Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition

P-T history and zircon geochronology of a felsic gneiss hosting ultrahigh pressure metamorphic rocks from the Tromso Nappe, Caledonian orogenic belt

第8回極域科学シンポジウム/個別セッション：[OG] 極域地圏12月6日（水）国立極地研究所 3階セミナー室The Eighth Symposium on Polar Science/Ordinary sessions: [OG] Polar GeosciencesWed. 6 Dec./3F Seminar room, National Institute of Polar Researc

Methane release from carbonate rock formations in the Siberian permafrost area during and after the 2020 heat wave

Anthropogenic global warming may be accelerated by a positive feedback from the mobilization of methane from thawing Arctic permafrost. There are large uncertainties about the size of carbon stocks and the magnitude of possible methane emissions. Methane cannot only be produced from the microbial decay of organic matter within the thawing permafrost soils (microbial methane) but can also come from natural gas (thermogenic methane) trapped under or within the permafrost layer and released when it thaws. In the Taymyr Peninsula and surroundings in North Siberia, the area of the worldwide largest positive surface temperature anomaly for 2020, atmospheric methane concentrations have increased considerably during and after the 2020 heat wave. Two elongated areas of increased atmospheric methane concentration that appeared during summer coincide with two stripes of Paleozoic carbonates exposed at the southern and northern borders of the Yenisey-Khatanga Basin, a hydrocarbon-bearing sedimentary basin between the Siberian Craton to the south and the Taymyr Fold Belt to the north. Over the carbonates, soils are thin to nonexistent and wetlands are scarce. The maxima are thus unlikely to be caused by microbial methane from soils or wetlands. We suggest that gas hydrates in fractures and pockets of the carbonate rocks in the permafrost zone became unstable due to warming from the surface. This process may add unknown quantities of methane to the atmosphere in the near future

Origin of andradite in the Quaternary volcanic Andahua Group, Central Volcanic Zone, Peruvian Andes

Euhedral andradite crystals were found in trachyandesitic (latitic) lavas of the volcanic Andahua Group (AG) in the Central Andes. The AG comprises around 150 volcanic centers, most of wich are monogenetic. The studied andradite is complexly zoned (enriched in Ca and Al in its core and mantle, and in Fe in this compositionally homogenous rim). The core-mantle regions contain inclusions of anhydrite, halite, S- and Cl-bearing silicate glass, quartz, anorthite, wollastonite magnetite and clinopyroxene. The chemical compositions of the garnet and its inclusions suggest their contact metamorphic to pyrometamorphic origin. The observed zoning pattern and changes in the type and abundance of inclusions are indicative of an abrupt change in temperature and subsequent devolatilization of sulfates and halides during the garnet growth. This process is interpreted to have taken place entirely within a captured xenolith of evaporite-bearing wall rock in the host trachyandesitic magma. The devolitilization of sediments, especially sulfur-bearing phases, may have resulted in occasional but voluminous emissions of gases and may be regarded as a potential hazard associated with the AG volcanism

Contrasting coronas : microscale fluid variation deduced from monazite breakdown products in altered metavolcanic rocks associated with the Grangesberg apatite-iron oxide ore, Bergslagen, Sweden

Three different types of secondary coronas developed around monazite-(Ce) were discovered in altered metavolcanic rocks closely associated with the Palaeoproterozoic apatite-iron oxide ore deposit in Grangesberg, Sweden. All three types of reaction coronas include fluorapatite that is either rimmed by allanite-(Ce), REE-fluorocarbonate(s), or hingganite-(Y). The latter mineral has not been previously observed among monazite breakdown products. A unique feature of the described reaction coronas around monazite is their spatial proximity to each other, not exceeding a few hundreds of micrometres. We infer that the observed, strongly contrasting monazite breakdown assemblages highlight the presence of a heterogeneous fluid that mediated these microscale decomposition reactions. Thus, it is emphasized that metasomatic fluid variability in natural systems may often be too large to be predicted and reproduced experimentally

The Ordovician Thores volcanic island arc of the Pearya Terrane from northern Ellesmere Island formed on Precambrian continental crust

Ion microprobe U & ndash;Pb zircon dating of intermediate to felsic rocks coupled with bulk-rock geochemistry analyses and compared to previously published data shows that the Thores Suite of the Pearya Terrane of northern Ellesmere Island (Arctic Canada) represents an Early Ordovician (c. 490 & ndash;470 Ma) suite formed in an island arc setting. Interestingly, three out of five dated samples contain abundant xenocrystic zircon that have ages spanning from c. 2690 Ma to c. 520 Ma. The vast majority of xenocrystic zircon are Precambrian in age and typical of Laurentia. The youngest well-pronounced age cluster around 580 & ndash;570 Ma is inferred to be an expression of the Timanide Orogen, traditionally ascribed to Baltica. This geochronological dataset provides new insight on the origin of the Thores Suite of the Pearya Terrane, which was traditionally thought to be formed due to the M'Clintock orogenic event and commonly treated as independent from Caledonian tectonism. We suggest that the Thores island arc formed on a sliver of continental crust within the Iapetus Ocean. The timing of igneous activity recorded by the Thores Suite is consistent with other island arcs and subduction-related metamorphic units that occur within the Caledonides of northern Scandinavia and Svalbard. Hence, we suggest that the Thores volcanic island arc was closely associated with age equivalent arcs developed within the northern Iapetus Ocean. Its juxtaposition with the other successions of the Pearya Terrane is explained by a large-scale, left lateral, strike-slip system operating along the northeastern margins of Baltica and Laurentia, coeval with the main collision between the two continents. This strike-slip system was responsible for the juxtaposition of multiple terranes with contrasting Precambrian histories that can be traced in the present day High Arctic, e.g. in southwest Svalbard and the Pearya Terrane

Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure(HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no,or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy,a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsundgranulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmeniteand titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT systemresulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths ofXMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt +plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effectof equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HTat lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previouslythought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR andother HP–UHP terranes worldwide

Neoproterozoic metamorphic evolution of the Isbjørnhamna Group rocks from south-western Svalbard

A metamorphosed volcano-sedimentary complex constitutes the Caledonian basement in the south-western part of Wedel Jarlsberg Land, Svalbard. Field, textural and previous thermochronologic data indicate a weak, localized metamorphic Caledonian overprint (M2). Deformed M1 isograds and variation in pressure–temperature estimates indicate a pervasive Neoproterozoic amphibolite-facies metamorphism that pre-dates large-scale Caledonian age folding. Garnet–biotite and garnet–Al silicate–plagioclase (GASP) geothermobarometry of the Isbjørnhamna Group mica schists, and their comparison with the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) petrogenetic grid, indicates a peak pressure of ca. 11 kbar, and a peak temperature of ca. 670°C during M1 metamorphism. A cooling rate of ca. 5°C My-1 is estimated on the basis of geothermobarometry and the available U–Th–total Pb and Ar–Ar data

Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgotabreen Complex at the Motalafjella Nunatak, Svalbard

The Vestgotabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar /39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430-400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the LowerUnit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365-344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgotabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsakkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgotabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia