Methane-bearing,aqueous, saline solutions in the Skaergaard intrusion,east Greenland

Solutions of H₂O–NaCl–CH₄ occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 m diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH₄, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C–O–H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.     

The trapped fluid phase in upper mantle xenoliths from Victoria,Australia: implications for mantle metasomatism

Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 m) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO₂. Small isolated inclusions may have densities 1.19 g/cm³, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N₂, Ar, H₂S, CO_s and SO₂ in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO₂ thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO₂-H₂O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO₂, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.     

Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic–ultramafic intrusions, Eastern Desert, Egypt

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr₂O₃ contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe³⁺-rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al₂O₃) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe³⁺-rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe³⁺, extending from the Fe³⁺–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations.     

Mineralogy and partial melt textures within an ultramafic-mafic body,Greece

Ultramafic-mafic rocks from Makrirrakhi, Central Greece exhibit features of an original ophiolite sequence which contains depleted mantle material, ultramafic containing partial melt textures and possibly the mafic pluton which resulted from the coalescing of these partial melt segregations. Considerable mineralogical variation exists: unzoned olivine crystals range in composition from Fo_78–84 (mafics) to Fo_88–92 (ultramafics), plagioclases An_64–79 (mafics) to An_80–90 (ultramafics) and spinel varies from a chromian spinel (ultramafics) to a more aluminous-titaniferous spinel (mafics). Pyroxenes from the ultramafics display a limited range: En_89–92 Fs_9–8 Wo_0–2 (orthopyroxene) and En_48–54 Fs_1–10 Wo_38–50 (clinopyroxene). Mafic rocks display a greater range being richer in ferrosilite En_36–65 Fs_3–20 Wo_33–51. Pyroxenes from within the partial melt segregations have chemical affinities with those from the gabbrotroctolite series. A model of partial melt within the upper mantle, and, a set of criteria to distinguish partial melt textures from cumulate textures, are developed from analytical data and textural evidence.     

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820?740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl₂ · KCl, CaCl₂, nMgCl₂ · mCaCl₂) and high-density gases (CO₂, H₂S, N₂, CH₄, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.     

Genesis of carbonate in pyrope from ultramafic diatremes on the Colorado Plateau,southwestern United States

Mineral associations and compositions of carbonates within pyrope crystals are clues to the genesis of mantle carbonate and to the character of metasomatic melts in depleted peridotite. The pyrope crystals are in ultramafic diatremes of the Navajo field on the Colorado Plateau. Although inclusions of olivine and pyroxene are typically monomineralic, 4 of 6 inclusions of carbonates and hydrates are polymineralic. Polymineralic assemblages include: pargasite-magnesite-dolomite-apatite-spinel; pargasite-dolomite-Ba phlogopite (with 10% BaO); olivine-dolomite-spinel; edenite-chlorite; and olivine-ilmenite-spinel. Magnesite and chlorite are present also as monomineralic inclusions. The two inclusions with pargasite plus carbonate are in the same garnet; the association of carbonates plus hydrates and the enrichment in Ba are evidence that the included minerals originated from melt trapped in pyrope. The pargasite and mica are F-poor and contain about 0.4 and 1.1 wt% Cl, respectively, more than any other analyzed mantle amphibole or mica. If the parent melts of such inclusions are similar to those responsible for trace-element metasomatism of continental lithosphere, then these melts have higher Cl/F ratios than those inferred from typical xenolith minerals. Amphibole-garnet and olivine-spinel equilibration temperatures are in the range 500–700° C, so the garnets cooled to low temperatures within the mantle following inclusion of melt. All the hydrates and carbonates may have formed from trapped melt, but evidence is strong only for the complex pargasite-carbonate-mica inclusions. Two garnets containing chlorite are more Cr-rich and Fe-poor than most other inclusion-bearing pyropes, and the chlorite may have been included during prograde metamorphism of subducted lithosphere.     

Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the Allende meteorite

Lightly altered Al-rich inclusions in amoeboid olivine aggregates have cores containing primary melilite + fassaite + spinel + perovskite and no secondary alteration products. In moderately altered inclusions, whose cores now contain only fassaite + spinel + perovskite, melilite was replaced by a fine-grained mixture of grossular + anorthite + feldspathoids and perovskite was partially replaced by ilmenite. In heavily altered inclusions, fassaite has been replaced by a mixture of phyllosilicates + ilmenite and the remaining primary phases are spinel ± perovskite. In very heavily altered inclusions, no primary phases remain, the spinel having reacted to form either phyllosilicates or a mixture of olivine + feldspathoids. This sequence of alteration reactions may reflect successively lower solar nebular equilibration temperatures. During alteration, SiO₂, Na₂O, K₂O, FeO, Cr₂O₃, H₂O and Cl were introduced into the inclusions and CaO was lost. MgO may have been lost during the melilite reaction and added during formation of phyllosilicates. Electron microprobe analyses indicate that the phyllosilicates are a mixture of Na-rich phlogopite and chlorite or Alrich serpentine. Thermodynamic calculations suggest that, at a solar nebular water fugacity of 10⁻⁶, Na-rich phlogopite could have formed from fassaite at ~470 K and chlorite from Na-rich phlogopite at ~328 K. Olivine mantling Al-rich inclusions is not serpentinized, suggesting that these objects stopped equilibrating with the nebular gas above 274 K.     

