Early Crust Research Articles

Quantitative estimates of impact induced crustal erosion during accretion and its influence on the Sm/Nd ratio of the Earth

Dynamical scenarios of terrestrial planets formation involve strong perturbations of the inner part of the solar system by the giant-planets, leading to enhanced impact velocities and subsequent collisional erosion. We quantitatively estimate the effect of collisional erosion on the resulting composition of Earth, and estimate how it may provide information on the dynamical context of its formation. The composition of the Bulk Silicate Earth (BSE, Earth’s primitive mantle) for refractory and lithophile elements (RLE) should be strictly chondritic as these elements are not affected by volatile loss nor by core formation. However, an excess in 142Nd compared to the 144Nd has been emphasized in terrestrial samples compared to most measurements in chondrites. In that case, the Samarium/Neodymium (Sm/Nd) ratio could be roughly 6% higher in the BSE than in chondrites, as suggested from the 146Sm/142Nd isotope system (Boyet and Carlson, 2005). This proposed chemical offset could be the consequence of preferential collisional erosion of the crust during the late stages of Earth’s accretion, leaving a BSE enriched in Sm due to its lower incompatibility compared to Nd (O’Neill and Palme, 2008; Boujibar et al., 2015; Bonsor et al., 2015; Carter et al., 2015, 2018). However, if the present 142Nd of the BSE arises from nucleosynthetic heterogeneities within the protoplanetary disk (Burkhardt et al., 2016; Bouvier and Boyet, 2016; Boyet et al., 2018), then the BSE has no excess in Sm compared to Nd and this hypothesis precludes any significant loss of relatively Nd-enriched component early in the Solar System. Here, we simulate and quantify the erosion of Earth’s crust in the context of Solar System formation scenarios, including the classical model and Grand Tack scenario that invokes orbital migration of Jupiter during the gaseous disk phase (Walsh et al., 2011; Raymond et al., 2018). We find that collisional erosion of the early crust is unlikely to explain the proposed superchondritic Sm/Nd ratio of the Earth for most simulations. Only Grand Tack simulations in which the last giant impact on Earth occurred later than 50 million years after the start of Solar System formation can account for this Sm/Nd ratio. This time frame is consistent with current cosmochemical and dynamical estimates of the Moon forming impact (Chyba, 1991; Walker, 2009; Touboul et al., 2007, 2009, 2015; Pepin and Porcelli, 2006; Norman et al., 2003; Nyquist et al., 2006; Boyet et al., 2015). However, such a late fractionation in the Sm/Nd ratio is unlikely to be responsible for a 20-ppm 142Nd excess in terrestrial rocks due to the half life of the radiogenic system. Additionally, such a large and late fractionation in the Sm/Nd ratio would accordingly induce non-observed anomalies in the 143Nd/144Nd ratio. Considering our results, the Grand Tack model with a late Moon-forming impact cannot be easily reconciled with the Nd isotopic Earth contents.

Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa

The nature of Earth's earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism.

Lu–Hf Isotope Composition of Zircon and Magma Sources of the Vendian–Early Paleozoic Granitoids in Tuva (by the Example of the Kaa-Khem and East Tannu-Ola Batholiths)

Abstract —We present results of geochemical and Sr–Nd isotope studies of rocks and of local dating and determination of the Lu–Hf isotope composition of zircons from late Vendian–early Cambrian and Cambrian–Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the εHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian–early Paleozoic intrusive associations formed at the island arc and accretion–collision stages. Late Vendian (572–562 Ma, Kopto and Buren massifs) and early Cambrian (522–518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian–early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro–monzodiorite–granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion–collision stage. Island arc complexes were the main source of Cambrian–Ordovician accretion–collision calc-alkalic plagiogranitoids (500–450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva–Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren’ massif (450 Ma), marking the completion of accretion–collision processes in this region. The isotope parameters of the Vendian–early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion–collision processes.

