Fluids as primary carriers of sulphur and copper in magmatic assimilation

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.</p

Colombia: Doctoral Excursion, Earth Sciences Department ETH Zurich, August 21 – September 5, 2018

Field guide of Doctoral Excursion to Colombia, co-organized by Luz M. Mejia, R.Ott, L. Echeverria-Pazos, E. Erlanger. Chapter 11: Climate Change Effects on Colombian Biodiversity written by Luz M. Mejia and B. Revels

Experimental Investigaion of High-Temperature Carbon Isotope Fractionation

Unlike in many other stable isotope systems, equilibrium isotope fractionation in the carbon system is predicted by statistical mechanics’ theory to be in the order of several permil at high temperatures corresponding to Earth’s deep and extreme environments. Experimental studies confirming such theoretical predictions are rare, and lacking for most mineral-mineral, mineral-fluid, and fluid-fluid pairs. This thesis investigates carbon isotope exchange experiments between 300 – 1500 °C in the systems (i) CH4-CO2-CO, (ii) graphite-Na2CO3-CaCO3 (melt) and (iii) CH4-CO2-CO-graphitic carbon. Experiments have been conducted with box- and gas-mixing furnaces, externally-heated pressure vessels (“cold seals”), and piston cylinder apparatuses. Sample characterization (before and after experiment) involved gas-chromatography (GC), elemental analyzer (EA) and gas bench (GB) systems connected to an isotope ratio mass spectrometer (IRMS). Project (i) examined the carbon isotope exchange in a gas phase by using a variety of organic starting materials that were, sealed under vacuum in quartz tubes, thermally decomposed (300 -1200 °C) to create a CH4-CO2-CO gas mix plus an elemental carbon residual. While the decomposition and gas generation is nearly instantaneous, chemical and isotopic equilibration is protracted: vials with an additional piece of Ni-foil catalyzed the equilibration reactions, whereas experiments without Ni yield only minor chemical and isotopic reactions, if any. Measured gas speciation of catalyzed gases correspond well to the expected range from thermodynamic calculations. The experiments define carbon isotope partition functions for the CO2/CH4, CO2/CO and CH4/CO pairs as (T in Kelvin) 10^3ln a (CO2/CH4) = 8.9(±0.6)*10^5 (1/T^2)^0.825(±0.005) 10^3ln a (CO2/CO) = 1.07(±0.05)*10^6 (1/T^2)^0.830(±0.003) 10^3ln a (CH4/CO) = 1.1(±0.2)*10^3 (1/T^2)^0.462(±0.001) Project (ii) explored the carbon isotope exchange during graphite crystallization in a Na2CO3-CaCO3 melt at 900-1500 °C, 1 GPa using a piston-cylinder device. Graphite was grown anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal micron-sized graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. As the experiments did not yield bulk-equilibrated graphite at lower temperatures, the >1200 °C data was combined with empirical fractionation factors for carbonate-graphite obtained from upper amphibolite and lower granulite facies carbonate-graphite pairs reported by Kitchen & Valley (1995; J. metamorphic Geol., 13: 577-594) and Valley & O’Neil (1981; Geochim. Cosmochim. Acta 45: 411-419) which results in the general fractionation function (T in Kelvin) ∆13C(carbonate-graphite) = (3.37(4)*10^6)/T^2 This function is usable as a geothermometer for solid or liquid carbonate at >600 °C. Similar to previous observations, lower-temperature experiments (<1100 °C) deviate from equilibrium. By comparing the experimental results to diffusion and growth rates in graphite, it is shown that at <1100 °C diffusion rates are slower than graphite growth and equilibrium surface isotope effects control the isotope fractionation between graphite and carbonate-melt. The competition between diffusive exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond, especially since in high-temperature systems isotope fractionation is often assumed to proceed at or near equilibrium. Project (iii) investigates the influence of a COH-fluids redox state on the carbon isotope composition of graphite/diamond precipitated from it. Time series experiments were run to examine the carbon isotope exchange between carbonaceous system-components during the progressive oxidization of an initially CH4-dominated fluid. Stearic acid, thermally decomposed at 800 °C and 2 kbar, produced a reduced COH-fluid together with an elemental carbon phase. Progressive hydrogen loss from the capsule caused continuous methane dissociation accompanied by the precipitation of elemental carbon. The precipitating carbon, which aggregates as globules, is always 6.8 ±0.3 ‰ lighter than the methane, the opposite of what is expected from equilibrium isotope fractionation. In dynamic environments, kinetic isotope fractionation may hence control the carbon isotope composition of graphite or diamond. The commonly observed 13C-enrichment trends in diamonds are then consistent with deep reduced fluids oxidizing upon their rise. Finally, with the experimentally-derived fractionation functions now available it is possible to calculate carbon isotope fractionation factors for the CO2 – graphite/ diamond and CH4 – graphite/diamond pairs, which is necessary, as their isotopic equilibration appears impossible in experiment. The fractionations are defined as (T in Kelvin): 10^3ln a (CO2/graphite) = -(9*10^4/T^2)+(9580/T)-2.72 and 10^3ln a (graphite/CH4) = (8.9*10^5/T^1.65)-(9*10^4/T^2)-(9580/T)+2.72 With this extended set of equilibrium carbon isotope fractionation factors, tools are provided that will be useful for applied geochemical and industrial problems, and help developing more sophisticated models on the origin, evolution and fate of Earth’s carbon reservoirs

