Search

International audience; Although fossil evidence suggests that various animal groups were able to move actively through their environment in the early stages of their evolution, virtually no direct information is available on the nature of their muscle systems. The origin of jellyfish swimming, for example, is of great interest to biologists. Exceptionally preserved muscles are described here in benthic peridermal olivooid medusozoans from the basal Cambrian of China (Kuanchuanpu Formation, ca. 535 Ma) that have direct equivalent in modern medusozoans. They consist of circular fibers distributed over the bell surface (subumbrella) and most probably have a myoepithelial origin. This is the oldest record of a muscle system in cnidarians and more generally in animals. This basic system was probably co-opted by early Cambrian jellyfish to develop capacities for jet-propelled swimming within the water column. Additional lines of fossil evidence obtained from ecdysozoans (worms and panarthropods) show that the muscle systems of early animals underwent a rapid diversification through the early Cambrian and increased their capacity to colonize a wide range of habitats both within the water column and sediment at a critical time of their evolutionary radiation. Editor's evaluation Based on exceptionally preserved fossils of olivooids, a group of early cnidarians, from the basal Cambrian of China (535 million years ago), Wang and colleagues reveal primitive muscles of early animals with well-developed system of circular fibers directly comparable with the myoepithelial muscles of modern medusae, representing the oldest record of a muscle system in cnidarians and more generally in animals. The manuscript will be of broad interest to scientists, including paleontologists and evolutionary biologists as well as the public.

The paper presents the results of geological, geochemical and isotope-geochronological studies of subvolcanic rhyolites of the Western Gorny Altai and Rudny Altai which have related to two large volcanic systems, the Altai-Minusinsk and Altai-Salair, respectively. The results of U-Pb isotopic dating of zircons revealed two groups of ages ~410-405 and 390-381 million years. Isotope-geochemical characteristics of rhyolites show relatively high values of εNd(T) = +2,7...6,0 with relatively young values of model ages T(DM) = 851-966 Ma in the Rudny Altai and more ancient - up to 1266 Ma in the West Gorny Altai. Geochemical characteristics indicate that its composition is consistent with transitional field between within-plate and island-arc felsic magmas. The results obtained correspond to the two-stage evolution of volcanism and its migration from the continent to the ocean.

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 diversity of small shelly fossils (SSF) demonstrates that multicellular organisms underwent large-scale radiation at the beginning of the Cambrian, which is highlighted by the coexistence of va...

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.

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 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.

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.

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.

The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U–Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, > 3.95 Ga, and forms the Iqaluk–Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk–Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the > 3.95 Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average δ 238 U of most samples is 0.24 ± 0.16‰ (2SD; relative to standard CRM145), which is slightly higher than the average δ 238 U of 0.02 ± 0.12‰ for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled δ 238 U value of 0.2‰ for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238 U is preferentially removed to euxinic sediments compared to 235 U, expanded ocean anoxia will deplete seawater of 238 U relative to 235 U, ultimately leading to deposition of ORM with low δ 238 U. Hence, the high δ 238 U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago. The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H 2 S] aq >11 μM) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV, δ 98 Mo approaches the modern seawater value of 2.34 ± 0.10‰. High δ 98 Mo points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O 2 -rich compared to O 2 -deficient marine environments. However, the δ 98 Mo value for most Member IV ORM is near 0‰ (relative to standard NIST SRM 3134 = 0.25‰), suggesting extensive anoxia. The low δ 98 Mo is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high δ 238 U. Hence, we propose that the low δ 98 Mo of most Member IV ORM was fractionated from contemporaneous seawater. Possible mechanisms driving this isotope fractionation include: (1) inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater-like δ 98 Mo signature in sediments or (2) delivery of isotopically light Mo to sediments via a particulate Fe–Mn oxyhydroxide shuttle. A compilation of Mo isotope data from euxinic ORM suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late Neoproterozoic and early Paleozoic time. Hence, Member IV does not capture irreversible deep ocean oxygenation. Instead, complex ocean redox variations likely marked the transition from O 2 -deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans.

金沢大学理工研究域地球社会基盤学系, Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse-grained, magnetically unstable oxides. In this study, the first example of tiny, needle-shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First-order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence. © 2014. American Geophysical Union. All Rights Reserved.

