Search

The paper presents new original data on the Devonian felsic volcanism of the NW Rudny Altai (Russia) in the west of Central Asian Orogenic Belt (CAOB) – the front part of the Altai convergent margin of the Siberian continent. Two geochemical types of subvolcanic rhyolites were emplaced synchronously with the bimodal rhyolite-basalt association, which began to form in the end-Emsian, and clearly manifested on the border of the Givetian and the Frasnian. The rhyolites yield zircon U-Pb ages of ca. 390 Ma (R1-type) and 380 Ma (R2- and R3-types), reflecting two peaks of the volcanic activity. Most of these rocks have extreme petrochemical characteristics of high SiO2 contents and have contrast Na/K ratios. Their compositions are transition between calc-alkaline and tholeiite series: (La/Yb)n ~ 2–7, Zr/Y ~ 4 (Zr 350 ppm), Zr (>250 ppm), and high Ga/Al (>2.6) values. The island-arc-like R1-rhyolite formed immediately after the beginning of rifting due to widespread crustal melting under reduced conditions. The generation of rift-like R2- and R3-rhyolites took place under non-equilibrium conditions, synchronously with the rise in the upper crust of Givetian-Frasnian basic magmas, as a result of the active lithospheric extension and high thermal input from the underlying hot mantle. We propose an extension regime in the transition area between the island-arc and back-arc basin for the origin of rhyolites. The study of the Devonian volcanism of the Rudny Altai gives important information about the processes that occurred at the initial stage of the formation of the Altai convergent margin.

The Ediacaran to Cambrian periods are characterized by the diversification and extinction of metazoans, carbon-cycle perturbation, and ocean oxygenation. Ocean nutrient cycles play a major role in the primary production and productivity-driven shallow water anoxia that was responsible for mass extinction in some Phanerozoic intervals. However, the relationships between these cycles and primary production and biological events during the Ediacaran to Cambrian periods have not previously been well-explored. This study presents stratigraphic profiles of nitrogen and organic carbon isotopes for sections belonging to the shelf and deep basin regions of the Yangtze Platform, South China. The nitrogen isotope profiles for these two regions share a similar trend, showing 1) relatively stable δ15N values of 5–6‰ in the early–middle Ediacaran (635 Ma–ca. 560 Ma) and 2) a shift towards lower δ15N values (3–4‰) since the Shuram δ13Ccarb Excursion. The δ15N values in the shelf section experienced a two-stage decrease from the late Ediacaran to Cambrian Stage 2 of the Terreneuvian series (down to −1‰) and then an increase from −1‰ to +4‰ across the Cambrian Stage 2/3 boundary (i.e., the Terreneuvian/Cambrian Series 2 boundary at 521 Ma). These data, in combination with previously reported nutrient profiles (δ15N, P, and Mo), allow the following new conclusions to be drawn. 1) The early–middle Ediacaran ocean was characterized by N Mo colimitation of primary production on a geological timescale that is different from the P limitation on long-term primary production in the modern ocean. 2) A shift in the ultimate limiting nutrient from N and Mo to P likely occurred in late Ediacaran–early Cambrian marine systems. This shift may have been a prerequisite for the extinction of small shelly fossils in response to the development of photic zone anoxia for several million years in Cambrian Stage 2, which was followed by the third and most prominent phase of metazoan diversification.

The paper reports the results of geological, geochemical, and isotope–geochronological studies of subvolcanic rhyolites of NW Gorny Altai and Rudny Altai, which belong to the large Hercynian volcanic systems: Altai-Minusa and Altai–Salair, respectively. U–Pb zircon dating revealed two age groups: ~410–405 and 390–381 Ma. The isotope–geochemical characteristics of the rhyolites demonstrate high eNd(Т) from +2.6 to 6.0 with relatively young model ages T(DM) = 851–966 Ma in Rudny Altai and older model ages, up to 1266 Ma, in NW Gorny Altai. The rhyolites show transitional geochemical signatures between within-plate and island-arc felsic magmas. The results obtained are consistent with two-stage evolution of volcanism and its migration from the continent to the ocean.