Ultrahigh-pressure corundum-rich garnetite in garnet peridotite,Sulu terrane,China

Corundum-rich garnetite occurs as an isolated lens in a garnet peridotite body in the Donghai area of the Sulu ultrahigh-pressure (UHP) terrane. This rock consists of garnet and corundum, along with minor crack-related zoisite, pargasite, Mg-staurolite, Mg-chloritoid, sapphirine and chlorite. Pyropic garnet (Prp_54–63Grs_26–36Alm_10–12) exhibits a sinusoidal REE pattern, positive Ta, Pb, and negative Nb, Ti anomalies due to metasomatism. Reddish corundum contains 1.1–1.7 wt% Cr₂O₃, and shows three oriented sets of exsolved rutile needles. Both garnet and corundum contain inclusions of apatite, Mg-allanite (MgO>4 wt%), and Ni-Fe sulfides formed as trapped Ni-Fe-S melt. The protolith of the corundum-rich garnetite could have been spinel websterite formed in the upper mantle. Both the websterite and the host garnet peridotite were subjected to subduction-zone UHP metamorphism at 800 °C and >4 GPa. Crack-related hydrous phases were formed by fluid infiltration during exhumation.Editorial responsibility: T.L. Grove     

Spinel inclusions in olivine of peridotite xenoliths from TUBAF seamount (Bismarck Archipelago/Papua New Guinea): evidence for the thermal and tectonic evolution of the oceanic lithosphere

Olivine in spinel peridotite xenoliths from the Bismarck Archipelago northeast of Papua New Guinea, which were transported to the surface by Quaternary basalts, shows spinel inclusions up to 25 μm long and 200 nm wide. These inclusions mainly occur as inhomogeneously distributed needles and subordinately as octahedral grains in olivine of veined metasomatic peridotites as well as peridotites without obvious metasomatism. The needles very often occur in swarms with irregular spacing in between them. Similar spinel inclusions in olivine have only previously been reported from ultramafites of meteoritic origin. Composition and orientation of the spinel inclusions were determined by transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Both the needles and the grains display a uniform crystallographic orientation in the host olivine with [001]^O1//[1ˉ10]_Spl and (100)_Ol// (111)_Spl. The needles eare elongated parallel [010] in olivine, which is the same in all olivine grains. As these needles have no relation to the metasomatic sections in the peridotite, it is concluded that they are primary features of the rock. Although the composition of the spinel needles is often very similar to the large chromian spinel octahedra in the matrix, the small octahedral spinel inclusions in olivine are in part Mg-rich aluminous spinel and sometimes almost pure magnetite. The spinel needles are suggested to have formed by exsolution processes during cooling of Al- and Cr-rich, high-temperature olivine during the initial formation of the lithospheric mantle at the mid-ocean ridge. The Al-rich spinel octahedra probably formed by the breakdown of an Al-rich phase such as phlogopite or by metasomatism, whereas the magnetite was generated by oxidizing fluids. These oxidizing fluids may either have been set free by dehydration of the underlying, subducted plate or by the Quaternary magmatism responsible for the transport of the xenoliths to the seafloor. Received: 25 May 2000 / Accepted: 12 July 2000     

Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite+pargasite lherzolite

A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at ≥1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite+garnet lherzolite mineralogy at 1195 °C and with garnet lherzolite mineralogy at 1250 °C are both olivine nephelinite with K/Na (atomic)=0.51 and K/Na=0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite+pargasite lherzolite at pressures <1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 °C (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite (5% melt fraction, 3% H₂O) to silica-oversaturated basaltic andesite (8% melt fraction, 4.5% H₂O). Both compositions may be classified as ‘shoshonites’ on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce primary ‘shoshonitic’ magmas by dehydration melting at 1 GPa, 1050–1150 °C. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil.     

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.     

