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Type-1 chromitites are characterized by high-Cr, low-Ti composition and are hosted in Irvine, T.N. Chromian spinel as a petrogenetic indicator. Part 2. inferences as to the petrogenetic origin of these rocks are made here and the podiform to banded chrome spinels occurring in a dunite fraction. An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. Chromian spinels. BET MGM SUPPORT
See text for explanation. Most common mineral assemblages and compositions of the PGM inclusions are given in Figure 7 and Table 4, respectively. Minerals , 8, 13 of 21 Most common mineral assemblages and compositions of the PGM inclusions are given in Figure7 and Table4, respectively. BackBack-scattered-scattered electron electron images images of of representative representative PGM PGM included included in chromite from the Alaskan-typeAlaskan-type chromitites of the Urals.
A Composite crystal composed of kashinite rimmed by bowieite, cuproiridsite, and an unnamed Ir an andd Ni sulfide. B PGM consisting of isoferroplatium, erlichmanite, and osmium in contact with clinopyroxene. C Complex grain of isoferroplatinum, cuprorhodsite, cuproiridsite and erlichmanite. DD Isoferroplatinum Isoferroplatinum in in contact contact with with erlichmanite, pentlandite,pentlandite, and and clinopyroxene.
E BiE -phase Bi-phase grain grainof isoferroplatinum of isoferroplatinum and cuprorhodsite. F PGM composedF PGM composed of isoferroplatinum, of isoferroplatinum, osmium, osmium, and erlichmanite and erlichmanite in contact in contact with with clinopyroxene. H H,I, I Bi Bi-phase-phase grains grains of tetraferroplatinum and and osmium osmium in in contact contact with with clinopyroxene.
Abbreviations Abbreviations for the for studied the studied chromitites chromitites see Figure see Figure 1. In contrast, isoferroplatinum and tetraferroplatinum both enriched in Ni and Cu, would appear to have formed at relatively low fS2 as suggested by failure to crystallize Ni—Cu Ir-thiospinels and erlichmanite Figure7G—I. Minerals , 8, 14 of 21 Table 4. Primary precipitation of sulfides at high temperature is reported exclusively from the Uktus chromitite where laurite with low Os content and kashinite with Ir—Ni—sulfide Figure 7A occur included in fresh chromite.
The study of Kytlym and Uktus chromitites  allows identification of three groups of primary Pt—Fe alloys Figure 8. This indicates that the alloys were a stable phase under relatively high fS2 capable of stabilizing the suite of PGM sulfides reported in the pictures. In contrast, isoferroplatinum and tetraferroplatinum both enriched in Ni and Cu, would Minerals , 8, 15 of 21 appear to have formed at relatively low fS2 as suggested by failure to crystallize Ni—Cu Ir-thiospinels and erlichmanite Figure 7G—I.
Figure 8. Under low sulfur sulfide-richfugacity PGM sulfide assemblages free assemblage from theNi and syngenetic Cu cannot chromitites form independent of Kytlym sulfides and but Uktus. According to the authors , the fS2 exerts strong influence on the composition and paragenetic assemblage of primary Pt—Fe alloys crystallizing at high temperature. At low fS2 both Ni and Cu are correlation in Figure 8. At relatively higher fS2 Ni and Cu tend to form independent sulfides with Ir forcedand to Rh enter overgrowing the alloy the structure Pt—Fe alloys.
The the osmium Pt—Fe alloys. Its texturalOs is mainly relations carried support in primary the inferred erlichmanite. The Figure5 shows that these conditions are comparable with those obtained for initial PGM precipitation within upper mantle chromitites of the ophiolites [22,53].
The Role of Oxygen Fugacity and Temperature Results of the olivine-spinel thermobarometer indicate that the crystallization-equilibration of syngenetic chromitite and accessory chromite in dunite follows a unique trend Figure9A indicating that oxygen fugacity fO2 was increasing during fractionation of dunite to massive chromitite .
The Role of Oxygen Fugacity and Temperature Results of the olivine-spinel thermobarometer indicate that the crystallization-equilibration of syngenetic chromitite and accessory chromite in dunite follows a unique trend Figure 9A indicating that oxygen fugacity fO2 was increasing during fractionation of dunite to massive chromitite . A A Variation Variation of of oxygen oxygen fugacityfugacity as as function function of of temperature temperature in syngenetic in syngenetic chromitite chromitite and and accessoryaccessory chrome chrome spinel spinel disseminated disseminated in the the host host dunite.
