Contact metamorphism of organic-rich mudstones and carbon release around a magmatic sill in the Basque-Cantabrian Basin,western Pyrenees

Recent studies have documented the expulsion of methane and oil to the Albian paleoseabed in the Basque-Cantabrian Basin. They interpret that hydrocarbon generation and expulsion were triggered by seismically recorded magmatic intrusions which metamorphosed organic-rich host sediments. An outcrop within the basin was selected to investigate organic matter evolution and sediment degassing due to an igneous body. This intrusion is a 5 m thick Late Albian basaltic sill that intruded mudstones of the Black Flysch Group, near Gorliz (north Iberia). Vitrinite and bitumen reflectance profiles and metamorphic mineral distribution in the overburden indicate that the sill produced a thermal effect that increases toward the intrusion, defining a 2 m thick (minimum) contact aureole.Geochemical profiles of TOC, S1, S2, HI and PC show a gradual decrease toward the sill indicating organic carbon loss and increase in the thermal maturity of the organic matter in the same direction. Concordantly, gas chromatograms show a loss of n-alkanes and a predominance of the shorter chain length homologues adjacent to the sill. Tmax and PI (S1/S1 + S2) values increase toward the sill which suggests an increase in the thermal stress and in the extent of kerogen pyrolysis, respectively.Organic carbon loss in the aureole was the result of carbon devolatilization and formation of CO₂ and CH₄ gases. The newly formed CO₂ reacted with pore waters to precipitate ¹³C depleted carbonate minerals in the aureole and in sill fractures. CH₄ escaped from the aureole via hydrofractures to the paleoseabed, where methane-derived authigenic carbonates were formed.     

Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50-90 km depths. The eclogites contain 2-20 ppm N with δ¹⁵N_air values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1 ppm N and δ¹⁵N_air ∼ −4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3-15 ppm N with δ¹⁵N_air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ¹⁵N_air ∼ −5‰). Based on their relatively high N contents and their homogeneous and positive δ¹⁵N values, admixing of sedimentary N is also indicated for the serpentinized peridotites.One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab-mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc-subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.     

Variations in Os isotope ratios of pyrrhotite as a result of water-rock and magma-rock interaction: Constraints from Virginia Formation-Duluth Complex contact zones

Os isotope ratios in pyrrhotite-bearing pelitic rocks of the ∼1.85 Ga Virginia Formation are variable, with perturbations linked to the emplacement of the ∼1.1 Ga Duluth Complex. Pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1.1 Ga involving a low ¹⁸⁷Os/¹⁸⁸Os fluid. However, because rocks with perturbed pyrrhotite Os isotope ratios occur 1½ km or more from the Duluth Complex, the fluid is unlikely to have been of magmatic origin. Fluid inclusions in layer-parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between ∼300 and 400 °C. Analyses of fluid inclusions via LA-ICP-MS show that the fluids contain up to 1.7 wt% Na, 1.1 wt% K, 4330 ppm Fe, 2275 ppm Zn, and 415 ppm Mg. The veins also contain pyrite or pyrrhotite, plus minor amounts of chalcopyrite, bornite, pentlandite, and sphalerite. The Re-Os isotopic ratios of pyrite from the veins indicate that they crystallized from low ¹⁸⁷Os/¹⁸⁸Os fluids (<0.2). δ¹⁸O values of vein quartz range from 7.7‰ to 9.5‰, consistent with an origin involving fluid with a relatively low δ¹⁸O value between 2‰ and 5‰. Meteoric water with such a low δ¹⁸O value could have interacted with the igneous rocks of the Complex and would have acquired Os with a low ¹⁸⁷Os/¹⁸⁸Os ratio. Strongly serpentinized olivine-rich rocks of the Complex are commonly characterized by such low δ¹⁸O values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the Os isotopic system recorded by pyrrhotite in the Virginia Formation. Two important observations are that only pyrrhotite-bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing Os exchange are spatially restricted, and not uniformly distributed. Os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite, and where time-integrated water-rock ratios in the aureole were high enough to provide a supply of Os.Troctolitic and gabbroic rocks of the Partridge River Intrusion, Duluth Complex, are characterized by Os isotope ratios that are indicative of variable degrees of crustal contamination (γ_Os values of ∼0-543). Xenoliths of carbonaceous and sulfidic pelitic rocks of the Virginia Formation found in the igneous rocks provide evidence that Os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle-derived magma. However, residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by ¹⁸⁷Os/¹⁸⁸Os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole. The Os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of Re-Os liberation during devolatilization, kerogen maturation, and pyrite to pyrrhotite conversion (processes that contribute to magma contamination), followed by Os uptake by pyrrhotite during back reaction involving magma and/or fluid characterized by a relatively low ¹⁸⁷Os/¹⁸⁸Os ratio. The extreme Os uptake recorded by pyrrhotite in the xenoliths, as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole, is in line with the high Os diffusivity in pyrrhotite experimentally determined by Brenan et al. (2000). Our data confirm that Os isotope ratios in pyrrhotite-bearing rocks may be readily perturbed. For this reason caution should be exercised in the interpretation of Os isotope ratios in rocks where pyrrhotite may be the primary host of Os.     

Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium -bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ¹⁵N_air between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ¹⁵N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ¹⁵N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in ¹⁵N, with δ¹⁵N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N₂). We suggest that the relatively high δ¹⁵N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity.     

Worldwide distribution and significance of secondary microbial methane formed during petroleum biodegradation in conventional reservoirs

Around half of world’s endowment of in-place oil and bitumen experienced biodegradation, which is now believed to be largely an anaerobic methanogenic process. However, the distribution and scale of methanogenic biodegradation in the world’s petroleum accumulations and the significance of its terminal product, secondary microbial methane, in the global gas endowment and carbon cycle are largely unknown. Here, I present geological and geochemical criteria to recognize secondary microbial methane in conventional petroleum reservoirs. These include the presence of biodegraded oil (as pools, legs or shows) in the reservoir or down-dip, the relatively dry (methane dominated) gas containing methane with δ¹³C values between −55‰ and −35‰ and, most importantly, CO₂ with δ¹³C > +2‰. Based on these criteria, the presence of secondary microbial methane is apparent in 22 basins, probable in 12 basins and possible in six basins worldwide. Reservoirs apparently containing secondary microbial methane are mostly Cenozoic and clastic and occur at depths of 37-1834 m below surface/mudline and temperatures of 12-71 °C. Using the current global endowment of in-place oil and bitumen and reasonable assumptions about conversion of oil into methane during biodegradation, I estimated that ∼65,500 tcf of secondary microbial methane could have been generated in existing worldwide accumulations of oil and bitumen through their geological history. From 1461-2760 tcf in-place (845-1644 tcf recoverable) of secondary microbial methane may be accumulated as free and oil-dissolved gas in petroleum reservoirs. I also updated the inventory of primary microbial methane and estimated that the global primary microbial gas endowment (free and oil-dissolved) is from 676-797 tcf in-place (407-589 tcf recoverable). Secondary microbial methane may account for ∼5-11% of the global conventional recoverable gas endowment and appears more abundant than primary microbial gas (∼3-4% of the global gas endowment). Most of the generated secondary microbial methane probably is aerobically and anaerobically oxidized to CO₂ in the overburden above petroleum reservoirs. However, some secondary microbial methane may escape from shallow reservoirs into the atmosphere and affect present and past global climate.     

Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

Fluid inclusions in quartz veins within Proterozoic metamorphic rocks in the Black Hills, South Dakota, were examined by microthermometry and Raman spectroscopy to assess the evolution of fluid compositions during regional metamorphism of organic-rich shales and late-orogenic magmatism, both of which were related to the collision of the Wyoming and Superior crustal blocks. Fluid inclusions occur in veins that began to be generated before or during regional compression and metamorphism that reached at least garnet-grade conditions, and in veins within the aureole of the Harney Peak Granite (HPG), where temperatures reached second-sillimanite grade conditions. Early veins in the schists have undergone recrystallization during heating and deformation that modified the composition of early CH₄ or CO₂ and N₂-dominated inclusions. These fluids were apparently trapped under conditions of immiscibility with a saline aqueous fluid phase. They are interpreted to represent components generated during maturation of organic matter and dehydration of phyllosilicates during incipient metamorphism at reducing fO₂ conditions. Most inclusions in the quartz veins are, however, secondary CO₂-bearing. They imply a transition to higher fO₂ conditions with increasing temperature of regional metamorphism. The fO₂ conditions may have been controlled by the mineral assemblage in the host metapelites. The prevalence of bimodal distributions of trapped CO₂-N₂ and aqueous endmembers in the biotite and garnet zones also suggests that two immiscible fluid phases existed during the regional metamorphism.In the aureole of the HPG, graphite was evidently consumed by influx of magmatic fluids. CO₂-H₂O fluid inclusions dominate, but they have significantly less N₂ than inclusions at lower metamorphic grades. All inclusions define secondary trails in mostly unstrained quartz. The bimodality of inclusion compositions is not as well defined as at lower grades, with many inclusions containing intermediate CO₂-H₂O compositions. This suggests that a single fluid phase existed at the high temperatures in the granite aureole, but then unmixed during cooling. A set of late quartz veins with graphitized and tourmalinized selvages in the granite aureole contains CH₄-bearing inclusions with little N₂. The existence of CH₄ in these inclusions is attributed to complexing of magmatic B with hydroxyl anions taken from the CO₂-H₂O fluid phase, effectively causing reduction in fO₂ and promoting precipitation of graphite.     

Stable carbon isotope behaviour of natural seepage of deep underground C-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area,Japan

Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ¹³C₁ = −44.3‰ was obtained for gas in the container and δ¹³C₁ = −36.3‰ for seeping gas in a fault zone. However, typical CH₄ in this dissolved-in-water gas-field is mainly depleted in ¹³C, and δ¹³C₁ values range from −66‰ to −68‰ owing to microbial degradation of organic material. ¹³C-rich CH₄ is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north–south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30 m. Gas seepages were not observed below a depth of 40 m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. ¹³C-rich CH₄ (adsorbed CH₄) was found to have desorbed from drilled mudstone core samples taken at depths of 1400–1900 m in the main gas-production strata. Similarly, ¹³C-rich CH₄ was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200–1500 m along a tectonic line. Methane generated by microbial degradation of organic material through CO₂ reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of ¹³C-rich CH₄, derived from thermogenic gas without oil components in strata. It is assumed that ¹²C-rich CH₄ is easily detached from core or pore water through gas production, whereas ¹³C-rich CH₄ is strongly adsorbed on the surfaces of particles. The ¹³C-rich CH₄ rises along the major tectonic line or up the 50 m wide normal fault zone from relatively deep sources in the Kanto region.     

Zircon U-Pb and Lu-Hf isotopic systematics of the Daping plutonic rocks: Implications for the Neoproterozoic tectonic evolution of the northeastern margin of the Indochina block, Southwest China

The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U-Pb dating on the zircon cores and rims of a hornblende-gabbro yield ²⁰⁶Pb/²³⁸U weighted means ages of 873 ± 9.1 Ma and 769 ± 7 Ma, respectively, and that for cores, mantles and rims of a granodiorite yield ²⁰⁶Pb/²³⁸U weighted means ages of 981-987 Ma, 829 ± 10 Ma and 761 ± 11 Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have ε_Hf^(t) values ranging from − 5.0 to − 5.8 and + 0.6 to + 6.4. Corresponding single-stage model ages range from 1626 to 1662 Ma and 1094 to 1311 Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) ε_Hf^(t) values of + 3.3 to + 12.3 with single-stage Hf model ages of 897 to 1235 Ma; and (2) ε_Hf^(t) values of − 1.9 to − 7.8 with single stage model ages of 1470-1667 Ma. The zircon rims are characterized by positive ε_Hf^(t) values (+ 5.4 to + 8.2) with single-stage model ages ranging from 977 to 1108 Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg^# (52.1-65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