The article presents new geological, geochronological, mineralogical, geochemical, and isotopic data on the Early Cretaceous granitic rocks of the southern part of the Zhuravlevka Terrane (Sikhote-Alin). It is shown that four intrusive complexes containing significant amounts of granitic rocks were formed almost simultaneously in this area in the Early Cretaceous (about 100 Ma). These magmatic associations differ in rock set, their mineralogical characteristics, and chemical composition, varying from medium-potassium tonalites and granodiorites depleted in incompatible elements to shoshonitic monzonitic rocks enriched in HFSE and REE. The geochemical and isotopic characteristics of the granitic rocks indicate that the source of their melts was dominated by essentially juvenile metabasite crust with a limited contribution of the upper-crustal metasedimentary rocks. The diversity of geochemical types of the granitic rocks is explained by variable metabasite and metapelite contributions to their source, upper crustal contamination during magma ascent, as well as the variable contribution of the mantle source and different mechanisms of mantle–crust interaction.

The metamorphic P–T conditions and processes of the mid-Archean Barberton Greenstone Belt (BGB), South Africa were examined with petrological and thermodynamic data from metamorphosed banded iron formation (BIF) and garnet amphibolite in the Inyoni shear zone (ISZ). Equilibrium mineral assemblages in these two rocks range from Grt + Cpx + Gru + Mag + Qz through Grt + Cpx + Gru + Amp + Mag + Qz to Amp + Gru + Mag + Qz, and from Grt + Cpx + Amp + Pl + Ep + Qz to Cpx + Amp + Pl + Ep + Qz, respectively. The metamorphic P–T conditions of these assemblages were estimated from thermodynamic calculations combined with their whole-rock and constituent mineral compositions. The results show that the metamorphosed BIF and garnet amphibolite samples record three and two different metamorphic P–T conditions, respectively, within a single period of metamorphism. The P–T conditions of the BIF changed from ca. 600–680 °C and >10 kbar through ca. 510–540 °C and 8–11 kbar to less than 500 °C and 4 kbar. The P–T conditions of the garnet amphibolite changed from ca. 9.5–10.5 kbar and 620–660 °C to less than 5 kbar and 500 °C. These new results combined with previously published data suggest that the geothermal gradients of the peak and retrograde metamorphisms were ca. 15–20 °C/km and 20–30 °C/km, respectively, and the possibility that the ISZ underwent slightly earlier prograde metamorphism than the high-pressure BGB. The difference between the gradients of 15–20 °C/km and 20–30 °C/km in the ISZ rocks was probably related to a shift from oceanic plate subduction to continental collision at ca. 3.23 Ga.

The Ediacaran–Cambrian transition is characterized by numerous events such as the emergence of large multi-cellular metazoans and surface environmental disturbances. Based on geological evidence, it has been proposed that this transition coincided with the increase in the atmospheric oxygen level that was key to the evolution of life. Even though ancient redox conditions can be inferred from the composition of sedimentary iron mineral species, this method is not necessarily applicable to all rocks. In the Earth system, the cycling of iron is of considerable interest owing to its sensitivity to redox conditions. Information regarding the paleo-oceanic iron cycle is revealed in the iron isotopic composition of iron-bearing minerals. Unfortunately, only limited iron isotopic data exists for Ediacaran- to Cambrian-period oceans. To circumvent this deficiency, we drilled a fossiliferous Ediacaran to Early Cambrian sedimentary succession in the Three Gorges region, South China. We analyzed the iron isotope ratios (δ56/54Fe) of pyrite grains in the drill cores using laser ablation multi collector inductively coupled plasma mass spectrometry. The results demonstrate large variations in δ56/54Fe, from −1.6 to 1.6‰, and positive iron isotope ratios are observed in many successions. The presence of positive δ56/54Fe in pyrite indicates that the ferrous iron in the seawater was partially oxidized, suggesting that seawater at Three Gorges was ferruginous during the Ediacaran and Early Cambrian periods. However, aggregated pyrite grains in organic carbon-rich black shales at Member 4 of the Doushantuo Formation and the base of the Shuijingtuo Formation yield near-zero δ56/54Fe values; this suggests that the ocean was transiently dominated by sulfidic conditions during these periods. Notably negative δ56/54Fe values, lower than −1‰, can be interpreted as a signature of DIR. The DIR also might contribute in part to the re-mineralization of organic matter during the largest negative carbon isotope anomaly in the Ediacaran.