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr₂O_3, and 20 to 220 ppm H₂O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM₂₁O₃₈, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi₂O₆), and a new oxide mineral, carmichaelite (MO_2−x(OH)_x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb₂O₅), Cr (up to ∼6 wt% Cr₂O₃), and OH (up to ∼0.9 wt% H₂O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998     

Mobility of metamorphic fluids inferred from infrared microspectroscopy on natural fluid inclusions: the example of Tinos Island,Greece

OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H₂O–NaCl–CaCl₂–CO₂ inclusions whereas greenschist-facies veins contain H₂O–NaCl ± CO₂ inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm^–1 °C^–1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm^–1 for blueschist inclusions and 3,585 cm^–1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a loose structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.Editorial responsibility: J. Hoefs     

Pressure-temperature and evolution of fluid compositions of Al2SiO5-bearing rocks,Mica Creek,B.C., in light of fluid inclusion data and mineral equilibria

Metamorphosed pelitic rocks from Mica Creek, British Columbia contain sillimanite, kyanite with minor fibrolite and andalusite-bearing quartz pods. Mineral equilibria were used to infer peak P-T conditions and fluid compositions in equilibrium with the solid phases. Fluid inclusions in three schist samples appear to be good indicators of conditions affecting those rocks during and after peak metamorphic conditions. In samples from two localities, fluid inclusions from schist and quartz-rich segregations have densities appropriate to the peak metamorphic conditions. The observed compositions for these fluids (low salinity with 12 mole % dissolved CO₂) agree with calculated values of 0.84 to 0.85, based upon paragonite-quartz-albite-Al₂SiO₅ equilibria. The fluids unmixed as the schists were uplifted and cooled; fluid inclusions trapped during this stage outline a solvus in the CO₂-H₂O-NaCl system. A later influx of fluids containing CH₄ and N₂ accompanied formation of andalusite-bearing plagioclaserich segregations. The restricted association of andalusite-bearing pods and low density fluids suggest a localized but pervasive fluid influx during uplift. Preservation of high density fluid inclusions during uplift and erosion, coupled with evidence for unmixing of H₂O- and CO₂-rich fluids on the solvus, provide constraints on the P-T uplift path.     

Metasomatism in subcontinental mantle beneath Northern Patagonia (Rio Negro Province), Argentina: evidence from silica-rich melt inclusions

Summary In an attempt to investigate metasomatic processes in the subcontinental upper mantle beneath Northern Patagonia, melt and fluid inclusions trapped in spinel lherzolite and harzburgite xenoliths have been studied. The xenoliths contain three types of genetically related inclusions hosted by olivine, orthopyroxene, clinopyroxene and spinel: silicate glass inclusions, multiphase silicate melt inclusions and C0₂ fluid inclusions. CO₂ densities of early fluid inclusions (0.93–1.02 g/cm³) and homogenization temperatures of melt inclusions (1220 °C) indicate that they were trapped at uppermantle depths.Silicate melt inclusions occur as isolated inclusions as trails along fractures and decorating lamellae deformation. They are composed of glass, or of glass with daughter crystals of clinopyroxene, amphibole, apatite and ilmenite. Glass inclusions are characterized by high contents of silica, alumina and alkali elements. The nature and chemistry of the daughter minerals indicate that melt inclusions can be considered as trapped metasomatic melts. Moreover, glass inclusions show chemical variations from high silica (68 wt%) melts trapped as isolated inclusions in olivine Fog, towards less siliceous (60 wt%) melts trapped as secondary inclusions in olivine FO_89-90, orthopyroxene and spinel. This chemical evolution cannot be reconciled with magmatic processes, like fractional crystallization or different degrees of partial melting. The existence of two stages in their evolution, could reflect the heterogenous nature of the mantle source of the melts and the presence of a deep-seated fluid phase during melting.

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Les inclusions vitreuses riches en silice: témoignages du métasomatisme du manteau subcontinental de la province de Rio Negro (Patagonie septentrionale, Argentine)

Résumé Les inclusions fluides et vitreuses piégées dans les minéraux des xénolithes de lherzolite á spinelle et de harzburgite ont fait l'objet d'études thermométriques et barochimiques afin de mieux cerner les processus métasomatiques qui ont affecté le manteau supérieur subcontinental du nord de la Patagonie. Les xénolithes contiennent trois types d'inclusions génétiquement reliées entre elles et piégées dans l'olivine, l'orthopyroxène, le clinopyroxène et les spinelles: 1) inclusions vitreuses, 2) inclusions vitreuses multiphasées, 3) inclusions fluides á CO₂. La densité des inclusions fluides précoces (0.93 á 1.02g/c3) et la température d'homogénéisation des inclusions vitreuses (Th = 1220°C) indiquent que les inclusions ont été piégées dans le manteau supérieur.Les inclusions vitreuses sont soit en individus isolés, soit en alignements disposés selon des fractures et des lamelles de déformation (inclusions secondaires). Elles sont formées par du verre et par une bulle pouvant contenir du CO₂ auxquels peuvent s'associer des minéraux fils (clinopyroxènes, amphiboles, apatites et ilménites). La phase vitreuse, caractérisée par une composition riche en silice, alumine, alcalins et volatils élevés et la nature des minéraux fils indiquent que les inclusions vitreuses résultent du piégeage de liquides métasomatiques.La composition des inclusions vitreuses varie. La teneur en SiO2 atteint 68% en poids dans les inclusions isolées des Fo 91, elle est par contre proche de 60% dans les inclusions secondaires des Fo 89-90, de l'orthopyroxène et du spinelle. La composition chimique des différents types d'inclusions ne peut être expliquée par les processus classiques de différenciation magmatique tels que la cristallisation fractionnée ou la fusion partielle. L'existence d'une bimodalité dans les compositions chimiques et dans les niveaux de piégeage plaident en faveur d'une source mantellique hétérogène et de l'intervention de fluides profonds lors du processus de fusion ayant généré les liquides métasomatiques.