Potential environmental issues of CO2 storage in deep saline aquifers: Geochemical results from the Frio-I Brine Pilot test,Texas, USA

Sedimentary basins in general, and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO₂ that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO₂ in such aquifers, 1600 t of CO₂ were injected at 1500 m depth into a 24-m-thick “C” sandstone unit of the Frio Formation, a regional aquifer in the US Gulf Coast. Fluid samples obtained before CO₂ injection from the injection well and an observation well 30 m updip showed a Na–Ca–Cl type brine with ∼93,000 mg/L TDS at saturation with CH₄ at reservoir conditions; gas analyses showed that CH₄ comprised ∼95% of dissolved gas, but CO₂ was low at 0.3%. Following CO₂ breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3000 mg/L as HCO₃) and in Fe (30–1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H₂O, DIC, and CH₄. These data, coupled with geochemical modeling, indicate corrosion of pipe and well casing as well as rapid dissolution of minerals, especially calcite and iron oxyhydroxides, both caused by lowered pH (initially ∼3.0 at subsurface conditions) of the brine in contact with supercritical CO₂.     

Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium () substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, δ¹⁵N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a δ¹⁵N-Cu relationship that does not match the δ¹⁵N-Cu correlation found in low-strain rocks. This δ¹⁵N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH₃)₂²⁺, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same δ¹⁵N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system.We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10¹¹ g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ∼80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ∼9.6 × 10¹¹ g/yr. Assuming a constant flux of subducted N over the Earth’s history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.     

Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

The well-studied Paleozoic Cooma metamorphic complex in southeastern Australia is characterized by a uniform siliciclastic protolith, of uniform age, with a continuous range of metamorphic grade from subgreenschist- to upper amphibolite-facies, and migmatite-grade in an annular pattern around the Cooma granodiorite. Those conditions are optimal for investigating variations of N concentrations and δ¹⁵N values during progressive metamorphism. Nitrogen concentrations decrease and δ¹⁵N increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, δ¹⁵N = 2.3‰; chlorite zone: 110 ppm N, δ¹⁵N = 3.0‰; biotite and andalusite zone: 85 ppm N, δ¹⁵N = 3.8 ‰; sillimanite and migmatite zones: 40 ppm N, δ¹⁵N = 10.7‰). Covariation of K and N contents is consistent with N substituting for K as NH₄⁺ in micas. Observed trends of increasing δ¹⁵N values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in ¹⁵N with progressive metamorphism, which causes an enrichment of ¹⁵N in the residual nitrogen of the rock. Equilibrium models for Rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by N₂-NH₄⁺ exchange at temperatures of 300-600 °C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH₃-NH₄⁺ exchanges at temperature of 650-730 °C. Lower values in the highest grade zones may also stem in part from input of ¹⁵N-depleted fluids from the granodiorite.The magnitude of isotope fractionation of nitrogen is about 1-2‰ during progressive metamorphism of metasedimentary rocks from sub-chlorite zone to biotite-andalusite zone, which is consistent with previous studies. Consequently, the large spread of δ¹⁵N values in Archean greenschist-facies metasedimentary rocks of −6‰ to 30‰ can be accounted for by variable mixtures of mantle plume-dominated volatiles with a δ¹⁵N of −5‰, and a ¹⁵N-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having δ¹⁵N of 30‰ to 42‰.     