We determined highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) concentrations and 187Os/188Os ratios for ultramafic rocks distributed over the Eoarchean gneiss complex of the Saglek-Hebron area in northern Labrador, Canada in order to constrain to what extent variations in HSE abundances are recorded in Early Archean mantle that have well-resolved 182W isotope anomalies relative to the present-day mantle (∼+11 ppm: Liu et al., 2016). The samples analysed here have been previously classified into two suites: mantle-derived peridotites occurring as tectonically-emplaced slivers of lithospheric mantle, and metakomatiites comprising mostly pyroxenitic layers in supracrustal units dominated by amphibolites. Although previous Sm-Nd and Pb-Pb isotope studies provided whole-rock isochrons indicative of ∼3.8 Ga protolith formation for both suites, our whole-rock Re-Os isotope data on a similar set of samples yield considerably younger errorchrons with ages of 3612 ± 130 Ma (MSWD = 40) and 3096 ± 170 Ma (MSWD = 10.2) for the metakomatiite and lithospheric mantle suites, respectively. The respective initial 187Os/188Os = 0.10200 ± 18 for metakomatiites and 0.1041 ± 18 for lithospheric mantle rocks are within the range of chondrites. Re-depletion Os model ages for unradiogenic samples from the two suites are consistent with the respective Re-Os errorchrons (metakomatiite TRD = 3.4–3.6 Ga; lithospheric mantle TRD = 2.8–3.3 Ga). These observations suggest that the two ultramafic suites are not coeval. However, the estimated mantle sources for the two ultramafics suites are similar in terms of their broadly chondritic evolution of 187Os/188Os and their relative HSE patterns. In detail, both mantle sources show a small excess of Ru/Ir similar to that in modern primitive mantle, but a ∼20% deficit in absolute HSE abundances relative to that in modern primitive mantle (metakomatiite 74 ± 18% of PUM; lithospheric mantle 82 ± 10% of PUM), consistent with the ∼3.8 Ga Isua mantle source and Neoarchean komatiite sources around the world (∼70–86% of PUM). This demonstrates that the lower HSE abundances are not unique to the sources of komatiites, but rather might be a ubiquitous feature of Archean convecting mantle. This tentatively suggests that chondritic late accretion components boosted the convecting mantle HSE inventory after core separation in the Hadean, and that the Eoarchean to Neoarchean convecting mantle was depleted in its HSE content relative to that of today. Further investigation of Archean mantle-derived rocks is required to explore this hypothesis.

We describe here Sinaster petalon gen. et sp. nov., a new embryonic form from the c. 535 million-year-old Kuanchuanpu Formation of South China (Ningqiang, Shaanxi Province). The excellent three-dimensional, phosphatic preservation of these microfossils allowed us to use x-ray microtomographic techniques to make accurate reconstructions of their internal structures and to compare their anatomy point-by-point with that of extant cnidarians and other animal groups. Sinaster petalon has anatomical features typical of extant Medusozoa (Cnidaria), such as coronal muscles, perradial and adradial frenula, interradial septa, accessory septa, gonad-lamellae, tentacle buds and perradial pockets. Although Sinaster cannot be straightforwardly assigned to any crown-group within Medusozoa, the presence of marginal lappets and endodermal lamellae suggests that it is closer to Cubozoa and Scyphozoa than to any other group of modern cnidarians. The tentative placement of Sinaster within the stem-group Cubozoa is justified by the presence of a velarium supported by a frenulum. The cubozoan affinities of Sinaster are also supported by cladistic analysis.

The early Cambrian Carinachitidae, a family in the subclass Conulata, are intriguing and important small shelly fossils. Their gently tapering, tube-shaped skeletons consist of convex faces separated from each other by broad, deep corner sulci, and they exhibit triradial, pentaradial, or predominantly tetraradial symmetry. However, the morphology of the aperture and the modes of growth of carinachitid skeletons as well as the anatomy of their soft parts are unknown. Examination of a single new, exceptionally well-preserved specimen of tetramerousCarinachites spinatusQian, 1977, collected from the lower Cambrian Kuanchuanpu Formation in South China, reveals: (1) that its aperture is connected to a small mass of relic soft tissue and (2) that the apertural end of each of the four faces is developed into a subtriangular lappet or oral lobe that is smoothly folded toward the long axis of the tube, partially closing the tube aperture. Similarities between thorn-like spines on the faces and the oral lobes indicate that the transverse ribs were periodically displaced from the perradial portion of the aperture during formation of new ribs. In addition, the tube walls may have undergone secondary thickening during growth. The growth pattern of the tube and the spatial relationships between the tube aperture and soft parts are analogous to those of co-occurring olivooids. These findings further strengthen the previously proposed hypothesis that coeval carinachitids, olivooids, hexangulaconulariids, and Paleozoic conulariids are closely related taxa within the subphylum Medusozoa. Finally, carinachitids most likely represent an evolutionary intermediate between olivooids and hexangulaconulariids.

