The low 18O/16O stable isotope ratios (δ18O) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models ...suggesting lower δ18Osw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ'17O) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ18Osw of seawater on the early Earth without affecting the Δ'17O. Hence, the controversial long-term trend of increasing δ18O in chemical sediments over Earth's history partly reflects increasing δ18Osw due to decreasing atmospheric pCO2. We suggest that δ18Osw increased from about −5‰ at 3.2 Ga to a new steady-state value close to −2‰ at 2.6 Ga, coinciding with a profound drop in pCO2 that has been suggested for this time interval. Using the moderately low δ18Osw values, a warm but not hot climate can be inferred from the δ18O of the most pristine chemical sediments. Our results are most consistent with a model in which the "faint young Sun" was efficiently counterbalanced by a high-pCO2 greenhouse atmosphere before 3 Ga.
We present new major and trace element, high-precision high-field-strength-element, hafnium and neodymium isotope data for well preserved Eoarchean TTGs within the Itsaq Gneiss Complex (IGC) of ...southern West Greenland. These data are combined with thermodynamic model predictions of partial melting and fractional crystallization to gain new insights into continental crust formation in the Archean. Our results show that the observed compositional range of Eoarchean TTGs can be explained by a combination two processes: (1) 5–25% partial melting of amphibolite within thickened mafic crust and (2) subsequent fractional crystallization processes. The Eoarchean TTG suite of SW Greenland probably formed through mixing of melt batches that originally formed at different source depths between 10 and 14 kbar and ponded as plutons at mid-crustal levels. The trace element compositions of some TTGs point to subsequent fractional crystallization processes involving plagioclase, clinopyroxene, amphibole and garnet. Our model is consistent with recent studies proposing that the Eoarchean Itsaq Gneiss Complex TTGs from the IGC formed by re-working of mafic protocrust that stabilized as accreted juvenile crustal terranes in the Eoarchean. The model is also in good agreement with field observations from the area.
•TTGs form as mixtures of pooled melts generated under a variety of P/T conditions.•Partial melting and fractional crystallization explain TTG chemistry.•We present a coherent model for Archean crust formation.
Currently accepted mechanistic models describing aqueous corrosion of borosilicate glasses are based on diffusion-controlled hydrolysis, hydration, ion exchange reactions, and subsequent ...re-condensation of the hydrolyzed glass network, leaving behind a residual hydrated glass or gel layer. Here, we report results of novel oxygen and silicon isotope tracer experiments with ternary Na borosilicate glasses that can be better explained by a process that involves the congruent dissolution of the glass, which is spatially and temporally coupled to the precipitation and growth of an amorphous silica layer at an inwardly moving reaction interface. Such a process is thermodynamically driven by the solubility difference between the glass and amorphous silica, and kinetically controlled by glass dissolution reactions at the reaction front, which, in turn, are controlled by the transport of water and solute elements through the growing corrosion zone. Understanding the coupling of these reactions is the key to understand the formation of laminar or more complex structural and chemical patterns observed in natural corrosion zones of ancient glasses. We suggest that these coupled processes also have to be considered to realistically model the long-term performance of silicate glasses in aqueous environments.
•A wide range of geological tests are applied to the oldest proposed suture in the world.•High strain in the proposed suture is late and characterised by vertical sheath folds.•Garnet-bearing ...amphibolites are restricted to the vertical sheath folds.•Hf isotopes reveal a genetic link between rocks across the proposed suture.•Combined, the data indicate that the proposed suture is just another drip of greenstones.
Structural analysis of a proposed Mesoarchean suture located immediately to the southwest of the Barberton Greenstone Belt, South Africa – known as the Inyoni Shear Zone (ISZ) – reveals that the main, steeply dipping, NNE-striking fabric is distributed across only 1km width, is late (D3), and clearly overprints two earlier sets of fabric elements (D1, D2) that were originally oriented at right angles to the direction of proposed collision. Dating of a S2 foliated meta-trondhjemite is interpreted to indicate that D2 deformation was at, or younger than, 3238.2±0.9 Ma. The D3 high strain fabric of the ISZ is localised around tightly folded, kilometre-scale supracrustal rafts, but dissipates in all directions away from the rafts, along and across strike. The kinematic asymmetry of S3 shear foliations changes across opposing limbs of tight, upright F3 folds of supracrustal rocks in the ISZ, indicative of simple shear related to buckle folding of more competent supracrustal rocks in a matrix of less competent granitic gneisses. F3 buckle folding was highly non-cylindrical on steep to subvertical axes that plunge to the south in the northern part of the zone and to the north in the southern part of the zone, with nests of vertically plunging sheath folds along amphibolite/metagranite contacts in the central area. High-pressure (HP), garnet-bearing amphibolites are restricted to close association with the vertical D3 sheath folds and do not extend along strike.
Combined, the data indicate that the ISZ is not the product of west-directed subduction, as previously suggested. Rather, the strain and kinematic data indicate that the ISZ is just another drip of relatively cool, relatively rigid greenstones into hot, ductile granitic gneisses, formed during partial convective overturn of the upper (greenstone) and middle (granitic) crust. Uplift of the HP rocks was along a late (D4), brittle–ductile mylonite zone associated with emplacement of the c. 3.1Ga Kees Zyn Doorns syenite.
