Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.     

Genesis of mineralized cavities (Miaroles) in granitic pegmatites and granites

Analysis the development of large fluid segregations in a flux of small fluid bubbles during the degassing of granitic (pegmatitic) melts indicates that the velocity of the buoyant ascent of fluid bubbles depends on their sizes, the viscosity and density of the melts, and the duration of melt flow. Possible variants of the primary and secondary boiling of magma are discussed depending on the P-T conditions and concentrations of H₂O, F, B, and other components dissolved in the magma. The possible density ranges of the fluid phases are considered, along with the viscosity and density of granitic (pegmatitic) melts, velocities of the buoyant ascent of fluid bubbles in them, and the processes of their coalescence and accumulation in the temperature range of 650–850°C. Provisional evaluates are obtained for the duration of melt crystallization and the development of intrusive massifs and dikes of granites and syngenetic intragranite and epigenetic (intruded into the host rocks) granite pegmatites. Simulation data and geological observations suggest that large fluid segregations were formed already in the magma chambers in which the heterogeneous granite (pegmatitic) magma was derived, before its emplacement into the host rocks. These generation regions could be magma chamber areas within granite intrusions, in which melts enriched in volatiles were accumulated and then degassed with the release of fluid phases of various composition and density. The crystallization of fluid-rich melts under favorable conditions gives rise to granites with miarolitic structures. The emplacement of heterogeneous pegmatitic magma (which consists of immiscible silicate melts and large fluid segregations) into the host rocks results in that these segregations (would-be miaroles) occur in any part of the pegmatite-hosting chamber. This explains why miaroles of significantly different composition and with broadly varying proportions of their filling minerals may occur in various parts of pegmatite veins or their swells, as well as near contacts with the host rocks.     

Shonkinites and minettes of the Ryabinovyi massif (Central Aldan): composition and crystallization conditions

Dikes of biotitic shonkinites and minettes of the complex Ryabinovyi alkaline massif (Central Aldan) have been studied. The dikes are localized in a neck of K-picrites in the northeast of the massif, which intrudes gold-bearing microcline–muscovite metasomatites (Muscovitovyi site). The obtained data on the chemical and trace-element compositions of the rocks and minerals and study of melt inclusions in clinopyroxenes indicate that the biotitic shonkinites and minettes crystallized from the same deep-seated high-pressure alkaline ultrabasic magma during its evolution. Apparently, at the early stage of crystallization of diopside in the biotitic shonkinites, homogeneous carbonate–silicate melt was separated into immiscible fractions of silicate, carbonate–salt, and carbonate melts. The temperature of melt immiscibility was > 1120–1190 °C, i.e., higher than the homogenization temperature of silicate inclusions in the diopside. The contents of trace elements in the biotitic shonkinites and rock-forming clinopyroxenes were one or two orders of magnitude higher than the mantle values. The Eu/Eu* ratios of both the considered rocks and the clinopyroxenes were close to those of chondrites, which testifies to their crystallization from mantle magma. The HREE/LREE ratio indicates that the magma source was localized at the depths where garnet-spinel assemblages existed. The negative Nb and Ti anomalies in the trace-element spectra and the high (> 5) La/Nb ratios in the rocks and clinopyroxenes point to the influence of crustal material on the parental magma. Crystallization of magma took place in reducing conditions, which is evidenced by the low (4–7) Ti/V ratios in clinopyroxenes and the presence of chloride–sulfate inclusions in them. Since gold in the Ryabinovyi massif is associated with late sulfate–chloride and sulfate–carbonate fluids, it might have been transported by alkaline chloride–sulfate and carbonate (carbonatite) melts, found as inclusions in clinopyroxenes of the biotitic shonkinites, at the early stages of Mesozoic magmatism.     

