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.     

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation,remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases.Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions.The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.     

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.     

Segregation of magmatic fluids and their potential in the mobilization of platinum-group elements in the South Kawishiwi Intrusion,Duluth Complex,Minnesota — Evidence from petrography,apatite geochemistry and coexisting fluid and melt inclusions

Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO₂-rich (95 mol%) and contain small amounts of H₂O (4.5 mol%), CH₄ (0.4 mol%) and trace N₂, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Th_tot = ~ 225 ± 10 °C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO₂ in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO₂-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO₂-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.     

X-ray Spectromicroscopy of Mineral Intergrowths in the Santa Catharina Meteorite

This work describes the application of microfocus X-ray absorption spectroscopy (XAS) and X-ray photo-emission electron microscopy (XPEEM) to the study of the complex mineralogical intergrowths within the Santa Catharina meteorite. The Santa Catharina meteorite of this study (BM52283 from the meteorite collection of the Natural History Museum, London, UK) primarily comprises a taenite bulk host phase (Fe:Ni ratio = 70.9 ± 0.8%:29.1 ± 0.8%) with a set of oxide-bearing cloudy zone textured regions (Fe:Ni:O ratio = 40.4 ± 0.3%:49.0 ± 0.7%:10.6 ± 0.8% at the core and Fe:Ni:O ratio = 34.4 ± 1.5%:42.7 ± 0.6%:22.9 ± 1.8% towards the rims) and numerous schreibersite (Fe:Ni:P ratio = 38.6 ± 1.6%:38.4 ± 0.9%:23.0 ± 0.5%) inclusions. Between the schreibersite and the taenite are rims up to 50 μm across of Ni-rich kamacite (Fe:Ni ratio = 93.4 ± 0.4%:6.6 ± 0.5%). No chemical zoning or spatial variations in the Fe and Ni speciation was observed within either the schreibersite or the kamacite phases. The oxide-bearing cloudy zone textured regions mostly comprise metallic Fe–Ni alloy, predominantly tetrataenite. Within the oxide phases, the Fe is predominantly, but not entirely, tetrahedrally co-ordinated Fe³⁺ and the Ni is octahedrally co-ordinated Ni²⁺. Structural analysis supports the suggestion that non-stoichiometric Fe₂NiO₄ trevorite is the oxide phase. The trevorite:tetrataenite ratio increases at the edges of the oxide-bearing cloudy zone textured regions indicating increased oxidation at the edges of these zones. The spatial resolution of the XPEEM achieved was between 110 and 150 nm, which precluded the study of either the previously reported ∼ 10 nm precipitates of tetrataenite within the bulk taenite or any antitaenite.     

The origin of spongy texture in minerals of mantle xenoliths from the Western Qinling, central China

Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, Al^VI, and Al^IV/Al^VI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.     

车里雅宾斯克(Chelyabinsk)陨石的岩石矿物特征与冲击变质作用

2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。     

Homogenization of carbonate-bearing microinclusions in diamond at <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> parameters of the upper mantle

The staged high-pressure annealing of natural cubic diamonds with numerous melt microinclusions from the Internatsional’naya kimberlite pipe was studied experimentally. The results mainly show that the carbonate phases, the daughter phases in partially crystallized microinclusions in diamonds, may undergo phase transformations under the mantle P–T conditions. Most likely, partial melting and further dissolution of dolomite in the carbonate–silicate melt (homogenization of inclusions) occur in inclusions. The experimental data on the staged high-pressure annealing of diamonds with melt microinclusions allow us to estimate the temperature of their homogenization as 1400–1500°C. Thus, cubic diamonds from the Internatsional’naya pipe could have been formed under quite high temperatures corresponding to the lithosphere/asthenosphere boundary. However, it should be noted that the effect of selective capture of inclusions with partial loss of volatiles in relation to the composition of the crystallization medium is not excluded during the growth. This may increase the temperature of their homogenization significantly between 1400 and 1500°C.     

Refractory chromitites recovered from the Eretria mine,East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe²⁺) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.     

阿尔泰伟晶岩中流体熔融包裹体成分的研究

对阿尔泰可可托海、柯鲁木特和库威伟晶岩锂辉石及绿柱石中单个流体熔融包裹体各相成分，借助激光拉曼探针进行分析，鉴定出固体相为不同硅酸盐子晶矿物，定量给出了流体相成分。根据子晶矿物和流体相成分估算了整个流体熔融包裹体的成分，并据此进一步讨论了熔体中流体的溶解度问题。流体熔融包裹体成分研究表明熔体中流体已达饱和或过饱和，流体相与熔体相发生分离，相应残余伟晶岩浆体系进入晶体＋熔体＋流体三相共存的岩浆－热液     

Thin intergranular melt films and melt pockets in spinel peridotite xenoliths from the Rhön area (Germany): early stage of melt generation by grain boundary melting

