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1.
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.  相似文献   

2.
Djerfisherite, a Cl-bearing potassium sulfide (K6Na(Fe,Ni,Cu)24S26Cl), is a widespread accessory mineral in kimberlite-hosted mantle xenoliths. Nevertheless, the origin of this sulfide in nodules remains disputable. It is usually attributed to the replacement of primary Fe–Ni–Cu sulfides when xenoliths interact with a K-and Cl-enriched hypothetical melt/fluid. The paper is devoted to a detailed study of the composition and morphology of djerfisherite from a representative collection (22 samples) of the deepest mantle xenoliths—sheared garnet peridotite, taken from the Udachnaya-East kimberlite pipe (Yakutia). Four types of djerfisherite were distinguished in the mantle rocks on the basis of morphology, spatial distribution, and relationships with the rock-forming and accessory minerals in the nodules. Type 1 was found in the rims of polysulfide inclusions in the rock-forming minerals of the xenoliths; there, it was younger than the primary sulfide assemblage pyrrhotite + pentlandite ± chalcopyrite. Type 2 formed rims around large polysulfide segregations (pyrrhotite+ pentlandite) in the xenolith interstices. Type 3 formed individual grains in the xenolith interstices together with other sulfides, silicates, oxides, phosphates, and carbonates. Type 4 was present as a daughter phase in the secondary melt inclusions which occurred in healed cracks in the rock-forming minerals of the xenoliths. Along with djerfisherite, the inclusions contained silicates, oxides, phosphates, carbonates, alkaline sulfates, chlorides, and sulfides. The results indicate that djerfisherite from the xenoliths is consanguine with kimberlite. Djerfisherite both in the sheared-peridotite xenoliths from the Udachnaya-East pipe and in different xenoliths from other kimberlite pipes worldwide formed owing to the interaction between the nodules and kimberlitic melts. Djerfisherite forming individual grains in the melt inclusions and xenolith interstices crystallized directly from the infiltrating kimberlitic melt. Djerfisherite bounding the primary Fe–Ni ± Cu sulfides formed by their replacement as a result of a reaction with the kimberlitic melt.  相似文献   

3.
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.  相似文献   

4.
Diamond crystallization in multicomponent melts of variable composition is studied. The melt carbonates are K2CO3, CaCO3?MgCO3, and K-Na-Ca-Mg-Fe-carbonatites, and the melt silicates are model peridotite (60 wt.% olivine, 16 wt.% orthopyroxene, 12 wt.% clinopyroxene, and 12 wt.% garnet) and eclogite (50 wt.% garnet and 50 wt.% clinopyroxene). In the experiments carried out under the PT-conditions of diamond stability, the carbonate-silicate melts behave like completely miscible liquid phases. The concentration barriers of diamond nucleation (CBDN) in the melts with variable proportions of silicates and carbonates have been determined at 8.5 GPa. In the system peridotite–K2CO3–CaCO3?MgCO3–carbonatite they correspond to 30, 25, and 30 wt.% silicates, respectively, and in the analogous eclogite–carbonate system, 45, 30, and 35 wt.%. In the silicate-carbonate melts with higher silicate contents seed diamond growth occurs, which is accompanied by the crystallization of thermodynamically unstable graphite phase. In the experiments with melts compositionally corresponding to the CBDN at 7.0 GPa and 1200–1700 °C, a full set of silicate minerals of peridotite (olivine, orthopyroxene, clinopyroxene, garnet) and eclogite (garnet, clinopyroxene) parageneses was obtained. The minerals occur as syngenetic inclusions in natural diamonds; moreover, the garnets contain an impurity of Na, and the pyroxenes, K. The experimental data indicate that peridotite-carbonate and eclogite-carbonate melts are highly effective for the formation of diamond (or unstable graphite) together with syngenetic minerals and melts, which agrees with the carbonate-silicate (carbonatite) model for the mantle diamond formation.  相似文献   