An Amphibole Perspective on the Recent Magmatic Evolution of Mount St. Helens

Compositional variations of amphibole stratigraphically recovered from multiple eruptions at a given volcano have a great potential to archive long-term magmatic processes in its crustal plumbing system. Calcic amphibole is a ubiquitous yet chemically and texturally diverse mineral at Mount St. Helens (MSH), where it occurs in dacites and in co-magmatic enclaves throughout the Spirit Lake stage (last ~4000 years of eruptive history). It forms three populations with distinct geochemical trends in key major and trace elements, which are subdivided into a high-Al (11–14.5 wt% Al2O3), a medium-Al (10–12.5 wt% Al2O3), and a low-Al (7.5–10 wt% Al2O3) amphibole population. The oldest investigated tephra record (Smith Creek period, 3900–3300 years BP) yields a bimodal amphibole distribution in which lower-crustal, high-Al amphibole cores (crystallized dominantly from basaltic andesite to andesite melts) and upper-crustal, low-Al amphibole rims (crystallized from rhyolitic melt) document occasional recharge of a shallow silicic mush by a more mafic melt from a lower-crustal reservoir. The sudden appearance of medium-Al amphiboles enriched in incompatible trace elements in eruptive periods younger than 2900 years BP is associated with a change in reservoir conditions toward hotter and drier magmas, which indicates recharge of the shallow silicic reservoir by basaltic melt enriched in incompatible elements. Deep-crystallizing, high-Al amphibole, however, appears mostly unaffected by such incompatible-element-enriched basaltic recharge, suggesting that these basalts bypass the lower crustal reservoir. This could be the result of the eastward offset position of the lower crustal reservoir relative to the upper crustal storage zone underneath the MSH edifice. Amphibole has proven to be a sensitive geochemical archive for uncovering storage conditions of magmas at MSH. In agreement with geophysical observations, storage and differentiation have occurred in two main zones: an upper crustal and lower crustal reservoir (the lower one being chemically less evolved). The upper crustal silicic reservoir, offset to the west of the lower crustal reservoir, has captured compositionally unusual mafic recharge (drier, hotter, and enriched in incompatible trace elements in comparison to the typical parental magmas in the region), resulting in an increased chemical diversity of amphiboles and their carrier intermediate magmas, in the last ~3000 years of MSH’s volcanic record

A Global Assessment of the Controls on the Fractionation of Arc Magmas

During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co-crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near-primary fractionating magmas evolve along the olivine-clinopyroxene cotectic, which is pressure-sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene-plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O-poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.ISSN:1525-202

Kinetic carbon isotope fractionation links graphite and diamond precipitation to reduced fluid sources

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by -6.5±0.3‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ13C and δ15N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon.ISSN:0012-821XISSN:1385-013

Intramolecular hydrogen isotope exchange inside silicate melts – The effect of deuterium concentration

