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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.

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.

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.

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.

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

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.

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.

The timing of the emergence of life remains one of the principal unresolved questions in the Earth sciences. Putative relicts of microorganisms in the Eoarchean ( ca. 3.6–3.85 Ga) high-grade metamorphic terranes do not preserve morphological evidence for early life, but some relicts can be identified by their geochemical signatures created by metabolic processes. Among the oldest rocks of sedimentary origin ( ca . 3.8 Ga) occur in the Isua supracrustal belt (ISB), southern West Greenland; these have undergone metamorphism up to the amphibolite facies. Despite intense metamorphism, the carbon isotope compositions of graphite clots from the Isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process. Microbial dissimilatory iron reduction (DIR) is considered to be an early form of metabolism. The microbial DIR produced Fe 2 + aq with a lower δ 56 Fe value from a precursor Fe 3 + -bearing iron mineral. However, δ 56 Fe values lower than − 1‰ are not found in sedimentary rocks prior to about 2.9 Ga. Here, we report in-situ iron isotope analysis of pyrites in sedimentary rocks from the ISB, using a near infrared-femtosecond-laser ablation-multicollector-ICP-MS (NIR-fs-LA-MC-ICP-MS). A large variation of δ 56 Fe values from − 2.41 to + 2.35‰, was documented from 190 points within pyrite grains from 11 rock specimens, including those interpreted to be banded iron-formations (BIFs), chert, amphibole-rich chert, quartz-rich clastic sedimentary rocks, mafic clastic sedimentary rocks, carbonate rocks and conglomerates. We found that the distribution of δ 56 Fe values depends on the lithology, whereas there is no correlation between their δ 56 Fe values and the metamorphic grade. The δ 56 Fe values of pyrites in BIFs range from + 0.25 to + 2.35‰, indicating partial oxidation in the deep ocean. Especially, the high δ 56 Fe values, up to + 2.35‰, suggest that the BIF was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions. Pyrite grains in a conglomerate, carbonate rocks, mafic clastic sedimentary rocks, and amphibole-rich cherts show negative δ 56 Fe values around − 1.5‰, down to − 2.41‰, pointing to microbial DIR in the Eoarchean shallow sea. In addition, the relatively low δ 56 Fe values of pyrites in the shallow water sediments suggest anoxic, anoxygenic photoautotrophic iron oxidation in the photic zone.

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.

The Neoproterozoic Era records several important events in Earth history. The associations between BIF deposition, Snowball Earth events and the redox state of seawater are the key to explain the re-appearance of banded iron formations (BIF) during the Neoproterozoic. Unraveling ancient iron cycles provides important information about the linkage between the precipitation of BIF and the redox condition in the ocean, but current iron isotopic data are limited to sediments deposited only during the Sturtian glaciations and Ediacaran. We conducted a detailed geological survey of the Tonian to Ediacaran sedimentary succession in Nordaustlandet island, Svalbard. Our chemostratigraphies of δ 13 C carb from pre-Sturtian to post-Marinoan sedimentary successions in the Polarisbreen Group are consistent with those in coeval sediments. We first analyzed iron isotope ratios (δ 56/54 Fe) of individual pyrite grains with diverse shapes in carbonates, black shales, quartz arenites and diamictites in order to constrain the redox condition in seawater. We found large variations in iron isotope ratios; from ca. −2 to ca. +4‰. Positive δ 56/54 Fe values, over +1.0‰, are only preserved in euhedral and aggregated pyrite grains in black shales of the Backlundtoppen Formation and the MacDonaldryggen Member of the Elbobreen Formation. A partial oxidation of ferrous iron in seawater is necessary to explain such high δ 56/54 Fe values, which means that deep seawater was ferruginous (ferrous, iron-rich) before and after the Sturtian glaciation. This stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in Neoproterozoic seawater. This study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the Sturtian glaciation. If the deep ocean was intrinsically iron-rich, it is likely that Neoproterozoic BIFs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation.

