Carbonatite melts and genesis of apatite mineralization in the Guli pluton (northern East Siberia)

Inclusions of mineral-forming environments in apatite-containing ijolites and magnetite–phlogopite–apatite ores in carbonatites were studied to elucidate the genesis of apatite mineralization in the Guli alkaline ultramafic carbonatite massif. Primary inclusions of carbonate–salt and carbonate melts have been discovered and studied. The carbonate–salt melt inclusions are of alkaline high-Ca composition and are enriched in P, Sr, SO₃, and F (wt.%): CaO—30–40, Na₂O—5–12, K₂O—2–4, P₂O₅—1–3, SO₃—1.5–3, and SrO—1–3. They also contain minor MgO, FeO, BaO, and SiO₂ (tenths and hundredths of percent). The homogenization temperature of these inclusions is 850–970 °C. The carbonate inclusions contain predominant CaO (54–67 wt.%) and minor MgO, FeO, SrO, Na₂O, and P₂O₅ (tenths of percent). Their homogenization temperature is 840–860 °C. Similar primary carbonate–salt and carbonate inclusions were found in garnet, and secondary ones were detected in silicate minerals (clinopyroxene and nepheline) of ijolites. Clinopyroxenes of ijolites also contain primary inclusions of alkaline ultramafic high-Ca melts similar in composition to melilitite-melanephelinites highly enriched in P, SO₃, and CO₂ (wt.%): SiO₂—41–46, Al₂O₃—8–16, FeO—2–8, MgO—3–6, CaO—12–20, Na₂O—2–9, K₂O—1–6, P₂O₅—0.4–2.1, SO₃—0.2–2.3, and Cl—0.02–0.35. According to the obtained data, apatite of the magnetite–phlogopite–apatite ores and ijolites of the Guli pluton crystallized from phosphorus-rich alkaline carbonate–salt melts at 850–970 °C. The generation of these melts was, most likely, due to the silicate–salt immiscibility in melilitite-melanephelinite melts highly enriched in salts, which occurred either at the final stages of clinopyroxene crystallization or during the formation of melilite. The presence of alkalies, S, F, and CO₂ in spatially separated carbonate–salt melts contributed to the concentration and preservation of phosphorus in them at low temperatures, which led to the formation of apatite mineralization in ijolites and ore deposit in carbonatites.© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.     

Calcio-carbonatite melts and metasomatism in the mantle beneath Mt. Vulture (Southern Italy)

At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5–150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2–20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO₂ bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8–5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr–Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate–carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation.     

Magmatic evolution of the material of the Earth’s lower mantle: Stishovite paradox and origin of superdeep diamonds (Experiments at 24–26 GPa)

The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO₂ and periclase–wüstite solid solutions, (MgO · FeO)_ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO₂ react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO₂–CaSiO₃ system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO₃ (bridgmanite) + L = SiO₂ + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO₃ (bridgmanite) + (MgO · FeO)_ss (ferropericlase), and basic minerals, (FeO · MgO)_ss (magnesiowüstite) + SiO₂ (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.     

Stable (C,O) and radiogenic (Sr,Nd) isotopes of carbonates as indicators of magmatic and post-magmatic processes of phoscorite-series rocks and carbonatites from Catalão I,central Brazil

The Late-Cretaceous Catalão I contains stockworks of thin dykes of phoscorite-series rocks, which can be subdivided into P1 (olivine-bearing, phoscorites) and P2/P3 (olivine-lacking, nelsonites). Dolomite carbonatites (DC) are intimately associated with nelsonites, as pockets and dykes. The P2 apatite nelsonite, the P3 magnetite nelsonite, and, to a lesser extent DC, host the Catalão I niobium mineralization. C–O isotopes signatures in carbonates reveal several distinct magmatic and post-magmatic processes. Limpid carbonates with stable isotopic mantle-like composition show Rayleigh fractionation and are interpreted as primary, while those with brittle-turbid aspect, and higher oxygen isotope composition, probably underwent recrystallization by interaction with H₂O-rich fluids. A group of samples shows higher oxygen compositions and lower carbon values, which could be explained by degassing of carbonatite magma during cooling. A degassing pattern, parallel to magmatic degassing but at higher oxygen and lower carbon compositions, observed in carbonate veins, may indicate degassing of fracture filling fluids. Furthermore, C–O isotopes of carbonate from monazite-bearing carbonatite have a positive correlation, indicating a distinct, late-stage carbo-hydrothermal event. Though the Catalão I nelsonites and phoscorites are of igneous origin, they underwent several post-magmatic events, which sometimes overprinted partially or entirely the magmatic isotope signature.     