Textural, geochemical, and isotopic data from silicified rocks and associated chemical sedimentary rocks in the ~ 2.7 Ga Abitibi greenstone belt, Canada: Insight into the role of silicification

Silica-rich Precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust. Because these rocks are important geochemical and paleontological archives, we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving Archean silica cycle. Here, we investigate a series of sub- to greenschist facies Si-rich Archean rocks from the ~ 2.7 Ga Abitibi greenstone belt, Canada, that represent chemical sedimentary rocks and rocks formed via silica-addition through the process of silicification. We report data for major and trace element geochemistry, multi-crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry, and texture-specific silicon isotope values measured using secondary ion mass spectrometry on Neoarchean chemical sedimentary rocks, their silicified equivalents, and associated silicified volcanic rocks. We find that in such a well-preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin, distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification. Chemical sedimentary rocks display a wide range of 30Si-depleted silicon isotopes values that vary with stratigraphy similar to other Archean iron formation. Silicified volcanic rocks possess 30Si-enriched values, similar to Archean silicified basalts. We conclude that because silicon isotope values of iron formation shift toward 30Si-enriched values up stratigraphy, basinal changes in the composition of the silicon isotope reservoir may be preserved. Silicon isotope values of silicified volcanic rocks by contrast, likely represent precipitation from an isotopically heavy silicon reservoir, influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock (basalt or andesite). Overall, we confirm that Neoarchean silicified rocks are 30Si-enriched like their Paleoarchean counterparts.

Inclusions in impact-formed zircon as a tracer of target rock lithology: Implications for Hadean continental crust composition and abundance

Impacts play an essential role in the formation and possible habitability on early Earth and are the dominant mechanism controlling surface geomorphology on many solar system bodies. Without a significant >4 Ga rock record, geochemical variations and micron-scale inclusions within detrital Hadean zircon (Earth's oldest material - primarily from the Jack Hills, Western Australia) have provided valuable insights into the near-surface conditions of this crucial period in Earth's evolution. We report primary inclusion mineralogy of impact-formed zircon and the host impact melt from the Sudbury and Morokweng impact structures with estimates of target rock lithology to probe the composition of Earth's earliest crust given the possibility that Hadean grains were created in ancient impacts. Applying the theory of uniformitarianism between preserved and ancient impactites and utilizing geochemical proxies, we highlight the compositional evolution of an impact melt from a given target rock and the associated primary inclusion assemblages. Whole-rock X-ray Fluorescence and thick-section analyses coupled with primary inclusion populations from known terrestrial impact-formed zircon yield proportional mineralogy of the impact melt sheet from which they crystallized. Additionally, target rock lithology consistently plots within an area on a Total Alkali Silica (TAS) diagram centered amongst the granophyre and noritic units of an impact melt. Deducing similar trends of the impact melt mineralogy combined with TAS and Quartz-Alkali-Plagioclase ternary diagram melt pathways, we infer that if Hadean zircons from Jack Hills were impact-formed, the target rock lithology (i.e. Earth's earliest crust) would be between the composition of quartz-rich granitoid and tonalite-trondhjemite-granodiorite.

Mineral chemistry of high-Al chromian spinel from ultramafic rocks of the Babina–Prithvipur transect, Bundelkhand Craton, central India: Implication for petrogenesis and tectonic setting

