Accretion, dispersal, and reaccumulation of the Bishunpur (LL3.1) brecciated chondrite: Evidence from troilite-silicate-metal inclusions and chondrule rims

A set of troilite-silicate-metal (TSM) inclusions and chondrule rims in the Bishunpur (LL3.1) chondrite provide information regarding impact brecciation of small bodies in the early solar system. The TSM inclusions and chondrule rims consist of numerous angular to subrounded silicate grains that are individually enclosed by fine networks of troilite. FeNi metal also occurs in the troilite matrix. The silicates include olivine (Fo_55-98), low-Ca pyroxene (En_78-98), and high-Ca pyroxene (En_48-68Wo_11-32). Al- and Si-rich glass coexists with the silicates. Relatively coarse silicate grains are apparently fragments of chondrules typical of petrologic type-3 chondrites. Troilite fills all available cracks and pores in the silicate grains. Some of the TSM inclusions and rims are themselves surrounded by fine-grained silicate-rich rims (FGR).The TSM inclusions and rims texturally resemble the troilite-rich regions in the Smyer H-chondrite breccia. They probably formed by shock-induced mobilization of troilite during an impact event on a primitive asteroidal body. Because silicates in the TSM inclusions and rims have highly unequilibrated compositions, their precursor was presumably type-3 chondritic material like Bishunpur itself. The TSM inclusions and the chondrules with the TSM rims were fragmented and dispersed after the impact-induced compaction, then reaccreted onto the Bishunpur parent body. FGR probably formed around the TSM inclusions and rims, as well as around some chondrules, during the reaccumulation process. Components of most type-2 and 3 chondrites probably experienced similar processing, i.e., dispersal of unconsolidated materials and subsequent reaccumulation.     

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.     

Apatite-hosted melt inclusions from the Panzhihua gabbroic-layered intrusion associated with a giant Fe–Ti oxide deposit in SW China: insights for magma unmixing within a crystal mush

The ～?2-km-thick Panzhihua gabbroic-layered intrusion in SW China is unusual because it hosts a giant Fe–Ti oxide deposit in its lower zone. The deposit consists of laterally extensive net-textured and massive Fe–Ti oxide ore layers, the thickest of which is ～?60 m. To examine the magmatic processes that resulted in the Fe enrichment of parental high-Ti basaltic magma and the formation of thick, Fe–Ti oxide ore layers, we carried out a detailed study of melt inclusions in apatite from a ～?500-m-thick profile of apatite-bearing leucogabbro in the middle zone of the intrusion. The apatite-hosted melt inclusions are light to dark brown in color and appear as polygonal, rounded, oval and negative crystal shapes, which range from ～?5 to ～?50 µm in width and from ～?5 to ～?100 µm in length. They have highly variable compositions and show a large and continuous range of SiO₂ and FeO_t with contrasting end-members; one end-member being Fe-rich and Si-poor (40.2 wt% FeO_t and 17.7 wt% SiO₂) and the other being Si-rich and Fe-poor (74.0 wt% SiO₂ and 1.20 wt% FeO_t). This range in composition may be attributed to entrapment of the melt inclusions over a range of temperature and may reflect the presence of µm-scale and immiscible Fe-rich and Si-rich components in different proportions. Simulating results for the motion of Si-rich droplets within a crystal mush indicate that Si-rich droplets would be separated from Fe-rich melt and migrate upward due to density differences in the interstitial liquid when the magma unmixed. Migration of the Si-rich, immiscible liquid component from the interstitial liquid caused the remaining Fe-rich melt in the lower part to react with plagioclase primocrysts (An_59–60), as evidenced by fine-grained lamellar intergrowth of An-rich plagioclase (An_79–84)?+?clinopyroxene in the oxide gabbro of the lower zone. Therefore, magma unmixing within a crystal mush, combined with gravitationally driven loss of the Si-rich component, resulted in the formation of Fe-rich, melagabbro and major Fe–Ti oxide ores in the lower part and Si-rich, leucogabbro in the upper part of the intrusion.     

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.     

