Petrology最新文献

The high activity of the Klyuchevskoy group volcanoes in the Holocene suggests that considerable volumes of partly solidified magma (cumulates) and mafic–ultramafic intrusions have accumulated in the crust. Together with extensive fluid flow typical the zones of rapid subduction of an old oceanic plate, this provides conditions for the formation of a fluid–magma ore-forming system. Olivine with sulfide inclusions was found in the eruption products of Tolbachik Volcano. Its investigation may provide insight into the composition of crustal fluid of such ore-magmatic systems. The interaction of reduced water-poor fluid with oxidized basaltic melt (NNO + 1.5) containing 2000–3000 ppm sulfur was theoretically modeled. It was shown that at a local fluid content higher than ~1–2 wt %, sulfur in the melt is reduced and sulfide droplets are formed. Sulfur reduction in the melt can also be caused by the dissolution of SO₂, which is the main sulfur species in fluid at log fO₂ ≥ NNO + 1.5. This effect is related to the higher degree of sulfur oxidation (S⁺⁶) in melt, where ({text{SO}}_{4}^{{2 - }}) is the only oxidized sulfur species, compared with SO₂ (S⁺⁴) in fluid. According to calculations, sulfide formation begins after dissolution of approximately 2000–3000 ppm sulfur in the SO₂ form in melt at log fO₂ ≥ NNO + 1. Interaction with fluid with small contents of precious metals (PM) produces sulfide melt droplets with PM contents corresponding to the background values in the melt. According to experimental evidence, Pt and Pd are highly soluble in reduced water-poor fluids in the form of carbonyls, whereas Au is low soluble; in contrast, Au solubility is very high in oxidized fluids (NNO + 1 to NNO + 1.5). Reaction with mineralized fluid containing up to tens of ppm PM produces sulfide melt enriched in Au (oxidized fluid) or Pt (reduced fluid). Interaction of melt with water-poor fluid causes local dehydration and an increase in liquidus temperature, which results in rapid olivine crystallization at high overcooling. The localization of phase transitions at the boundary of fluid bubbles facilitates the entrapment of sulfide droplets by olivine. The rare occurrence of sulfide inclusions in olivine from Tolbachik Volcano can be related to the rapid dissipation of the local effect of magma interaction with small amounts of fluid and dissolution of the precipitated sulfide phase in the melt.

The Norilsk–Talnakh magmatic sulfide Cu–Ni–PGE (platinum-group elements) deposits were formed by the accumulation of metals in immiscible sulfide melt comagmatic with the parental mafic–ultramafic magma. In this study, the main types of magmatic sulfide ores of the Norilsk–Talnakh deposits are considered as manifestations of different stages in the evolution of the initial sulfide melts. In the context of the overall evolution of Norilsk sulfide melts, the earliest ores are Cu-poor pyrrhotite ores with high concentrations of Rh and IPGE (Os, Ir, and Ru), which were discovered at the Talnakh deposit. The second stage of sulfide melt evolution was marked by the formation of most disseminated ores and Cu- and PGE-poor massive pyrrhotite ores. The massive and disseminated ores were formed independently from each other, but generally correspond to the melts with identical compositions. The only exception is low-sulfur PGE-rich ores from the Upper Gabbroid rocks of the differentiated intrusions, which were affected by wall rock assimilation and early magmatic degassing. It has been shown that the concentrations of ore components in the disseminated sulfides, which are examples of in-situ crystallized droplets of immiscible sulfide melt, vary depending on the composition and degree of fractionation of the parental silicate magma. During the final stage, the crystallization of the residual sulfide melts led to the formation of Cu-rich ores with high Pt and Pd contents. The compositions of these main ore types are compared with the compositions (including trace elements) of their base metal sulfides (BMS). All element dependencies in the massive ores follow the fractional crystallization trend of the sulfide melt. PGE in Norilsk ores are concentrated in distinct platinum-group minerals (PGM) and occur as trace elements in BMS. Rhodium and IPGE are concentrated in pyrrhotite, pentlandite, and pyrite; Pt is occasionally found in pyrite; whereas Pd is found predominantly in pentlandite. The concentration of Pd in pentlandite increases from the Cu-poor to Cu-rich ores. Based on a detailed analysis with the application of several methods, the Pd-rich pentlandite (containing 4.84 wt % Pd) from massive primary magmatic Cu-rich MSS–ISS ores is thought to have been formed by a high-temperature mechanism involving a reaction with sulfide melt. Using X-ray absorption spectroscopy (XAS), the oxidation state of Pd in pentlandite (2⁺) and its occurrence in the form of a solid solution, in which Pd apparently replaces Ni in the pentlandite structure, were identified for the first time.