Tracing the deep geological water cycle requires knowledge of the hydrogen isotope systematics between and within hydrous materials. For quenched hydrous alkali-silicate melts, hydrogen NMR reveals a distinct heterogeneity in the distribution of stable hydrogen isotopes (D, H) within the silicate tetrahedral network, where deuterons concentrate strongly in network regions that are associated with alkali cations. Previous hydrogen NMR studies performed in the sodium tetrasilicate system (Na2O x 4SiO2, NS4) with a 1:1 D2O/H2O ratio showed on average 1300 ‰ deuterium enrichment in the alkali-associated network, but the effect on varying bulk D2O/H2O ratios on this intramolecular isotope effect remained unconstrained. Experiments in the hydrous sodium tetrasilicate system with 8 wt% bulk water and varying bulk D2O/H2O ratios were performed at 1400 °C and 1.5 GPa. It is found that both hydrogen isotopes preferably partition into the silicate network that is associated with alkali ions. The partitioning is always stronger for the deuterated than for the protonated hydrous species. The relative enrichment of deuterium over protium in the alkali-associated network, i.e., the intramolecular isotope effect, correlates positively with the D2O/H2O bulk ratio of the hydrous NS4 system. Modeled for natural deuterium abundance (D/H near 1.56 × 10−4), a 1.4-fold (c. 340 ‰) deuterium enrichment in the alkali-associated silicate network is predicted. The partitioning model further predicts a positive correlation between the bulk water content of the silicate melt and the intramolecular deuterium partitioning into the alkali-associated silicate network. Such heterogeneities may explain the magnitude and direction of hydrogen isotope fractionation in hydrous silicate melts coexisting with silicate-saturated fluids. As such, this intramolecular isotope effect appears to be an effective mechanism for deuterium separation, particularly in hydrous magmatic settings, such as subduction zones.ISSN:0009-2541ISSN:1872-683

Experimental determination of equilibrium CH4–CO2-CO carbon isotope fractionation factors (300–1200 °C)

Carbon isotope fractionation in the CO2–CO–CH4–C system was investigated at 300–1200 °C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO2/CH4, CH4/CO, and CO2/CO at ≤600 °C or without a catalyzing agent for CO2/CO at ≥800 °C. The experiments define carbon isotope fractionation factors for the CO2/CH4, CO2/CO and CH4/CO pairs as (i) 103lnαCO2/CH4 = 8.9 (±0.6) ⋅ 105⋅ (1/T2)0.825(±0.005) (200–1200 °C) (ii) 103lnαCO2/CO = 1.07 (±0.05) ⋅ 106⋅ (1/T2)0.830(±0.003) (300–1200 °C) (iii) 103lnαCH4/CO = 1.1 (±0.2) ⋅ 103⋅ (1/T2)0.462(±0.001) (300–1200 °C), which reproduce the experimental values within 0.2‰ for CO2/CH4 and CO2/CO and within 0.12‰ for CH4/CO (T in K, 1σ fit uncertainties in brackets, CO2/CH4 includes the ≤600 °C experimental data of Horita, 2001). Carbon isotope fractionation factors at 1000 °C are still large for CO2/CH4 and CO2/CO (6.6 and 7.5‰ respectively) but only 1.5‰ for CH4/CO. Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ13C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO2/CO system, probably due to the activation of the silicate surface by H+ and OH− ions at >650 °C. Our experimental results yield improved calibrations of the CO2/CH4 equilibria and the first experimental calibration of CO2/CO and CH4/CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes.ISSN:0012-821XISSN:1385-013

Experimental carbonatite/graphite carbon isotope fractionation and carbonate/graphite geothermometry

Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900–1500 °C and 1 GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200–1500 °C) equilibrium isotope fractionation with type (3) graphite can be described by (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the ≥1200 °C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function usable as a geothermometer for solid or liquid carbonate at ≥600 °C. Similar to previous observations, lower-temperature experiments (≤1100 °C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ≤1100 °C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite = 2 × 10−17 m2s−1 for T > 1150 °C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (∼1.6 × 10−29 m2 s−1) but in good agreement with the value calculated for mono-vacancy migration (∼2.8 × 10−16 m2 s−1).ISSN:0016-7037ISSN:1872-953

Controls on lithium concentration and diffusion in zircon

ISSN:0009-2541ISSN:1872-683