Metals in Avachinsky peridotite xenoliths with implications for redox heterogeneity and metal enrichment in the Kamchatka mantle wedge

Kepezhinskas, Pavel; Бердников, Н. В.; Kepezhinskas, Nikita; Konovalova, N. S.

doi:10.1016/j.lithos.2022.106610

Cited by 17 publications

(27 citation statements)

References 113 publications

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“…Gold and other noble metal enrichments were previously reported for both sub-arc mantle sources [19,50,[81][82][83] and subduction-related crustal magmatic conduits [19,36], as well as primitive basaltic and andesitic magmas in volcanic arcs [54,84,85]. The distribution of gold in differentiated arc-related plutonic and volcanic suites suggests that Au behaves as an incompatible element during the fractionation of primitive convergent zone magmas [36,54,85,86].…”

Section: Geologic Conditions Of the Formation Of Cu-ag-au Alloys In M...mentioning

confidence: 65%

“…and utilization of the "fresh fracturing" sample preparation method immediately prior to loading of the freshly broken sample surface into the sample chamber of the SEM instrument. We previously described these sample preparation methods in much detail [50]. The data set used to in this paper is based on the samples collected by the Khingan Minerals AS (Oslo, Norway; personal communication, 2021) over the entire area of the relatively well-exposed central part of the Ildeus intrusion (Figure 1c).…”

Section: Methodsmentioning

confidence: 99%

“…Numerous relatively large (50-200 µm) particles of Au-bearing intermetallic compounds were recovered from olivine websterite sample IL-5 (most enriched in Au = 596.18 ppm) by rough crushing and gravitational separation, following the general methods described in [50]. These are irregularly shaped, partially facetted Cu-Ag-Au grains with multiple crystalline inclusions and specific inner structure (Figure 6).…”

Section: Gold Alloys In the Ildeus Intrusionmentioning

confidence: 99%

“…Another controlling factor, which can greatly influence the formation of Cu-Ag-Au alloys during the crustal fractionation of convergent zone magmas, is low oxygen fugacity. Although current petrologic models suggest that most arc magmas and their ultramafic cumulates are variably oxidized in comparison to mid-ocean ridge basalts (MORB) [69,70], some recently published data indicate that modern and fossil subduction zones are characterized by substantial redox heterogeneity, with some portions of the subducted slab and mantle wedge associated with reduced conditions (negative ∆FMQ values) [36,50,71,72]. Moreover, various geochemical proxies suggest that early fractionates of convergent zone magmas (primitive arc cumulates) can be generally reduced (similar to MORB) and increase in oxygen fugacity during their further crustal magmatic differentiation [73,74].…”

Section: Physico-chemical Conditions Of Cu-ag-au Alloys Formationmentioning

confidence: 99%

“…Consequently, the search for "primary magmatic" gold should concentrate on mineralized systems hosted in deep-seated, relatively reduced crustal and mantle rocks, which were not previously subjected to the upper crustal oxidation. Primary magmatic gold particles (clustering around Cu 30 Ag 10 Au 60 compositions) were documented in pyroclastic rocks associated with iron mineralization in the Lesser Khingan Range [65], mantle wedge peridotite xenoliths from Kamchatka [50] and adakites from the Stanovoy suture zone [54]. Oen and Kieft [80] reported native gold particles with 11 wt.% copper in the hydrothermal Ni-Co arsenide ores from the chromitites in the serpentinized Beni Bousera Ultramafic Massif (Morocco).…”

Section: Geologic Conditions Of the Formation Of Cu-ag-au Alloys In M...mentioning

confidence: 99%

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Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

et al. 2022

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Gold is typically transported by mafic and evolved magmas into the upper crust to be deposited in shallow oxidized porphyry and epithermal environments. However, the magmatic behavior of gold is still poorly understood and warrants further attention. Additional insights into the magmatic evolution of gold and other noble metals can be provided by investigations of primitive convergent zone magmas and products of their differentiation that contain primary-textured Au-alloys. One of the best examples of such Au-rich ultramafic cumulates is the Triassic (232–233 Ma) Ildeus intrusion, which was emplaced within the Mesozoic Stanovoy subduction zone in the Russian Far East. Some websterites from the Ildeus intrusion, representing cumulates crystallized from a primitive convergent zone magma, are enriched in Au (up to 596 ppm) and contain abundant Cu-Ag-Au micro-particles. Most of these Au-alloy micro-particles display compositions similar to those previously found in explosive pyroclastic rocks in the Lesser Khingan iron district, mantle wedge peridotites in Kamchatka and Cretaceous adakites in the Stanovoy suture zone. Textural and compositional characteristics suggest that Cu-Ag-Au alloys precipitated from a primitive calc-alkaline melt during its crustal differentiation in a Mesozoic paleo-subduction zone. Some large Cu-Ag-Au grains display an internal honeycomb-like structure with alternating Cu-rich and Cu-poor zones. Heating experiments under atmospheric conditions recorded a substantial loss of Cu from primary magmatic Cu-Ag-Au alloys, which appears to be a process characteristic of oxidized hydrothermal ore systems. We suggest that the later-stage hydrothermal alteration of differentiated igneous conduits containing magmatic gold alloys results in the formation of Cu-free gold mineralization comparable to the upper crustal porphyry and epithermal environments.