These emphasizes the anomaluos behavior of Pt that co-precipitates with the refractory Ir, and is not removed from the melt together with the companion Rh and Pd as a result of the segregation of a magmatic sulfide liquid [24,57]. The discrepancy with chromitites from other geological settings i.
Among others, one possible model assumes that the increase in f O2 required for the crystallization of chromite might have been responsible for the sharp drop of Pt solubility in the silicate melt, causing precipitation of the Pt—Fe alloys [58,59]. However, reversing this cause-effect order,  proposed that it was the strong tendency of Pt to combine with Fe to form Pt—Fe alloys that caused the Pt-solubility falling down.
According to this mechanism, the extensive stabilization of Pt—Fe alloys at high temperature may actually reflect the anomalous increase of the FeO and Fe2O3 activity in the magma parent to Alaskan-type chromitites, that is a major consequence expected from the SiO2-undersaturation condition of these melts. The effect on chromite composition would be the incorporation of larger amounts of magnetite component FeOFe2O3 in the chromite structure, thus simulating an increase of f O2 in the system .
Conclusions The review based on examination of more than analyses of PGM associated with ophiolitic and Alaskan-type chromitites of the Urals reveals that mineralogy of PGM crystallizing at high temperature is controlled by: 1 the nature of the parent melt and relative concentrations of PGE; 2 Minerals , 8, 17 of 21 5. Conclusions The review based on examination of more than analyses of PGM associated with ophiolitic and Alaskan-type chromitites of the Urals reveals that mineralogy of PGM crystallizing at high temperature is controlled by: 1 the nature of the parent melt and relative concentrations of PGE; 2 the presence of melt-soluble clusters of PGE in the parent melt; and 3 by the chemical-physical conditions such as temperature, sulfur- and oxygen-fucagity, prevailing during their precipitation.
The most important factors controlling the precipitation of PGM in ophiolitic chromitites are temperature and sulfur fugacity. The chromitites that have suffered compositional re-equilibration in a wide thermal range e. In this post-magmatic stage, all the PGE that were present in solid chromite as dispersed atomic clusters could easily be converted into discrete PGM inclusions splitting off the chromite structure.
The key factor for the precipitation of abundant Pt—Fe alloys in the Alaskan-type chromitites is the SiO2-undersaturation condition of the parent melt and the oxygen fugacity that was increasing during fractionation of dunite Minerals , 8, 18 of 21 to massive chromitite.
Only a few high refractory PGM might have formed during partial melting in the deep mantle source, being transported as suspended solid particles to the site of chromite deposition Author Contributions: F. All the authors contributed to the interpretation of the data and to organize the manuscript. Funding: E. The comments of two anonymous referees improved the manuscript.
References 1. Barnes, S. The origin of the fractionation of Platinum-group elements in terrestrial magmas. Cabri, L. The nature, distribution, and concentrations of platinum-group elements in various geological environments. Crocket, J. Geochemistry of the Platinum-Group Elements. Legendre, O. Mineralogy of platinum-group mineral inclusions in chromitites from different ophiolite complexes.
Page, N. Palladium , platinum, rhodium , ruthenium and iridium in chromitites from the Massif du Sud and Tiebaghi massif, New Caledonia. Talkington, R. Platinum-group minerals and other solid inclusions in chromite of ophiolitic complexes: Occurrence and petrological significance. Tschermaks Mineralogische Petrogr. Rudashevskiy, N. Garuti, G. In situ alteration of platinum-group minerals at low temperature: evidence from serpentinized and weathered chromitite of the Vourinos Complex, Greece.
Capobianco, C. Partitioning of ruthenium, rhodium, and palladium between spinel and silicate melt and implications for platinum-group element fractionation trends. Acta , 54, — Constantinides, C. The occurrence of platinum group minerals in the chromitites of the Kokkinorotsos chrome mine, Cyprus.
Stockman, H. Platinum-group minerals in Alpine chromitites from southwestern Oregon. Tredoux, M. The fractionation of platinum-group elements in magmatic systems, with the suggestion of a novel causal mechanism. Chromite-platinum-group element magmatic deposits. Razin, L. Geologic and genetic features of forsterite dunites and their platinum-group mineralization. Makeyev, A. Earth Sci. Anikina, Ye.