Before continents attained a critical aerial dimension on the early Earth, hydrothermal carbonation of subseafloor crust is considered to have played the dominant role in fixing CO 2 from the CO 2 -rich ocean. However, it is uncertain how and when the seawater CO 2 level decreased and the strong carbonation of oceanic crust ceased. Here we report the depth profiles of the volume concentration and the carbon isotopes of calcites in the Late Archean/Paleoproterozoic volcanic rocks (Fortescue and Hamersley groups), exposed in the southwestern Pilbara Craton, Western Australia. The depth profiles indicate that 2.6 Ga seafloor hydrothermal carbonation is well preserved in the study area and that the CO 2 content of subseafloor crust per seafloor unit area is estimated to be clearly lower than those in the Early and Middle Archean and similar to the Phanerozoic equivalents. This suggests that the CO 2 concentration in seawater decreased from the Middle Archean to the Late Archean. This period broadly corresponds to the time of the first appearance of supercontinent on Earth. The amalgamation of continents has the potential to decrease seawater CO 2 concentration due to the removal of platform carbonate to continental interior. Subsequent fragmentation of supercontinent likely cause the carbonate deposition around newly created continental shelves. It is therefore implied that seawater CO 2 concentration in the early Earth was lowered by not only the hydrothermal carbonation of subseafloor crust but also through the formation and breakup of supercontinent in the Late Archean.

[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.

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.

The biogeochemical cycle of the Late Archean ocean is important for understanding the relationships between biological activity and oxygenation of the atmosphere and ocean. Based on the detailed geological survey of the Fortescue Group in the Redmont area in Pilbara Craton, Western Australia, we carefully selected 44 samples for iron isotope analyses, which consist of sandstones, stromatolitic carbonate rocks, alternating mudstone/sandstone rocks, mudstones and cherts. Our in situ analyses of δ56Fe values of 210 pyrite grains in these samples show a large variation from −4.2‰ to +3.0‰. We also analyzed 128 and 40 carbon isotope compositions of organic (δ13Corg: −51.8 to −10.3‰) and inorganic (δ13Ccarb: −6.1 to 0.6‰) carbons, respectively. Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive δ56Fe values show hexagonal, rectangular, and parallelogram shapes, which may replace former iron-oxide crystal systems: hematite, magnetite, and goethite, respectively. In contrast, more than half the pyrite grains with negative δ56Fe values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive δ56Fe values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower δ56Fe values below −2.2‰ suggesting a biological origin, probably due to microbial iron reduction. On the other hand, the pyrite is accompanied by isotopically very light organic carbon (δ13Corg: −51.8‰ to −40‰), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low δ56Fe values and low δ13C values in the some rocks suggests anoxic oxidation of methane by iron-reduction (AOM/IR). The iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a Late Archean shallow marine environment.

Middle Archean greenstones with mid-ocean ridge basalt affinity and overlying bedded chert/banded iron formation (BIF) are exposed in the Cleaverville area, Pilbara Craton, Western Australia. On the basis of the hydrothermal carbonation of these Cleaverville greenstones, we estimated the potential CO2 flux from ocean to oceanic crust and the physical–chemical conditions of the subseafloor hydrothermal system for the middle Archean. The greenstones exhibit various extents of carbonation, and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals. The degreeof carbonation correlates with stratigraphy; the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert/BIF horizon. Our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle Archean mid-ocean ridge. Both δ13C and δ18O values of calcite also change with the depth below the chert/BIF horizon. The δ18O value of calcite decreases with increasing depth, reflecting increasing temperature, while the δ13C value of calcite changes from positive to negative stratigraphically downward. A model reconstructing the δ13C of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater. These mineralogical, geochemical, and geological features of calcite provide the amount of CO2 in the whole oceanic crust per seafloor unit area (1.2 × 107 mol/m2). Based on this value, CO2 flux from the ocean to the oceanic crust in the middle Archean can be estimated to be 1.5 × 1014 mol/yr when the spreading rate (m2/yr) of Archean oceanic crust is assumed to have been three times higher than it is today. The estimated CO2 flux into the oceanic crust is two orders of magnitude higher than the modern value, which points to the significance of sea-floor hydrothermal carbonation in the Archean carbon cycle.

Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350 °C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO2 condition, and predict a generation of SiO2-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life.

The Neoproterozoic–Cambrian Dalradian Supergroup of Scotland contains three glaciogenic sedimentary units, the Port Askaig Formation, the Stralinchy-Reelan Formation, and the Loch na Cille Boulder Bed, in ascending order. Although the idea, that relatively cold climates occurred during the latest Proterozoic, is widely accepted, the cause of the onset of glaciation is still controversial; we use new carbonate 87 Sr/ 86 Sr data to better constrain this environmental change. Some Dalradian limestones have high strontium contents (>1000 ppm), low manganese contents and Mn/Sr ratios ( 87 Sr/ 86 Sr values of paleoseawater in the Neoproterozoic. The 87 Sr/ 86 Sr values of limestones of the Islay Formation ( ca. 0.7067), sampled on the island of Islay, are similar to the values of previous studies. However, the 87 Sr/ 86 Sr values of limestones just below the Port Askaig Formation ( ca. 0.7064), collected on the island of Garbh Eileach, have the lowest values recorded in the Dalradian Supergroup. Comparing these 87 Sr/ 86 Sr values with coeval 87 Sr/ 86 Sr values of other regions in the world, such low 87 Sr/ 86 Sr ( i87 Sr/ 86 Sr ratio from the Tonian through the Cryogenian and Ediacaran to the earliest Cambrian. The results show that the 87 Sr/ 86 Sr ratio of seawater decreased just before the older Cryogenian glaciation, that it increased by two steps in the early and middle Ediacaran, and that there were at least two abrupt positive excursions in the middle Ediacaran and at the Precambrian–Cambrian boundary. The decline of the 87 Sr/ 86 Sr ratio of seawater just before the older Cryogenian Sturtian glaciation suggests that enhancement of weathering, related to break-up of the Rodinia supercontinent and continental flood basalt volcanism, was a likely driver for generating global cooling for the Snowball Earth conditions ( Godderis et al., 2003; Donnadieu et al., 2004 ).

Mt. Narryer and Jack Hills meta-sedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia are of particular importance because they yield Hadean detrital zircons. To better understand the tectonothermal history and provenance of these ancient sediments, we have integrated backscattered scanning electron images, in situ U–Pb isotopic and geochemical data for monazites from the meta-sediments. The data indicate multiple periods of metamorphic monazite growth in the Mt. Narryer meta-sediments during tectonothermal events, including metamorphism at ~3.3–3.2 and 2.7–2.6 Ga. These results set a new minimum age of 3.2 Ga for deposition of the Mt. Narryer sediments, previously constrained between 3.28 and ~2.7 Ga. Despite the significant metamorphic monazite growth, a relatively high proportion of detrital monazite survives in a Fe- and Mn-rich sample. This is likely because the high Fe and Mn bulk composition resulted in the efficient shielding of early formed monazite by garnet. In the Jack Hills meta-sediments, metamorphic monazite growth was minor, suggesting the absence of high-grade metamorphism in the sequence. The detrital monazites provide evidence for the derivation of Mt. Narryer sediments from ca. 3.6 and 3.3 Ga granites, likely corresponding to Meeberrie and Dugel granitic gneisses in the Narryer Gneiss Complex. No monazites older than 3.65 Ga have been identified, implying either that the source rocks of >3.65 Ga detrital zircons in the sediments contained little monazite, or that >3.65 Ga detrital minerals had experienced significant metamorphic events or prolonged sedimentary recycling, resulting in the complete dissolution or recrystallization of monazite.