Geochemical specifics of ongonites in the Ary-Bulak Massif, eastern Transbaikalia

A genetic model for magmatic rocks of the Ary-Bulak Massif is discussed based on a detailed geological map of the massif (prepared by the authors) and on original data of the authors on the petrography of the massif, its compositional zoning, trace-element geochemistry, physicochemical parameters of its crystallization, and melt inclusions in its minerals. The Ary-Bulak Massif was determined to be zonal, with the predominance (approximately 70% by area) of porphyritic topaz ongonites (central facies), which grade toward contacts into weakly porphyritic ongonites bearing topaz and, occasionally, fluorite (margin facies). Aphyric rocks with fluorite (inner-contact facies) occur as a stripe 50–80 m wide at the southwestern inner contact of the massif. Analysis of petrographic and geochemical data indicates that subvolcanic rocks of the Ary-Bulak Massif differ from typical elvanes (as they occur in the Cornwall province) but are similar to classic ongonites in the central and marginal facies of the massif. Rocks in the southwestern inner-contact zone are unusual high-F and high-Ca varieties, whose analogues have never been found in any rare-metal provinces with ongonites and which provide evidence of a complicated evolutionary history of the Ary-Bulak Massif. The geochemical evolution of this massif was determined to be characterized by the enrichment of the older inner-contact facies rocks in CaO, K₂O, F, and Rb, Cs, B, Ba, Sr, Sn, and Ta, whose concentrations decrease in the ongonites of the central facies. The central-facies ongonites thereby have much higher Na₂O and Li concentrations than those in the inner-contact facies rocks. It is demonstrated that the intense heating and melting of crustal material in this region at the Jurassic-Cretaceous boundary could have been induced by subalkaline basaltic magma. The chemical composition of the rocks, which is unusual for typical ongonites in, for example, high Ca and Sr concentrations, could be caused by the possible assimilation by the magma of limestones, which occur in the territory at a certain depth in the Ust’-Borzya Formation that hosts the Ary-Bulak Massif. The genesis of most rocks in the massif was controlled by the magmatic differentiation of crustal granitic magma, with the residual melts forming Li-F granites enriched in several trace elements (Li, Rb, Cs, B, Ba, Sr, etc.) and ongonites as their subvolcanic analogues.     

俄罗斯Chukchi半岛新生代碱性岩的形成和演化：熔体和流体包裹体证据

Melt and fluid inclusions were studied in the minerals of Cenozoic olivine melanephelinites from the Chukchi Peninsula, Russia.The rock contain several generations of olivine phenocrysts varying in composition at mg=0.88～0.77.The phenocrysts bear fluid and melt inclusions recording various stages of melt crystallization in volcanic conduits and shallow magma chambers.Primary fluid inclusions are CO_2-dominated with a density of up to O.93 g/cm~3.All fluid inclusions are partially leaked,which is indicated by haloes of tiny fluid bubbles around large fluid inclusions in minerals.Melt inclusions contain various daughter crystals,which were completely resorbed in thermometric experiments at about 1230℃.Assuming that this temperature corresponds to the entrapment conditions of the CO_2 fluid inclusions,the minimum pressure of the beginning of magma degassing is estimated as 800MPa.Variations in the compositions of homogenized silicate melt inclusions indicate that olivine was the earliest crystalline phase followed by clinopyroxene,nepheline and orthoclase.This sequence is in agreement with the mineralogy of the rocks.The melts are strongly enriched in incompatible trace elements and volatiles(in addition to CO_2,high C1,F,and S contents were detected).There are some differences between the compositions of melts trapped in minerals from different samples.Variations in SiO_2,FeO,and incompatible element contents are probably related to melt generations at various levels in a homogeneous mantle reservoir.     

Evidence for the magmatic origin of quartz-topaz rocks from the New England batholith,Australia

Quartz-topaz rocks from the New England district, New South Wales, have mineralogical, textural and field relationships suggesting a magmatic origin. These rocks (called topazites) occur as dykes and sills intruding a biotite granite and sediments in a roof pendant. Where they have intruded into sediments, the topazites have a narrow aureole of induration or hornfels. One type of primary solid inclusion, thought to be silicate glass, has a composition ranging from that of the topazite towards that of nearby granite. Primary fluid inclusions contain an aqueous solution of alkali chlorides with concentrations of total salts to 57 wt%. These fluid inclusions indicate crystallization temperatures in the range 570–620° C, close to the experimentally determined solidus of a vapour-saturated, topaz-normative melt. The presence of primary fluid inclusions indicates crystallization of topazite following saturation of a granitic magma with water and the formation of immiscible silicate and aqueous phases. Partitioning of alkali metals into the aqueous phase left a silicate melt that could only crystallize quartz and topaz.     

Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy

Melt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 – 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite.     

Compositions of magmas and carbonate–silicate liquid immiscibility in the Vulture alkaline igneous complex, Italy

This paper presents a study of melt and fluid inclusions in minerals of an olivine-leucite phonolitic nephelinite bomb from the Monticchio Lake Formation, Vulture. The rock contains 50 vol.% clinopyroxene, 12% leucite, 10% alkali feldspars, 8% hauyne/sodalite, 7.5% nepheline, 4.5% apatite, 3.2% olivine, 2% opaques, 2.6% plagioclase, and < 1% amphibole. We distinguished three generations of clinopyroxene differing in composition and morphology. All the phenocrysts bear primary and secondary melt and fluid inclusions, which recorded successive stages of melt evolution. The most primitive melts were found in the most magnesian olivine and the earliest clinopyroxene phenocrysts. The melts are near primary mantle liquids and are rich in Ca, Mg and incompatible and volatile elements. Thermometric experiments with the melt inclusions suggested that melt crystallization began at temperatures of about 1200 °C. Because of the partial leakage of all primary fluid inclusions, the pressure of crystallization is constrained only to minimum of 3.5 kbar. Combined silicate–carbonate melt inclusions were found in apatite phenocrysts. They are indicative of carbonate–silicate liquid immiscibility, which occurred during magma evolution. Large hydrous secondary melt inclusions were found in olivine and clinopyroxene. The inclusions in the phenocrysts recorded an open-system magma evolution during its rise towards the surface including crystallization, degassing, oxidation, and liquid immiscibility processes.     

Conditions of crystallization of olivine shonkinites in the Inagli massif (Central Aldan)

The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)_n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.     

Physicochemical conditions of crystallization of dunites of the Nizhnii Tagil Pt-bearing massif (Middle Urals)

Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 °C and then olivine formation continued to 1280 °C during the evolution of melts.     

Tantalite-(Mn) from the Borborema Pegmatite Province,northeastern Brazil: conditions of formation and melt- and fluid-inclusion constraints on experimental studies

Carbon dioxide-rich fluid and carbonate-rich aluminosilicate melt inclusions in tantalite-(Mn) from the Alto do Giz pegmatite in the Borborema Pegmatite Province, northeastern Brazil were investigated to constrain the formation of the host crystals. The results demonstrate that in the Alto do Giz pegmatite, water- and alkaline carbonate-rich fluids and melts are responsible for the transport and deposition of tantalite-(Mn) at temperatures around 600°C and about 4 kbar. Moreover, evidence is presented to show that during crystallization of the tantalite-(Mn), three different components coexisted, which are now trapped as separate inclusions: two immiscible silicate melts (types A and B melt inclusions) and a CO₂-rich aqueous fluid. We hypothesize that immiscible fluid separation may have been a critical factor in producing the water- and alkaline carbonate-rich fluids and melts necessary for Ta and Nb transport. Since the tantalite-(Mn) crystallized during pegmatite formation, this mechanism must also have implications for pegmatite genesis in general.     