[1] Analysis of Hadean and Archean rocks for 16O-17O-18O isotopes demonstrates that the Terrestrial Mass Fractionation Line of oxygen isotopes has had the same slope and intercept for at least the past 4.0 and probably for as long as 4.2 Ga. The homogenization of oxygen isotopes required to produce such long-lived consistency was most easily established by mixing in a terrestrial magma ocean. The measured identical oxygen isotope mass fractionation lines for Earth and Moon suggest that oxygen isotope reservoirs of both bodies were homogenized at the same time during a giant moon-forming impact. But other sources of heat for global melting cannot be excluded such as bolide impacts during early accretion of proto-Earth, the decay of short-lived radioactive isotopes, or the energy released during segregation of core from mantle.

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.

Geochemical compositions are reported for Late Miocene to Pliocene granitoids from the Taitao Peninsula near the Chile ridge subduction zone. Major element compositions of Taitao granitoids show a resemblance with those of TTG suites. However, trace element compositions are characterized by low Sr (50‐300 ppm), moderately high Y (10‐45 ppm) and Yb concentrations (1‐5 ppm), and low Sr/Y and La/Yb ratios compared with those of typical adakites, which are presumably produced by melting of young and hot oceanic crust under eclogite to garnet amphibolite conditions. Instead, trace element composition of the Taitao granitoids resembles that of typical calc‐alkaline arc magmas. Based on trace element compositions, together with tectonic constraints, we infer that the Taitao granitoids were generated by partial melting of the subducted oceanic crust in garnet-free amphibolite conditions at depths shallower than 30 km. Our results indicate that slab-melting-related magmas do not necessarily show a HREE-depleted signature, which was used as evidence for slab-melting for granitic rocks of the TTG suites.

A new concept of continental construction based on four main terms: (1) crustal growth, (2) crustal formation, (3) continental growth and (4) continental formation is presented here. Each of these terms reflects a certain process responsible for the formation of what we call now "continental crust". This concept is applied to the Central Asian Orogenic Belt (CAOB), which is a global major accretionary orogen formed after the closure of the Paleo-Asian Ocean, and to its actualistic analogues - orogenic belts and accretionary complexes of the Western Pacific. The main focuses of the paper are the state of activities in the study of the CAOB, the theoretical basics of the new concept of continental construction, its challenges, prospects and social impacts, main methods of investigation. The main issues of the paperare what has been done in this field of geoscience, which questions remained unaddressed and which problems should be solved. The most important challenges are: (a) dominantly Phanerozoic formation of the CAOB continental crust versus its dominantly Archean growth; (b) to what extent the CAOB continental crust was juvenile or recycled; © whether magmatic arcs or Gondwana-derived terranes were accreted to the Siberian, Kazakhstan, Tarim and North China cratons; (d) what was the balance between continental formation and tectonic erosion based on modern examples from the Western Pacific; (e) what social benefits (mineral deposits) and geohazards (seismicity and volcanism) can be inferred from the study of orogenic belts formed in place of former oceans.

The presence of pillow basalts in the 3.8-3.7 Ga Isua supracrustal belt, Southern West Greenland indicates the presence of liquid water on early Earth around 3.8-3.7 Ga. The total thickness of pillow basalts is up to 1.0 km for the thickest unit, suggesting an ocean depth of at least 1.0 km. The rigidity of the Early Archean oceanic lithosphere can be calculated from the petrologically estimated mantle potential temperature of 1450°C and ocean surface temperature below 100°C. The results show that the oceanic lithosphere was sufficiently rigid to operate modern-style plate tectonics only 0.7-0.8 Ga after the formation of the Earth.

Water contents estimates based on infrared spectroscopy and deformation microstructures are used to constrain the origin of textural variations observed in mantle xenoliths from Kimberley, South Africa. Infrared spectra indicate water contents of 10–370 ppm (H2O by weight) in olivine, 20–370 ppm in orthopyroxene, up to 340 ppm in garnet and 30–550 ppm in clinopyroxene, although no systematic differences are detected among texturally distinct xenoliths (i.e. granular and sheared peridotites). In contrast, active slip systems in olivine, as inferred from tilt boundaries, are different between the granular and sheared peridotites; the former deformed via the [100](010) slip system, which is commonly active under hot, dry conditions, whereas the latter deformed via the [001](100) slip system, which is commonly activated in water-rich environments. The discrepancy in water contents estimates based on infrared spectroscopy and deformation microstructures may reflect different stages of the evolution of water content: the present water contents, as analysed by infrared spectroscopy, may have been modified by the most recent events (e.g. eruption by kimberlite magma), whereas the water contents inferred from tilt boundaries may have resulted from the various deformation processes in the deep cratonic mantle. Accordingly, we suggest that the observed heterogeneity in water content within cratonic roots facilitates local deformation, and gives rise to the range of textures seen in cratonic mantle xenoliths. Copyright © 2010 John Wiley & Sons, Ltd.

