Petrology最新文献

{"title":"New Data on the Rock and Mineral Composition of Kharchinsky and Zarechny Volcanoes, Central Kamchatka Depression: Heterogeneity of the Mantle Source and Peculiarities of Magma Evolution in the Crust","authors":"N. V. Gorbach,&nbsp;N. A. Nekrylov,&nbsp;M. V. Portnyagin,&nbsp;K. Hoernle","doi":"10.1134/S0869591123030050","DOIUrl":"10.1134/S0869591123030050","url":null,"abstract":"<p>Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific plate subducting beneath Kamchatka. This paper presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are magnesian (Mg# = 60–75 mol %) medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = 52–59 mol %) basaltic andesites are present among the monogenic cones of Kharchinsky Lake. The rare rock varieties include the high-K basalts–basaltic andesites of dikes in the center of Kharchinsky volcano and the magnesian andesites (Mg# = 58–61 mol %) of the extrusions of Zarechnу volcano. The distribution of trace-element contents in these samples demonstrates enrichment in large-ion lithophile elements and light REEs at depletion in high field strength elements and heavy REEs, as is typical of arc rocks. The high-K basalts and basaltic andesites show anomalous enrichment in Ba (&gt;1000 ppm), Th (&gt;3.8 ppm), U (&gt;1.8 ppm), Sr (&gt; 800 ppm, Sr/Y &gt; 50), and light REE (La &gt; 20 ppm), and their compositions are close to those of low-Si adakites. The basalts and basaltic andesites contain phenocrysts of high-Mg olivine (up to <i>Fo</i>_92.6) and clinopyroxene (Mg # up to 91 mol %). The rocks show petrographic and geochemical evidence of fractional crystallization, along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and an elevated Fe/Mn ratio (up to 80), which were interpreted as evidence of the participation of a pyroxenite source in the magma generation processes. The use of the Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivine associated with different sources: peridotite and pyroxenite, which were formed by a reaction between mantle-wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other lines of evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka Depression at the Kurile–Kamchatka and Aleutian subduction zone junction and testify to a significant heterogeneity of the mantle in this area.</p>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"320 - 337"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5088961","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Magnesian Basalts of the Medvezhia Caldera: Dominant Magmas and Their Sources, as Exemplified by Menshiy Brat Volcano, Iturup Island, Kuriles","authors":"D. V. Kuzmin,&nbsp;I. R. Nizametdinov,&nbsp;S. Z. Smirnov,&nbsp;T. Yu. Timina,&nbsp;A. Ya. Shevko,&nbsp;M. P. Gora,&nbsp;A. V. Rybin","doi":"10.1134/S0869591123030062","DOIUrl":"10.1134/S0869591123030062","url":null,"abstract":"<p>The paper presents new data on the formation conditions of basalts from Menshiy Brat postcaldera volcano in the Medvezhia caldera, Iturup Island. The liquidus mineral assemblage consists of olivine (<i>Fo</i> 85.3–90.1 mol %) and chromium spinel (Cr# = 0.46–0.6), which crystallized at 1090–1170°C and oxygen fugacity at NNO + 0.6 (σ = 0.2) to NNO + 0.2 (σ = 0.14) in the course of the eruption. Data on melt inclusions in the liquidus olivine demonstrate that its parental melts were low-Al₂O₃ and low-K₂O, with up to 15.5 wt % MgO, and with an average H₂O content of 5.5 wt %. The newly obtained data on volatile contents in the olivine-hosted melt inclusions suggest that the mafic melts were derived by the partial melting of a peridotitic-rich source with a small admixture of an olivine-free component at 1225°C, under active influence of the slab-derived fluids. These fluids were separated from the subducting slab at 670–705°C and depths of 95–105 km beneath Iturup Island. Our results enhance our understanding of the evolution of basic magmas that serve as a heat and volatile sources during the formation of large calderas.