Bundelkhand Craton is an important Archaean cratonic nuclei of the Central Indian Shield and comprises two greenstone complexes, i.e., the Babina–Mauranipur Greenstone Belt and the Girar–Madawara Greenstone Belt. The E–W trending Babina–Mauranipur Greenstone Belt in the central part of the craton, encloses several isolated lensoid shaped ultramafic bodies which has suffered various degrees of alteration and metamorphism. As per modal mineral analysis, the ultramafic rocks of the Babina–Prithvipur section belong to harzburgite and also contain high-Al chromian spinel along with olivine, ortho-pyroxene and amphibole. Mineral chemistry reveals that the spinels are of high Al and Cr poor variety, where Al2O3 and Cr2O3 contents range from 40.06 to 54.34 wt.% and 9.05 to 14.89 wt.%, respectively. The TiO2 content is extremely low (average ≈0.07 wt.%). The Cr# value of the spinel is <0.2, whereas Mg# ranges from 0.495 to 0.633. The forsterite contents of the olivine ranges from 86.088 to 88.105 wt.%. Average CaO and NiO contents of the olivine stand 0.03 and 0.24 wt.%, respectively. Composition of the analyzed ortho-pyroxene belongs to En84.20–87.75Wo0.15–0.39 with low CaO content of 0.080 to 0.207 wt.%. As per mineral chemistry, these harzburgite rocks of the Babina–Prithvipur section belong to mantle peridotite. Melt calculation for the spinel also suggests a least differentiated magmatic product, which is also supported by the olivine spinel mantle array diagram as all the samples are plotted within the mantle array field very close to the fertile mantle source. Low TiO2 and high Al contents of spinel also reflect the MORB type peridotite characteristics for these ultramafic rocks which probably originated from least differentiated plagioclase free mantle derived harzburgite/lherzolitic magma in a rift related spreading centre. As a whole these ultramafic rocks appear to be the remnant of the early crust that existed during the Archaean time.

Modification of the composition and density of Mercury from late accretion

Late accretion is a process that strongly modulated surface geomorphic and geochemical features of Mercury. Yet, the fate of the impactors and their effects on Mercury's surface through the bombardment epoch are not clear. Using Monte Carlo and analytical approaches of cratering impacts, we investigate the physical and thermodynamical outcomes of late accretion on Mercury. Considering the uncertainties in late accretion, we develop scaling laws for the following parameters as a function of impact velocity and total mass of late accretion: (1) depth of crustal erosion, (2) the degree of resurfacing, and (3) mass accreted from impactor material. Existing dynamical models indicate that Mercury experienced an intense impact bombardment (a total mass of ~8 × 1018 − 8 × 1020 kg with a typical impact velocity of 30 − 40 km s−1) after 4.5 Ga. For this parameter range, we find that late accretion could remove 50 m to 10 km of the early (post-formation) crust of Mercury, but the change to its core-to-mantle ratio is negligible. Alternatively, the mantles of putative differentiated planetesimals in the early solar system could be more easily removed by impact erosion and their respective core fraction increased, if Mercury ultimately accreted from such objects. Although the cratering is notable for erasing the older geological surface records on Mercury, we show that ~40 − 50wt. % of the impactor's exogenic materials, including the volatile-bearing materials, can be heterogeneously implanted on Mercury's surface as a late veneer (at least 3 × 1018 − 1.6 × 1019 kg in total). About half of the accreted impactor's materials are vaporized, and the rest is completely melted upon the impact. We expect that the further interplay between our theoretical results and forthcoming surface observations of Mercury, including the BepiColombo mission, will lead us to a better understanding of Mercury's origin and evolution.

Archean lithospheric differentiation: Insights from Fe and Zn isotopes

Abstract The Archean continental lithosphere consists of a dominantly felsic continental crust, made of tonalite-trondhjemite-granodiorite (TTG) and subordinate granitoids, and a cratonic lithospheric mantle, made of highly refractory peridotites. Whether they stemmed from the same process of differentiation from the primitive mantle, or were two distinct components that were physically juxtaposed, remains debated. Metal stable isotope ratios are sensitive to magmatic and metamorphic processes and do not evolve with time. Therefore, stable isotope ratios are complementary to radiogenic isotope ratios, and they allow direct comparisons to be made between different terrestrial components without age corrections. Isotopes of iron and zinc, metals ubiquitous in Earth’s lithosphere, can be tracers of lithospheric formation and evolution because they are affected by partial melting (Fe, Zn), redox state (Fe), and the presence of sulfides (Fe, Zn). Here, using stable Fe and Zn isotopic data from Archean samples of the lithospheric mantle and the continental crust, we show that Fe and Zn isotopes define a linear array, best explained by their coupled fractionation behavior during magmatic processes. Our data show that high degrees of partial melting (&amp;gt;30%) during the formation of the cratonic mantle and mafic protocrust, and reworking of the early crust significantly fractionate Fe and Zn isotopes. Conversely, Fe and Zn isotope ratios in the TTG are similar to those in Archean mafic rocks, suggesting an origin by fractional crystallization of basalt, and implying limited Fe and Zn isotopic fractionation, instead of partial melting of mafic crust. Moreover, the absence of Fe and Zn isotope decoupling due to redox effects, melt (fluid)–rock or sediment-rock interaction, and decarbonation indicates that subduction, at least as we understand it now, is not required to explain the Fe and Zn isotope composition of the Archean lithosphere.