application/pdf, The Taitao granites are distributed around the Late Miocene Taitao ophiolite (5.66 ± 0.33 Ma to 5.19 ± 0.15 Ma) exposed at the western tip of the Taitao peninsula, southern Chile, ~ 50 km southeast from the present day Chile triple junction. In this paper, we report sensitive high mass-resolution ion microprobe (SHRIMP) U–Pb ages for the Taitao granites to elucidate the temporal relationship between the ophiolite and granites, and discuss the origin of the granitic melts. Five intrusive bodies of the Taitao granites have U–Pb ages ranging from 5.70 ± 0.25 Ma (Tres Montes pluton in southeast) to 3.92 ± 0.07 Ma (Cabo Raper pluton in southwest). The Estero Cono, Seno Hoppner and Bahia Barrientos intrusions that fringe eastern margin of the ophiolite have U–Pb ages ranging from 5.17 ± 0.09 Ma to 4.88 ± 0.3 Ma. Recycled zircon cores are common only in the Tres Montes pluton. Our data indicate that the generation of the granitic melts started in the Tres Montes area when a short segment of the Chile ridge system started to subduct ca. 6 Ma ago. This magmatism involved contamination with sediments/basement rocks. A part of the subducting ridge center was emplaced to form the present Taitao ophiolite at ~ 5.6 Ma. Generation of granitic melts continued as the spreading center of the same ridge segment subducted, due perhaps to partial melting of the ophiolite and/or oceanic crust enhanced by heat from upwelling mantle beneath the ridge. Granitic magmas with various compositions developed during subduction of the ridge. Emplacement of the ophiolite and formation of continental crust took place almost simultaneously.

The volatile content of komatiite is a key to constrain the thermal and chemical evolution of the deep Earth. We report the volatile contents with major and trace element compositions of ~ 80 melt inclusions in chromian spinels (Cr-spinels) from beach sands on Gorgona Island, Colombia. Gorgona Island is a ~ 90 Ma volcanic island, where picrites and the youngest komatiites known on the Earth are present. Melt inclusions are classified into three types on the basis of their host Cr-spinel compositions: low Ti (P type), high Ti with high Cr # (K1 type) and high Ti with low Cr # (K2 type). Chemical variations of melt inclusions in the Cr-spinels cover all of the island's lava types. P-type inclusions mainly occur in the picrites, K1-type in high-TiO 2 komatiites (some enriched basalts: E-basalts) and K2-type in low-TiO 2 komatiites. The H 2 O and CO 2 contents of melt inclusions within Cr-spinels from the beach sand are highly variable (H 2 O: 0.03–0.9 wt.%; CO 2 : 40–4000 ppm). Evaluation of volatile content is not entirely successful because of compositional alterations of the original melt by degassing, seawater/brine assimilation and post-entrapment modification of certain elements and volatiles. However, the occurrence of many melt inclusions with low H 2 O/K 2 O ratios indicates that H 2 O/K 2 O of Gorgona komatiite is not much different from that of modern mid-oceanic ridge basalt (MORB) or oceanic island basalt. Trend of CO 2 /Nb and Zr/Y ratios, accounted for by two-component mixing between the least degassed primary komatiite and low-CO 2 /Nb evolved basalt, allow us to estimate a primary CO 2 /Nb ratio of 4000 ± 2200 or a CO 2 content of 0.16 ± 0.09 wt.%. The determined CO 2 /Nb ratio is unusually high, compared to that of MORB (530). Although the presence of CO 2 in the Gorgona komatiite does not affect the magma generation temperature, CO 2 degassing may have contributed to the eruption of high-density magmas. High CO 2 /Nb and the relatively anhydrous nature of Gorgona komatiite provide possible resolution to one aspect of the hydrous komatiite debate.