Abundant microstructures have been discovered in small skeletal fossils (SSFs) and embryo-like fossils collected from the Lower Cambrian Kuanchuanpu Formation (ca. 535 Ma) in Xixiang County, Shaanxi Province, China. These involve two co-occurring structures: (1) long, unbranched cylindrical filaments, which are comparable to phosphatic casts of microborings constructed by euendolithic cyanobacteria (Endoconchia lata) in morphology and preservation pathway; and (2) meandering micro-tubes or grooves on fossil moulds (and steinkerns) of a wide range of sizes and morphological diversities, perceived as ambient inclusion trails (AITs). Herein, we also report a new occurrence of organic carbon spherules as AIT-propelled material, which is rare in comparable fossils. From direct comparison of endolith fossils and AITs, we propose a mechanism to account for their notably different preservation, and further attempt to offer an explanation for their co-occurrence. Their differential preservation suggests a chronological, taphonomic sequence of their formation. We interpret that E. lata microborings formed prior to phosphate sedimentation, whereas AITs are likely generated in a later phase of (or after) phosphorite precipitation but before calcareous re-cementation. Dissecting the sequence of formation of these structures, in conjunction with detailed morphological observations, assists in distinguishing true biologically produced endoliths from otherwise abiogenically produced microstructures.

The Archean continental crusts account for ca. 20% of the present volume, but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean. Because the Archean continental crust underwent severe metamorphism, it is important to avoid influence by the later thermal events. We carried out a comprehensive geochronological work of Cathodoluminescence (CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages. The zircons were classified into three domains of core, mantle and rims, and the cores were further classified into three groups of inherited, altered and zoned cores based on the zonation on the CL images. We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents. We made a detailed sketch of a small outcrop in St. John's Harbour South (SJHS) area, and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence. The first and second generations comprise mafic rocks and lack magmatic zircons. We conducted CL imaging and U-Pb dating of zircons from the third, sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 ± 25, 3892 ± 33 and 3897 ± 33 Ma for each, supporting the presence of the over 3.9 Ga Iqaluk Gneiss. The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world. Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to >3.9 Ga, and tentatively classified into six groups based on their ages. The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area, and the orthogneisses become young as it is away. The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.

The earliest fossil record of animal biomineralization occurs in the latest Ediacaran Period (c. 550 Ma).CloudinaandSinotubulitesare two important tubular taxa among these earliest skeletal fossils. The evolutionary fate ofCloudina-type fossils across the Ediacaran–Cambrian transition, however, remains poorly understood. Here we report a multi-layered tubular microfossilFeiyanella manicagen. et sp. nov. from a phosphorite interval of the lowest Cambrian Kuanchuanpu Formation, southern Shaanxi Province, South China. This newly discovered fossil is a conical tube with a ‘funnel-in-funnel’ construction, showing profound morphological similarities toCloudinaandConotubus. On the other hand, the outer few layers, and particularly the outermost layer, ofFeiyanellatubes are regularly to irregularly corrugated, a feature strikingly similar to the variably folded/wrinkled tube walls ofSinotubulites. TheFeiyanellatubes additionally exhibit two orders of dichotomous branching, similar to branching structures reported occasionally inCloudinaand possibly indicative of asexual reproduction. Owing to broad similarities in tube morphology, tube wall construction and features presumably indicative of asexual reproduction,Cloudina, Conotubus, Sinotubulitesand the here describedFeiyanellamay thus constitute a monophyletic group traversing the Ediacaran–Cambrian boundary. The tube construction and palaeoecological strategy ofFeiyanellaputatively indicate evolutionary continuity in morphology and palaeoecology of benthic metazoan communities across the Ediacaran–Cambrian transition.

With millions of extant species, ecdysozoans (Scalidophora, Nematoida and Panarthropoda) constitute a major portion of present-day biodiversity. All ecdysozoans secrete an exoskeletal cuticle which must be moulted periodically and replaced by a larger one. Although moulting (ecdysis) has been recognized in early Palaeozoic panarthropods such as trilobites and basal groups such as anomalocaridids and lobopodians, the fossil record lacks clear evidence of ecdysis in early scalidophorans, largely because of difficulties in recognizing true exuviae. Here, we describe two types of exuviae in microscopic scalidophoran worms from the lowermost Cambrian Kuanchuanpu Formation (ca535 Ma) of China and reconstruct their moulting process. These basal scalidophorans moulted in a manner similar to that of extant priapulid worms, extricating themselves smoothly from their old tubular cuticle or turning their exuviae inside out like the finger of a glove. This is the oldest record of moulting in ecdysozoans. We also discuss the origin of ecdysis in the light of recent molecular analyses and the significance of moulting in the early evolution of animals.