Early crustal evolution of the Superior craton – A U–Pb, Hf and O isotope study of zircon from the Assean lake complex and a comparison to early crust in other cratons

We report U–Pb/Hf/O isotopic measurements of zircon extracted from meta-igneous rocks exposed in the Assean Lake crustal complex, northwestern Superior craton (Canada). With crystallization ages of ca. 3.22 Ga, zircon from the Assean Lake crustal complex are among the oldest reported in the Superior craton. The Hf and O isotope compositions of zircon indicate that these meta-igneous rocks derive from a mantle component with a relatively primitive Hf isotope composition, similar to chondrites, that was uncontaminated by elevated δ18O material (εHf,initial ~ 0 and δ18OVSMOW ~ 5.5‰). These features are shared by the large majority of the zircon extracted from the oldest (meta)-igneous rocks exposed in Archean terranes. Positive εHf deviations in Archean zircons are typically observed in magmas that post-date the oldest magmatic activity in a given terrain. This temporal variability in magma source regions may reflect a sequence of events comprising: (i) felsic magmatism derived from undepleted mantle components followed by, (ii) re-melting of the same mantle component producing the depleted Hf signatures in zircon and then, (iii) accretion of the proto-craton and reworking of the whole-system enabling stabilization.

Inhalable formulations of rifampicin by spray drying of supersaturated aqueous solutions

Tuberculosis is still one of the leading causes of death from a single infectious agent (i.e. Mycobacterium tuberculosis). First line therapy includes per oral administration of high doses of rifampicin over several months and is often times accompanied by the occurrence of unwanted side effects that might limit the patient’s adherence to the therapy. Thus, local antibiotic treatment at the site of infection i.e. the lungs is desirable. Amongst other approaches, spray drying of solutions of rifampicin has been shown as suitable method to produce respirable dry powders. In this work, we present inhalable formulations manufactured via spray drying of aqueous solutions of rifampicin. Powders manufactured were characterized for their aerodynamic and solid state properties, as well as their physical and chemical stability. The main focus of this study was to investigate the mechanism of particle formation using an acoustic levitator. Fine particle fractions of the test formulations ranged from 80 to 89% whereas a reference formulation (a spray dried isopropyl alcoholic solution of rifampicin) showed a lower fine particle fraction of 37%. Acoustic levitator and surface tension experiments showed that interfacial properties of rifampicin lead to early crust formation upon drying of the droplets, which eventually decoupled from the liquid core and formed highly collapsed, low apparent density powders with excellent aerosol properties.

Early crust building enhanced on the Moon’s nearside by mantle melting-point depression

The Moon’s Earth-facing hemisphere hosts a geochemically anomalous region, the Procellarum KREEP Terrane, which is widely thought to have provided radiogenic heat for mantle melting from ~3.9 to ~1 billion years ago. However, there is no agreement on such a link between this region and the earliest pulse of post-differentiation crust-building magmatism on the Moon at ~4.37 billion years ago; whether this early magmatism was global or regional has been debated. Here we present results of high-temperature experiments that show the nearside geochemical anomaly may have caused asymmetric early crust building via mantle melting-point depression. Our results demonstrate that the anomalous enrichment in incompatible elements of this nearside reservoir dramatically lowers the melting temperature of the source rock for these magmas and may have resulted in 4 to 13 times more magma production under the nearside crust, even without any contribution from radioactivity. From thermal numerical modelling, we show that radiogenic heating compounds this effect and may have resulted in an asymmetric concentration of post-magma-ocean crust building on the lunar nearside. Our findings suggest that the nearside geochemical anomaly has influenced the thermal and magmatic evolution of the Moon over its entire post-differentiation history. Early magmatism on the Moon’s nearside may have been enhanced by a geochemical anomaly lowering the melting point of the mantle source region, according to high-temperature experiments and thermal numerical modelling.