We have determined U–Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U–Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7–2.5, 2.2–1.9, 1.7–1.6, 1.2–1.0, 0.9–0.4, and The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3–2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than active margin settings and that the sedimentary mass significantly grew through addition of first-cycle sediments from young igneous basements, until after ∼1.3 Ga when sedimentary recycling became the dominant feature of sedimentary evolution. These findings, coupled with the lack of zircons older than 3.3 Ga in river sands, imply the emergence of large-scale continents at about 3.3 Ga with further rapid growth at around 1.3 Ga. This resulted in the major growth of the sedimentary mass between 3.3 and 1.3 Ga and the predominance of its cannibalistic recycling later.

The Ediacaran period was one of the most important times for the evolution of life. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution. The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling through the Doushantuo Fm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed 87 Sr/ 86 Sr and 88 Sr/ 86 Sr ratios of the fresh carbonate rocks, selected on the basis of microscopic observations and the geochemical signatures of Sr contents, Mn/Sr and Rb/Sr ratios, and δ 18 O values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The chemostratigraphy of the 87 Sr/ 86 Sr ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of δ 13 C, δ 18 O and 87 Sr/ 86 Sr values and Mn and Fe contents enables us to decode the surface environmental changes and their causes in the Ediacaran. The first large positive excursion of 87 Sr/ 86 Sr occurred together with negative δ 13 C and positive δ 18 O excursions. The higher 87 Sr/ 86 Sr values indicate an enhancement of continental weathering, whereas the positive δ 18 O excursion suggests global cooling. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering. Accelerated influx of nutrients promoted primary productivity, resulting in oxidation of dissolved organic carbon (DOC), whereas active sulfate reduction due to a higher sulfate influx from the continents caused remineralization of the large DOC, both of which caused a negative δ 13 C anomaly. The 580 Ma Gaskiers glaciation accounts for the close correlation among the positive 87 Sr/ 86 Sr, negative δ 13 C and positive δ 18 O excursions. The second large positive shift of 87 Sr/ 86 Sr firstly accompanied a positive δ 13 C excursion, and continued through the Shuram δ 13 C negative excursion. The positive correlation of δ 13 C and 87 Sr/ 86 Sr values is consistent with an enhanced continental weathering rate due to continental collisions that built Trans-Gondwana mountain chains, and with a higher primary activity due to the enhancement of continental weathering and consequent higher nutrient contents in seawater. The accompanied increase in Mn and Fe contents implies a gradual decline of the seawater oxygen content due to more active aerobic respiration and oxidation of reductive materials flowing in the oceans. In the Shuram excursion, higher 87 Sr/ 86 Sr values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative δ 13 C excursion. The higher 87 Sr/ 86 Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ 13 C anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx. Higher Mn and Fe contents in the early and middle stages of the excursion suggest a decline in the oxygen content of seawater due to oxidative decay of the DOC, whereas in the late stages the decrease in Mn and Fe contents is consistent with oceanic oxygenation. The emergence of Ediacara biota after the Gaskiers glaciation and the prosperity of the latest Ediacaran is concomitant with the formation of more radiogenic seawater with high 87 Sr/ 86 Sr values, suggesting that enhanced continental weathering, and the consequent higher influx of nutrients, played an important role in biological evolution.

The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS). We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ56Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ56Fe value (1‰) and a rim with a crustal δ56Fe value (0‰). The observed isotopic zoning suggests that the positive δ56Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis. The positive δ56Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of 56Fe-enriched ferric oxyhydroxide (Type 1) and 56Fe depleted Fe2+aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ56Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe2+aq steady-state model. The model calculation shows that half the Fe2+aq influx was oxidized in the seawater. This implies that O2 produced by photosynthesis would have been completely consumed by oxidation of the Fe2+aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.

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.