The Acasta Gneiss Complex, located in the western part of the Slave Province, Canada, is widely recognized as the oldest Eoarchean terrane. In addition to felsic gneisses with the ages of 3.6–4.0 Ga, minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses. Despite serving as potential sources of geochemical information on the Hadean mantle, the mafic rocks have received less attention in previous studies. Thus, we conducted a comprehensive geological petrological and geochemical investigation on the Acasta mafic rocks to constrain their petrogenesis and geodynamic setting. The mafic rocks comprise massive to weakly foliated amphibolite, garnet amphibolite and hornblendite, with variable abundances of hornblende, plagioclase, chlorite and quartz and subordinate clinopyroxene, garnet and cummingtonite. They commonly underwent high-grade metamorphic recrystallization under amphibolite to upper-amphibolite facies conditions. The observed variations in mineral assemblages, abundances and compositions reflect large differences in whole-rock compositions, likely caused by crustal anatexis during the Eoarchean thermal events responsible for the generation of the surrounding felsic gneisses. Infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between Al 2 O 3 and MgO contents and variable enrichments in the incompatible elements. Despite the widespread influence of anatexis on the geochemistry of Acasta mafic rocks, we identified the precursor compositions of the least-modified amphibolites as basaltic magmas. They are characterized by intermediate Al 2 O 3 and MgO contents on the observed array and by near chondritic patterns for incompatible trace elements, except for slightly negative Nb and Ta anomalies. We considered two scenarios to explain the origin of Eoarchean basaltic rocks with Nb–Ta anomalies: (1) generation of Nb–Ta deficient basaltic magma in a suprasubduction setting, analogous to modern arcs-derived magmas, and (2) generation of Nb–Ta deficient basaltic magma from the melting of a Nb–Ta deficient primitive mantle, possible if the core contains significant proportions of the Earth’s Nb and Ta budget. Although the operation of plate tectonics and the presence of subduction zones at the end of Hadean may be an attractive explanation for the observed Nb–Ta depletions, the chondritic relative proportions of other immobile trace elements for Acasta mafic rocks leave open the possibility of their formation from an Nb–Ta deficient primitive mantle.

Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of δ 13 C and δ 18 O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The δ 13 C chemostratigraphy possesses some unique characteristics: (1) stable δ 13 C values as a whole, but ubiquitous low δ 13 C anomalies through the Cap Carbonate, (2) increase of the δ 13 C values from −3 to +5‰ across the C2/C3 boundary, (3) no δ 13 C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high δ 13 C value (+2.3‰) and a faint positive correlation between δ 13 C and δ 18 O values in the C1 unit. Evidence of quite low δ 13 C anomalies (with a nadir of −41‰), ubiquitous negative δ 13 C anomalies through the Cap Carbonate, and a high δ 13 C anomaly accompanied with a faint positive correlation between δ 13 C and δ 18 O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of δ 13 C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of δ 13 C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.

Understanding Earth’s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on Earth. The Saglek–Hebron Block in the North Atlantic Craton is one of the oldest terrains in the world. To better understand the Eoarchean history of the Saglek–Hebron Block, we undertook a comprehensive geological and geochronological investigation of this terrain. The Saglek–Hebron locality contains orthogneisses and supracrustal rocks, which are classified into two groups of the Eoarchean and Mesoarchean suites based on field associations defined by intrusion of the Mesoarchean mafic Saglek dykes. The Eoarchean suites contain the Nulliak supracrustal rocks and Uivak Gneisses. In this paper, we report cathodoluminescence (CL) observations and U–Pb ages of zircons from the Eoarchean orthogneisses to constrain the evolution of the Saglek–Hebron Block. We made detailed sketch maps (1:20) of critical outcrops to establish relative chronological relationships between lithologies. At least seven generations of mafic and felsic units were recognized in an outcrop of the St John’s Harbour South area. The first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses, which are intruded into the supracrustal units. We study CL observation, geochemistry and U–Pb dating of zircons from the oldest and youngest tonalitic gneisses. The CL observation of internal structures of the zircons showed they typically comprise three domains of core, mantle and rim. The cores have clear oscillatory zoning, whereas the mantles lack oscillatory zoning and exhibit dark CL emission and structureless CL images. The obvious correlation between the ages and chemical compositions of U contents and Th/U ratios indicates that the cores with low U contents and high Th/U ratios preserve the protolith age. The cores with clear oscillatory zoning, low U contents and high Th/U ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 Ma. The oldest and average ages of the six oldest spots on the concordia line are 3953 ± 54 and 3920 ± 49 Ma, respectively. On the other hand, the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 ± 63 Ma. The geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other, confirming an Eoarchean age of ca. 3.9 Ga for the oldest suite of the Uivak Gneisses. We name this oldest suite the Iqaluk Gneiss. As the protolith of the Iqaluk Gneiss was clearly intruded into the Nulliak supracrustal rocks, the Nulliak supracrustals must have a minimum age of ca. 3.9 Ga, indicating that they are the oldest supracrustal rocks on Earth. But, further studies should be necessary to obtain more precise age of the Iqaluk Gneiss and Nulliak supracrustal rocks.