Geochemistry and zircon U-Pb ages of adakitic rocks from the Dulan area of the North Qaidam UHP terrane, north Tibet: Constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny

Geochemistry and U-Pb ages of leucosomes and tonalites in a high pressure granulite unit of the Dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the North Qaidam Mountains (NQD). Leucosomes and tonalites show a marked chemical resemblance to adakites: (1) high La/Yb and Sr/Y, and low Y and HREE; (2) high Al₂O₃ and low Mg^# values with obvious positive Eu anomalies; and (3) slightly positive ε_Nd(t) values. Zircon U-Pb analysis of leucosomes and tonalites yielded ²⁰⁶Pb/²³⁸U ages of 428-437 Ma and 436-437 Ma respectively, which constrain the emplacement ages of the adakitic rocks. Petrological, geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust (> 50 km), suggesting a dominating source regime of synchronous high-pressure mafic granulites. Contemporary magmatism in other units of the NQD shows evidence of a widespread tectonothermal event during early Silurian (420-450 Ma) that includes metamorphism, magmatism and anatexis related to collision and thickening of lower crust.     

Al in plagioclase-rich chondrules in carbonaceous chondrites: Evidence for an extended duration of chondrule formation

The ²⁶Al-²⁶Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) - one ungrouped (Acfer 094), six CR, five CV, and one CO - reveal large variations in the initial ²⁶Al/²⁷Al ratio, (²⁶Al/²⁷Al)₀. Well-resolved ²⁶Mg excesses (δ²⁶Mg) from the in situ decay of the short-lived nuclide ²⁶Al (t_1/2 ∼ 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with (²⁶Al/²⁷Al)₀ values ranging from ∼3 × 10⁻⁶ to ∼1.5 × 10⁻⁵. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of ²⁶Al but do not yield well defined values for (²⁶Al/²⁷Al)₀, while the remaining chondrules do not contain excess radiogenic ²⁶Mg and yield corresponding upper limits of (11-2) × 10⁻⁶ for (²⁶Al/²⁷Al)₀. The observed range of (²⁶Al/²⁷Al)₀ in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the (²⁶Al/²⁷Al)₀ values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of (²⁶Al/²⁷Al)₀ are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower (²⁶Al/²⁷Al)₀ values in PRCs from CCs, relative to most CAIs, are consistent with a model in which ²⁶Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of (²⁶Al/²⁷Al)₀ values in PRCs from CCs is most plausibly explained in terms of an extended duration of ∼2-3 Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 10⁵ years. The active solar nebula appears to have persisted for a period approaching 4 Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system.     

Hf-W chronometry of primitive achondrites

Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived ¹⁸²Hf-¹⁸²W radionuclide system. Here we present new ¹⁸²Hf-¹⁸²W data for major types of primitive achondrites including acapulcoites, winonaites and one lodranite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites.For acapulcoites we derived a ¹⁸²Hf-¹⁸²W age of Δt_CAI = 4.1 ^+1.2/_−1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of Δt_CAI = 4.8 ^+3.1/_−2.6 Ma (assuming a bulk Hf/W ratio of ∼1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of Δt_CAI = 14.3 ^+2.7/_−2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between ∼1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf-W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between ∼35 and ∼4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were ∼40 K/Ma to ∼720 K and then ∼3 K/Ma to ∼550 K.Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse correlation between accretion-age of asteroids and intensity of heating in their interiors as expected from heating by ²⁶Al and ⁶⁰Fe decay. However, the early accretion of the parent asteroid of primitive IAB silicates (∼1.0 Ma after CAIs; Schulz et al., 2009) and the possibly impact-induced melting-history of winonaites show that this concept is too simplistic. Parent body size, impact-driven melting as well as heat-insulating regolith cover also need to be considered in the early history of asteroid differentiation.     