     

High-pressure minerals in mafic microgranular enclaves: evidences for co-mingling between lamprophyric and syenitic magmas at mantle conditions

Mafic microgranular enclaves, composed of diopside and rare magnesium biotite phenocrysts in a groundmass of diopside, biotite, apatite, Fe-Ti-oxides, and alkali feldspar, are associated with Neoproterozoic Piquiri potassic syenite in southern Brazil. Co-genetic mica and clinopyroxene cumulates present inclusions of pyrope-rich garnet in diopside phenocrysts. Textural evidence, as well as the chemical and mineralogical composition, suggest that enclaves crystallized from a lamprophyric magma and co-mingled with the host syenitic magma. The contrasting temperature between both magmas and the consequent chilling was important for the preservation of some early-crystallized minerals in the mafic magma. Diopside groundmass grains contain micro-inclusions of K-rich augite and phlogopite, and some clinopyroxene phenocrysts and elongate groundmass crystals have potassium-rich cores. The pyrope-rich garnet have high #mg number (67–68), with appreciable amounts of Na₂O and K₂O comparable to pyrope synthesized at 5 GPa. The extremely high K₂O contents of K-rich augite micro-inclusions suggest non-equilibrium with the parental magma, whereas the other K-rich clinopyroxenes are similar to K-clinopyroxenes produced at 5–6 GPa. K-clinopyroxene and garnet in mafic microgranular enclaves suggest that lamprophyric magma started its crystallization at upper mantle conditions, and chilled clinopyroxenes with measurable amounts of K₂O are taken as evidence that co-mingling began still at mantle pressures.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

The Banhad?o Alkaline Complex, Southeastern Brazil: source and evolution of potassic SiO2-undersaturated high-Ca and low-Ca magmatic series

The Cretaceous Banhad?o alkaline complex in southeastern Brazil presents two potassic SiO₂-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo_92-91) + diopside (Wo_48-43En_49-35Ae_0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo_47-38En_46-37Ae_0-8) + phlogopite + apatite + perovskite (Prv_>92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo_47-41En_32-24 Ae_3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne_74.8-70.1Ks_26.3-21.2Qz_7.6-0.9) and leucite (Lc_65-56) and subsequently by melanite and potassic feldspar (Or_85-99Ab_1-7) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo_36-29En_25-4Ae_39-18) + alkali feldspar (Or_57-96Ab_3-43) + nepheline (Ne_76.5-69.0Ks_19.9-14.4Qz_15.1-7.7) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO₂ and P₂O₅ contents and lower SiO₂ contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb initial ratios for the high-Ca series (0.70407–0.70526, 0.51242–0.51251, 17.782–19.266 and 38.051–39.521, respectively) were slightly different from those of the low-Ca series (0.70542–0.70583, 0.51232–0.51240, 17.758–17.772 and 38.021–38.061, respectively). For both series, a CO₂-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.     

Redox state of the upper mantle

The oxygen fugacity condition of equilibration has been carefully determined from a spinel lherzolite from Mongolia, olivine xenocrysts from chrome pyrope-bearing peridotite nodules from kimberlites of Yakutia, and basaltic samples from ocean floor, iron arcs and the continental areas. These indicate that the spinel lherzolites occurring within alkali basalts from Mongolia, equilibrated under an \(f_{O_2 } \) condition similar to that of WM buffer. The diamond and chrome pyrope-bearing peridotites from the kimberlite pipes equilibrated between IW and WM buffers. Some of the ilmenite-bearing peridotite crystallized under \(f_{O_2 } \) conditions similar to that between WM and QFM buffers and chondrites equilibrated below the QFI buffer. It is concluded that during geochemical processes in the upper mantle the \(f_{O_2 } \) conditions vary broadly, and are similar to that between FMQ and IW buffers. There is a dramatic change in the composition of the kimberlitic fluid, which is CH₄-bearing at an early stage, but is in equilibrium with H₂O and CO₂ at a later stage. This is related to mass transfer of fluids from the lower part of the mantle with a low oxidation state to the upper part having a higher \(f_{O_2 } \) condition.     

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.