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Section: Geologic Conditions Of the Formation Of Cu-ag-au Alloys In M...mentioning

confidence: 65%

Section: Methodsmentioning

confidence: 99%

Section: Gold Alloys In the Ildeus Intrusionmentioning

confidence: 99%

Section: Physico-chemical Conditions Of Cu-ag-au Alloys Formationmentioning

confidence: 99%

Section: Geologic Conditions Of the Formation Of Cu-ag-au Alloys In M...mentioning

confidence: 99%

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Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

et al. 2022

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H2-H2O immiscibility in Earth’s upper mantle

Vlasov

Audétat

Keppler

2023

Contrib Mineral Petrol

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Immiscibility between water and hydrogen-rich fluids may be responsible for the formation of super-reduced mineral assemblages and for the early oxidation of Earth´s upper mantle. In the current study, we present new data on the critical curve in the H2-H2O system to 1400 ℃ and 4 GPa. We utilized a synthetic fluid inclusion method to trap fluids at high P–T conditions within quartz and olivine crystals. Experiments were performed in a piston-cylinder type apparatus, employing a double-capsule technique. The inner capsule contained the crystal and fluids of interest, while the outer served as oxygen fugacity buffer, maintaining f(O2) at the iron-wüstite (Fe-FeO) equilibrium. Our results suggest that below ~ 2.5 GPa, the critical curve has a mostly linear slope of 200 ℃/GPa, while at more elevated pressure it becomes significantly steeper (400 ℃/GPa). This implies that in most of the modern, reduced upper mantle, water and hydrogen are immiscible, while localized heating events, such as rising plumes, may close the miscibility gap. The steep increase of the critical curve at high pressure observed in this study implies that even for very hot geotherms in the early Archean or the late Hadean, H2-H2O immiscibility likely occurred in the deeper parts of the upper mantle, thus making a plausible case for rapid H2 loss as a mechanism of early mantle oxidation. To constrain the geochemical fingerprint of this process, we performed a series of element partitioning experiments to unravel how the H2-H2O unmixing may affect element transfer. Noble gases such as Xe as well as methane are preferentially incorporated in the hydrogen-rich phase, with a XeH2O/XeH2 ratio of ~ 8. This partitioning pattern may, for example, explain the underabundance of Xe isotopes produced by fission of Pu in the mantle. These Xe isotopes may have been removed by a primordial H2-H2O unmixing event during the early stages of planetary evolution.

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Mantle-to-crust metal transfer by nanomelts

Schettino

Gonzáléz-Jiménez

Marchesi

et al. 2023

Commun Earth Environ

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The transfer of chalcophile metals across the continental lithosphere has been traditionally modeled based on their chemical equilibrium partitioning in sulfide liquids and silicate magmas. Here, we report a suite of Ni-Fe-Cu sulfide droplets across a trans-lithospheric magmatic network linking the subcontinental lithospheric mantle to the overlying continental crust. Petrographic characteristics and numerical calculations both support that the sulfide droplets were mechanically scavenged from the mantle source during partial melting and transported upwards by alkaline magmas rising through the continental lithosphere. Nanoscale investigation by high-resolution transmission electron microscopy (HR-TEM) documents the presence of galena (PbS) nanoinclusions within the sulfide droplets that are involved in the mantle-to-crust magma route. The galena nanoinclusions show a range of microstructural features that are inconsistent with a derivation of PbS by exsolution from the solid products of the Ni-Fe-Cu sulfide liquid. It is argued that galena nanoinclusions crystallized from a precursor Pb(-Cu)-rich nanomelt, which was originally immiscible within the sulfide liquid even at Pb concentrations largely below those required for attaining galena saturation. We suggest that evidence of immiscibility between metal-rich nanomelts and sulfide liquids during magma transport would disrupt the classical way by which metal flux and ore genesis are interpreted, hinting for mechanical transfer of nanophases as a key mechanism for sourcing the amounts of mantle-derived metals that can be concentrated in the crust.

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Metals in Avachinsky peridotite xenoliths with implications for redox heterogeneity and metal enrichment in the Kamchatka mantle wedge

Cited by 17 publications

References 113 publications

Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

H2-H2O immiscibility in Earth’s upper mantle

Mantle-to-crust metal transfer by nanomelts

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