</p>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"279 - 303"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5088127","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Origin of Olivine Basalts from Medvezhya Mount (Avachinsky Group of Volcanoes, Kamchatka): The Evidence for Assimilation of Sulfide-Bearing Cumulates","authors":"D. P. Savelyev,&nbsp;N. V. Gorbach,&nbsp;M. V. Portnyagin,&nbsp;V. D. Shcherbakov","doi":"10.1134/S0869591123030074","DOIUrl":"10.1134/S0869591123030074","url":null,"abstract":"<div><p>The role and conditions of liquid immiscibility or crystallization of sulfide phase during evolution of subduction-related magmas remains a debated topic, which bears relevance to the genesis of porphyry copper deposits and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine—the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol %) middle-K island-arc olivine basalts. Olivine with normal zoning predominates (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt % and decrease of Fe/Mn from 60 to 35 with a change of olivine composition from <i>Fo</i>_87.8 to <i>Fo</i>_78.2. Olivine of this group contains numerous inclusions of spinel-group minerals varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence of or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures are 1036–1241°C for olivine of the prevailing type and 1010–1062°C for sulfide-bearing olivine. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong magma oxidation. On the contrary, the zoning pattern and compositional features of sulfide-bearing olivine suggest its xenogenic origin and the probable crystallization under deep-crustal conditions from low-temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.</p></div>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"263 - 278"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5086054","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Conditions and Magmas Sources of the Summit and Flank Eruptions of Klyuchevskoy Volcano in 2020–2021: Isotope (Sr–Nd–Pb–O)-geochemical data","authors":"R. I. Cherkashin,&nbsp;O. V. Bergal-Kuvikas,&nbsp;A. V. Chugaev,&nbsp;Yu. O. Larionova,&nbsp;I. N. Bindeman,&nbsp;A. L. Khomchanovsky,&nbsp;E. Yu. Plutakhina","doi":"10.1134/S0869591123030037","DOIUrl":"10.1134/S0869591123030037","url":null,"abstract":"<div><p>In 2021, a unique event occurred on Klyuchevskoy volcano (Kamchatka). After over 30-year prevalence of summit eruptions, a flank vent was formed. It was named after the Corresponding Member of the USSR Academy of Sciences G.S. Gorshkov. The eruption began immediately after the end of the summit crater activation in 2020–2021 at an altitude of 2850 m in the northwestern part of the volcano, where manifestations of flank volcanism were not observed earlier. This paper presents geochemical and isotopic Sr–Nd–Pb–O data on lavas of the summit and flank eruptions of Klyuchevskoy volcano in 2020–2021. A comparative petrographic analysis was carried out and the chemical composition of the <i>Ol</i>, <i>Cpx</i>, and <i>Pl</i> phenocrysts in these lavas was analyzed. The lavas of both eruptions are alumina andesitic basalts of normal alkalinity. Variations of major oxides in the lavas of the summit eruption and G.S. Gorshkov vent are SiO₂ 53.1–53.2 wt % and 51.6–53.2 wt %, MgO 5.6 wt % and 5.5–6.0 wt %; respectively. Temperature and pressure estimates showed that plagioclase crystallization occurred at 1210–1118°С and 12.3–3.6 kbar in lavas of the summit eruption and at 1203–1119°С and 9.0–3.3 kbar in lavas of the flank vent. The contents of major elements, similar conditions of plagioclase generations and compositional variations of <i>Ol</i>, <i>Cpx</i>, and <i>Pl</i> phenocrysts in the lavas of both eruptions indicate a genetic relationship of the magmas that fed the summit and flank eruptions. The lavas of the 2016 and 2020–2021 summit eruptions, as well as the lavas of the previous summit eruptions of Klyuchevskoy volcano are characterized by the persistent Sr–Nd–Pb isotopic characteristics: ⁸⁷Sr/⁸⁶Sr = 0.703625–0.703626, ¹⁴³Nd/¹⁴⁴Nd = 0.513085–0.513102, ²⁰⁶Pb/²⁰⁴Pb = 18.3148–18.3179. Isotopic ratios ²⁰⁷Pb/²⁰⁴Pb (15.5022–15.5107) and ²⁰⁸Pb/²⁰⁴Pb (37.9597–38.0143) are significantly higher for the lavas of the last summit and flank eruptions than for all Klyuchevskoy lavas of the past eruptions, which indicates more complex magma evolution at crustal levels. The values of δ¹⁸O = 6.49–7.39‰ (SMOW)-in the lavas of the considered eruptions are consistent with previously published data on Klyuchevskoy volcano. The lavas of the Gorshkov vent are enriched with Ba, Zr, Sr and other incompatible elements at constant MgO values in comparison with the lavas of the last summit eruptions, which points to the different evolutionary paths of their magmas. Sharply increased values of the ⁸⁷Sr/⁸⁶Sr ratio (0.703673–0.703743) in the lavas of the G.S. Gorshkov vent, which were not previously observed in the lavas of Klyuchevskoy volcano, testify to the intense crustal assimilation of initial melts in the northwestern part of the volcano.