Magmatic and hydrothermal inclusions in carbonatite of the Magnet Cove Complex,Arkansas

The carbonatite at Magnet Cove, Arkansas, USA contains a great variety and abundance of magmatic and hydrothermal inclusions that provide an informative, though fragmentary, record of the original carbonatite melt and of late hydrothermal solutions which permeated the complex in postmagmatic time. These inclusions were studied by optical and scanning electron microscopy. Primary magmatic inclusions in monticellite indicate that the original carbonatite melt contained approximately 49.7 wt% CaO, 16.7% CO₂, 15.7% SiO₂, 11.4% H₂O, 4.4% FeO+Fe₂O₃, 1.1% P₂O₅ and 1.0% MgO. The melt was richer in SiO₂ and iron oxides than the carbonatite as now exposed; this is attributed to crystal settling and relative enrichment of calcite at shallower levels. The density of the carbonatite melt as revealed by the magmatic inclusions was approximately 2.2–2.3 g/cc. Such a light melt should separate rapidly from any denser parent material and could be driven forcibly into overlying crustal rocks by buoyant forces alone. Fluid inclusions in apatite suggest that a separate (immiscible) phase composed of supercritical CO₂ fluid of low density coexisted with the carbonatite magma, but the inclusion record in this mineral is inconclusive with respect to the nature of any other coexisting fluids. Maximum total pressure during CO₂ entrapment was about 450 bars, suggesting depths of 1.5 km or less for apatite crystallization and supporting earlier proposals of a shallow, subvolcanic setting for the complex. Numerous secondary inclusions in the Magnet Cove calcite contain an intriguing variety of daughter minerals including some 19 alkali, alkaline earth and rare earth carbonates, sulfates and chlorides few of which are known as macroscopic phases in the complex. The exotic fluids from which the daughter minerals formed are inferred to have cooled and diluted through time by progressive mixing with local groundwaters. These fluids may be responsible for certain late veins and elemental enrichments associated with the complex.     

Magmatic fluids in the Fen carbonatite complex,S.E. Norway

Three different types of carbonatite magma may be recognized in the Cambrian Fen complex, S.E. Norway: (1) Peralkaline calcite carbonatite magma derived from ijolitic magma; (2) Alkaline magnesian calcite carbonatite magma which yielded biotite-amphibole søvite and dolomite carbonatite; and (3) ferrocarbonatite liquids, related to (2) and/or to alkaline lamprophyre magma (damjernite). Apatite formed during the pre-emplacement evolution of (2) contains inclusions of calcite and dolomite, devitrified mafic silicate glass and aqueous fluid. All of these inclusions have a magmatic origin, and were trapped during a mid-crustal fractionation event (P4 kbars, T625° C), where apatite and carbonates precipitated from a carbonatite magma which coexisted with a mafic silicate melt. The fluid inclusions contain water, dissolved ionic species (mainly NaCl, with minor polyvalent metal salts) and in some cases CO₂. Two main groups of fluid inclusions are recognized: Type A: CO₂-bearing inclusions, of approximate molar composition H2O _88–90 CO _27-5 NaCl ₅ (d=0.85–0.87 g/ cm³). Type B: CO₂-free aqueous inclusions with salinities from 1 to 24 wt% NaCl_eq and densities betwen 0.7 and 1.0 g/cm³. More strongly saline type B inclusions (salinity ca. 35wt%, d=1.0 to 1.1 g/cm³) contain solid halite at room temperature and occur in overgrowths on apatite. Type A inclusions probably contain the most primitive fluid, from which type B fluids have evolved during fractionation of the magmatic system. Type B inclusions define a continuous trend from low towards higher salinities and densities and formed as a result of cooling and partitioning of alkali chloride components in the carbonatite system into the fluid phase. Available petrological data on the carbonatites show that the fluid evolution in the Fen complex leads from a regime dominated by juvenile CO₂ + H₂O fluids during the magmatic stage, to groundwater-derived aqueous fluids during post-magmatic reequilibration.     

Torngat ultramafic lamprophyres and their relation to the North Atlantic Alkaline Province

Geological mapping and diamond exploration in northern Quebec and Labrador has revealed an undeformed ultramafic dyke swarm in the northern Torngat Mountains. The dyke rocks are dominated by an olivine-phlogopite mineralogy and contain varying amounts of primary carbonate. Their mineralogy, mineral compositional trends and the presence of typomorphic minerals (e.g. kimzeyitic garnet), indicate that these dykes comprise an ultramafic lamprophyre suite grading into carbonatite. Recognized rock varieties are aillikite, mela-aillikite and subordinate carbonatite. Carbonatite and aillikite have in common high carbonate content and a lack of clinopyroxene. In contrast, mela-aillikites are richer in mafic silicate minerals, in particular clinopyroxene and amphibole, and contain only small amounts of primary carbonate. The modal mineralogy and textures of the dyke varieties are gradational, indicating that they represent end-members in a compositional continuum.

The Torngat ultramafic lamprophyres are characterized by high but variable MgO (10–25 wt.%), CaO (5–20 wt.%), TiO₂ (3–10 wt.%) and K₂O (1–4 wt.%), but low SiO₂ (22–37 wt.%) and Al₂O₃ (2–6 wt.%). Higher SiO₂, Al₂O₃, Na₂O and lower CO₂ content distinguish the mela-aillikites from the aillikites. Whereas the bulk rock major and trace element concentrations of the aillikites and mela-aillikites overlap, there is no fractional crystallization relation between them. The major and trace element characteristics imply related parental magmas, with minor olivine and Cr-spinel fractionation accounting for intra-group variation.

The Torngat ultramafic lamprophyres have a Neoproterozoic age and are spatially and compositionally closely related with the Neoproterozoic ultramafic lamprophyres from central West Greenland. Ultramafic potassic-to-carbonatitic magmatism occurred in both eastern Laurentia and western Baltica during the Late Neoproterozoic. It can be inferred from the emplacement ages of the alkaline complexes and timing of Late Proterozoic processes in the North Atlantic region that this volatile-rich, deep-seated igneous activity was a distal effect of the breakup of Rodinia. This occurred during and/or after the rift-to-drift transition that led to the opening of the Iapetus Ocean.     

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe,Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO₂, H₂O, F, Cl, and S) at lower concentrations of SiO₂, MgO, Al₂O₃, Cr₂O₃, and TiO₂. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.     

Compositions of magmas,formation conditions,and genesis of carbonate-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex,eastern Sayan

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO₂, from 9 to 21 wt % Al₂O₃, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism.     

Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone,Purulia, West Bengal: A saga of primary magmatic carbonatite

The Purulia carbonatite, ‘carbonatite’-‘alkali-pyroxenite’-‘apatite-magnetite rock’ association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The ⁸⁷Sr/⁸⁶Sr ratio of the carbonatite and apatite separated from the carbonatite (～0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.     

Influence of oxygen fugacity on mineral compositions in peralkaline melts: The Katzenbuckel volcano, Southwest Germany

The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I comprises the main rock bodies of phonolite and nepheline syenite, which were later intruded by different peralkaline dyke rocks (tinguaites and alkali feldspar syenite dykes) of phase II. The dyke assemblage was accompanied by magnetite and apatite veins and was followed by a late-stage pneumatolytic activity causing autometasomatic alterations.

As is typical for alkaline to peralkaline igneous rocks, early mafic minerals of phase I rocks comprise olivine, augite and Fe–Ti oxides, which are substituted in the course of fractionation by Na-amphibole and Na-pyroxene. For the early magmatic stage, calculated temperatures range between 880 and 780 °C with low silica activities (0.4 to 0.6) but high relative oxygen fugacities between 0.5 and 1.9 log units above the FMQ buffer. Even higher oxygen fugacities (above the HM buffer) are indicated for the autometasomatic alteration, which occurred at temperatures between 585 and 780 °C and resulted in the formation of pseudobrookite and hematite.

The unusually high oxygen fugacities (even during the early magmatic stage) are recorded by the major element compositions of the mafic minerals (forsterite content in olivine between 68 and 78 mol%, up to 6.2 wt.% ZrO₂ and 8.5 wt.% TiO₂ in clinopyroxene), the unusual mineral assemblages (pseudobrookite, freudenbergite) and by the enrichment of Fe³⁺ in the felsic minerals (up to 2.8 wt.% Fe₂O₃ in alkali feldspar and up to 2.6 wt.% Fe₂O₃ in nepheline). These observations point to a metasomatically enriched and highly oxidized lithospheric mantle as a major source for the Katzenbuckel melts.     

Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr₂O₃ contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr₂O₃ (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr₂O₃ and <0.5 wt.% TiO₂. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr₂O₃ contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K₂O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).     

Fluid-rock interaction in the Kangankunde Carbonatite Complex,Malawi: SEM based evidence for late stage pervasive hydrothermal mineralisation

The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO₂-rich and alkali chloride-carbonate-bearing fluid at ～250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis.     