An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean

The composition and origin of Earth’s early crust remains hotly debated. Here we use partition coefficients to invert the trace element composition of 4.3–3.3 Gyr Jack Hills zircons to calculate the composition of the melts from which they crystallised. Using this approach, the average SiO2 content of these melts was 59 ± 6 wt. % with Th/Nb, Dy/Yb and Sr/Y ratios of 2.7 ± 1.9, 0.9 ± 0.2 and 1.6 ± 0.7, respectively. Such features strongly indicate that the protolith for the Jack Hills zircons was not an intra-plate mafic rock, nor a TTG (tondjhemite-tonalite-granodiorite) or a Sudbury-like impact melt. Instead, the inferred equilibrium melts are much more similar to andesites formed in modern subduction settings. We find no evidence for any secular variation between 4.3 and 3.3 Gyr implying little change in the composition or tectonic affinity of the Earth’s early crust from the Hadean to Mesoarchaean.

A mushy Earth's mantle for more than 500 Myr after the magma ocean solidification

SUMMARYIn its early evolution, the Earth mantle likely experienced several episodes of complete melting enhanced by giant impact heating, short-lived radionuclides heating and viscous dissipation during the metal/silicate separation. After a first stage of rapid and significant crystallization (Magma Ocean stage), the mantle cooling is slowed down due to the rheological transition, which occurs at a critical melt fraction of 40–50%. This transition first occurs in the lowermost mantle, before the mushy zone migrates toward the Earth's surface with further mantle cooling. Thick thermal boundary layers form above and below this reservoir. We have developed numerical models to monitor the thermal evolution of a cooling and crystallizing deep mushy mantle. For this purpose, we use a 1-D approach in spherical geometry accounting for turbulent convective heat transfer and integrating recent and solid experimental constraints from mineral physics. Our results show that the last stages of the mushy mantle solidification occur in two separate mantle layers. The lifetime and depth of each layer are strongly dependent on the considered viscosity model and in particular on the viscosity contrast between the solid upper and lower mantle. In any case, the full solidification should occur at the Hadean–Eoarchean boundary 500–800 Myr after Earth's formation. The persistence of molten reservoirs during the Hadean may favor the absence of early reliefs at that time and maintain isolation of the early crust from the underlying mantle dynamics.

Tungsten isotopic constraints on homogenization of the Archean silicate Earth: Implications for the transition of tectonic regimes

The short-lived 182Hf-182W system has been used to constrain the early Earth’s accretion and differentiation. This paper reports high precision W isotopic compositions and trace element concentrations for Archean (4.0–3.0 Ga) intermediate to felsic rocks from the Slave, North China, and Kaapvaal cratons to discuss early Earth differentiation and mantle mixing processes. The 4.0–3.8 Ga diorite and trondhjemite samples from the Acasta Gneiss Complex (AGC) in the Slave Craton and Anshan Complex in the North China Craton have positive μ182W values ranging from 8.3 ± 3.6 to 14.5 ± 4.0, whereas the 3.36–2.95 Ga TTG rocks from the North China Craton have a uniform modern mantle-like μ182W value of 0.7 ± 3.9 (2 SD, n = 11); with one exception, the 3.32 Ga old sample F28-2, which has a positive μ182W anomaly (7.3 ± 3.9 and 13.0 ± 3.2 for two individual analyses). The Earth’s oldest potassic granites, occurring as conglomerates in the Moodies Group, an intrusive trondhjemite, and an amphibolite with ages of ~3.55 Ga in the Kaapvaal Craton exhibit W isotopic compositions indistinguishable from what is proposed for the modern mantle. The positive 182W anomalies in the 4.0–3.8 Ga rocks could possibly be the result of either early mantle differentiation that occurred within the lifetime of 182Hf or a partial lack of late accreted material. The signature of 182W excess in the 3.32 Ga sample (F28-2) is inherited from earlier crust, presumably similar to the 3.8 Ga TTGs, either by melting or crustal contamination. The transition in μ182W values from positive to near-zero would indicates that a significant event occurred at ∼3.6 Ga which caused efficient mixing of early differentiated mantle and late accreted material, possibly indicating the transition of tectonic regimes from plume to plate tectonics on the Earth.

