Composition of Fluids Responsible for Gold Mineralization in the Pechenga Structure-Imandra-Varzuga Greenstone Belt, Kola Peninsula, Russia.

This study presents the first fluid inclusion data from quartz of albite–carbonate–quartz altered rocks and metasomatic quartzite hosting gold mineralization in the Pechenga structure of the Pechenga– Imandra–Varzuga greenstone belt. A temperature of 275–370°C, pressure of 1.2–4.5 kbar, and the fluid composition of gold-bearing fluid are estimated by microthermometry, Raman spectroscopy, and LA-ICP-MS of individual fluid inclusions, as well as by bulk chemical analyses of fluid inclusions. In particular, the Au and Ag concentrations have been determined in fluid inclusions. It is shown that albite–carbonate–quartz altered rocks and metasomatic quartzite interacted with fluids of similar chemical composition but under different physicochemical conditions. It is concluded that the gold-bearing fluid in the Pechenga structure is similar to that of orogenic gold deposits

In situ determination of sulfur speciation and partitioning in aqueous fluid-silicate melt systems

International audienceCurrent knowledge of sulfur behaviour in magmas is based exclusively on ex situ analyses. Here we report the first in situ measurement of sulfur speciation and partitioning between coexisting aqueous fluids and silicate melts. These data were acquired using Raman spectroscopy in a diamond anvil cell at 700 °C, 0.3–1.5 GPa, and oxygen fugacity in the vicinity of the sulfide-sulfate transition, conditions relevant to subduction zone magmatism. Results show that sulfate and sulfide are dominant in the studied systems, together with the and radical ions that are absent in quenched fluid and silicate glass products. The derived fluid/melt partition coefficients for sulfide and sulfate are consistent with those from available ex situ data for shallow crust conditions (<10 km), but indicate stronger partitioning of these sulfur species into the silicate melt phase with increasing depth. In contrast, both radical ions partition at least 10 times more than sulfate and sulfide into the fluid phase. Thus, by enhancing the transfer of sulfur and associated chalcophile metals from melt into fluid upon magma degassing, and may be important players in the formation of economic metal resources within the redox window of the sulfate-sulfide transition in subduction zone settings

Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones

Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the geochemical cycle of these volatiles and associated trace metals, as well as to quantify the halogen fluxes to the atmosphere via volcanic degassing. Here, the partitioning of bromine between coexisting aqueous fluids and hydrous granitic melts and its speciation in slab-derived fluids have been investigated in situ up to 840 ∘C and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-ray absorption spectroscopy (XAS) in diamond anvil cells. The partition coefficients range from ∼2 to ∼15, with an average value of 6.7±3.6 (1σ) over the whole pressure–temperature (P–T) range, indicating a moderate Br enrichment in aqueous fluids, in agreement with previous work. Extended x-ray-absorption fine-structure (EXAFS) analysis further evidences a gradual evolution of Br speciation from hydrated Br ions [Br(H2O)6]− in slab dehydration fluids to more complex structures involving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greater depth (> 200 km). In denser fluids (ρ > 1.5 g cm−3) containing 60 wt % dissolved alkali–silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), Br is found to be in a “salt-like” structure involving the six nearest Na ions and several next-nearest O neighbors that are either from water molecules and/or the silicate network. Bromine (and likely chlorine and iodine) complexing with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and silica-rich supercritical fluids. Our results suggest that both shallow dehydration fluids and deeper silicate-bearing fluids efficiently remove halogens from the slab in the sub-arc region, thus favoring an efficient transfer of halogens across subduction zones

<i>In situ</i> X-ray absorption spectroscopy using the FAME autoclave: a window into fluid-mineral-melt interactions in the Earth’s crust

International audienceWe discuss the X-ray absorption spectroscopy (XAS) methods elaborated using the high pressure and high-temperature autoclaves installed at FAME and FAME-UHD beamlines. These methods are based on the in situ acquisition of X-ray transmission and fluorescence spectra of hydrothermal fluids and silicate melts and enable the derivation of both solubility and speciation information about metal complexes. The technological assets of our autoclaves are described across a wide range of experimental conditions spanning from different types of hydrothermal fluids, from liquid-like to vapor-like densities having metal concentrations of &lt;1–10,000 ppm, to magmatic fluid-melt systems at 1000°C. Scientific examples are presented to illustrate the use and the advantages of our spectroscopic ‘micro-batch’ reactors. Finally, the integration of our autoclave setups on the FAME-UHD crystal analyzer spectrometer and the benefits of this technique are demonstrated

Redox dynamics of subduction revealed by arsenic in serpentinite

International audienceRedox dynamics of subduction processes remain poorly constrained owing to the lack of direct geochemical tracers. We studied, using X-ray absorption spectroscopy, the chemical and redox state of arsenic in the Tso Morari serpentinites that are witnesses of the Himalayan subduction. Our measurements reveal remarkably contrasting redox speciation, from arsenide (As-III) to arsenite (As III) and arsenate (As V). Combined with physical-chemical constraints, these data enable reconstruction of the 'redox travel' of arsenic in the subduction process. Upon early serpentinisation of mantle peridotite, arsenic was scavenged from the fluid and dragged down as insoluble nickel arsenide. Partial deserpentinisation close to the peak metamorphism (550-650 °C) resulted in oxidative dissolution of arsenide to aqueous As III and As V and their non-specific intake by antigorite. The As V /As III ratios (∼0.1-10) analysed in the mineral are ∼10 4 times higher on average than predicted assuming bulk system thermodynamic equilibrium. These findings reflect a transient out-of-equilibrium release of highly oxidised fluids, with f O 2 reaching ∼10 log units above the fayalite-magnetite-quartz buffer (FMQþ10). Arsenic in serpentinite is thus a sensitive record of subduction redox dynamics inaccessible when using traditional equilibrium approaches applied to bulk fluid-mineral systems