This paper reports new experimental results on the chemical counterdiffusion of major components (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the ({text{CO}}_{3}^{{2 - }}) anion during interaction of basalt and kimberlite melts under upper-mantle pressure. The method of diffusion couples was employed on a BARS split-sphere apparatus at 5.5 GPa and 1850°C. It was shown that the rates of chemical counterdiffusion of all major melt species (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the ({text{CO}}_{3}^{{2 - }}) anion are almost identical during interaction of model basalt and carbonate-bearing kimberlite melts and approximately an order of magnitude higher than the diffusion rates of these components during melt interaction under moderate pressures (100 MPa). The equal diffusion rates of CaO and ({text{CO}}_{3}^{{2 - }}) indicate that molecular CaCO₃ diffusion from the kimberlitic to basaltic melt (model and natural) occurs also at the high pressure. The diffusion patterns are dramatically different during interaction of natural magnesian basalt and model kimberlite, which was observed for the interaction of these melts at moderate pressure. In addition to the molecular diffusion of CaCO₃ into the magnesian basalt, the diffusion rates of other melt species increase significantly. All diffusing components show weak exponential dependence on concentrations approaching D_i = const, similar to that observed during interaction of such melts at moderate pressures.

In this paper, we examine the compositions of basaltic melts from ocean islands using our regularly updated database of analyses of melt inclusions in minerals and glasses of igneous rocks. Mean concentrations of major, trace, and volatile components in melts were calculated for the global dataset (22 550 analyses from 33 island systems) and for the most extensively studied complexes (Iceland, Hawaii, Canaries, Galapagos, and Reunion). It was found that the mean contents of most elements fall between the mean compositions of magmas from mid-ocean ridges (most depleted) and intraplate continental complexes (most enriched). Variations of element ratios in particular complexes were considered in detail, and it was found that they can be described by mixing magmas from a depleted source and variably enriched materials. The abundances of trace elements in the supposed mantle sources were estimated. The depleted source is most clearly manifested in Iceland and is almost identical to the depleted mantle the melting of which produces mid-ocean ridge basalts.

Metabasites within the Jandaq Metamorphic Complex (JMC), Iran, offer valuable insights into the region’s magmatic and metamorphic history. Whole-rock geochemical data (major, trace, and rare earth elements) coupled with Sm-Nd isotopes were used to decipher the protolith origin and tectonic setting of formation of these metabasites. Our results demonstrate a predominantly ortho-amphibolitic nature for the JMC metabasites, with igneous protoliths ranging from basalt to andesite based on geochemical discrimination diagrams (Zr versus MgO and Sm/Nd). They exhibit geochemical affinities closer to enriched mid-oceanic ridge basalts (E-MORB) rather than normal MORB, implying a nascent oceanic basin within an intracontinental extensional setting. Trace element signatures (LILE enrichment, HFSE depletion) suggest a metasomatized subcontinental lithospheric mantle (SCLM) or a metasomatized lithospheric mantle beneath the oceanic crust as the parental magma source. Sm-Nd isotopic data suggest a potential plume source for the protoliths. These rocks were metamorphosed further by at least three metamorphic events: M1 (regional metamorphism, Barrovian-type; 616–687°C, 8–11 kbar), M2 (a brittle deformation event), and a later retrograde metamorphism (M3). These findings provide a comprehensive understanding of the geochemical characteristics, tectonic setting, and metamorphic evolution of JMC metabasites, shedding light on the geological history of the Jandaq region as a Paleo-Tethyan remnant.

A diagram is proposed for discriminating between at least some A-type intraplate and postcollisional granitoids. The diagram is based on data of the discriminant analysis of geochemically similar Phanerozoic A-type granitoids and is demonstrated to be able to identify the types not only of Phanerozoic but also of Precambrian A-type granitoids.

The paper presents experimental data on melting model compositions of basaltic komatiite (BK) and enstatite chondrite (ECH) at a temperature of T = 1300°C and hydrogen pressure ({{P}_{{{{{text{H}}}_{{text{2}}}}}}}) = 100 MPa. The experiments modeled interaction between the magma ocean and the early Earth’s hydrogen atmosphere. The experimental products consist of silicate glass (quenched melts), which is notably depleted in FeO but enriched in lithophile oxides and H₂O, and iron with minor Si and O admixtures. The equilibrium oxygen fugacity in the experimental runs was approximately two logarithmic units below the Fe−FeO buffer. Calculation of the fractional crystallization of the melts indicates that the complete crystallization products are granodiorite, which consists of two feldspars, clinopyroxene, and quartz with a minor amount of black mica (for the starting composition obtained in the run with BK), or quartz−two feldspars granite with minor amounts of biotite and muscovite (for the starting composition obtained in the run with ECH). Crystallization of zircon from the ECH melt might occur at T = 730−750°C. Our proposed model is the first that explains generation of melts enriched in SiO₂ and H₂O by internal processes of planetary evolution and does not invoke pre-hydrated upper crust for generating the Earth’s first felsic material.