Dissolution of orthopyroxene in basanitic magma between 0.4 and 2 GPa: further implications for the origin of Si-rich alkaline glass inclusions in mantle xenoliths

A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO₂-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO₂-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO₂-bearing experiments is only slightly enriched in SiO₂ and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO₂-bearing experiments is less SiO₂ rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO₂ enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor. Received: 24 August 1998 / Accepted: 2 December 1998     

肇东和亳县陨石中的黑包体及其成因意义

肇东、毫县陨石中的黑包体在总体成分、形状、大小上与陨石球粒相似,但两者的内部结构以及矿物组合不同。黑包体中矿物呈密堆状,主要由细粒橄榄石以及其它硅酸盐微晶组成,不含火成玻璃等特点表明黑包体未经历过熔融,它们可能是形成球粒的毛坯。因此认为球粒的形成有三个阶段:星云凝聚形成尘粒—尘粒吸积形成黑包体—黑包体熔融形成球粒。     

TEM study of a silicate-carbonate-microbe interface prepared by focused ion beam milling

The biogeochemical alteration of an Mg-Fe orthopyroxene, reacted for 70 yr under arid conditions in a desert environment, was studied by transmission electron microscopy. For this purpose, an electron transparent cross-section of the interface between a single microorganism, an orthopyroxene and nanometer-sized calcite crystals, was prepared with a focused ion beam system. X-ray energy dispersive spectrometry and electron energy loss spectroscopy allowed one to clearly distinguish the microorganism en route to fossilization from the nanometer-sized calcite crystals, showing the usefulness of such a protocol for identifying unambiguously traces of life in rocks. A 100-nm-deep depression was observed in the orthopyroxene close to the microorganism, suggesting an enhanced dissolution mediated by the microbe. However, an Al- and Si-rich amorphous altered layer restricted to the area just below the microorganism could be associated with decreased silicate dissolution rates at this location, suggesting complex effects of the microorganism on the silicate dissolution process. The close association observed between silicate dissolution and carbonate formation at the micrometer scale suggests that Urey-type CO₂ sequestration reactions could be mediated by microorganisms under arid conditions.     

River water quality in weathered limestone: A case study in upper Mahanadi basin, India

Stromatolitic limestone and calcareous shale belonging to Chattisgarh Supergroup of Proterozoic age dominate the upper part of the Mahanadi river basin. X-ray diffractogram (XRD) of limestone rocks show presence of a significant amount of calcite, dolomite and ankerite. Shales of various colours contain calcite and dolomite. It is observed that congruent dissolution of carbonate minerals in the Charmuria pure limestone has given rise to a typical karst topography. On the other hand, limestones are also seen to support red and black soil profiles. This indicates that the limestone bedrock undergoes a parallel incongruent weathering, which leaves a residue of decomposed rock. The XRD analyses reveal that the limestone soils thus formed contain an assemblage of quartz, clays and Fe-oxides. It is likely that the silicate component trapped during deposition of the stromatolitic limestone weathers incongruently resulting in diverse soil profiles. Carbonate and silicate mineral weathering schemes have been worked out to explain the soil formation, fixation of Al in clay minerals, and Fe in goethite. The water quality parameters such as Ca, Mg and HCO₃ in the river water suggest under saturation with respect to calcite and dolomite. The mineral stability diagrams indicate that kaolinite and Ca-smectite are stable in the river water environment, hence they occur in suspended sediments and soils. The dominant influence of carbonate weathering on the water quality is observed even in the downstream part of the river outside the limestone terrain.     

Cryptic crustal contamination of MORB primitive melts recorded in olivine-hosted glass and mineral inclusions

The origin of compositional heterogeneities among the magmas parental to mid-ocean ridge basalts (MORB) was investigated using a single rock piece of the olivine-phyric basalt from 43°N, Mid-Atlantic Ridge (AII D11-177). The exceptional feature of this sample is presence of very primitive olivine crystals (90–91 mol% Fo) that are significantly variable in terms of CaO (0.15–0.35 wt%). A population of low-Ca olivine (0.15–0.25 wt% CaO) is also notably distinct from high-Ca olivine population in AII D11-177, and primitive MORB olivine in general, in having unusual assemblage of trapped mineral and glass inclusions. Mineral inclusions are represented by high-magnesian (Mg# 90.7–91.1 mol%) orthopyroxene and Cr-spinel, distinctly enriched in TiO₂ (up to 5 wt%, c.f. <1 wt% in common MORB spinel). Glass inclusions associated with orthopyroxene and high-Ti Cr-spinel have andesitic compositions (53–58 wt% SiO₂). Compared to the pillow-rim glass and “normal” MORB inclusions, the Si-rich glass inclusions in low-Ca olivine have strongly reduced Ca and elevated concentrations of Ti, Na, K, P, Cl, and highly incompatible trace elements. Strong variability is recorded among glass inclusions within a single olivine phenocrysts. We argue that the observed compositional anomalies are mineralogically controlled, and thus may arise from the interaction between hot MORB magmas and crystal cumulates in the oceanic crust or magma chamber.     