Inclusions of magmatic fluids: P-V-T-X properties of aqueous salt solutions of various types and petrological implications

The P-V-T-X properties of H₂O-salt systems are compared depending on the solubility coefficient of compounds contained in these systems and the presence or absence of critical phenomena in the saturated solutions. Data on synthetic and natural inclusions captured in minerals at elevated temperatures and pressures and employed to discuss the principal features of phase diagrams of the H₂O-NaCl system (type I) and H₂O-NaF system (type II or P-Q type). It is demonstrated how characteristics of magmatic fluids of various types are manifested during the development of miarolitic pegmatites (Malkhan field in Transbaikalia) and during the crystallization of F-rich ongonitic melts (Ary-Bulak Massif in eastern Transbaikalia). Characteristics of solutions and gas-rich (gaseous) fluid inclusions in quartz phenocrysts from porphyritic ongonites (disappearance of the liquid regardless of its density and the overall salinity near the critical point of water, distinctive features of the dissolution of the crystalline phase, and the ability of the inclusions to withstand heating to 1400°C without decrepitation), and the richness of the fluid-magmatic system as a whole in F suggest that the ongonite melt crystallized in the presence of low-density NaF-bearing fluids of the P-Q type with a minor admixture of chlorides. It is important to identify the type of solutions in the fluid inclusions, because without knowing this type, it is impossible to accurately enough calculate the pressure at the temperatures of inclusion capture. For example, the unwarranted classification of solutions of type II (P-Q) in inclusions with the chloride system results in a significant overestimation of the calculated fluid pressures. A technique is proposed for studying the high-temperature immiscibility region in P-Q systems based on data obtained on gaseous fluid inclusions.     

The role of carbonate-fluoride melt immiscibility in shallow REE deposit evolution

The Lugiin Gol nepheline syenite intrusion, Mongolia, hosts a range of carbonatite dikes mineralized in rare-earth elements(REE). Both carbonatites and nepheline syenite-fluorite-calcite veinlets are host to a previously unreported macroscale texture involving pseudo-graphic intergrowths of fluorite and calcite. The inclusions within calcite occur as either pure fluorite, with associated REE minerals within the surrounding calcite, or as mixed calcite-fluorite inclusions, with associated zirconosilicate minerals. Consideration of the nature of the texture, and the proportions of fluorite and calcite present(～29 and 71 mol%,respectively), indicates that these textures most likely formed either through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonatefluoride melt. Laser ablation ICP-MS analyses show the pure fluorite inclusions to be depleted in REE relative to the calcite. A model is proposed, in which a carbonate-fluoride melt phase enriched in Zr and the REE, separated from a phonolitic melt, and then either unmixed or underwent cotectic crystallization to generate an REE-rich carbonate melt and an REE-poor fluoride phase. The separation of the fluoride phase(either solid or melt) may have contributed to the enrichment of the carbonate melt in REE, and ultimately its saturation with REE minerals. Previous data have suggested that carbonate melts separated from silicate melts are relatively depleted in the REE, and thus melt immiscibility cannot result in the formation of REE-enriched carbonatites. The observations presented here provide a mechanism by which this could occur, as under either model the textures imply initial separation of a mixed carbonate-fluoride melt from a silicate magma. The separation of an REEenriched carbonate-fluoride melt from phonolitic magma is a hitherto unrecognized mechanism for REE-enrichment in carbonatites, and may play an important role in the formation of shallow magmatic REE deposits.     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

Immiscibility of Fluoride–Calcium and Silicate Melts in Trachyrhyolitic Magma: Data on Acidic Volcanic Rocks from the Nyalga Basin,Central Mongolia

