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Abstract: A natural crystal with NaMnSi2O6 composition (space group C2/c) belonging to the family of clinopyroxene structures was investigated by in-situ high-pressure single-crystal X-ray diffraction up to 9 GPa in order to verify if the compressibility can be predicted from the unit-cell volume V 0 at 1 bar applying an established volume–compressibility relationship. Together with data for isostructural analogue phases the equation-of-state parameters of NaMnSi2O6 showed that such simplified relationship can not be longer applied. The findings demonstrate that prediction models for isostructural series are consistent not only for the aspect of structural isotopy, and valid relationships presuppose similar character of the electronic structure. In case of pyroxenes containing transition elements the established relationship is only valid for series of compounds with cations of identical oxidation state according to isovalent electronic character. [Copyright &y& Elsevier]

Abstract: The equation of state of a single-crystal synthetic silicate NaInSi2O6 was determined from the unit cell parameters measured by X-ray diffraction using a diamond-anvil cell at 12 different pressures up to 7.83(1) GPa in order to provide a definitive model capable of predicting the high-pressure behavior of NaMe3 +Si2O6 silicate compounds. The data were fitted by a third-order Birch–Murnaghan equation of state obtaining the following coefficients: , . Our results show similar results for NaMe3 +Si2O6 compounds and confirm that for this mineral family, the empirical relationship proposed in previous investigations is valid for isostructural compounds only if they share the same valence electron character. In addition, we collected intensity data on the same compound at 16 different pressures up to 9.467 GPa using a diamond-anvil cell with diamond backing plates. The main structural compression mechanism is played by the tetrahedral chain kinking, which is related to the shortening of Na–O3long distances. Comparing our equation of state and high-pressure crystal structural results with previous data on NaMe3 +Si2O6 pyroxenes, we show that their compressibility increases much faster with the size of Me3 + cation when this one is a 3d transition metal. [Copyright &y& Elsevier]

Abstract: An in situ high-pressure X-ray diffraction study has been carried out at room temperature up to 9.23 GPa on a crystal of Ca2Sb2O7 having a weberite-2O structure synthesized by solid-state reaction. A 2nd-order Birch–Murnaghan Equation of State (EoS) was used to refine the pressure–volume data. The refinement of the unit-cell volume and of the isothermal bulk modulus at room pressure leads to: V 0 = 555.8 (1), KT 0 = 154.6 (9) GPa. Unit-cell parameters decrease gradually as a function of pressure with a bulk modulus anisotropy scheme Ka 0 > Kb 0 > Kc 0, with c being the softest direction. The compressional behavior of Ca2Sb2O7 is compared with that shown by ingersonite, Ca3MnSb4O14, a mineral showing a weberite-3T structure. [Copyright &y& Elsevier]

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle f O 2 over a vast portion of Earth's history. Diamonds' inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ∼3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ∼3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total δ13C data range from −17.1 to −1.9 ‰ (P = −8.4 to −1.9 ‰; E = −17.1 to −2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in δ13C of peridotitic diamonds has been at −5 (±2) ‰ since the earliest diamond growth ∼3.5 Ga, and this mode is also observed in the eclogitic diamond record since ∼3 Ga. The skewness of eclogitic diamonds' δ13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal δ13C mode of −5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in f O 2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga. [ABSTRACT FROM AUTHOR]

When and how Earth's ancient crust – the cratons – became underpinned by cool, thick lithospheric mantle roots capable of hosting diamonds are among the most controversial aspects of Archean geology. Alluvial diamonds in cratonic sedimentary cover rocks, whose minimum age is determined by detrital-zircon geochronology, provide a unique perspective on this topic. A new discovery of a diamond-bearing quartz-pebble conglomerate from the northern Slave craton, Canada contains detrital zircon with a restricted U-Pb age distribution that has a dominant peak at ∼2.94 Ga and depositional age of ∼2.83 Ga. Pressure-temperature constraints derived from an olivine-diamond host pair lie on a conductive Mesoarchean geotherm of ∼36–38 mW/m2, comparable to the coolest modern lithospheric geotherms. This result is at odds with a hotter geothermal gradient related to nearby Mesoarchean komatiites. We propose a model whereby early building blocks for cratons were small but with deep cool roots that formed by slab-stacking, and were subsequently juxtaposed with regions of thinner, hotter lithosphere. This heterogeneous initial architecture later amalgamated and thickened through lateral accretion forming the more uniformly thick cratonic lithosphere observed today. Thermal modelling indicates that stacking/thickening of cool initial lithosphere into a lithospheric keel thick enough to stabilise diamonds is the most likely way of generating the observed geotherm by Mesoarchean times. • Detrital zircon in host sedimentary rock has a Mesoarchean U-Pb age of >2.83 Ga. • Oxidizing diamond-forming fluids had crustal component (non-mantle δ 13 C- δ 15 N). • PT constraints show cool conductive Mesoarchean mantle geotherm of ∼36–38 mW/m2. • Early cratonic building blocks were small with deep cool roots. • These blocks likely formed through slab-stacking. [ABSTRACT FROM AUTHOR]

• The Moncuni (Lanzo Massif, W-Alps) ophiolite experienced subduction to 600 C ∘ and 2.1 GPa at dry conditions. • Ductile flow at temperatures > 700 C ∘ prior to subduction. • Seismic fracturing at eclogite-facies conditions recorded by pseudotachylyte. • Preservation of pre-subduction assemblages and pseudotachylyte glass during subduction. • The Moncuni ophiolite is the proxy for the rheology of subducting dry oceanic slab. The ophiolitic peridotite and gabbro of Moncuni (Southern Lanzo Massif, Western Alps) retain pre-subduction mantle-to-oceanic, high-temperature (> 700 C ∘) ductile fabrics. These fabrics are overprinted by seismic fracturing and faulting associated with pseudotachylytes. Within the gabbro, the pseudotachylytes preserve dry glass and pristine microlites. The occurrence of rare, minute garnet and the static development of eclogite-facies assemblages in local hydrated domains indicate that pseudotachylytes experienced subduction conditions of 600 C ∘ and 2.1 GPa. The exceptional survival of glass and the absence of post-oceanic ductile deformation demonstrate prevailing dry conditions during the entire Alpine subduction and exhumation path. Dry conditions inhibited reaction kinetics and viscous flow. In contrast, the majority of the Alpine ophiolites, derived from the upper hydrated portions of the oceanic lithosphere, show pervasive fluid-assisted metamorphism and ductile deformation. The Moncuni body can, therefore, be regarded as representative for the rheological behaviour during subduction of seismic, dry, deeper oceanic lithosphere that is rarely exhumed to the Earth's surface. In Moncuni, the brittle-ductile transition of dry oceanic rocks is constrained to be between 600 and 750 C ∘. This temperature range corresponds to the observed cut-off of intermediate-depth seismicity within subducting slabs. We infer that the base of the seismic layer corresponds to the brittle-ductile transition of a dry slab rather than the locus of antigorite breakdown triggering earthquakes by dehydration embrittlement. [ABSTRACT FROM AUTHOR]