Evolution of the Ca isotopic composition of the mantle

Calcium stable isotopes are mainly fractionated during low-temperature geochemical processes and have, therefore, the potential to trace weathering and incorporation of recycled carbonated material in the mantle. However, behavior of Ca isotopes during mantle processes over Earth's history is not well understood. In this study, we present Ca isotopic compositions of 27 spinifex-textured and olivine cumulate komatiites ranging in age from 2.41 to 3.55 Ga. The Ca isotopic composition of all spinifex textured komatiites analyzed is similar, with an average δ44/40Ca = 0.92 ± 0.16‰ (n = 7; 2SD) (permil deviation of the 44Ca/40Ca ratio from SRM915a). This value is identical, within error, to the previous estimates of the composition of the mantle. In contrast, olivine cumulates from the Weltevreden Formation in the Barberton Greenstone Belt are isotopically heavier compared to the rest of the komatiites studied. While igneous processes and sea-floor alteration unlikely account for this Ca isotopic signature, crystallization of an early crust from a global magma ocean is a possibility, in agreement with previously obtained O, Os, Nd, Hf isotopic data and trace elements systematics.

A new cache of Eoarchaean detrital zircons from the Singhbhum craton, eastern India and constraints on early Earth geodynamics

The dominant geodynamic processes that underpin the formation and evolution of Earth's early crust remain enigmatic calling for new information from less studied ancient cratonic nuclei. Here, we present U–Pb ages and Hf isotopic compositions of detrital zircon grains from ∼2.9 Ga old quartzites and magmatic zircon from a 3.505 Ga old dacite from the Iron Ore Group of the Singhbhum craton, eastern India. The detrital zircon grains range in age between 3.95 Ga and 2.91 Ga. Together with the recently reported Hadean, Eoarchean xenocrystic (up to 4.24 Ga) and modern detritus zircon grains from the Singhbhum craton, our results suggest that the Eoarchean detrital zircons represent crust generated by recycling of Hadean felsic crust formed at ∼4.3–4.2 Ga and ∼3.95 Ga. We observe a prominent shift in Hf isotope compositions at ∼3.6–3.5 Ga towards super-chondritic values, which signify an increased role for depleted mantle and the relevance of plate tectonics. The Paleo-, Mesoarchean zircon Hf isotopic record in the craton indicates crust generation involving the role of both depleted and enriched mantle sources. We infer a short-lived suprasubduction setting around ∼3.6–3.5 Ga followed by mantle plume activity during the Paleo-, Mesoarchean crust formation in the Singhbhum craton. The Singhbhum craton provides an additional repository for Earth's oldest materials.

Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.

The Exotic Inim Block of the Argun Superterrane of the Central Asian Fold Belt: Results of U–Th–Pb Geochronological (LA-ICP-MS) and Sm–Nd Isotopic–Geochemical Studies

The results of studies indicate that the age of the protoliths of garnet-bearing biotite–sericite–muscovite schists of the Inim Block is <991 Ma, and they are derived from rocks of the Neo-, Meso-, and Paleoproterozoic (as well as Archean) crust. This suggests that the Inim Block is exotic relatively to the Argun Superterrane due to the formation of the protolith of its metasedimentary rocks largely from erosion products of the Early Precambrian continental crust, the presence of which within the Argun Superterrane is not proven. It is not excluded that the Inim Block is a fragment of the Dzhugdzhur–Stanovoi Superterrane implanted in the structure of the Argun Superterrane as a result of Mesozoic tectonic events.

