Widespread occurrences of variably crystalline C-13-depleted graphitic carbon in banded iron formations

Almost all evidence for the oldest traces of life on Earth rely on particles of graphitic carbon preserved in rocks of sedimentary protolith. Yet, the source of carbon in such ancient graphite is debated, as it could possibly be non-biological and/or non-indigenous in origin. Here we describe the co-occurrence of poorly crystalline and crystalline varieties of graphitic carbon with apatite in ten different and variably metamorphosed banded iron formations (BIF) ranging in age from 1,800 to >3,800 Myr. In Neoarchean to Palaeoproterozoic BIF subjected to low-grade metamorphism, C-13-depleted graphitic carbon occurs as inclusions in apatite, and carbonate and arguably represents the remineralisation of syngenetic biomass. In BIF subjected to high-grade metamorphism, C-13-depleted graphite co-occurs with poorly crystalline graphite (PCG), as well as apatite, carbonate, pyrite, amphibole and greenalite. Retrograde minerals such as greenalite, and veins cross-cutting magnetite layers contain PCG. Crystalline graphite can occur with apatite and orthopyroxene, and sometimes it has PCG coatings. Crystalline graphite is interpreted to represent the metamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline graphite was precipitated from C-O-H fluids partially sourced from the syngenetic carbon, along with fluid-deposited apatite and carbonate. The isotopic signature of the graphitic carbon and the distribution of fluid-deposited graphite in highly metamorphosed BIF is consistent with carbon in the fluids being derived from the thermal cracking of syngenetic biomass deposited in BIF, but, extraneous sources of carbon cannot be ruled out as a source for PCG. The results here show that apatite + graphite is a common mineral assemblage in metamorphosed BIF. The mode of formation of this assemblage is, however, variable, which has important implications for the timing of life's emergence on Earth. (C) 2019 Elsevier B.V. All rights reserved.Peer reviewe

Early differentiation of silicate soil recorded in Archean rocks of Isua (Greenland) : implications on mantle dynamics over time

L’enregistrement géologique de l’Hadéen et de l’Eoarcheén étant limité, la composition et l’évolution de la Terre silicatée au cours de ces périodes reste très débattue et peu connue. La première évidence d’un événement de différenciation précoce du manteau provient de l’étude du système éteint 146Sm-142Nd. En effet, les excès en 142Nd détectés dans les échantillons âgés de 3.7 milliard d’années (Ga) d’Isua (Sud-Ouest du Groenland) par rapport aux échantillons modernes, impliquent que leur source fût appauvrie en éléments incompatibles et formée durant l’Hadéen. Dans cette thèse nous avons étudié les systèmes isotopiques 146Sm-142Nd, 147Sm–143Nd et 176Lu–176Hf d’une collection d’échantillons mantelliques âgés de 3.8 Ga, 3.7 Ga, 3.4 Ga et 3.3 Ga et provenant de différentes unités géologiques de la région d’Isua. Cette collection d’échantillons couvre ~500 millions d’années de l’histoire précoce terrestre et a permis d’apporter des informations précieuses sur l’évolution de la composition et la dynamique du manteau au début de l’histoire de la Terre. Nous avons démontré un découplage des systèmes isotopiques de longue-vie Sm-Nd et Lu-Hf dans la source des roches âgées de 3.7 Ga, alors que ces deux systèmes sont généralement couplés lors des processus magmatiques de basse pression. Pour expliquer ce découplage, nous proposons que la source des laves d’Isua avait une origine profonde (manteau inférieur). Nous avons également confirmé les excès en 142Nd dans ces échantillons, ce qui implique que leur source a été appauvrie en éléments incompatibles dans les premiers 100 Ma de l’histoire de la Terre. L’évènement responsable de la différenciation du réservoir appauvri aurait également formé un réservoir complémentaire enrichi, enregistré dans les roches de 3.4 Ga. Cependant, les échantillons de 3.3 Ga ne montrent pas d’anomalies en 142Nd par rapport aux standards terrestres. Ceci suggère que les hétérogénéités chimiques hadéennes aient résisté au mélange convectif pendant plus de 1 Ga, puis qu’elles aient disparu et soient remélangées au reste du manteau à 3.3 Ga. Le mélange à 3.3 Ga pourrait aussi s’expliquer par les processus de subduction initiées par la tectonique de plaques récemment proposée comme ayant démarrée autour de 3.2 Ga.The composition and the evolution of the silicate Earth during the Hadean-Eoarchean is widely debated and largely unknown due to the limited geological record. The first undisputable evidence for a very early differentiation of the mantle came from the extinct 146Sm-142Nd chronometer. The 142Nd excesses measured in 3.7 billion year (Ga) old rocks from Isua (Southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean. We have studied 146,147Sm–142,143Nd and 176Lu–176Hf isotope systematics in 3.8 Ga, 3.7 Ga, 3.4 Ga and 3.3 Ga mantle-derived samples from different tectonic domains of the Isua region. This dataset covers ~ 500 million years of the early history of the Earth and revealed precious information about the compositional evolution and dynamics of the early Earth mantle. Combined Lu-Hf and Sm-Nd of the 3.7 Ga Isua samples revealed that these isotopic systematics were decoupled, suggesting a deep-seated source for the Isua lavas. We have further expanded the dataset and confirmed the 142Nd excesses in these rocks, implying that their source was depleted and differentiated during the first 100 Ma of Earth’s history. The differentiation event that created this early-depleted reservoir had to also form a complementary enriched component. We have found the first evidence for this reservoir recorded in the 3.4 Ga samples that yielded negative 142Nd anomalies. However, the 3.3 Ga analyzed samples showed no resolvable 142Nd anomalies compared to terrestrial Nd standards. This suggests that the Hadean heterogeneities were likely isolated for more than 1 Ga before being completely rehomogeneized by 3.3 Ga. The remixing by 3.3 Ga could have been achieved by subduction processes due to the onset of modern plate tectonics at ~3.2 Ga, as proposed by other recent studies