Enigmatic association of the carbonatite and alkali-pyroxenite along the Northern Shear Zone, Purulia, West Bengal: A saga of primary magmatic carbonatite

The Purulia carbonatite, ‘carbonatite’-‘alkali-pyroxenite’-‘apatite-magnetite rock’ association, is located at Beldih area of Purulia district, West Bengal and falls within the 100 km long Northern Shear Zone (NSZ). Published literature suggests that the Purulia carbonatite was formed by the process of liquid immiscibility from under-saturated silicate parent magma. However, no silica under-saturated rocks like ijolite, nepheline-syenite etc. is known from the area. The trace element geochemistry (Ba/La, Nb/Th, Nb/Pb and Y/Ce ratios in the present study) also does not support this view. Present study indicates that the Purulia carbonatite is enriched in ΣREE and incompatible elements but the carbonatite is also poorer in Nb, Th and Pb compared to the world average of calicocarbonatites. The lower value of Nb is characteristics of carbo(hydro)thermal carbonatite where carbonatite is associated with alkali-pyroxenite and suggests probable origin of the carbonatite as carbothermal residua evolved from an unknown parentage. However, the field, petrographic and geochemical data indicate the genesis of this carbonatite from a primary carbonatitic magma of mantle decent. The ⁸⁷Sr/⁸⁶Sr ratio of the carbonatite and apatite separated from the carbonatite (∼0.703) implies primary magmatic derivation of the Purulia carbonatite. Close similarity of the apatite of the apatite-magnetite rock with the mantle apatite (of type Apatite B) indicates that they are also of primary magmatic origin. The present work portrays a unique example where primary magmatic carbonatite is associated with the alkali-pyroxenite.     

藏南过铝花岗岩中电气石的矿物化学特征及成因意义

讨论了藏南过铝花岗岩中电气石的地质产状、矿物学和矿物化学特征。结果表明:(1)在以氧原子数为24.5计算的化学式中,电气石的(Fe+Mg)/Mg比值在2.32~5.37之间,指示花岗岩和伟晶岩中的电气石均为黑电气石系列,而且属镁电气石—铁电气石系列中的较富铁电气石的成员;(2)电气石的FeO/(FeO+MgO)值高达0.70~0.89,与贫Li花岗岩接近,Al-Al50Fe50-Al50Mg50图解和Fe-Mg-Ca图解投点均位于贫Li花岗岩区,属于贫Li花岗岩有关的电气石;(3)TiO2-MnO/CaO-MgO/FeO三元图解可判定属于第Ⅰ类,即MgO和FeO含量同步消长,且较贫Mg富Fe,而MnO和TiO2含量为异步消长,这与电气石的FeO/(FeO+MgO)值所反映的性质相同;(4)地质产状、矿物学及矿物化学揭示的成因信息表明藏南过铝花岗岩中的电气石为酸性侵入体岩浆期后热液成因。     

A reassessment of models for hydrocarbon generation in the Khibiny nepheline syenite complex, Kola Peninsula, Russia

Although hydrocarbon-bearing fluids have been known from the alkaline igneous rocks of the Khibiny intrusion for many years, their origin remains enigmatic. A recently proposed model of post-magmatic hydrocarbon (HC) generation through Fischer-Tropsch (FT) type reactions suggests the hydration of Fe-bearing phases and release of H₂ which reacts with magmatically derived CO₂ to form CH₄ and higher HCs. However, new petrographic, microthermometric, laser Raman, bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of HC generation. The gas phase is dominated by CH₄ with only minor proportions of higher hydrocarbons. No remnants of the proposed primary CO₂-rich fluid are found in the complex. The majority of the fluid inclusions are of secondary nature and trapped in healed microfractures. This indicates a high fluid flux after magma crystallisation. Entrapment conditions for fluid inclusions are 450–550 °C at 2.8–4.5 kbar. These temperatures are too high for hydrocarbon gas generation through the FT reaction. Chemical analyses of rims of Fe-rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation. Furthermore, there is no clear relationship between the presence of Fe-rich minerals and the abundance of fluid inclusion planes (FIPs) as reported elsewhere. δ¹³C values for methane range from − 22.4‰ to − 5.4‰, confirming a largely abiogenic origin for the gas. The presence of primary CH₄-dominated fluid inclusions and melt inclusions, which contain a methane-rich gas phase, indicates a magmatic origin of the HCs. An increase in methane content, together with a decrease in δ¹³C isotope values towards the intrusion margin suggests that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.     