Genesis of kalsilite melilitite at Cupaello,Central Italy: Evidence from melt inclusions

The paper presents data on primary carbonate–silicate melt inclusions hosted in diopside phenocrysts from kalsilite melilitite of Cupaello volcano in Central Italy. The melt inclusions are partly crystalline and contain kalsilite, phlogopite, pectolite, combeite, calcite, Ba–Sr carbonate, baryte, halite, apatite, residual glass, and a gas phase. Daughter pectolite and combeite identified in the inclusions are the first finds of these minerals in kamafugite rocks from central Italy. Our detailed data on the melt inclusions in minerals indicate that the diopside phenocrysts crystallized at 1170–1190°C from a homogeneous melilitite magma enriched in volatile components (CO₂, 0.5–0.6 wt % H₂O, and 0.1–0.2 wt % F). In the process of crystallization at the small variation in P-T parameters two-phase silicate-carbonate liquid immiscibility occurred at lower temperatures (below 1080–1150°C), when spatially separated melilitite silicate and Sr-Ba-rich alkalicarbonate melts already existed. The silicate–carbonate immiscibility was definitely responsible for the formation of the carbonatite tuff at the volcano. The melilitite melt was rich in incompatible elements, first of all, LILE and LREE. This specific enrichment of the melt in these elements and the previously established high isotopic ratios are common to all Italian kamafugites and seem to be related to the specific ITEM mantle source, which underwent metasomatism and enrichment in incompatible elements.     

Mineralogy of silicate inclusions in the Elga IIE iron meteorite

The petrography, mineral modal data and major and trace element compositions of 15 silicate inclusions in the Elga iron meteorite (chemical group IIE) show that these inclusions represent chemically homogeneous zoned objects with highly variable structures, reflecting the sequence of crystallization of a silicate melt during cooling of the metal host. The outer zones of inclusions at the interface with their metal host have a relatively medium-grained hypocrystalline texture formed mainly by Cr-diopside and merrillite crystals embedded in high-silica glass, whereas the central zones have a fine-grained hypocrystalline texture. Merrillite appears first on the liquidus in the outer zones of the silicate inclusions. Na and REE concentrations in merrillite from the outer zones of inclusions suggest that it may have crystallized as α-merrillite in the temperature range of 1300–1700°С. Merrillite tends to preferentially accumulate Eu without Sr. Therefore, strongly fractionated REE patterns are not associated with prolonged differentiation of the silicate melt source but depend on crystallization conditions of Н-chondrite droplets in a metallic matrix. The systematic decrease in Mg# with increasing Fe/Mn in bronzite may indicate partial reduction of iron during crystallization of the inclusion melt. The modal and bulk compositions of silicate inclusions in the Elga meteorite, as well as the chemical composition of phases are consistent with the model equilibrium crystallization of a melt, corresponding to 25% partial melting of H-chondrite, and the crystallizing liquidus phase, merrillite, and subsequent quenching at about 1090°С. Despite a high alkali content of the average weighted bulk inclusion composition, La/Hf and Rb/Th fall within the field of H chondrites, suggesting their common source. Our results reveal that silicate inclusions in the Elga (IIE) iron meteorite originated by mixing of two impact melts, ordinary chondrite and Ni-rich iron with а IIE composition, which were produced by impact event under near-surface conditions on a partially differentiated parent asteroid.     