An Early Cretaceous (120 ± 5 Ma) trachyrhyolite lava sheet in the Nyalga basin, Central Mongolia, includes a domain (～0.5 km²) of unusual fluorite-enriched rocks with anomalously high concentrations of CaO (1.2–25.7 wt %) and F (0.6–15 wt %). The textures and structures of the rocks suggest that they were produced by two immiscible melts: fluoride–calcium (F–Ca) and trachyrhyolitic. Data on mineral-hosted inclusions and SEM EDS studies of the matrixes of the rocks indicate that a F–Ca melt occurred in the trachyrhyolitic magmas during its various evolutionary episodes, starting from the growth of minerals in a magmatic chamber and ending with eruptions on the surface. Elevated fluorine concentrations (up to 1.5–2 wt %) in local domains of the trachyrhyolitic melt may have resulted in the onset of its liquid immiscibility and the exsolution of a F–Ca liquid phase. This was associated with the redistribution of trace elements: REE, Y, Sr, and P were preferably concentrated in the F–Ca melt, while Zr, Hf, Ta, and Nb were mostly redistributed into the immiscible silicate liquid. The F–Ca melt contained oxygen and aqueous fluid and remained mobile until vitrification of the trachyrhyolitic magma. The oxygen-enriched F–Ca phase was transformed into fluorite at 570–780°? and a high oxygen fugacity Δlog fO₂ (0.9–1.7) relative to the NNO buffer. Ferrian ilmenite, monazite-group As-bearing minerals, and cerianite crystallized under oxidizing conditions, and the titanomagnetite was replaced by hematite. The Ca- and F-enriched rocks were affected by low-density (0.05–0.1 g/cm³) aqueous fluid, which was released from the crystallizing trachyrhyolitic melt, and this led to the partial removal of REE from the F–Ca phase. The chondrite-normalized REE and Y patterns of the fluidmodified rocks show positive Y anomalies and W-shaped minima from Gd to Ho. A composition of the F–Ca phase close to the original one is conserved in mineral-hosted inclusions and in relict isolations in the rocks matrix. It is so far unclear why fluorite did not crystallize from the F–Ca melt contained in the trachyrhyolitic magma. Conceivably, this was favored by high-temperature oxidizing conditions under which the melt accommodated oxygen and aqueous fluid. The possible origin of mobile oxygen-bearing fluorite–calcic melt at subsolidus temperature should be taken into account when magmatic rocks and ores are studied. Fluorite and accompanying ore mineralization might have been formed in certain instances not by hydrothermal–metasomatic processes but during the fluid–magmatic stage as a result of the transformation of F–Ca melt enriched in REE, Y, and other trace elements.     

Magmatic Evolution of Changbaishan Tianchi Volcano,China–North Korea: Evidence from Mineral-Hosted Melt and Fluid Inclusions

Data on mineral-hosted melt, fluid, and crystalline inclusions were used to study the composition and evolution of melts that produced rocks of Changbaishan Tianchi volcano, China–North Korea, and estimate their crystallization parameters. The melts crystallized within broad ranges of temperature (1220–700°C) and pressure (3100–1000 bar), at a drastic change in the redox potential: Δ log \(f_{O_2}\) from NNO + 0.92 to +1.42 for the basalt melts, NNO –1.61 to –2.09 for the trachybasaltic andesite melts, NNO –2.63 to –1.89 for the comendite melts, and NNO –1.55 to –3.15 for the pantellerite melts. The paper reports estimates of the compositions of melts that produced the continuous rock series from trachybasalt to comendite and pantellerite. In terms of trace-element concentrations, all of the mafic melts are comparable with OIB magmas. The silicic melts are strongly enriched in trace elements and REE. The most strongly enriched melts contain concentrations of certain elements almost as high as in ores of these elements. The paper reports data on H2O concentrations in melts of different composition. It is demonstrated that the variations in the H₂O concentrations were controlled by magma degassing. Data are reported on the Sr and Nd composition of the rocks. The deviations in the Sr isotopic composition are proportional to the ⁸⁷Sr/⁸⁶Sr ratio and could be produced in a melt with a high enough ⁸⁷Sr/⁸⁶Sr ratio during a geologically fairly brief time period. The evolution of melts that produced rocks of the volcano was controlled by crystallization differentiation of the parental basalt magmas at insignificant involvement of melt mixing and liquid immiscibility of silicate and sulfide melts. The alkaline salic rocks were generated in shallow-sitting (13–3.5 km) magmatic chambers in which the melts underwent profound differentiation that gave rise to pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition of the magmas and parameters of their derivation are used to develop a generalized petrologic–geodynamic model for the origin of Changbaishan Tianchi volcano.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

A Preliminary Research on Skarns of Magmatic Origin

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Carbonatite melt inclusions in coexisting magnetite,apatite and monticellite in Kerimasi calciocarbonatite,Tanzania: melt evolution and petrogenesis

