were observed on the fifth week of lactation while the highest rates of cortisol and reduction of NBT occurred on the fourth week after weaning. A positive correlation (r = 0.52; P<0.01) was observed between the neutrophils capacity to reduce the NBT and the serum cortisol concentration in the pre-parturition period. Otherwise, the neutrophils oxidative metabolism decreased near to the parturition. A negative correlation (r = -0.39; P<0.01) between EPG and NBT reduction test was observed after weaning, which was coincident with the increase in the neutrophils capacity to reduce NBT, indicating that animals showing better immune response after weaning have neutrophils with higher oxidative metabolism and lower worm burden