5.
Gem-quality chrysolite (peridot) from a phlogopite deposit related to the Kovdor ultrabasic-alkaline massif in the Kola Peninsula, Russia, was studied using a variety of techniques (optical mineralogical microscopy, chemical, Mössbauer spectroscopy, and photoluminescence) to determine its chemical composition, the Fe2+/Fe3+ ratio, refraction indexes, density, as well as to examine inclusions in it. Much attention was devoted to the microprobe identification of crystalline inclusions in the host chrysolite (apatite, tetraferriphlogopite, amphibole, and magnetite), its exsolution products (diopside and magnetite), and the daughter phases of melt inclusions in this mineral (which were subdivided into primary and secondary genetic types). The daughter phases of these melt inclusions are silicates (forsterite, diopside, tetraferriphlogopite, clinohumite, and serpentine), various carbonates (Ca-dominated carbonates are characteristic of the primary inclusions, whereas Mg-rich carbonates were found only in the secondary inclusions), magnetite, djerfisherite (alkali sulfide), and Ba-Sr-REE carbonates. The presence of melt inclusions testifies to a magnatic genesis of the gem, and the simultaneous occurrence of these inclusions with crystalline inclusions can be used as an additional identification feature of gem chrysolite from the Kovdor Massif.  相似文献   

6.
At Jacupiranga (Brazil) the latest manifestations of C3 carbonatite are veins and impregnations of s?vite composition containing carbonates, Fe-oxides, apatite, phlogopite, olivine, clinohumite, pyrochlore, zirconolite. Apatite contains fluid inclusions l.s. of two types: (i) two-phase fluid inclusions of complex composition (H2O±CO2±NaCl±KCl±NaHCO3) with 17–50?vol.% of gas bubble, (ii) multi-phase melt inclusions of two types, carbonate-H2O rich and silicate-carbonate-H2O rich with very high homogenization temperatures up to 584°C and up to 1194°C, respectively. The coeval origin of fluid and melt inclusions allows to calculate the trapping pressures which indicate approximate depths of 30 to 60?km for the origin of the veins. Carbonatite veins contain Ba-poor tetra-ferriphlogopite of extreme composition which extends the compositional trend of the micas of Jacupiranga. The occurrence of zirconolite, Nb-rich pyrochlore and Ta-rich U-pyrochlore is mutually exclusive and related, probably, to the variation of fluorine content in the magmatic fluids.  相似文献   

7.
 Investigations of peridotite xenolith suites have identified a compositional trend from lherzolite to magnesian wehrlite in which clinopyroxene increases at the expense of orthopyroxene and aluminous spinel, and in which apatite may be a minor phase. Previous studies have shown that this trend in mineralogy and chemical composition may result from reaction between sodic dolomitic carbonatite melt and lherzolite at pressures around 1.7 to 2 GPa. This reaction results in decarbonation of the carbonatite melt, releasing CO2-rich fluid. In this study, we have experimentally reversed the decarbonation reaction by taking two natural wehrlite compositions and reacting them with CO2 at a pressure of 2.2 GPa and temperatures from 900 to 1150° C. Starting materials were pargasite-bearing wehrlites, one with minor apatite (composition 71001*) and one without apatite (composition 70965*). At lower temperatures (900° C) the products were apatite+pargasite+magnesite harzburgite for runs using composition 71001*, and pargasite+dolomite lherzolite for runs using composition 70965*. At and above 1000° C, carbonatite melt with harzburgite residue (olivine+orthopyroxene+spinel) and with lherzolite residue (olivine+orthopyroxene+clinopyroxene+ spinel) were produced respectively. Phase compositions in reactants and products are consistent with the documented carbonatite/lherzolite reactions, and also permit estimation of the carbonatite melt compositions. In both cases the melts are sodic dolomitic carbonatites. The study supports the hypothesis of a significant role for ephemeral, sodic dolomitic melts in causing metasomatic changes in the lithosphere at P≤2 GPa. The compositions of wehrlites imply fluxes of CO2, released by metasomatic reactions, which are locally very large at around 5 wt% CO2. Received: 15 December 1995/Accepted: 14 February 1996  相似文献   