The origin of metal clasts in the Bencubbin meteoritic breccia

Bencubbin is a breccia containing metal and silicate clasts, along with occasional chondritic fragments. The breccia is cemented together by a small amount of shock-melted metal-silicate matrix. There is no evidence, however, for complete melting of either metal or silicate clasts after their incorporation within the breccia. The main metal phase occurs as rounded and angular clasts of Fe-Ni. Each clast is chemically homogeneous, but systematic chemical variations between clasts are observed with Ni concentrations varying from ? 5.3 wt% in some clasts up to 7.5 wt% in others. Cobalt concentrations vary between clasts from 0.25 to 0.35 wt% and are positively correlated with Ni. The Co and Ni concentrations are consistent with the metal condensation path (pressure ? 10^-3 atm) predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187). The P vs Ni concentrations are consistent with the metal condensation path (pressure ? 10^?4atm) predicted by Wai et al. (1978, Lunar and Planetary Science IX, pp. 1193–1195). Thus we believe that Bencubbin metal clasts may record chemical information imparted during condensation of the metal from the nebula. Chromium concentrations in Bencubbin metal (0.05–0.30 wt%) greatly exceed concentrations observed in iron meteorites as predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187) for metal condensates from the solar nebula. The Cr-Ni trend in Bencubbin, however, is positively correlated with Ni, in contrast to the predicted condensation trend. This unexpected correlation may be the result of subsequent redistribution of Cr between metal and micron-sized troilite blebs. The incorporation of these troilite blebs within the metal clasts is difficult to explain in terms of low temperature (? 700 K) condensation of troilite. The possible explanations for the presence of the troilite may or may not be consistent with an unaltered primitive composition for the metal clasts. High temperature equilibrium condensation of Bencubbin metal, however, is also supported by the low Ga and Ge contents reported by Kallemeynet al. (1978, Geochim. Cosmochim. Acta42, 507–515). Three of the metal clasts were found to contain ?2.3wt% Si in alloy with the metal. The compositions of these clasts are consistent with equilibrium condensation at a pressure of ? 1 atm from a gas of cosmic composition. The Si-rich clasts could also have condensed at lower pressures from a gas with a fractionated $\text{C}\text{O}$ ratio relative to cosmic abundances.     

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.     

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.     

Silicate-silicate liquid immiscibility and graphite ribbons in Libyan desert glass

Transmission electron microscopic (TEM) investigation of the dark (brown or bluish) streaks occurring in Libyan Desert Glass reveals the common presence of small glass spherules. The spherules, mostly 100 nm in size, are homogeneously dispersed within the silica-glass matrix. The complete absence of electron diffraction effects confirms their amorphous nature. The spherules are Al-, Fe- and Mg-enriched with respect to the surrounding silica matrix and their (Mg + Al + Fe) : Si ratio is close to 1.The silica-glass matrix and amorphous spherules form an emulsion texture (i.e., globules of one glass in a matrix of another glass), which originates from silicate-silicate liquid immiscibility. This texture has also been observed in other impact-derived glasses.The silica glass also contains carbonaceous inclusions consisting of 5-50 nm thick, polygonalized graphite ribbons that form closed structures up to 200 nm in diameter.These observations are in agreement with an impact origin for Libyan Desert Glass.     

Melt inclusions in scoria and associated mantle xenoliths of Puy Beaunit Volcano,Chaîne des Puys,Massif Central,France

In order to characterize the composition of the parental melts of intracontinental alkali-basalts, we have undertaken a study of melt and fluid inclusions in olivine crystals in basaltic scoria and associated upper mantle nodules from Puy Beaunit, a volcano from the Chaîne des Puys volcanic province of the French Massif Central (West-European Rift system). Certain melt inclusions were experimentally homogenised by heating-stage experiments and analysed to obtain major- and trace-element compositions. In basaltic scoria, olivine-hosted melt inclusions occur as primary isolated inclusions formed during growth of the host phase. Some melt inclusions contain both glass and daughter minerals that formed during closed-system crystallisation of the inclusion and consist mainly of clinopyroxene, plagioclase and rhönite crystals. Experimentally rehomogenised and naturally quenched, glassy inclusions have alkali-basalt compositions (with SiO₂ content as low as 42 wt%, MgO>6 wt%, Na₂O+K₂O>5 wt%, Cl~1,000–3,000 ppm and S~400–2,000 ppm), which are consistent with those expected for the parental magmas of the Chaîne des Puys magmatic suites. Their trace-element signature is characterized by high concentration(s) of LILE and high LREE/HREE ratios, implying an enriched source likely to have incorporated small amounts of recycled sediments. In olivine porphyroclasts of the spinel peridotite nodules, silicate melt inclusions are secondary in nature and form trails along fracture planes. They are generally associated with secondary CO₂ fluid inclusions containing coexisting vapour and liquid phases in the same trail. This observation and the existence of multiphase inclusions consisting of silicate glass and CO₂-rich fluid suggest the former existence of a CO₂-rich silicate melt phase. Unheated glass inclusions have silicic major-element compositions, with normative nepheline and olivine components, ~58 wt% SiO₂, ~9 wt% total alkali oxides, <3 wt% FeO and MgO. They also have high chlorine levels (>3,000 ppm) but their sulphur concentrations are low (<200 ppm). Comparison with experimental isobaric trends for peridotite indicates that they represent high-pressure (~1.0 GPa) trapped aliquots of near-solidus partial melts of spinel peridotite. Following this hypothesis, their silica-rich compositions would reflect the effect of alkali oxides on the silica activity coefficient of the melt during the melting process. Indeed, the silica activity coefficient decreases with addition of alkalis around 1.0 GPa. For mantle melts coexisting with an olivine-orthopyroxene-bearing mineral assemblage buffering SiO₂ activity, this decrease is therefore compensated by an increase in the SiO₂ content of the melt. Because of their high viscosity and the low permeability of their matrix, these near-solidus peridotite melts show limited ability to segregate and migrate, which can explain the absence of a chemical relationship between the olivine-hosted melt inclusions in the nodules and in basaltic scoria.     