Formation and abundance of doubly-substituted methane isotopologues (CH3D) in natural gas systems

Formation of the Carbon-13 (¹³C) and deuterium (D) doubly-substituted methane isotopologues (¹³CH₃D) in natural gases is studied utilizing both first-principle quantum mechanism molecular calculation and direct FTIR laboratorial measurements of specifically synthesized high isotope concentration methane gas. For ¹³CH₃D, the symmetrically breathing mode A₀ emerges as IR-detectable attributed to the molecular symmetry lowering to C_3v from T_d of the non-isotopic methane (CH₄), along with a large vibrational frequency shift from ∼3000 to ∼2250 cm⁻¹. Our studies also indicate that the concentration of ¹³CH₃D is dependent on the environmental temperature through isotope exchanges among methane isotopologues; and the Gibbs’ Free Energy difference due to Quantum Mechanics Zero-Point vibrational motions has the major contribution to this temperature dependency. Potential geologic applications of the ¹³CH₃D measurement to natural gas exploration and assessments are also discussed. In order to detect the ¹³CH₃D concentration change of each 50 °C in the natural gas system, a 10⁻⁹ resolution is desirable. Such a measurement could provide important add-on information to distinguish natural gas origin and distribution.     

Contact metamorphism in the Los Santos W skarn (NW Spain)

Summary The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (±clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite ± wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 °C. ¹³C and ¹⁸O depletions in calcite marbles are consistent with hydrothermal fluid–rock interaction during metamorphism. Calcites are depleted in both ¹⁸O (δ¹⁸O = 12.74‰) and ¹³C (δ¹³C = −5.47‰) relative to dolomite of unmetamorphosed dolostone (δ¹⁸O = 20.79‰ and δ¹³C = −1.52‰). The δ¹³C variation can be interpreted in terms of Rayleigh distillation during continuous CO₂ fluid removal from the carbonates. The δ¹⁸O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO₂ (±CH₄ ± N₂) suggesting a metamorphic origin. Fluorine-bearing minerals such as chondrodite, norbergite and F-rich phlogopite indicate that contact metamorphism was accompanied by fluorine metasomatism. Metasomatism was more intense in the inner-central portion of the contact aureole, where access to fluids was extensive. The irregular geometry of the contact with small aplitic intrusives between the metasediments and the Variscan granitoids probably served as pathways for fluid circulation.     

Transport and thermodynamics constrain belowground carbon turnover in a northern peatland

Rates of anaerobic respiration are of central importance for the long-term burial of carbon (C) in peatlands, which are a relevant sink in the global C cycle. To identify constraints on anaerobic peat decomposition, we determined detailed concentration depth profiles of decomposition end-products, i.e. methane (CH₄) and dissolved inorganic carbon (DIC), along with concentrations of relevant decomposition intermediates at an ombrotrophic Canadian peat bog. The magnitude of in situ net production rates of DIC and CH₄ was estimated by inverse pore-water modeling. Vertical transport in the peat was slow and dominated by diffusion leading to the buildup of DIC and CH₄ with depth (5500 μmol L⁻¹ DIC, 500 μmol L⁻¹ CH₄). Highest DIC and CH₄ production rates occurred close to the water table (decomposition constant k_d ∼ 10⁻³-10⁻⁴ a⁻¹) or in some distinct zones at depth (k_d ∼ 10⁻⁴ a⁻¹). Deeper into the peat, decomposition proceeded very slowly at about k_d = 10⁻⁷ a⁻¹. This pattern could be related to thermodynamic and transport constraints. The accumulation of metabolic end-products diminished in situ energy yields of acetoclastic methanogenesis to the threshold for microbially mediated processes (−20 to −25 kJ mol⁻¹ CH₄). The methanogenic precursor acetate also accumulated (150 μmol L⁻¹). In line with these findings, CH₄ was formed by hydrogenotrophic methanogenesis at Gibbs free energies of −35 to −40 kJ mol⁻¹ CH₄. This was indicated by an isotopic fractionation α_CO2-CH4 of 1.069-1.079. Fermentative degradation of acetate, propionate and butyrate attained Gibbs free energies close to 0 kJ mol⁻¹ substrate. Although methanogenesis was apparently limited by some other factor in some peat layers, transport and thermodynamic constraints likely impeded respiratory processes in the deeper peat. Constraints on the removal of DIC and CH₄ may thus slow decomposition and contribute to the sustained burial of C in northern peatlands.     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.