8.
The paper presents data on the composition of olivine macrocrysts from two Devonian kimberlite pipes in the Arkhangelsk diamond province: the Grib pipe (whose kimberlite belongs to type I) and Pionerskaya pipe (whose kimberlite is of type II, i.e., orangeite). The dominant olivine macrocrysts in kimberlites from the two pipes significantly differ in geochemical and isotopic parameters. Olivine macrocrysts in kimberlite from the Grib pipe are dominated by magnesian (Mg# = 0.92–0.93), Ti-poor (Ti < 70 ppm) olivine possessing low Ti/Na (0.05–0.23), Zr/Nb (0.28–0.80), and Zn/Cu (3–20) ratios and low Li concentrations (1.2–2.0 ppm), and the oxygen isotopic composition of this olivine δ18O = 5.64‰ is higher than that of olivine in mantle peridotites (δ18O = 5.18 ± 0.28‰). Olivine macrocrysts in kimberlite from the Pionerskaya pipe are dominated by varieties with broadly varying Mg# = 0.90–0.93, high Ti concentrations (100–300 ppm), high ratios Ti/Na (0.90–2.39), Zr/Nb (0.31–1.96), and Zn/Cu (12–56), elevated Li concentrations (1.9–3.4 ppm), and oxygen isotopic composition δ18O = 5.34‰ corresponding to that of olivine in mantle peridotites. The geochemical and isotopic traits of low-Ti olivine macrocrysts from the Grib pipe are interpreted as evidence that the olivine interacted with carbonate-rich melts/fluids. This conclusion is consistent with the geochemical parameters of model melt in equilibrium with the low-Ti olivine that are similar to those of deep carbonatite melts. Our calculations indicate that the variations in the δ18O of the olivine relative the “mantle range” (toward both higher and lower values) can be fairly significant: from 4 to 7‰ depending on the composition of the carbonate fluid. These variations were formed at interaction with carbonate fluid, whose δ18O values do not extend outside the range typical of mantle carbonates. The geochemical parameters of high-Ti olivine macrocrysts from the Grib pipe suggest that their origin was controlled by the silicate (water–silicate) component. This olivine is characterized by a zoned Ti distribution, with the configuration of this distribution between the cores of the crystals and their outer zones showing that the zoning of the cores and outer zones is independent and was produced during two episodes of reaction interaction between the olivine and melt/fluid. The younger episode (when the outer zone was formed) likely involved interaction with kimberlite melt. The transformation of the composition of the cores during the older episode may have been of metasomatic nature, as follows from the fact that the composition varies from grain to grain. The metasomatic episode most likely occurred shortly before the kimberlite melt was emplaced and was related to the partial melting of pyroxenite source material.  相似文献   

9.
 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995  相似文献   

10.
Based on experimental and mineralogical data, the model of mantle carbonate-silicate (carbonatite) melts as dominating parental media for natural diamonds was substantiated. It was demonstrated that the compositions of silicate constituents of parental melts were variable and saturated with respect to mantle rocks, namely pyrope peridotite, garnet pyroxenite, and eclogite. Based on concentration contributions and role in diamond genesis, major (carbonate and silicate) and minor (admixture) components were distinguished. The latter components may be both soluble (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble (sulfides, metals, and carbides) in silicate-carbonate melts. This paper presents the results of a study of diamond crystallization in multicomponent melts of variable composition with carbonate components (K2CO3, CaCO3 · MgCO3, and K-Na-Ca-Mg-Fe carbonatite) and silicate components represented by model peridotite (60 wt % olivine, 16 wt % orthopyroxene, 12 wt % clinopyroxene, and 12 wt % garnet) and eclogite (50 wt % garnet and 50 wt % clinopyroxene). Carbonate-silicate melts behave like completely miscible liquid phases in experiments performed under the P-T conditions of diamond stability. The concentration barriers of diamond nucleation (CBDN) in melts with variable proportions of silicates and carbonates were determined at 8.5 GPa. In the peridotite system with K2CO3, CaCO3 · MgCO3, and carbonatite, they correspond to 30, 25, and 30 wt % silicates, respectively, and in the eclogite system, the CBDN is shifted to 45, 30, and 35 wt % silicates. In the silicate-carbonate melts with higher silicate contents, diamond grows on seeds, which is accompanied by the crystallization of thermodynamically unstable graphite. At P = 7.0 GPa and T = 1200−1800°C, we studied and constructed phase diagrams for the multicomponent peridotite-carbonate and eclogite-carbonate systems as a physicochemical basis for revealing the syngenetic relationships between diamond and its silicate (olivine, ortho- and clinopyroxene, and garnet) and carbonate (aragonite and magnesite) inclusions depending on the physicochemical conditions of growth media. The results obtained allowed us to reconstruct the evolution of diamond-forming systems. The experiments revealed similarity between the compositions of synthetic silicate minerals and inclusions in natural diamonds (high concentrations of Na in garnets and K in clinopyroxenes). It was experimentally demonstrated that the formation of Na-bearing majoritic garnets is controlled by the P-T parameters and melt alkalinity. Diamonds with inclusions of such garnets can be formed in alkalic carbonate-silicate (aluminosilicate) melts. A mechanism was suggested for sodic end-member dissolution in majoritic garnets, and garnet with the composition Na2MgSi5O12 and tetragonal symmetry was synthesized for the first time.  相似文献   