Cr-spinel assemblage from the Upper Triassic gritstones of the northeastern Siberian Platform

An assemblage of Cr-spinels widespread in Carnian (Upper Triassic) diamondiferous deposits in the northeastern Siberian Platform is studied. Analysis of their morphology and chemical composition has revealed two dominant varieties of Cr-spinels and has demonstrated certain regularities in their distribution in the study area. Correlations have been established between the areal distribution of the recognized types of Cr-spinels and diamond varieties typical of kimberlite sources and between the distribution of Cr-spinels and rounded diamond dodecahedrons. The phase and chemical compositions of polyphase inclusions in the Cr-spinels are studied. The spatial arrangement of inclusions along the crystal growth zones indicates their primary genesis and trapping from the melt during crystallization. Compositional features of some minerals in the inclusions—SiO₂ impurity in apatite and high CaO contents (0.2-0.8 wt.%) in olivines—point to a nonkimberlite source of these Cr-spinels. The presence of K- and Na-containing phases and calcite in the inclusions indicates saturation of the initial melt with alkalies, Ca, and CO₂. The data obtained suggest that the numerous Late Vendian diatremes in K-rich alkaline basites of the Olenek Uplift area are the source of the dominant Cr-spinel variety.     

Nitrogen-rich fluid in the upper mantle: fluid inclusions in spinel dunite from Lanzarote,Canary Islands

Fluid inclusions, ranging from pure N₂ to pure CO₂, occur in olivine porphyroclasts in spinel dunite xenoliths (chrome-diopside suite) from two localities within the Quaternary to Historic alkaline lavas of Lanzarote, Canary Islands. This is the first report of fluid inclusions containing major amounts of N₂ in mantle xenoliths. The nitrogen-rich fluid inclusions predate at least one generation of nitrogen-free carbon dioxide inclusions; textural evidence indicates that the inclusions were trapped within the upper mantle. Some of the nitrogen-rich fluid inclusions are intimately associated with solid inclusions of spinel. The nitrogen-rich fluid was most likely produced in-situ, by oxidation-dehydration reactions destabilizing ammonium-bearing silicate minerals (phlogopite, amphibole), increasing oxygen fugacity or, possibly, increasing temperature of the mantle. This process could be related to an event of CO₂ and silicate melt injection at 6–8 kbar (Neumann et al., in press), or to some earlier event in the evolution of the mantle beneath Lanzarote. The existence of a N₂-rich fluid phase in at least some mantle lithology(ies) at certain conditions is demonstrated by these data. This discovery has consequences for the understanding of the evolution of the mantle below the Canary Islands, as well as for the global nitrogen budget.     