</p></div>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"304 - 319"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5088123","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sulfide Mineralization in Pyrometamorphosed Upper Crustal Xenoliths, Bezymianny Volcano, Kamchatka","authors":"V. O. Davydova,&nbsp;V. D. Shcherbakov,&nbsp;N. A. Nekrylov,&nbsp;P. Yu. Plechov,&nbsp;V. O. Yapaskurt","doi":"10.1134/S0869591123030049","DOIUrl":"10.1134/S0869591123030049","url":null,"abstract":"<div><p>Bezymianny volcano eruptions transport numerous xenoliths to the surface. Crustal xenoliths contain unique information about the crust structure and composition of crustal rocks located around the active magmatic system. We describe the chemical and mineral composition of upper crustal xenoliths that pyrometamorphosed (recrystallized and partially melted) in the Bezymianny shallow chamber. We reconstructed protoliths and hydrothermal processes for several rocks, which were previously altered, based on pre-pyrometamorhic relics (primary igneous associations in some xenoliths and rare hydrothermal relics). Moderate-K andesites, basaltic andesites, and basalts of Kamen and Bezymianny volcanoes dominate among the xenoliths. During pyrometamorphism, a microgranoblastic assemblage composed of homogenous pyroxenes, plagioclase, Fe-Ti oxides, and interstitial glass is formed in these xenoliths. Less common xenoliths are presented by high-K basaltic trachyandesite (plateau basalt from the basement of the Klyuchevskaya group of volcanoes). Quartz–carbonate–sulfide mineralization is present in some of them, which formed prior to xenolith entrapment and pyrometamorphism. When xenoliths were entrapped by magma, recrystallization of hydrothermally altered rock produced Fe-wollastonite–hedenbergite association (in some cases with garnet), untypical for Bezymianny. Some of these xenoliths have extremely high copper content (up to 1500 ppm).</p></div>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"358 - 382"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5088124","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Subduction and Oceanic Magmatism Records in Plutonic Rocks of the Kamchatsky Mys Ophiolite, Eastern Kamchatka","authors":"B. A. Bazylev,&nbsp;M. V. Portnyagin,&nbsp;D. P. Savelyev,&nbsp;G. V. Ledneva,&nbsp;N. N. Kononkova","doi":"10.1134/S0869591123030025","DOIUrl":"10.1134/S0869591123030025","url":null,"abstract":"<p>The paper presents petrographic, mineralogical, and geochemical data on dunites, pyroxenites, peridotites, and gabbroids of the Kamchatsky Mys ophiolite. These data were acquired to distinguish cogenetic assemblages of igneous rocks, gain an insight into their geodynamic settings, and test various criteria of genetic links between the different magmatic rocks of ophiolites. The ultramafic and mafic rocks are shown to belong to two series, which differ in the compositions of the primary minerals, bulk rocks, and estimated trapped melts. The rocks of these series are found out to have been produced by geochemically different melts in different geodynamic settings, and during different episodes of mantle magmatism. The rocks of the high-Ti series (gabbro of the Olenegorsk massif, dunite and melanogabbro xenoliths in them, and vein gabbro in these xenoliths) crystallized from N-MORB melts in an oceanic spreading center. The rocks of the low-Ti series (dunite, pyroxenite, and gabbro veins in the residual spinel peridotites of the Mount Soldatskaya massif, as well as pyroxenite, peridotite, and gabbro alluvium and diluvium in the central and western parts of the peninsula) crystallized from water-rich boninite melts in relation to initial subduction magmatism. Taken into account the absence of boninite lavas from the Kamchatsky Mys ophiolite, the plutonic ultramafic rocks (including the rocks of the veins) might be the only evidence of subduction boninitic magmatism in the ophiolites. It was demonstrated that conclusions about the geodynamic settings of plutonic ultramafic and mafic rocks and recognition of cogenetic relations of these rocks with spatially associated basalts are more reliable when derived from the compositions of the trapped melts, which are estimated from their bulk geochemistry and primary mineral compositions, than when they are based on the mineral compositions only.