碳酸盐岩石成因类型的判别分析和白云鄂博“白云岩”的成因讨论

本文根据对世界上20多个国家和我国21个省碳酸盐岩石化学全分析数据的统计和换算结果,发现岩浆碳酸岩与沉积碳酸盐岩之间常量元素的一般含量无显著不同,但采用两组或逐步判别分析的方法,仍能将岩浆碳酸岩与沉积碳酸盐岩很好的区分开。判别效率可达90%以上。将这一方法应用到白云”鄂博矿区的“白云岩”和灰岩,90%以上的“白云岩”样品判为岩浆碳酸岩,灰岩的样品全部判为沉积碳酸盐岩,与它们的地质产状和地球化学特征一致。     

碳酸盐岩石成因类型的判别分析和白云鄂博“白云岩”的成因讨论

本文根据对世界上20多个国家和我国21个省碳酸盐岩石化学全分析数据的统计和换算结果,发现岩浆碳酸岩与沉积碳酸盐岩之间常量元素的一般含量无显著不同,但采用两组或逐步判别分析的方法,仍能将岩浆碳酸岩与沉积碳酸盐岩很好的区分开。判别效率可达90%以上。将这一方法应用到白云”鄂博矿区的“白云岩”和灰岩,90%以上的“白云岩”样品判为岩浆碳酸岩,灰岩的样品全部判为沉积碳酸盐岩,与它们的地质产状和地球化学特征一致。     

Composition of a carbonatitic melt in equilibrium with lherzolite at 5.5–6.3 GPa and 1350°C

Generation of ultra-alkaline melts by the interaction of lherzolite with cardonatites of various genesis was simulated at the P–T parameters typical of the base of the subcratonic lithosphere. Experiments with a duration of 150 h were performed at 5.5 and 6.3 GPa and 1350°C. The concentrations of CaO and MgO in melts are buffered by the phases of peridotite, and the concentrations of alkalis and FeO depend on the composition of the starting carbonatite. Melts are characterized by a low (<7 wt %) concentration of SiO₂ and Ca# from 0.40 to 0.47. It is demonstrated that only high-Mg groups of carbonatitic inclusions in fibrous diamonds have a composition close to that of carbonatitic melts in equilibrium with lherzolite. Most likely, the formation of kimberlite-like melts relatively enriched in SiO₂ requires an additional source of heat from mantle plumes and probably H₂O fluid.     

磁铁矿和钛铁矿成分对四川太和富磷灰石钒钛磁铁矿床成因的约束

产于层状镁铁质-超镁铁质岩体中的太和岩浆型Fe-Ti氧化物矿床是峨眉山大火成岩省内带几个超大型Fe-Ti氧化物矿床之一。太和岩体长超过3km,宽2km,厚约1.2km。根据矿物含量和结构等特征,整个岩体从下向上可划分为下部岩相带、中部岩相带、上部岩相带。下部岩相带主要以(橄榄)辉长岩和厚层不含磷灰石的块状Fe-Ti氧化物矿层组成。中部岩相带韵律旋回发育,(磷灰石)磁铁辉石岩主要位于旋回的底部,旋回上部为(磷灰石)辉长岩。上部岩相带主要是贫Fe-Ti氧化物的磷灰石辉长岩。太和中部岩相带磷灰石磁铁辉石岩含有5%~12%磷灰石、20%~35%Fe-Ti氧化物、50%~60%硅酸盐矿物,且硅酸盐矿物与磷灰石呈堆积结构。磷灰石磁铁辉石岩中磁铁矿显示高TiO2、FeO、MnO、MgO,且变化范围与趋势接近于攀枝花岩体。钛铁矿FeO分别与TiO2、MgO显示负相关,而FeO分别与Fe2O3、MnO显示正的相关,且TiO2、FeO、MnO、MgO含量变化较大,这些特征都暗示磁铁矿和钛铁矿是从富Fe-Ti-P岩浆中分离结晶。因此,可以推断太和磷灰石磁铁矿辉石岩形成于矿物重力分选和堆积。太和下部岩相带包裹在橄榄石中磁铁矿含有相对较高Cr2O3(0.07%~0.21%),而中部岩相带包裹在橄榄石中磁铁矿Cr2O3(0.00%~0.03%)显著降低,且这些磁铁矿Cr2O3含量变化与单斜辉石Cr含量和斜长石An牌号呈正相关。这些特征印证了形成中部岩相带的相对演化的富Fe-Ti-P母岩浆可能是源自中部岩浆房的混合岩浆。上部岩相带磁铁矿和中部岩相带顶部少量磁铁矿显示较低Ti+V可能是由于岩浆房中累积的岩浆热液对磁铁矿成分进行了改造。