汉诺坝地幔捕虏体中富硅熔体的成因及其意义

近年来地幔捕虏体中富硅熔体的研究受到广泛关注,富硅熔体不仅具有多种产出状态,其成因来源也具有多样性。本文选取汉诺坝地区地幔捕虏体矿物中的熔体包裹体和斜方辉石反应边中的熔体玻璃为研究对象,探索富硅熔体在岩石圈地幔演化中的作用。通过详细的岩相学观察并结合电子探针分析发现,二者成因不同。熔体包裹体玻璃成分富硅(SiO₂ 61%~65%),相对贫Na₂O(1%~3%)、K₂O(<1%),富含挥发份(约3%~6%),为地幔交代成因;斜方辉石反应边玻璃富硅(SiO₂ 64%~67%)、碱(Na₂O 5%~7%,K₂O 6%~9%),几乎不含挥发份,是地幔捕虏体被寄主玄武岩浆快速携带上升至地表的过程中两者反应的产物。地幔交代作用产生的富硅熔体包裹体深刻影响了岩石圈地幔的地球化学性质;而玄武岩浆在上升过程中与岩石圈地幔捕虏体中斜方辉石的反应,导致了岩石圈地幔由富硅向贫硅转变,为华北克拉通的破坏提供了证据。     

Reconciling fluid inclusion types,fluid processes,and fluid sources in skarns: an example from the Bismark Deposit,Mexico

The Bismark deposit (8.5 Mt at 8% Zn, 0.5% Pb, 0.2% Cu, and 50 g/t Ag) located in northern Mexico is an example of a stock-contact skarn end member of a continuum of deposit types collectively called high-temperature, carbonate-replacement deposits. The deposit is hosted by massive sulfide within altered limestone adjacent to the Bismark quartz monzonite stock (~42 Ma) and the Bismark fault. Alteration concurrently developed in both the intrusion and limestone. The former contains early potassic alteration comprising K-feldspar and biotite, which was overprinted by kaolinite-rich veins and alteration and later quartz, sericite, and pyrite with minor sphalerite and chalcopyrite. Prograde exoskarn alteration in the limestone consists of green andradite and diopside, and transitional skarn comprising red-brown andradite, green hedenbergite and minor vesuvinite, calcite, fluorite, and quartz. The main ore stage post-dates calc-silicate minerals and comprises sphalerite and galena with gangue pyrite, pyrrhotite, calcite, fluorite, and quartz. The entire hydrothermal system developed synchronously with faulting. Fluid inclusion studies reveal several distinct temporal, compositional, and thermal populations in pre-, syn- and post-ore quartz, fluorite, and calcite. The earliest primary fluid inclusions are coexisting vapor-rich (type 2A) and halite-bearing (type 3A; type 3B contain sylvite) brine inclusions (32 to >60 total wt% salts) that occur in pre-ore fluorite. Trapping temperatures are estimated to have been in excess of 400 °C under lithostatic pressures of ~450 bar (~1.5 km depth). Primary fluid inclusions trapped in syn-ore quartz display critical to near critical behavior (type 1C), have moderate salinity (8.4 to 10.9 wt% NaCl equiv.) and homogenization temperatures (Th) ranging from 351 to 438 °C. Liquid-rich type 1A and 1B (calcite-bearing) inclusions occur as primary to secondary inclusions predominantly in fluorite and show a range in Th (104–336 °C) and salinity (2.7–11.8 wt% NaCl equiv.), which at the higher Th and salinity ranges overlap with type 1C inclusions. Oxygen isotope analysis was carried out on garnet, quartz, and calcite (plus carbon isotopes) in pre-, syn-, post-ore, and peripheral veins. Pre-ore skarn related garnets have a δ¹⁸O_mineral range between 3.9 and 8.4‰. Quartz from the main ore stage range between 13.6 and 16.0‰. Calcite from the main ore stage has δ¹³C values of –2.9 to –5.1‰ and δ¹⁸O values of 12.3 to 14.1‰, which are clearly distinct from post-ore veins and peripheral prospects that have much higher δ¹⁸O (16.6–27.3‰) and δ¹³C (1.3–3.1‰) values. Despite the numerous fluid inclusion types, only two fluid sources can be inferred, namely a magmatic fluid and an external fluid that equilibrated with limestone. Furthermore, isotopic data does not indicate any significant mixing between the two fluids, although fluid inclusion data may be interpreted otherwise. Thus, the various fluid types were likely to have formed from varying pressure–temperature conditions through faulting during exsolution of magmatic fluids. Late-stage hydrothermal fluid activity was dominated by the non-magmatic fluids and was post-ore.