Différenciation précoce de la terre silicatée enregistrée dans les roches archéennes d'Isua (Groenland) : implications sur la dynamique du manteau au cours du temps

The composition and the evolution of the silicate Earth during the Hadean-Eoarchean is widely debated and largely unknown due to the limited geological record. The first undisputable evidence for a very early differentiation of the mantle came from the extinct 146Sm-142Nd chronometer. The 142Nd excesses measured in 3.7 billion year (Ga) old rocks from Isua (Southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean. We have studied 146,147Sm–142,143Nd and 176Lu–176Hf isotope systematics in 3.8 Ga, 3.7 Ga, 3.4 Ga and 3.3 Ga mantle-derived samples from different tectonic domains of the Isua region. This dataset covers ~ 500 million years of the early history of the Earth and revealed precious information about the compositional evolution and dynamics of the early Earth mantle. Combined Lu-Hf and Sm-Nd of the 3.7 Ga Isua samples revealed that these isotopic systematics were decoupled, suggesting a deep-seated source for the Isua lavas. We have further expanded the dataset and confirmed the 142Nd excesses in these rocks, implying that their source was depleted and differentiated during the first 100 Ma of Earth’s history. The differentiation event that created this early-depleted reservoir had to also form a complementary enriched component. We have found the first evidence for this reservoir recorded in the 3.4 Ga samples that yielded negative 142Nd anomalies. However, the 3.3 Ga analyzed samples showed no resolvable 142Nd anomalies compared to terrestrial Nd standards. This suggests that the Hadean heterogeneities were likely isolated for more than 1 Ga before being completely rehomogeneized by 3.3 Ga. The remixing by 3.3 Ga could have been achieved by subduction processes due to the onset of modern plate tectonics at ~3.2 Ga, as proposed by other recent studies.L’enregistrement géologique de l’Hadéen et de l’Eoarcheén étant limité, la composition et l’évolution de la Terre silicatée au cours de ces périodes reste très débattue et peu connue. La première évidence d’un événement de différenciation précoce du manteau provient de l’étude du système éteint 146Sm-142Nd. En effet, les excès en 142Nd détectés dans les échantillons âgés de 3.7 milliard d’années (Ga) d’Isua (Sud-Ouest du Groenland) par rapport aux échantillons modernes, impliquent que leur source fût appauvrie en éléments incompatibles et formée durant l’Hadéen. Dans cette thèse nous avons étudié les systèmes isotopiques 146Sm-142Nd, 147Sm–143Nd et 176Lu–176Hf d’une collection d’échantillons mantelliques âgés de 3.8 Ga, 3.7 Ga, 3.4 Ga et 3.3 Ga et provenant de différentes unités géologiques de la région d’Isua. Cette collection d’échantillons couvre ~500 millions d’années de l’histoire précoce terrestre et a permis d’apporter des informations précieuses sur l’évolution de la composition et la dynamique du manteau au début de l’histoire de la Terre. Nous avons démontré un découplage des systèmes isotopiques de longue-vie Sm-Nd et Lu-Hf dans la source des roches âgées de 3.7 Ga, alors que ces deux systèmes sont généralement couplés lors des processus magmatiques de basse pression. Pour expliquer ce découplage, nous proposons que la source des laves d’Isua avait une origine profonde (manteau inférieur). Nous avons également confirmé les excès en 142Nd dans ces échantillons, ce qui implique que leur source a été appauvrie en éléments incompatibles dans les premiers 100 Ma de l’histoire de la Terre. L’évènement responsable de la différenciation du réservoir appauvri aurait également formé un réservoir complémentaire enrichi, enregistré dans les roches de 3.4 Ga. Cependant, les échantillons de 3.3 Ga ne montrent pas d’anomalies en 142Nd par rapport aux standards terrestres. Ceci suggère que les hétérogénéités chimiques hadéennes aient résisté au mélange convectif pendant plus de 1 Ga, puis qu’elles aient disparu et soient remélangées au reste du manteau à 3.3 Ga. Le mélange à 3.3 Ga pourrait aussi s’expliquer par les processus de subduction initiées par la tectonique de plaques récemment proposée comme ayant démarrée autour de 3.2 Ga