In Russian Distler, V. Ore petrology of chromite-PGE mineralization in the Kempirsai ophiolite complex. Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex Central Urals, Russia. Deposita , 38, — Grieco, G. Platinum group elements zoning and mineralogy of chromitites from the cumulate sequence of the Nurali massif Southern Urals, Russia. Ore Geol. Ivanov, O. Melcher, F. Moloshag, V. Notes Rus. Pushkarev, E. In Russian Minerals , 8, 20 of 21 Low temperature origin of the Ural-Alaskan type platinum deposits: Mineralogical and geochemical evidence.
Chromium -Platinum deposits of Nizhny-Tagil type in the Urals: Structural-substantial characteristic and a problem of genesis. Sedler, I. Platinum group minerals and associated chrome- spinels of the Alaskan-type Nizhny Tagil massif, Middle Urals. Smirnov, S. Thalhammer, T. Thesis, University of Leoben, Leoben, Austria, Zaccarini, F.
Thesis, University of Bologna, Bologna, Italy, ; p. In Italian Platinum-group element mineralogy and geochemistry of chromitite of the Kluchevskoy ophiolite complex, central Urals Russia. Multi-analytical characterization of minerals of the bowieite-kashinite series from Svetly Bor complex, Urals Russia and comparison with worldwide occurrences.
Platinum-Group Minerals and other accessory phases in chromite deposits of the Alapaevsk ophiolite, Central Urals, Russia. Minerals , 6, Grove and Sack Calibration of spinel-melt geothermometers General approach Experimental data and criteria for equilibrium The thermodynamic background for calculating min- This work began with a detailed systematization of experimental eral-melt equilibria from empirically calibrated geo- data on spinel-melt equilibria in natural systems available in the thermometers for olivine, plagioclase and pyroxenes INFOREX This The current version of INFOREX is a computerized melting- approach is based on iterative solution of a system of experiment reference database containing 6, individual runs from equations, including temperature-compositional de- separate studies carried out between and The program can sort the data based on compositions, pressure, tem- pendencies of equilibrium constants for end-member perature, oxygen fugacity, experimental run duration, types of ex- components and a stoichiometry equation for each perimental containers, and resultant phase assemblages.
The IN- mineral. The main upon the postulated solid and liquid activity models. However, re- calculation of phase equilibria parameters distribution coefficients, gardless of whether a sub-ideal activity model e. This the use of these data for thermodynamic modeling is determining which experiments represent equilibrium compositions.
To solve the Reynolds A complete list of the experimental condi- independent variables has proved efficient in the devel- tions, including calculated spinel and melt molar concentrations, is opment of olivine-melt and plagioclase-melt geother- available from the authors upon request mometers Ariskin et al. These in- cluded single distribution coefficients e. For each permutation of the 28 equilib- a complete description of spinel compositions includ- Sp rium constants, 16 combinations of melt structure- ing above six cations, see Appendix and which best chemical parameters were investigated.
This seems to be important, because the strong stoichiometry. Mathematical processing for each of the complex than that given by Sack et al. In the first simple spinel stoichiometry calculations. We believe stage, the regression parameters were calculated for the that these coefficients might be considered as an em- entire set of coexisting spinel-melt compositions.
The effect of different atures are more than three times the average standard MSCPs on the equilibrium constants might be inter- deviation of the experimental values, these temper- preted as an efficient way to take into account the atures are considered to present non-equilibrium ex- production of some unknown cation activity coeffi- periments and excluded from the second stage process- cients in melts which should be present to the right of ing of the data.
As a rule, five to eight points with Eq. This constraint compositions. Comparison of the calculated and 2 3 Table 2 Regression constants for expressions which best describe the distribution of spinel-forming cations between chromian spinels and melts at atmospheric pressure! Five series of the regression constants correspond to 5 equilibrium constants K and 3 melt structure-chemical parameters R molar i L ratios , included in the calculations as per linear model 1. Standard deviations 1r for the constants are given in parentheses.
The inverse calculations for equilibrium constants lead us to expect that the final To develop a model for simulating naturally occurring spinel-melt equilibrium model based on the equations chromian spinel compositions, it is necessary to link the will also accurately reproduce spinel compositions empirical equations derived from Eq.
The first is 2 4 Sack ; Sack and Ghiorso b. The integration based on the algorithm described in the Appendix for of the empirical equations and the stoichiometry rela- the calculation of spinel temperature and composition tionships into a single algorithm can be used for calcu- in the equilibrium with a melt if oxygen fugacity is lating the saturation temperatures and spinel composi- known.