The redox state of the surface environment of early Earth is still controversial (e.g. Ohmoto, 1997). Many previous papers suggest that oxygen was free before 2.7 Ga, and then gradually increased due to oxygen-producing photosynthesis (e.g. Holland, 1999; Farquhar et al., 2000). But, a detailed and quantitative estimate is still lacking. It is well known that deposited carbonate minerals are equilibrated with ambient seawater in a microbial or abiotic environment. The composition and mineralogy allow us to estimate the physical and chemical properties of pale minerals with primary sedimentary structures in shallow and deep-sea deposits, in order to eliminate secondary carbonate and contamination of detrital materials, and to estimate the redox condition of seawater over time.We estimated the depositional environments from the field occurrence of coexisting basaltic lava and sedimentary rocks and the fabric of the carbonates themselves. The shallow marine deposits have included sedimentary carbonates with a stromatolite structure and elastic layers, and amygdaloidal and matrix carbonates of hot-spot basaltic lava since 3.5 Ga. Deep-sea carbonates have included interstitial carbonate minerals in a matrix of hyaloclastite and amygdaloidal carbonate minerals accompanied by MORB-type basalts since 3.5 Ga. In addition, we excavated at three localities in South China, and obtained the complete sequence from the Marinoan tillite to early Cambrian rocks. The carbonate rocks belong to shallow marine deposits.Deep-sea carbonates have only faint Ce and Eu anomalies between 3.5 and 1.9 Ga. The negative Ce anomaly of shallow carbonates has frequently deviated from those of deep-sea carbonate since 2.78 Ga. It fluctuated greatly, and was very large at 2.5 and 2.3 Ga. We calculated the oxygen activity of shallow and deep seawater respectively, based on Ce content and anomalies of carbonate minerals at given parameters of atmospheric carbon dioxide content (pCO2) and Ca. content of seawater. The results show that the oxygen content of the deep sea was low and constant until at least 1.9 Ga. The oxygen content of shallow seawater increased from 2.7 Ga, but fluctuated. In particular, it was at a minimum during and after Snowball Earth events. It became quite high at 2.5 and 2.3 Ga, but eventually increased after the Phanerozoic. We also calculated it under another condition of high pCO2 to show that the seawater was more oxic even in the Archean than at present. The calculation suggests a relatively low pCO2 through geologic time.

A varied suite of mantle xenoliths from Malaita, Solomon Islands, was investigated to constrain the evolution of the mantle beneath the Ontong Java Plateau. Comprehensive petrological and thermobarometric studies make it possible to identify the dominant processes that produced the compositional diversity and to reconstruct the lithospheric stratigraphy in the context of a paleogeotherm. P–T estimates show that both peridotites and pyroxenites can be assigned to a shallower or deeper origin, separated by a garnet-poor zone of 10 km between 90 and 100 km. This zone is dominated by refractory spinel harzburgites (Fo91–92), indicating the occurrence of an intra-lithospheric depleted zone. Shallower mantle (∼Moho to 95 km) is composed of variably metasomatized peridotite with subordinate pyroxenite derived from metacumulates. Deeper mantle (∼95–120 km) is represented by pyroxenite and variably depleted peridotites that are unevenly distributed; the least-depleted garnet lherzolite (Fo90–91) lies just below the garnet-poor depleted zone (∼100–110 km), whereas the presence of pyroxenite is restricted to the deepest region (∼110–120 km), together with relatively Fe-enriched garnet lherzolite (Fo87–88). This depth-related variation (including the depleted zone) can be explained by assuming that the degree of melting for a basalt–peridotite hybrid source was systematically different at each level of arrival depth within a single adiabatically ascending mantle plume: (1) the depleted zone at the top of the mantle plume, where garnet was totally consumed in the residual solid; (2) an intermediate part of the plume dominated by the least-depleted garnet lherzolite just above the depth of the peridotite solidus; (3) the deepest pyroxenite-rich zone, whose petrochemical variation is best explained by the interaction between peridotite and normative quartz-rich basaltic melt, below the solidus of peridotite and liquidus of basalt. We explain the obvious lack of pyroxenites at shallower depths as the effective extraction of hybrid melt from completely molten basalt through the partially molten ambient peridotite, which caused the voluminous eruption of the Ontong Java Plateau basalts. From these interpretations, we conclude that the lithosphere forms a genetically unrelated two-layered structure, comprising shallower oceanic lithosphere and deeper impinged plume material, which involved a recycled basaltic component, now present as a pyroxenitic heterogeneity. This interpretation for the present lithospheric structure may explain the seismically anomalous root beneath the Ontong Java Plateau.

The Isua supracrustal belt (ISB) rocks are dated at about 3.8 Ga and constitute the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of over 1,500 rock samples in ISB enabled us to estimate the extent of regional metamorphism, petrotectonic environment and subduction- zone geothermal gradient in the Archean. The following lines of evidence indicate the first discovery of progressive, prograde metamorphism from greenschist (Zone A) through Ab-Ep-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeast part of the Isua supracrustal belt: (1) systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) absence of any vestige of high-grade metamorphism even in the lowest metamorphic zone. Geology and chronological constraints of ISB indicate that the regional metamorphism was related to the subduction of Archean lithosphere. Metamorphic pressures and temperatures of the metamorphism are estimated to be 5 to 7 kbar from Grt-Hbl-Pl-Qz geobarometry and 380° to 550°C from the Grt-Bt geothermometry in Zones B to D. These P-T estimates indicate that ISB was affected by progressive metamorphism of an intermediate P/T ratio metamorphic facies series, and that it records a much higher geothermal gradient of a subduction zone in the Archean than is known from the Phanerozoic. The high geothermal gradient may have resulted from the subduction of young lithosphere and a high potential temperature of mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin oceanic plate, creating many huge granitic (tonalite, trondhjemite, and granodiorite) batholiths. The slab melting changed the oceanic crust (density = 3.07) into a denser Grt-bearing residue (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics. In addition to the preservation of the oldest accretionary complex, this suggests that Precambrian-type plate tectonics, whose driving force is slab-pull due to densification of the residue of oceanic crust as a consequence of slab melting, was already operating in the Early Archean. The transition from Precambrian-type to Phanerozoic-type plate tectonics may be caused by thinning of oceanic crust and thickening of oceanic lithosphere in the late Archean, due to decrease of mantle temperature.