Long-term preservation of early formed mantle heterogeneity by mobile lid convection: Importance of grainsize evolution

The style of tectonics on the Hadean and Archean Earth, particularly whether plate tectonics was in operation or not, is debated. One important, albeit indirect, constraint on early Earth tectonics comes from observations of early-formed geochemical heterogeneities: 142Nd and 182W anomalies recorded in Hadean to Phanerozoic rocks from different localities indicate that chemically heterogeneous reservoirs, formed during the first ∼500 Myrs of Earth's history, survived their remixing into the mantle for over 1 Gyrs. Such a long mixing time is difficult to explain because hotter mantle temperatures, expected for the early Earth, act to lower mantle viscosity and increase convective vigor. Previous studies found that mobile lid convection typically erases heterogeneity within ∼100 Myrs under such conditions, leading to the hypothesis that stagnant lid convection on the early Earth was responsible for the observed long mixing times. However, using two-dimensional Cartesian convection models that include grainsize evolution, we find that mobile lid convection can preserve heterogeneity at high mantle temperature conditions for much longer than previously thought, because higher mantle temperatures lead to larger grainsizes in the lithosphere. These larger grainsizes result in stronger plate boundaries that act to slow down surface and interior convective motions, in competition with the direct effect temperature has on mantle viscosity. Our models indicate that mobile lid convection can preserve heterogeneity for ≈0.4–1 Gyrs at early Earth mantle temperatures when the initial heterogeneity has the same viscosity as the background mantle, and ≈1–4 Gyrs when the heterogeneity is ten times more viscous than the background mantle. Thus, stagnant lid convection is not required to explain long-term survival of early formed geochemical heterogeneities, though these heterogeneities having an elevated viscosity compared to the surrounding mantle may be essential for their preservation

Différenciation précoce de la terre silicatée enregistrée dans les roches archéennes d'Isua (Groenland) (implications sur la dynamique du manteau au cours du temps)