We report in-situ petrological evidence of deep mantle origin from podiform chromitites in the Luobusa ophiolite, southern Tibet. Analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the Luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite, which indicate an ultrahigh-pressure origin of over 3 GPa (> 100 km deep). The presence of these lamellae, coupled with abundant micro-inclusions of clinopyroxene, requires high solubility of SiO2 and CaO in the host chromite, and suggests a precursor of chromite, a CaFe2O4-structured high-pressure polymorph, stable at pressures over 12.5 GPa (> 380 km deep). These nano-scale observations and geological occurrence indicate that the mantle peridotite under the Tibetan mid-ocean ridge was transported from the deep mantle (at least 100 km, probably more than 380 km deep) by the mantle convection. It implies that the root of mantle upwelling has a much deeper origin than previously believed.

Exposure of the ca . 6 Ma Taitao ophiolite, Chile, located ∼50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ 18 O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ 18 O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87 Sr/ 86 Sr and 143 Nd/ 144 Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187 Os/ 188 Os ratios that range widely from 0.1168 to 0.1288 ( γ Os = −8.0 to +1.1), averaging 0.1239 ( γ Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg # of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions. In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ 18 O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187 Os/ 188 Os ratios, calculated for 6 Ma, that range from 0.126 ( γ Os = −1) to as high as 0.561 ( γ Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187 Os/ 188 Os between MORB and abyssal peridotites. The collective results of this study suggest that this >500 km 3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.

We estimated metamorphic conditions for the ~ 6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite-actinolite facies, greenschist facies and the greenschist-amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge. Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ~ 230 [degree sign]C in the volcanic unit to ~ 380 [degree sign]C at the bottom of the gabbro unit, systematically ~ 200 [degree sign]C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration. Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U-Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130-142).

A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge. The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO2 in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO2 than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust.

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.

In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ13C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from –18 to +2‰ and from –7 to –3‰, respectively. The maximum δ13C value of graphite globules in the metasediment rises from –14 to –5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ13C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ13C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively 13C-depleted graphite indicates that the regional metamorphism increased the δ13C values of the Isua graphite. This is consistent with the regional trend of 13C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused 13C-enrichment of Isua graphite. The δ13C values of graphite reported here (δ13C > –18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ13C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13C-enrichment of Isua graphite, previously reported large 13C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).

Ultramafic melt inclusions were discovered in Cr-spinels of 2.7 Ga Al-undepleted komatiites from the Belingwe Greenstone Belt, Zimbabwe. The inclusions consist of glass and sub-micrometer-size quench crystals of olivine and clinopyroxene. Homogenized melt inclusions are highly magnesian, ranging from 12.5 to 19.5 wt% in MgO content, and are also close to host komatiitic magma in other compositions. This fact indicates entrapment of melt into host spinel during the early stages of crystallization. The water content of two melt inclusions was determined using an ion probe; the high magnesian melt inclusion, 17.5 wt% in MgO, contains 1.1 wt% H2O, whereas the moderately magnesian melt inclusion, 11.8 wt% in MgO, has 1.7 wt% H2O. This evidence suggests that the primary komatiite melt contained 0.8–0.9 wt% H2O and 23.4–25.0 wt% MgO. The water content is about five times greater than previous estimates from melt inclusions in olivine [McDonough and Danyushevsky, EOS Trans. AGU 76 (1995) S266]. In addition, even the high H2O content preserved in melt inclusions within Cr-spinel may represent the minimum estimates of the parental composition, because part of the water should be dehydrated from parental magma during crystallization in the magma chamber. If the komatiite melt was formed by a high degree of partial melting of a peridotite, the source mantle should contain considerable amounts of water (∼0.5 wt%). However, recent melting experiments of hydrous peridotite indicate that the addition of 0.5 wt% H2O to mantle peridotite would not significantly decrease komatiite liquidus temperature [Asahara et al., Geophys. Res. Lett. 25 (1998) 2201–2204]. Petrological study of melt inclusions together with experimental data suggest that Belingwe komatiites were formed from a hydrous plume at high temperatures.