</p>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"338 - 357"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5087076","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Magmatism in Kamchatka and the Kurile Islands","authors":"","doi":"10.1134/S0869591123030086","DOIUrl":"10.1134/S0869591123030086","url":null,"abstract":"","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 3","pages":"261 - 262"},"PeriodicalIF":1.5,"publicationDate":"2023-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"5088963","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Compositions of Kimberlite Melts: A Review of Melt Inclusions in Kimberlite Minerals","authors":"A. V. Golovin,&nbsp;V. S. Kamenetsky","doi":"10.1134/S0869591123020030","DOIUrl":"10.1134/S0869591123020030","url":null,"abstract":"&lt;p&gt;The paper presents a comprehensive review of currently available data on melt inclusions entrapped in minerals of kimberlites of different age and different provenance in ancient cratons. The crystallized melt inclusions represent snapshots of kimberlite melts at different stages of their evolution. All of the inclusions are completely crystallized and consist of daughter minerals and shrinkage bubbles, which sometimes contain low-density CO&lt;sub&gt;2&lt;/sub&gt;, but no aqueous fluids and quenched silicate glasses have been found so far. Although more than 60 mineral species have been identified among the daughter phases in the inclusions, all inclusions hosted in various minerals from different kimberlites have closely similar or even identical composition. The daughter minerals are various Na–K–Ca, Na–Ca, Na–Mg, K–Ca, Ca–Mg, Ca, Mg, and Na carbonates; Na–Mg and Na carbonates with additional anions Cl&lt;sup&gt;–&lt;/sup&gt;, &lt;span&gt;({text{SO}}_{4}^{{2 - }})&lt;/span&gt;, and &lt;span&gt;(text{PO}_{4}^{3 - })&lt;/span&gt;; and alkali sulfates, chlorides, phosphates, sulfides, oxides, and silicates. Alkali carbonates, sulfates, and chlorides are usually absent from among the groundmass phases of most kimberlites sampled worldwide, except the Udachnaya-East kimberlite in Siberia. However, this mineral assemblage, in association with such widespread kimberlite minerals as olivine, micas, monticellite, spinel-group minerals, perovskite, rutile, ilmenite, calcite, and dolomite, is common in the crystallized melt inclusions in all studied kimberlites. Carbonates (~30 to 85 vol %) always dominate over silicates (no more than 18 vol %) in all inclusions. All inclusions also contain variable (2 to 55 vol %.) amounts of chlorides (halite and sylvite). In cases where the abundance of carbonates is relatively low (30–50 vol %), the other major phases within inclusions are chlorides (18–55 vol %) rather than daughter silicates, as could be expected based on the traditional paradigm of the silicate composition of kimberlite melts. Published data on melt inclusions in the kimberlite minerals strongly imply that parental kimberlite melts were generated and further evolved within the Na&lt;sub&gt;2&lt;/sub&gt;O–K&lt;sub&gt;2&lt;/sub&gt;O–CaO–MgO–CO&lt;sub&gt;2&lt;/sub&gt;–Cl system, that is, they were alkali-rich carbonate/carbonate–chloride liquids. According to various estimates, SiO&lt;sub&gt;2&lt;/sub&gt; content in kimberlite melts could have varied during different stages of their evolution from a few to 19 wt %. Clearly, kimberlite bodies are altered in the crust via interaction with meteoric and/or connate waters, resulting in serpentinization of kimberlite olivine and dissolution of many bona fide magmatic minerals, such as alkali carbonates, sulfates, and chlorides. In the traditional approach to studying kimberlites, the role of such components as