L enregistrement géologique de l Hadéen et de l Eoarcheén étant limité, la composition et l évolution de la Terre silicatée au cours de ces périodes reste très débattue et peu connue. La première évidence d un événement de différenciation précoce du manteau provient de l étude du système éteint 146Sm-142Nd. En effet, les excès en 142Nd détectés dans les échantillons âgés de 3.7 milliard d années (Ga) d Isua (Sud-Ouest du Groenland) par rapport aux échantillons modernes, impliquent que leur source fût appauvrie en éléments incompatibles et formée durant l Hadéen. Dans cette thèse nous avons étudié les systèmes isotopiques 146Sm-142Nd, 147Sm 143Nd et 176Lu 176Hf d une collection d échantillons mantelliques âgés de 3.8 Ga, 3.7 Ga, 3.4 Ga et 3.3 Ga et provenant de différentes unités géologiques de la région d Isua. Cette collection d échantillons couvre ~500 millions d années de l histoire précoce terrestre et a permis d apporter des informations précieuses sur l évolution de la composition et la dynamique du manteau au début de l histoire de la Terre. Nous avons démontré un découplage des systèmes isotopiques de longue-vie Sm-Nd et Lu-Hf dans la source des roches âgées de 3.7 Ga, alors que ces deux systèmes sont généralement couplés lors des processus magmatiques de basse pression. Pour expliquer ce découplage, nous proposons que la source des laves d Isua avait une origine profonde (manteau inférieur). Nous avons également confirmé les excès en 142Nd dans ces échantillons, ce qui implique que leur source a été appauvrie en éléments incompatibles dans les premiers 100 Ma de l histoire de la Terre. L évènement responsable de la différenciation du réservoir appauvri aurait également formé un réservoir complémentaire enrichi, enregistré dans les roches de 3.4 Ga. Cependant, les échantillons de 3.3 Ga ne montrent pas d anomalies en 142Nd par rapport aux standards terrestres. Ceci suggère que les hétérogénéités chimiques hadéennes aient résisté au mélange convectif pendant plus de 1 Ga, puis qu elles aient disparu et soient remélangées au reste du manteau à 3.3 Ga. Le mélange à 3.3 Ga pourrait aussi s expliquer par les processus de subduction initiées par la tectonique de plaques récemment proposée comme ayant démarrée autour de 3.2 Ga.The composition and the evolution of the silicate Earth during the Hadean-Eoarchean is widely debated and largely unknown due to the limited geological record. The first undisputable evidence for a very early differentiation of the mantle came from the extinct 146Sm-142Nd chronometer. The 142Nd excesses measured in 3.7 billion year (Ga) old rocks from Isua (Southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean. We have studied 146,147Sm 142,143Nd and 176Lu 176Hf isotope systematics in 3.8 Ga, 3.7 Ga, 3.4 Ga and 3.3 Ga mantle-derived samples from different tectonic domains of the Isua region. This dataset covers ~ 500 million years of the early history of the Earth and revealed precious information about the compositional evolution and dynamics of the early Earth mantle. Combined Lu-Hf and Sm-Nd of the 3.7 Ga Isua samples revealed that these isotopic systematics were decoupled, suggesting a deep-seated source for the Isua lavas. We have further expanded the dataset and confirmed the 142Nd excesses in these rocks, implying that their source was depleted and differentiated during the first 100 Ma of Earth s history. The differentiation event that created this early-depleted reservoir had to also form a complementary enriched component. We have found the first evidence for this reservoir recorded in the 3.4 Ga samples that yielded negative 142Nd anomalies. However, the 3.3 Ga analyzed samples showed no resolvable 142Nd anomalies compared to terrestrial Nd standards. This suggests that the Hadean heterogeneities were likely isolated for more than 1 Ga before being completely rehomogeneized by 3.3 Ga. The remixing by 3.3 Ga could have been achieved by subduction processes due to the onset of modern plate tectonics at ~3.2 Ga, as proposed by other recent studies.CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Iron isotopes trace primordial magma ocean cumulates melting in the Earth’s upper mantle

The differentiation of the Earth ~ 4.5 Ga is believed to have culminated in magma ocean crystallization, crystal-liquid separation and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate due to the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (57Fe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7 Ga metabasalts from the Isua Supracrustal Belt (Greenland). The 57Fe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (182W). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the 57Fe and 182W heterogeneity of modern oceanic basalts.This project was funded by a NERC Consortia Grant (“Mantle Reservoirs, volumes and fluxes” NE/M000303/1

Geochemistry and petrogenesis of the early Archean mafic crust from the Saglek-Hebron Complex (Northern Labrador)