Kerimasi calciocarbonatite consists principally of calcite together with lesser apatite, magnetite, and monticellite. Calcite hosts fluid and S-bearing Na–K–Ca-carbonate inclusions. Carbonatite melt and fluid inclusions occur in apatite and magnetite, and silicate melt inclusions in magnetite. This study presents statistically significant compositional data for quenched S- and P-bearing, Ca-alkali-rich carbonatite melt inclusions in magnetite and apatite. Magnetite-hosted silicate melts are peralkaline with normative sodium-metasilicate. On the basis of our microthermometric results on apatite-hosted melt inclusions and forsterite–monticellite phase relationships, temperatures of the early stage of magma evolution are estimated to be 900–1,000°C. At this time three immiscible liquid phases coexisted: (1) a Ca-rich, P-, S- and alkali-bearing carbonatite melt, (2) a Mg- and Fe-rich, peralkaline silicate melt, and (3) a C–O–H–S-alkali fluid. During the development of coexisting carbonatite and silicate melts, the Si/Al and Mg/Fe ratio of the silicate melt decreased with contemporaneous increase in alkalis due to olivine fractionation, whereas the alkali content of the carbonatite melt increased with concomitant decrease in CaO resulting from calcite fractionation. Overall the peralkalinity of the bulk composition of the immiscible melts increased, resulting in a decrease in the size of the miscibility gap in the pseudoquaternary system studied. Inclusion data indicate the formation of a carbonatite magma that is extremely enriched in alkalis with a composition similar to that of Oldoinyo Lengai natrocarbonatite. In contrast to the bulk compositions of calciocarbonatite rocks, the melt inclusions investigated contain significant amount of alkalis (Na₂O + K₂O) that is at least 5–10 wt%. The compositions of carbonatite melt inclusions are considered as being better representatives of parental magma composition than those of any bulk rock.     

Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts

This paper reviews the results of investigations of melt inclusions in minerals of carbonatites and spatially associated silicate rocks genetically related to various deep-seated undersaturated silicate magmas of alkaline ultrabasic, alkaline basic, lamproitic, and kimberlitic compositions. The analysis of this direct genetic information showed that all the deep magmas are inherently enriched in volatile components, the most abundant among which are carbon dioxide, alkalis, halides, sulfur, and phosphorus. The volatiles probably initially served as agents of mantle metasomatism and promoted melting in deep magma sources. The derived magmas became enriched in carbon dioxide, alkalis, and other volatile components owing to the crystallization and fractionation of early high-magnesium minerals and gradually acquired the characteristics of carbonated silicate liquids. When critical compositional parameters were reached, the accumulated volatiles catalyzed immiscibility, the magmas became heterogeneous, and two-phase carbonate-silicate liquid immiscibility occurred at temperatures of ≥1280–1250°C. The immiscibility was accompanied by the partitioning of elements: the major portion of fluid components partitioned together with Ca into the carbonate-salt fraction (parental carbonatite melt), and the silicate melt was correspondingly depleted in these components and became more silicic. After spatial separation, the silicate and carbonate-silicate melts evolved independently during slow cooling. Differentiation and fractionation were characteristic of silicate melts. The carbonatite melts became again heterogeneous within the temperature range from 1200 to 800–600°C and separated into immiscible carbonate-salt fractions of various compositions: alkali-sulfate, alkali-phosphate, alkali-fluoride, alkali-chloride, and Fe-Mg-Ca carbonate. In large scale systems, polyphase silicate-carbonate-salt liquid immiscibility is usually manifested during the slow cooling and prolonged evolution of deeply derived melts in the Earth’s crust. It may lead to the formation of various types of intrusive carbonatites: widespread calcite-dolomite and rare alkali-sulfate, alkali-phosphate, and alkali-halide rocks. The initial alkaline carbonatite melts can retain their compositions enriched in P, S, Cl, and F only at rapid eruption followed by instantaneous quenching.