Na&lt;sub&gt;2&lt;/sub&gt;O, CO&lt;sub&gt;2&lt;/sub&gt;, Cl, and to a lesser extent K&lt;sub&gt;2&lt;/sub&gt;O, S, and F in the petrogenesis of kimberlite magmas and rocks have been largely underestimated, while the roles of o","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 2","pages":"143 - 178"},"PeriodicalIF":1.5,"publicationDate":"2023-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4618789","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Variations in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: a Consequence of Various Mantle Sources or Crustal Contamination","authors":"O. M. Turkina","doi":"10.1134/S0869591123020066","DOIUrl":"10.1134/S0869591123020066","url":null,"abstract":"<div><p>The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. The mafic granulites are characterized by the mineral association <i>Cpx + Pl ± Hbl ± Opx ± Qz</i> and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concentrations of TiO₂ and immobile trace elements (REE, Zr, Nb), and mainly positive ε_Nd(Т) values. The first type of mafic granulites show elevated (La/Sm)_n and enrichment in Th and LREE relative to Nb, which is typical of subduction-related or crustally contaminated basalts. The absence of negative correlation between (La/Sm)_n and ε_Nd(Т) and a clear positive correlation of TiO₂ with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of the first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental lithospheric mantle modified by melts derived from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO₂ content, positive ε_Nd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Mafic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)_n, depletion in Nb relative to Th and LREE, and negative ε_Nd(Т) values. Post-magmatic influence of granitoids leads to the enrichment of mafic granulites in biotite and apatite, an increase in concentrations of K₂O, P₂O₅, a significant enrichment in Zr, Nb, Th, LREE, and negative ε_Nd(Т) values. The difference between mafic granulites of the first and second types is not related to crustal contamination, but is caused by melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted at the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements was likely caused by felsic melts generated from the rocks of subducting plate immediately prior to mafic magmatism.</p></div>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 2","pages":"204 - 222"},"PeriodicalIF":1.5,"publicationDate":"2023-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4615613","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Native Iron in Siberian Traps","authors":"M. D. Tomshin,&nbsp;A. G. Kopylova,&nbsp;A. E. Vasilyeva","doi":"10.1134/S0869591123020054","DOIUrl":"10.1134/S0869591123020054","url":null,"abstract":"<p>The study of trap intrusions with a large-scale occurrence of native iron allowed us to identify general features in their composition and origin. Intrusive bodies are weakly differentiated and have similar structure and mineralogical, petrochemical and geochemical composition. Two associations of rock-forming minerals were found in all studied bodies: early deep-seated (pre-chamber) and intra-chamber. Native iron forms nodular segregations, with a subordinate amount of cohenite, troilite and magnetite–wüstite. Metallic iron can accumulate Ni, Co, Au, and PGE. Their content in metal increases by hundreds or even thousands of times compared to host silicate rock. The formation of native iron is based on the fluid-magmatic interaction between magma and reducing components of the fluid, mainly of methane–hydrogen composition. As a result, an initially homogeneous basalt liquid is dispersed into silicate and metallic components. In the course of transportation, finely dispersed iron phases form droplet-liquid segregations with a monomolecular gas layer on their surface, thus preventing enlargement of metallic droplets. In the hypabyssal chamber, magma, including metallic spherules, is degassed, and droplets are merged to form nodular segregations of native iron.</p>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"31 2","pages":"223 - 236"},"PeriodicalIF":1.5,"publicationDate":"2023-06-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"4615614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}