11.
New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O18 by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC13 values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C13 content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.  相似文献   

12.
山东昌乐新生代玄武岩内的刚玉巨晶(蓝宝石)中含有多种类型熔融包裹体,其成分对了解华北深部地幔交代过程中的流/熔体性质和刚玉母岩浆特点具有重要意义.详细的岩相学和激光拉曼分析鉴定出一类富碳酸盐和硫酸盐成分的原生熔融包裹体以及一类含硫酸盐和氯化物等成分的次生熔融包裹体,二者同时还含有CO2和H2O.碳酸盐和硫酸盐成分在世界范围玄武岩内刚玉巨晶中是首次发现,结合已有的包裹体稀有气体同位素和测温资料,反映两种成分可能来源于交代地幔的碳酸岩熔体,预示着华北深部地幔不仅经历了硅酸盐成分的交代还经历了富碳酸盐和硫酸盐成分(碳酸岩)的交代,同时也显示刚玉母岩浆成分复杂,至少有富这两类成分物质的参与,刚玉很可能是硅酸盐岩浆/岩石和幔源碳酸岩岩浆相互作用的产物,后被玄武岩喷发携带至地表.  相似文献   

13.
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.  相似文献   

14.
The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe2+), 1.01–0.68 wt% TiO2) and are comparable to other proposed parental magmas except in having higher Al2O3 and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An78.3-An83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo89-Fo91) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo89 but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine, plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts and in the cores of zoned plagioclases, any model must account for all phases present.  相似文献   

15.
Experiments have been performed in the multicomponent (natural) bulk system to constrain the conditions of generation and differentiation of a K-rich group II kimberlite (now also referred to as orangeite). The group II composition examined was saturated in olivine, orthopyroxene, and garnet at near liquidus conditions in the pressure range 4 to 10 GPa. In the range 2 to 3 GPa, the liquidus phase was olivine only. The potassic nature of the melts in the bulk compositions studied was ensured by the absence of any K-bearing phase in the residual assemblage at P > 4 GPa. Phlogopite is destabilized toward higher pressures by a carbonation reaction of the type phlogopite + CO2 = enstatite + garnet + K2CO3 (liquid) + H2O leading to alkalic, carbonatitic liquids coexisting with a garnet-peridotite (harzburgite or lherzolite) residue over a wide pressure-temperature space at pressures in excess of 4 GPa. Evidently, CO2-bearing systems do not favor the stability of phlogopite and/or K-richterite amphibole at pressures in excess of 4 to 5 GPa, and it is suggested that the carbonate-bearing and potassic character of any mantle melt originating from this depth is most likely the product of a two-stage process: either a carbonate-bearing protolith is invaded by a potassic melt or fluid (probably supercritical), or a potassic protolith (after metasomatism) has been invaded by a carbonatite melt.  相似文献   