The structure of the Western and Central parts of the Ukrainian schield. Controversial issues

Some controversial issues of composing the Scheme of the fault megablock tectonics of the western half of the Ukrainian shield in connection with the decision of the Interdepartmental Tectonic Committee of Ukraine on drawing up the Tectonic map of the Ukrainian shield at a scale of 1:500 000 have been considered. Geophysical data must be more fully taken into account and used in detail and objectively reflecting both the surface and the deep structure of the Earth’s crust of the shield while drawing up such a Scheme. The latest maps of anomalous magnetic and gravitational fields, a map of the topography of the Moho surface and other materials are given, the analysis of which gives grounds for a number of conclusions on specific structures of the western and central parts of the region. It has been established that the formation of a typical megablock structure of the Ukrainian shield began not earlier than in the Neoarchean. It is difficult unambiguously to establish and trace the investigated Archean fault zones in the part of the shield at the present time, and sometimes practically it is almost impossible, since the Early Proterozoic processes of regional granitization and dynamometamorphism radically changed the composition and structure of the earth’s crust. Geophysical data, supported by geological and tectonophysical observations, clearly record the Proterozoic fault zones, which began to form after 2.5 billion years ago. They cross and deform both the Archean and the Early Proterozoic granitoid complexes, accompanied by intensive diffusion, metasomatism and ore formation. It is proved that the Archean-Early Proterozoic structural-formation complexes (SFC), mapped within the shield, cannot and should not be artificially linked with the Proterozoic fault-megablock structure, which was formed at the end of the Early Proterozoic and superimposed on the SFC. It seems like the dubious and outdated thesis about the subhorizontal-floor structure of the Early Precambrian crust of the Ukrainian shield, according to which the structural floors of the crust are formed one above another by stratometamorphic complexes characterized by a directly proportional relationship of age and the degree of regional metamorphism of the rocks. This concept does not take into account the significant role of horizontal motions of lithospheric blocks, which cause the redistribution and localization of PT conditions in the earth’s crust.

Geochemistry of Paleo to Mesoproterozoic Metasedimentary Units of Chandil Formation, North Singhbhum Crustal Province: Implications for Provenance and Source Area Weathering

ABSTRACT The Mesoproterozoic metasedimentary units of Chandil Formation belong to the northern part of Singhbhum crustal province, eastern India. The Formation lies north of Dalma volcano-sedimentary belt and is separated from the Meso-Neoproterozoic Chhotanagpur granite gneissic complex (CGGC) by Tamar-Porapahar shear zone (TPSZ), i.e. South Purulia shear zone, (SPSZ) at its northern boundary. The metasedimentary unit comprises of quartz-mica-sericite-schist, quartzite, amphibolites and carbonaceous phyllites, which have not been studied well, so far in terms of their geochemistry, source rock and provenance characterization. In this work, the existing gaps in all those aspects are studied to infer the provenance of these meta-sedimentary sequences. The high silica contents of the Chandil Formation (∼95% for the quartzites and ∼70% for the metapelites) and low trace element concentrations indicate silica-rich source. The REE patterns of the quartzites and the metapelites exhibit LREE enrichment and HREE depletion with moderate negative Eu-anomalies which strongly favor felsic nature of the provenance. The significant enrichment of LREE, flat HREE patterns and negative Eu-anomalies of the metapelites suggest their derivation from an old upper continental crust, which is composed of felsic components and the negative Eu-anomalies indicate intra-crustal differentiation. The higher concentrations of the HFSE i.e. Zr, Hf and Ta, within the Chandil metasediments relative to those of EPC (Early Proterozoic Continental Crust), also suggests a felsic provenance. The geochemical signatures of the Chandil metasediments indicate that the sediments were derived during a prolonged period of weathering due to the slow upliftment and unroofing of the southern evolving Singhbhum granitic complex (SGC).