Optical microscopy and transmission electron microscopy (TEM) on a porphyroclastic high temperature spinel peridotite from the Rhön area reveal fine, irregular glass layers and pockets along mineral interfaces, cracks in olivine, inside olivine crystals and in spongy rims of clinopyroxene. The chemical composition of the glass deviates significantly from the composition of the host basanite. Electron diffraction technique confirms the amorphous nature of the glass, thus classifying it as a former melt. Every grain or phase boundary shows amorphous intergranular glass layers of variable thickness and characteristic chemical composition with distinct chemical inhomogeneities. Olivine grain boundaries, as the most common type of interfaces, exhibit two different types of melt glasses: (1) Type I melt at olivine grain boundaries, which is characterized by low contents of SiO₂ (～37?wt%) and Al₂O₃ (～5?wt%) and elevated contents of MgO (～31?wt%) and FeO (～22?wt%), is supposed to have formed prior to or during the thermal overprint and the dynamic recrystallisation of the xenolith in the mantle. Melt inclusions inside olivine grains with an average composition of type I melt are suggested to be earlier melt droplets at olivine interfaces, overgrown by migrating olivine grain boundaries during recrystallization in the mantle prior to the uplift of the xenolith. (2) Type II melt, the most common type of melt in the xenolith, shows higher contents of SiO₂ (～48?wt%) and Al₂O₃ (～17?wt%) but lower contents of MgO (～20?wt%) and FeO (～11?wt%). The observation of different types of glass within a single xenolith indicates the development of different chemical melt equilibria at interfaces or triple junctions in the xenolith. The absence of geochemical trends in bivariate plots excludes a unifying process for the genesis of these glasses. Melt inclusions in the spongy rims of clinopyroxene are interpreted to be the product of a potassium-rich metasomatism. The formation of most amorphous intergranular melt layers and pockets at the mineral interfaces including type II melt at olivine grain boundaries is suggested to result from decompression melting during the uplift with the basalt magma. We suggest that these glasses were produced by grain boundary melting due to lattice mismatch and impurity segregation. The observed intergranular amorphous layers or melts represent the very beginning of mineral melting by grain boundary melting.     

Unique mineral assemblages of shock-induced titanium-rich melt pockets in eucrite Northwest Africa 8003

Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ˜0.9–˜10 GPa, ˜1300–˜1730 °C, and < 1 ms (5–50 μm in size), respectively.     

Origin of spongy textures in clinopyroxene and spinel from mantle xenoliths, Hessian Depression, Germany

Summary Spongy textures are observed in anhydrous Group 1 mantle xenoliths (harzburgite, lherzolite and wehrlite) hosted in Tertiary alkali basaltic lavas from the Hessian Depression, Germany. These textures are developed only on clinopyroxene and spinel, and occur as rims or cross-cutting veinlets and patches showing optical continuity with the host grain. They are often associated with pools of amorphous glassy material. There is no preferential development of spongy domains against the xenolith-lava contact suggesting that the host magma did not play any significant role in their formation. Spongy clinopyroxene and spinel occur in all rock types, but, are more pervasive in wehrlite. Chemically, spongy domains of clinopyroxene and spinel are more refractory than unaffected areas, which is consistent with their formation through a partial melting event. The associated glassy material shows chemical characteristics which suggest that the melt pools are genetically related to the development of the spongy textures. The partial melting event was probably triggered by the infiltration of a low-density fluid. The fluid may have evolved from a silicate melt responsible for the metasomatic Fe-enrichment recorded in wehrlite. In this context, the more pervasive development of spongy clinopyroxene in wehrlite may be explained by a higher concentration of the evolved fluid phase at proximity to its silicate melt source. Received March 15, 2000; revised version accepted September 6, 2001     

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO₂, CH₄, and N₂, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO₂ and reach the highest H₂O contents (6.5–10 wt%). In the transition zone melts have intermediate H₂O (4.8–8.5 wt%), CO₂ (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO₂ (up to 2,400 ppm), with H₂O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe³⁺‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H₂O contents of the melts throughout the three zones are higher than usually assumed for initial H₂O contents of anatectic melts. The CO₂ contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H₂O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.     