The Skaergaard intrusion in East Greenland hosts mineralisation of platinum group elements and gold that is characterised by a remarkably low amount of sulphides (about 0.02 wt% sulphide) and a very ...high metal/sulphur ratio. Here we present the first quantitative analyses of sulphur (S), copper (Cu), lithium (Li) and other trace elements from melt inclusions in plagioclase to trace their evolution in the magma and to constrain the formation of the mineralisation. The concentration of these elements in the melt inclusions varies considerably within each sample. The S content in melt inclusions ranges from 165 to 320 ppm in the lower part of the intrusion, but increases to 349–512 ppm immediately below the mineralisation interval. In the same interval, the Li content increases from 22–43 to 40–69 ppm and Cu ranges from 118 to 1586 ppm with no systematic variations. Across the mineralisation, the Li content drops an order of magnitude to 4–7 ppm and S drops to 250–326 ppm. We explain these compositional changes as reflecting the magmatic evolution as the result of fractional crystallisation below, and coinciding aqueous fluid exsolution and sulphide saturation across the mineralisation. Normally the S content of basaltic magmas is much higher at sulphide saturation. We propose that sulphide saturation is possible, nevertheless, mostly due to the high Cu content and high Cu/Fe in the evolved ferrobasaltic melt resulting in a lowering of the S concentration required for sulphide saturation. The relationship between exsolutions of aqueous fluids and sulphides remains unknown.
Permian basin formation and magmatism in the Southern Alps of Italy have been interpreted as expressions of a WSW‐ENE‐trending, dextral megashear zone transforming Early Permian Pangea B into Late ...Permian Pangea A between ~285 and 265 Ma. In an alternative model, basin formation and magmatism resulted from N‐S crustal extension. To characterize Permian tectonics, we studied the Grassi Detachment Fault, a low‐angle extensional fault in the central Southern Alps. The footwall forms a metamorphic core complex affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, the synmylonitic Val Biandino Quartz Diorite and the postmylonitic Valle San Biagio Granite. U‐Pb zircon dating yielded crystallization ages of 289.1 ± 4.5 Ma for the former and 286.8 ± 4.9 Ma for the latter. Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S. Even though a dextral continental wrench system has long been regarded as a viable model at regional scale, the local kinematic evidence is inconsistent with this and, rather, supports N‐S extensional tectonics. Based on a compilation of >200 U‐Pb zircon ages, we discuss the evolution and tectonic framework of Late Carboniferous to Permian magmatism in the Alps.
Key Points
Early Permian metamorphic core complex and extensional detachment fault are reconstructed in the Orobic Anticline (Southern Alps)
Two synkinematic to postkinematic plutons in the metamorphic core complex are dated at ~289 and ~287 Ma by U‐Pb LA‐ICP‐MS on zircon
Early Permian tectonics represent N‐S extension rather than dextral wrenching related to the Pangea B to A transformation
High‐ and ultrahigh‐pressure rocks occur in the Austroalpine Nappes in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure ...increase was dated using Lu‐Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short‐lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh‐pressure metamorphism. The P‐T conditions of the eclogites generally lie on the ‘warm’ side of the global range of subduction‐zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe‐Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan‐Eoalpine mixed age indicating re‐subduction of Variscan eclogite‐bearing continental crust during the Eoalpine orogeny. Jurassic blueschist‐facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite‐facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift.
The Eo-Alpine high-pressure belt in the Austroalpine nappes consists of pre-Mesozoic basement rocks overprinted by eclogite-facies metamorphism during the Late Cretaceous. Parts of this basement were ...already eclogitized during the Variscan orogeny. Lu–Hf geochronology allowed to identify two high-pressure events in an eclogite body in the Schobergruppe, an Austroalpine basement complex south of the Tauern Window. Two samples from closely neighboring outcrops were studied. Both contain two garnet generations. In one sample, PRI3, garnet belongs almost exclusively to the younger (Alpine) generation with only rare preservation of relic cores. In the other sample, PRI4, Variscan garnet is merely coated by a thin rim of the second, Alpine generation, which is in equilibrium with the high-pressure matrix assemblage. In PRI3, two-point garnet-whole rock ages scatter between ~ 97 and ~ 104 Ma, reflecting Alpine garnet growth with minor contamination by a Variscan component. In PRI4, two-point isochrons yield ages between ~ 300 and ~ 313 Ma. The limited spread in these ages suggests minor contamination by Alpine garnet rims. We propose 97 Ma as the maximum age for Alpine metamorphism, which is close to previously determined ages from other parts of the high-pressure belt, and 313 Ma as a minimum age for Variscan metamorphism. Thermodynamic modelling infers that eclogite-facies conditions were reached in both events; during the Late Cretaceous, these were ca. 1.9 GPa/650 °C. Variscan high-pressure conditions in PRI4 are inferred from the amount of garnet in the sample, which indicates at least 1.6 GPa. We propose that the occurrence of Alpine versus Variscan garnet in eclogites depends on the intensity of Variscan and post-Variscan retrogression: the more prograde, Variscan garnet was removed during retrogression, the more garnet grew during the Alpine cycle.
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