Du, W., Wang, X.L., Komiya, T., Zhao, R. & Wang, Y., April 2016. Dendroid multicellular thallophytes preserved in a Neoproterozoic black phosphorite in southern China. Alcheringa 40, xxx–xxx. ISSN 0311-5518.A new form of dendroid multicellular thallophyte is documented in the Ediacaran Doushantuo phosphorite at Weng’an, Guizhou Province, southern China. The dendroid thallophytes have variable forms, possibly owing to heteromorphic variation. Many lateral branches extend from the upper portions of the main axes; the lateral branches bear terminal vegetative vesicles, reproductive vesicles, monosporangium-like discoidal vesicles and urn-shaped pseudoparenchymatous structures. The vegetative vesicles give rise to clavate pseudoparenchymatous structures, characterized by differentiation of the thallus medulla/cortex, which might represent an early stage of thallus development. An oogamous conceptacle arising from one carpogonial vesicle forms a highly specialized goblet-shaped conceptacle. The discovery of th...

The Ongeluk Formation mainly consists of submarine volcanics (pillow lavas and sheet flows) composed of basaltic andesites that are between underlying and overlying glaciogenic deposits (i.e., the Makganyene diamictite and a dropstone layer at the base of the Hotazel Formation, respectively). The stratigraphic position of the Ongeluk Formation indicates that the Ongeluk volcanism occurred during a period of global glaciation. The Ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids. The hydrothermal quartz contains many primary (Type 1) and secondary (Type 2) liquid–vapor fluid inclusions, as well as inclusions that are randomly distributed without a trace of secondary healed cracks (Type 3). All these fluid inclusions types were individually analyzed with microthermometry and particle-induced X-ray emission (PIXE) methods. The results show that Type 1 fluid inclusions are highly saline, whereas Type 2 fluid inclusions are relatively less saline. Type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of Type 1 and Type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types. Among the various fluid inclusion salinities of fluid inclusions, a wide range in Na/Ca values was identified in the high-salinity fluid inclusions (i.e., Type 1 and a subset of Type 3) which are thought to represent subseafloor fluids circulated by the Ongeluk submarine volcanism. The wide range in Na/Ca values can be explained by a mixing process between Na-rich and Ca-rich fluids associated with albitization of the host basaltic andesites. In the albitization process, Na is removed from the fluids and fixed by the host rocks; by contrast Ca is released from the host rocks into the fluids. PIXE analysis also showed two distinct trends (i.e., vertical and horizontal) on variation diagram of Ca/Cl versus Cu/Cl and positive correlations between Mn/Cl, Cu/Cl, Zn/Cl and Pb/Cl. These data are best explained by the presence of other mixing processes between; (1) a Na-rich, Ca- and Cu-poor fluid, (2) a Ca-rich, Na- and Cu-poor fluid, and (3) a Na- and Cu-rich, Ca-poor fluid. The Cu-, Mn-, Zn- and Pb-rich fluid likely represents a high-temperature hydrothermal fluid from a deep reaction zone in the Ongeluk subseafloor hydrothermal system. By contrast, the (1) Na- (and K-) rich, Ca- and Cu-poor endmember for the high-salinity primary inclusions is considered to represent the composition of 2.2 Ga Ongeluk seawater. We propose the estimation of 2.2 Ga Ongeluk seawater composition is >∼2237 mmol/kg of Na, 200–1000 mmol/kg of K