A melt and fluid inclusion assemblage in beryl from pegmatite in the Orlovka amazonite granite, East Transbaikalia, Russia: implications for pegmatite-forming melt systems

Beryl crystals from the stockscheider pegmatite in the apical portion of the Li-F granite of the Orlovka Massif in the Khangilay complex, a tantalum deposit, contain an assemblage of melt and fluid inclusions containing two different and mutually immiscible silicate melts, plus an aqueous CO₂-rich supercritical fluid. Pure H₂O and CO₂ inclusions are subordinate. Using the terminology of Thomas R, Webster JD, Heinrich W. Contrib Mineral Petrol 139:394–401 (2000) the melt inclusions can be classified as (i) water-poor type-A and (ii) water-rich type-B inclusions. Generally the primary trapped melt droplets have crystallized to several different mineral phases plus a vapor bubble. However, type-B melt inclusions which are not crystallized also occur, and at room temperature they contain four different phases: a silicate glass, a water-rich solution, and liquid and gaseous CO₂. The primary fluid inclusions represent an aqueous CO₂-rich supercritical fluid which contained elemental sulfur. Such fluids are extremely corrosive and reactive and were supersaturated with respect to Ta and Zn. From the phase compositions and relations we can show that the primary mineral-forming, volatile-rich melt had an extremely low density and viscosity and that melt-melt-fluid immiscibility was characteristic during the crystallization of beryl. The coexistence of different primary inclusion types in single growth zones underlines the existence of at least three mutually immiscible phases in the melt in which the large beryl crystals formed. Moreover, we show that the inclusions do not represent an anomalous boundary layer.     

Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)

Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO₂ calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO₂. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO₂-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass     

Rare earth element geochemistry and petrogenesis of miles (IIE) silicate inclusions

An ion probe study of rare earth element (REE) geochemistry of silicate inclusions in the Miles IIE iron meteorite was carried out. Individual mineral phases among inclusions have distinct REE patterns and abundances. Most silicate grains have homogeneous REE abundances but show considerable intergrain variations between inclusions. A few pyroxene grains display normal igneous REE zoning. Phosphates (whitlockite and apatite) are highly enriched in REEs (50 to 2000 × CI) with a relatively light rare earth element (LREE)-enriched REE pattern. They usually occurred near the interfaces between inclusions and Fe host. In Miles, albitic glasses exhibit two distinctive REE patterns: a highly fractionated LREE-enriched (CI normalized La/Sm ∼15) pattern with a large positive Eu anomaly and a relatively heavy rare earth element (HREE)-enriched pattern (CI-normalized Lu/Gd ∼4) with a positive Eu anomaly and a negative Yb anomaly. The glass is generally depleted in REEs relative to CI chondrites.The bulk REE abundances for each inclusion, calculated from modal abundances, vary widely, from relatively depleted in REEs (0.1 to 3 × CI) with a fractionated HREE-enriched pattern to highly enriched in REEs (10 to 100 × CI) with a relatively LREE-enriched pattern. The estimated whole rock REE abundances for Miles are at ∼ 10 × CI with a relatively LREE-enriched pattern. This implies that Miles silicates could represent the product of a low degree (∼10%) partial melting of a chondritic source. Phenocrysts of pyroxene in pyroxene-glassy inclusions were not in equilibrium with coexisting albitic glass and they could have crystallized from a parental melt with REEs of ∼ 10 × CI. Albitic glass appears to have formed by remelting of preexisting feldspar + pyroxene + tridymite assemblage. Yb anomaly played an important role in differentiation processes of Miles silicate inclusions; however, its origin remains unsolved.The REE data from this study suggest that Miles, like Colomera and Weekeroo Station, formed when a molten Fe ball collided on a differentiated silicate regolith near the surface of an asteroid. Silicate fragments were mixed with molten Fe by the impact. Heat from molten Fe caused localized melting of feldspar + pyroxene + tridymite assemblage. The inclusions remained isolated from one another during subsequent rapid cooling.     

Graves Nunataks 95209: A snapshot of metal segregation and core formation

GRA 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies. This lodranite preserves a physically, chemically, and mineralogically complex set of metal-sulfide veins. High-resolution X-ray computed tomography revealed three distinct lithologies. The dominant mixed metal-silicate-sulfide matrix is cut by metal-rich, graphite-bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal-poor region. Silicate compositions and modal abundances are typical for lodranites, while the mineralogy of the metal-sulfide component is complex and differs among the three lithologies. Kamacite and troilite occur with chromite, tetrataenite, schreibersite, graphite, and a range of phosphates. An ³⁹Ar-⁴⁰Ar age of 4.521 ± 0.006 Ga measures the time of closure of the K-Ar system. Carbon rosettes within the metal-rich vein are nitrogen-poor, well crystallized, include kamacite sub-grains of composition comparable to the host metal, and are essentially isotopically homogeneous (δ¹³C ∼ −33‰). In contrast, carbon rosettes within metal of the metal-poor lithology are N-poor, poorly crystallized, include kamacite grains that are Ni-poor compared to their host metal, and are isotopically heterogeneous (δ¹³C ranging from −50 to +80‰) even within a single metal grain. The silicate portion of GRA 95209 is similar to the lodranite EET 84302, sharing a common texture, silicate mineral compositions, and Ar-Ar age. GRA 95209 and EET 84302 are intermediate between acapulcoites and lodranites. Both experienced Fe,Ni-FeS melting with extensive melt migration, but record only the onset of silicate partial melting with limited migration of silicate melt. The complex metal-sulfide veins in GRA 95209 resulted from low-degree partial melting and melt migration and intruded the matrix lithology. Reactions between solid minerals and melt, including oxidation-reduction reactions, produced the array of phosphates, schreibersite, and tetrataenite. Extensive reduction in the metal-rich vein resulted from its origin in a hotter portion of the asteroid. This difference in thermal history is supported by the graphite structures and isotopic compositions. The graphite rosettes in the metal-rich vein are consistent with high-temperature igneous processing. In contrast, the carbon in the metal-poor lithology appears to preserve a record of formation in the nebula prior to parent-body formation. Carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal-sulfide melts that form a core, but does not achieve isotopic homogeneity until extensive thermal processing occurs.     