16.
《Chemical Geology》2007,236(3-4):303-322
In order to better understand what controls sulfur speciation in melt inclusions, and how that pertains to the original basalt composition, we have conducted a series of heating experiments on naturally quenched and crystalline olivine-hosted melt inclusions. Sulfur speciation was determined from S Kα peak shift measurements by electron microprobe on the experimentally heated inclusions as well as a series of naturally quenched inclusions, and matrix glasses.Naturally quenched olivine-hosted melt inclusions record a similar but more variable sulfur speciation relative to matrix glasses, (up to 45–50% variation in S6+/Stotal). Much of this range can be attributed to the effect of degassing which may either increase or decrease the S6+/Stotal. In addition, olivine melt re equilibration and H diffusion out of the inclusion both potentially result in the oxidation of melt inclusions. Heating of melt inclusions can have different effects on the sulfur speciation under different conditions. A slight decrease in S6+/Stotal and oxygen fugacity (∼0.1 log units) can occur from overheating of inclusions (above the temperature of entrapment), resulting from excess ferrous iron in the melt. An increase in heating times should result in an oxidation of the inclusion generated by increased H diffusion out of the inclusion. However, results of heating experiments on melt inclusions from an Izu backarc basalt for less than 30  min do not show a significant increase in sulfur oxidation. In addition, experiments conducted at both IW and FMQ have measured sulfur speciation consistent with naturally quenched inclusions suggesting that at experimental temperatures near that of olivine crystallization the furnace atmosphere does not exert significant control on the melt fO2. By taking these parameters into account, sulfur speciation and oxidation state of basaltic melt trapped within inclusions can be accurately determined from both naturally quenched and heated olivine hosted melt inclusions.  相似文献   

17.
Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09-1.1 mm2 areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new “chemical point-counting” technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26-29.5 wt% SiO2, ∼7 wt% of FeOT, 25.7-28.7 wt% MgO, 11.3-15 wt% CaO, 8.3-11.3 wt% CO2, and 7.6-9.4 wt% H2O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (η = 101-103 Pa s at 1100-1000 °C) and depolymerized (NBO/T = 2.3-3.2), but entrains with 40-50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190-350 ppm of water suggesting crystallization from a low SiO2 magma (aSiO2 below the olivine-orthopyroxene equilibrium) at 30-50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO2-rich residual liquids. The melt crystallised phlogopite (6-10%), monticellite (replaced by serpentine, ∼10%), calcite rich in Sr, Mg and Fe (19-27%), serpentine (29-31%) and minor amounts of apatite, ulvöspinel-magnetite, picroilmenite and perovskite. The observed content of H2O can be fully dissolved in the primitive melt at pressures greater than 0.8-1.2 kbar, whereas the amount of primary CO2 in the kimberlite exceeds CO2 soluble in the primitive kimberlite melt. A mechanism for retaining CO2 in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.  相似文献   

18.
In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe, northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rh?nite, clinopyroxene, a sulfide globule, and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions (QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite. Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation. Original Russian Text ? T.Yu. Timina, V.V. Sharygin, A.V. Golovin, 2006, published in Geokhimiya, 2006, No. 8, pp. 814–833.  相似文献   

19.
Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo77-73); Type-II inclusions occur in phenocryst mantles (Fo71-66); Type-III inclusions occur in phenocryst rims (Fo61-51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150-250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25-75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo77 composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo77-73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo77. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine.The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites.  相似文献   

20.
The Let?eng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86?88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24–26 wt% SiO2, 20–21 wt% MgO, 16–19 wt% CaO and 1.9–2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09–0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5–5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10–12 wt% CO2, H2O is estimated to 4–5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i)?=?0.703602–0.703656, 143Nd/144Nd(i)?=?0.512660 and 176Hf/177Hf(i)?=?0.282677–0.282679 indicate that the Let?eng kimberlite originates from the convective upper mantle. U–Pb dating of groundmass perovskite reveals an emplacement age of 85.5?±?0.3 (2σ) Ma, which is significantly younger than previously proposed for the Let?eng kimberlite.  相似文献   

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