We present a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy (OPS), and on the petrology and geochemistry of basalts from mid-oceanic ridges (MORB) and oceanic islands (OIB) using examples from four accretionary complexes (AC) in SW Japan: Akiyoshi, Mino–Tamba, Chichibu and Shimanto. The key to the problem is the model of OPS, which includes an association of igneous and sedimentary rocks that form on an oceanic plate during its travel from a mid-oceanic ridge to a subduction zone. We propose the reconstruction of the tectonic settings of basalts according to their relationships with associated OPS sediments, their petrogenesis and their geochemical features. Five types of OPS are recognized in the accretionary complexes of SW Japan: (1) sandstone/shale; (2) sandstone/shale and chert; (3) sandstone/shale, chert and MORB; (4) sandstone/shale, chert, MORB and gabbro (± peridotite); (5) seamount OPS including OIB, cap carbonates, slope clastics and basal shale/chert. The alkaline, tholeiitic or calc-alkaline composition of basaltic melts, which are typical of oceanic islands, mid-oceanic ridges and island-arcs, respectively, can be identified by the sequence in crystallization of their major phenocrysts, i.e. olivine (ol), clinopyroxene (cpx) and plagioclase (pl), and by their compositions. Alkaline and calc-alkaline mafic lavas are characterized by an ol → cpx → pl succession, whereas tholeiitic melts by their ol → pl ± cpx succession. Titanium-rich minerals, e.g., Ti–augite, kaersutite, Ti–biotite, are typical of alkaline lavas. The application of geochemistry-based tectonic discrimination diagrams is also a powerful tool, if not supported by geological and petrological data, may result in confusion due to magma contamination, post-magmatic alteration, and secular change of mantle thermal conditions. We propose that a direct comparison of normalized multi-element patterns and key binary plots from older volcanic rocks with their modern analogues provides a more viable and reliable method of basalt discrimination. Our OPS–petrology–geochemistry method allows us to confirm the above conclusions that the lavas of the Akiyoshi, Mino–Tamba and Southern Chichibu AC formed in oceanic islands, because they are associated with seamount OPS sediments, crystallized from ol to cpx and pl, contain Ti–augite and kaersutite and are enriched in TiO2, LREE and Nb. In this paper we present geochemical data from the Inuyama basalts of the Mino–Tamba AC and from the Toba complex in the huge Mikabu greenstone belt of the Chichibu AC. The Inuyama basalts are in contact with Jurassic pelagic cherts, but their geochemical features are confusing; they contain phenocrysts of ol, Ti–augite and kaersutite and therefore probably formed in seamounts. The Toba volcanic rocks are a part of the huge ophiolite belt; they have flat to slightly LREE-enriched REE patterns, are characterized by an ol → cpx succession of phenocrysts and they plot in the OIB field in binary plots suggesting they formed in an oceanic plateau.

Compositional variability found in modern mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) reflects differentiation processes associated with material recycling in the mantle-crust system. To investigate the timing at which this recycling system was established and how it transformed into the present-day system, we present geochemical analyses of the Archean basalts from North Pole (NP) in the East Pilbara Craton, Western Australia, and the Isua supracrustal belt (ISB), southern West Greenland. These rocks represent Archean accretionary complexes with ages of ~3.5 Ga and 3.7–3.8 Ga, respectively. We analyzed the trace element contents including rare earth elements (REEs), and Sr and Nd isotopic compositions of the basalts, which may represent MORBs and OIBs, from NP and ISB. Their trace-element compositions are broadly similar, but show distinct geochemical characteristics particularly with respect to REEs that probably reflect differences in both the source mantle and degree of melting. Such differences are also evident in their initial Nd isotopic compositions, which were estimated based on equilibrium partitioning of REEs and well-defined isochron ages. In contrast, the Sr isotopic compositions of the NP and ISB basalts are highly variable and their isochron ages are inconsistent with previous studies. Furthermore, the partitioning of Rb and Sr in the NP basalts indicates disequilibrium, suggesting that the Rb-Sr system has been disturbed by post-igneous alteration and metamorphism. Based on these observations, we propose the following model to explain the temporal variations in the geochemical composition of the Archean mantle: (i) ~3800 Ma: recycling of plate material and melting occurred quite readily and, therefore, MORBs and OIBs were produced from differentiated mantle sources; (ii) 3460 Ma to ~3800 Ma: mantle-crust mixing occurred as the result of an extreme event, such as mantle overturning, reducing the compositional variation of the mantle; and (iii) after ~3460 Ma: mantle heterogeneity gradually developed in the material-recycling system, re-establishing the compositional differences between MORBs and OIBs. This model requires an extreme event to drive the homogenization during stage (ii), which may provide new insights into the evolution of the crust-mantle system.