The Saglek-Hebron Complex located in Northern Labrador, Canada, is one of the oldest granite-greenstone terrains on Earth. It is dominated by granitoids as old as 3.9 Ga and several enclaves of supracrustal rocks. We present here the largest whole-rock major and trace element geochemical dataset (over 100 samples)on the mantle-derived rocks from the Saglek-Hebron Complex to constrain their petrogenesis. The mafic amphibolites are metavolcanic rocks basaltic in composition, have tholeiitic affinities, and exhibit relatively homogeneous compositions with flat rare-earth element profiles. They include incompatible element enriched and depleted compositions, both which appear to be related by fractional crystallization of a gabbroic assemblage within volcanic flows. The enriched mafic rocks exhibit higher Ti contents compared to the more depleted mafic rocks, representing, respectively, evolved liquids and pyroxene-rich cumulate-liquid mixtures. Previous work has suggested the occurrence of two distinct supracrustal units in the Saglek-Hebron Complex, the Eoarchean Nulliak assemblage and the Mesoarchean Upernavik assemblage. Here we report that the petrological and geochemical composition of both assemblages is indistinguishable. This suggests either that the Nulliak and Upernavik metavolcanic rocks represent a single unit, or that they were formed by identical processes in a comparable context, despite an age difference of ∼400 Ma. Contrary to some other early metavolcanic belts, such as the Isua supracrustal belt and Nuvvuagittuq greenstone belt, none of the metavolcanic rocks from the Saglek-Hebron Complex studied here exhibit geochemical compositions that could be reminiscent of suprasubduction environments. The Saglek-Hebron Complex also includes two compositionally distinct groups of ultramafic rocks characterized by different Al/Ti ratios, Fe contents and controlled by olivine fractionation with different Fo contents. Ultramafic rocks from both groups appear to be cumulates derived from distinct parental magmas with a komatiitic basalt composition. One of these parental magmas may be genetically linked through fractional crystallization with the mafic metavolcanic rocks, whereas the other group is more difficult, at least geochemically, to relate to the Saglek-Hebron basaltic rocks

146Sm–142Nd systematics measured in enstatite chondrites reveals a heterogeneous distribution of 142Nd in the solar nebula

The short-lived 146Sm–142Nd chronometer (T1/2 = 103 Ma) is used to constrain the early silicate evolution of planetary bodies. The composition of bulk terrestrial planets is then considered to be similar to that of primitive chondrites that represent the building blocks of rocky planets. However for many elements chondrites preserve small isotope differences. In this case it is not always clear to what extent these variations reflect the isotope heterogeneity of the protosolar nebula rather than being produced by the decay of parent isotopes. Here we present Sm–Nd isotopes data measured in a comprehensive suite of enstatite chondrites (EC). The EC preserve 142Nd/144Nd ratios that range from those of ordinary chondrites to values similar to terrestrial samples. The EC having terrestrial 142Nd/144Nd ratios are also characterized by small 144Sm excesses, which is a pure p-process nuclide. The correlation between 144Sm and 142Nd for chondrites may indicate a heterogeneous distribution in the solar nebula of p-process matter synthesized in supernovae. However to explain the difference in 142Nd/144Nd ratios, 20% of the p-process contribution to 142Nd is required, at odds with the value of 4% currently proposed in stellar models. This study highlights the necessity of obtaining high-precision 144Sm measurements to interpret properly measured 142Nd signatures. Another explanation could be that the chondrites sample material formed in different pulses of the lifetime of asymptotic giant branch stars. Then the isotope signature measured in SiC presolar would not represent the unique s-process signature of the material present in the solar nebula during accretion

Combined Nd and Hf isotope evidence for deep-seated source of Isua lavas

International audienceWe present the first combined Sm-146.147-Nd-142,Nd-143 and Lu-176-Hf-176 study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm-Nd and Lu-Hf whole-rock isochrons yield identical ages within error: 3.72 +/- 0.08 and 3.67 +/- 0.07 Ga, respectively. Nd-142 measurements confirm and extend the database of Nd-142 excesses in Isua samples (mu Nd-142 = 7-16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined Sm-146,Sm-147-Nd-142,Nd-143 systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of similar to 4.53, similar to 4.47, and similar to 4.32 Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm-Nd whole-rock isochron is in accordance with Nd-142 results and consistent with a superchondritic initial Nd-143/Nd-144 ratio (epsilon Nd-143(3.7Ga) = +1.41 +/- 0.98). In contrast, the corresponding initial epsilon Hf-176(3.7Ga) = -1.41 +/- 0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm-Nd and Lu-Hf isotope systems display incongruent relative parent-daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies Sm-147-Nd-143, Lu-176-Hf-176, and Nd-142 results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53-4.32 Ga) deep mantle cumulate. Since the similar to 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic Hf-176/Hf-177 (Scherer et al., 2010), dissociation of the Lu-Hf and Sm-Nd isotope systems may not be a ubiquitous feature of the Hadean mantle, but rather reflects that a deep-seated source was involved in the formation of ISB lava