BackgroundExtant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved.MethodsMicroscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating.ResultsThe figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans. A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils. Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps, whcih indicates a direct development in these fossil taxa, lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.

A 1rcolon;5000 scale mapping was performed in the Isukasia area of the ca. 3.8-Ga Isua supracrustal belt, southern West Greenland. The mapped area is divided into three units bounded by low-angle thrusts: the Northern, Middle, and Southern Units. The Southern Unit, the best exposed, is composed of 14 subunits (horses) with similar lithostratigraphy, bound by layer-parallel thrusts. Duplex structures are widespread in the Isua belt and vary in scale from a few meters to kilometers. Duplexing proceeded from south to north and is well documented in the relationship between link- and roof-thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, of greenstone with low-K tholeiitic composition with or without pillow lava structures, chert/banded iron-formation, and turbidites. The cherts and underlying low-K tholeiites do not contain continent- or arc-derived material. The lithostratigraphy is quite similar to Phanerozoic "oceanic plate stratigraphy," except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present-day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100 degrees C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half-space cooling at given parameters of surface and mantle temperatures of 100 degrees and 1450 degrees C, respectively, suggesting that the Archean oceanic lithosphere was rigid. These conclusions-rigidity and lateral plate movement-support the idea that the modern style of plate tectonics was in operation only 0.7-0.8 G.yr. after the formation of the Earth.

sequence has a fault contact with the Triassic flysch-type sedimentary rocks. The northern and the southern boundaries dip gently to the south. Mesoscopic structures were observed on outcrop surfaces which are nearly perpendicular to the foliation and nearly parallel to the lineation. Microscopic observations were made on polished slabs and thin sections of oriented samples taken from the shear zones near to the top and bottom boundaries of the Luobusa ophiolite. The slabs and thin sections were made on the section perpendicular to the foliation and parallel to the mineral lineation. A serpentine melange zone, which lies along the northern margin of ophiolite sequence occupies the structural bottom of Luobusa ophiolite. The serpentine melange zone contains lenses of gabbroic rocks, pillow lavas and Cretaceous marine sedimentary rocks. These rocks occur as blocks in a partly serpentinized ultramafic matrix. Individual lithologies are irregularly distributed and the serpentinite matrix is intensively deformed. In the outcrops and oriented samples sets of subparallel shear bands and minor shear zones occur in the serpentinite. The shear bands are oblique to foliation in serpentinite and are regarded as "C'-type shear bands" (Figure 1). Consistent top-to-the-north displacements are observed at three localities in the serpentine melange zone.

Evidence for the existence of continental crust older than 4.06 Ga has so far been obtained only from zircons in the Yilgarn Craton of Western Australia. In this paper we report the first occurrence of a very old zircon with a U-Pb age of 4.2 Ga in the Acasta Gneiss Complex of northwestern Canada, based on a laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe study. The U-Pb data reveal that the 4.2 Ga zircon occurs as a xenocryst in a 3.9 Ga granitic rock. Trace element compositions of the xenocryst suggest that it crystallized from a granitic magma. Our results, suggesting the existence of granitic rocks outside the Yilgarn Craton at 4.2 Ga, imply that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust.

We carried out in situ U-Pb and Lu-Hf isotope analyses of detrital zircons from the Mississippi River in order to understand crustal reworking and continental growth rates. The U-Pb analyses for 416 zircons reveal three major peaks of crust formation at 2.8–2.6 Ga, 1.8–0.9 Ga, and after 0.2 Ga. Initial Hf isotope ratios were obtained for 402 of the dated zircons, and only 8% of the zircons have \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\ \_{Hf(\mathit{T})\_{DM}}\) \end{document} values less negative than −2.5. These data correspond to a crustal residence time of