Fluid inclusions in hydrothermal quartz in the 2.4 Ga Ongeluk Formation, South Africa, are expected to partially retain a component of the ancient seawater. To constrain the origin of the fluid and the quartz precipitation age, we conducted Ar–Ar dating for the quartz via a stepwise crushing method. The obtained argon isotopes show two or three endmembers with one or two binary mixing lines as the crushing proceeds, suggesting that the isotopic compositions of these endmembers correspond to fluid inclusions of each generation, earlier generated smaller 40 Ar- and K-rich inclusions, moderate 40 Ar- and 38 Ar Cl (neutron-induced 38 Ar from Cl)-rich inclusions and later generated larger atmospheric-rich inclusions. The K-rich inclusions show significantly different 40 Ar/ 38 Ar Cl values compared to the 38 Ar Cl -rich inclusions, indicating that it is difficult to constrain the quartz formation age using only fluid inclusions containing excess 40 Ar. The highest obtained 40 Ar/ 36 Ar value from the fluid inclusions is consistent with an expected value of the Ongeluk plume source, suggesting that the quartz precipitation was driven by Ongeluk volcanism. Considering the fluid inclusion generations and their compositions, the hydrothermal system was composed of crustal fluid and magmatic fluid without seawater before the beginning of a small amount of seawater input to the hydrothermal system.

It was previously revealed that the total CO2 concentration in seawater decreased during the Late Archean. In this paper, to assess the secular change of total CO2 concentration in seawater, we focused on the Paleoproterozoic era when the Earth experienced its first recorded global glaciation. The 2.4 Ga Ongeluk Formation outcrops in the Kaapvaal Craton, South Africa. The formation consists mainly of submarine volcanic rocks that have erupted during the global glaciation. The undeformed lavas are mostly carbonate-free but contain rare disseminated calcites. The carbon isotope ratio of the disseminated calcite (δ13Ccc vs. VPDB) ranges from − 31.9 to − 13.2 ‰. The relatively low δ13Ccc values clearly indicate that the carbonation was partially contributed by 13C-depleted CO2 derived from decomposition of organic matter beneath the seafloor. The absence of δ13Ccc higher than − 13.2‰ is consistent with the exceptionally 13C-depleted CO2 in the Ongeluk seawater during glaciation. The results suggest that carbonation occurred during subseafloor hydrothermal circulation just after the eruption of the lavas. Previously, it was reported that the carbonate content in the uppermost subseafloor crust decreased from 3.2 to 2.6 Ga, indicating a decrease in total CO2 concentration in seawater during that time. However, the average CO2 (as carbonate) content in the Ongeluk lavas (

The authors provide evidence for the existence of life on Earth in the earliest known sedimentary rocks and suggest that the presence of organic carbon, and low stable-isotope values of graphite from sedimentary rocks in Labrador pushes back the existence of organic life to beyond 3.95 billion years. The beginning of organic life on Earth is being pushed back by evidence from the earliest known sedimentary rocks. Tsuyoshi Komiya and colleagues argue that the presence of organic carbon and stable-isotope excursions in graphite from sedimentary rocks in Labrador, Canada, pushes back the existence of organic life to more than 3.95 billion years ago. Together with recent work showing evidence for a diverse range of living organisms around 3.7 billion years ago, including stromatolites living in sunlit surface waters and bacteria living in deep-sea hydrothermal vents, the work shows that life has been around almost as long as there has been a planet that it can call home. The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible1,2,3, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt4,5,6,7. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador8,9. Some pelitic rocks have low δ13Corg values of −28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

Molecular phylogenetic analysis reveals that the splitting of the phylum Cnidaria into Anthozoa and Medusozoa probably took place in deep time as early as Cryogenian–Ediacaran (720–635 Ma), a long period with low dissolved oxygen concentration in the ocean. However, the pelagic life-style of medusae and the skeleton construction of anthozoans are expensive oxygen-consuming behaviours. Thus the advent of these evolutionary events, including the medusoid phase with statoliths necessary for medusa swimming, should be constrained by Neoproterozoic–Cambrian ocean geochemical environments, particularly the progressive oxygenation of Neoproterozoic–Cambrian ocean. This is well consistent with the dominance of Ediacaran–Cambrian cnidarian fossils by polypoid forms and later rise of medusae.