This subroutine can easily be integrated into tions as a function of magma composition at a the empirical phase equilibria models simulating crys- given pressure and oxygen fugacity. A complete de- tallization of silicate minerals Nielsen ; Ariskin scription of the formalization technique is presented in et al.
The second option represents a slight modification Based on the algorithm proposed, a special petrol- of the basic algorithm. The program is written in Microsoft Fortran temperature for spinel-melt equilibrium is indepen- 5 for IBM-PC or compatibles and can be applied to dently estimated. This could be temperature obtained a wide range of mafic igneous systems for the numerical experimentally for rehomogenized melt inclusions in investigations of the effect of oxygen fugacity and natural crystals e.
Results of testing of the program equilibrium such as the olivine-melt equilibrium. The experimental tem- data. These data pendently available. Based on the results, simulation of chromite saturation was carried out at temperatures slightly For a practical demonstration of the SPINMELT pro- lower but considerably above the olivine liquidus.
The proposed parental oxygen fugacity. The calculations were performed with magma corresponds to a high-magnesium, high-Ti Cr O contents of 0. For the tholeiite that is saturated in silica. Simulation of the 2 3 modeling of chromite saturation temperatures and crystallization sequence for this magma using the compositions at a given oxygen fugacity, we used the COMAGMAT program Ariskin et al.
Results of the calculations olivine is the first silicate liquidus phase at atmospheric are given in Figs. Figure 4A provides pressure.
|Khl betting advice cs||However,limit see arrow some T1 in Figure 5. The most important factors controlling the precipitation of PGM in ophiolitic chromitites are temperature and sulfur fugacity. Chromian spinel as a petrogenetic indicator. Barnes, S. The glassy rims of pillow fragments, the glassy goundmass of large volcanic clasts, and the tuffaceous go here of the sediment are altered to palagonite. Calibration of spinel-melt geothermometers General approach Experimental data and criteria for equilibrium The thermodynamic background for calculating min- This work began with a detailed systematization of experimental eral-melt equilibria from empirically calibrated geo- data on spinel-melt equilibria in natural systems available in the thermometers for olivine, plagioclase and pyroxenes INFOREX|
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|Chromium spinel as a petrogenetic indicator forex||Paragenesis and composition of laurite from chromitites of Othrys Greece : Implication for Os—Ru fractionation in ophiolitic upper mantle of the Balkan Peninsula. A revised and internally consistent thermodynamic ria in natural anhydrous mafic systems. Nikolaev An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. A larger number of detrital particles toward the top of thick crusts record the increasing influence of active volcanoes of the Aleutian arc during northwestward movement of the Pacific plate. Sedler, I.|
|Desktop ethereum wallet||The the osmium Pt—Fe alloys. Standard deviations 1r for the constants are given in parentheses. These data pendently available. In this post-magmatic stage, all the PGE that were present in solid chromite as dispersed atomic clusters could easily be converted into discrete PGM inclusions splitting off the chromite structure. For example, if we assume mental lines obtained by Roeder and Reynoldsa Cr O content of 0. The desulfidation paragenesis of and primary composition PGM during of these serpentinization PGM can be modelled .|
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|Chromium spinel as a petrogenetic indicator forex||Makeyev, A. Comparison of PGE-PGM data with chromite composition allows us to explore the possible correlation between the type of PGM mineralizationMinerals8and conditions invoked for the precipitation of chromite at high magmatic3 of 21 temperature, or the influence of chromite on-cooling equilibration in the subsolidus stage. D LauriteD Laurite rimmed rimmed by erlichmanite. Zaccarini, F. The second option represents a slight modification Based on the algorithm proposed, a special petrol- of the basic algorithm. Multi-analytical characterization of minerals of the bowieite-kashinite series from Svetly Bor complex, Urals Russia and comparison with worldwide occurrences.|
|Chromium spinel as a petrogenetic indicator forex||Petrological applications. The Role of Oxygen Fugacity and Temperature Results of the olivine-spinel thermobarometer indicate that the crystallization-equilibration of syngenetic chromitite and accessory chromite in dunite follows a unique trend Figure 9A indicating that oxygen fugacity fO2 was increasing during fractionation of dunite to massive chromitite . These authors suggested that in natural magmas the PGE do not occur as free cations or any other molecular species, but form disordered clusters consisting of a few hundred atoms, suspended in the melt. Sedler, I. Chromite-platinum-group element magmatic deposits.|
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