The nature and origin of ureilites

Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Haverö, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating.     

Magmatic-Hydrothermal Evolution in the “Bottoms” of Porphyry Copper Systems: Evidence from Silicate Melt and Aqueous Fluid Inclusions in Granitoid Intrusions in the Gyeongsang Basin,South Korea

The Gyeongsang Basin of southeastern Korea contains numerous Cretaceous-early Tertiary (120–40 Ma) granitoid intrusions formed at a convergent plate boundary. The geotectonic setting is similar to that associated with porphyry-type mineralization elsewhere in the Circumpacific region. However, erosion has removed higher-level economic mineralization and exposed deeper levels of the granitoids, representing the poorly mineralized “bottoms” of porphyry copper systems. The intrusions of the Gyeongsang Basin thus provide a unique opportunity to advance our understanding of magmatic-hydrothermal evolution in the roots of porphyry-type systems, below the level of economic mineralization.

The physical and chemical environment during crystallization of the magmas has been characterized through studies of silicate melt and aqueous fluid inclusions in the granitoids. Two different types of silicate melt inclusions were recognized based on occurrence and room-temperature appearance. Type-I inclusions contain one or more crystalline phases and vapor; type-II inclusions consist of a cluster of small crystals, partially devitrified glass, and vapor. Petrographic and Raman analyses indicate that most silicate melt inclusions contain muscovite daughter crystals. Some also contain feldspar. Solidus temperatures of type-I inclusions in quartz phenocrysts range from ≈630to 650°C, whereas solidus temperatures of type-I and type-II inclusions in vug quartz are slightly higher (640–670°C). Liquidus temperatures span a much wider range compared to solidus temperatures, with maximum liquidus temperatures of melts in phenocrysts being slightly higher (≤930°C) than those in vug quartz (≤910°C).

Three types of aqueous inclusions were observed based on occurrence and room temperature phase proportions. Type-A inclusions are liquid rich and low to moderate in salinity; type-B inclusions are vapor rich and low in salinity; type-C inclusions are liquid rich and contain a halite daughter mineral. Some type- A inclusions with a salinity of approximately 25 wt% NaCl equivalent are spatially associated with silicate melt inclusions in phenocrysts, where they occur as three-dimensional clusters of tiny inclusions surrounding the silicate melt inclusion. Type-A inclusions also occur along fractures in quartz phenocrysts. Non-fracture-controlled type-C inclusions are rare in phenocryst quartz, but are common in vug quartz, where they are associated with silicate melt inclusions. Type-C inclusions that coexist with silicate melt inclusions generally homogenize by halite dissolution after the vapor bubble and show a wide range in salinity, from about 30 to >60 wt% NaCl equivalent. Coexisting halite-bearing (Type-C) and vapor-rich (Type-B) inclusions in phenocryst quartz suggest local immiscibility in the late-or post-magmatic fluid.

Pressure-temperature conditions during the final stages of magmatic-hydrothermal activity associated with the granitoid intrusions of the Gyeongsang Basin were approximately 630° to 670° C and 1.9 to 2.5 kbars. These results suggest that the granitoids do not contain economic porphyry coppertype mineralization because the magmas crystallized at high pressures (relative to typical porphyry copper magmas) and did not become saturated in water until a relatively late stage in the crystallization history. Failure to reach water saturation resulted in most of the copper in the original melt being sequestered as a trace component in earlier-crystallizing silicate and sulfide phases to produce anomalous but subeconomic copper grades. Furthermore, owing to the depth of emplacement, less energy was available to fracture the rocks when water did exsolve from the magma, and the pressure remained too high for aqueous fluid immiscibility to be an important metal-concentrating or depositing mechanism. Geological, petrographic, and geochemical characteristics suggest that the granitoid rocks of the Gyeongsang Basin represent ethroot zones of porphyry-type systems, and any higher-grade mineralization that may have been present higher in the system has since been removed by erosion.