M. E. Varela

Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO2 and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr2O3 and poorer in SiO2, K2O, Na2O, Al2O3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Abstract Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al 2 O 3 ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al 2 O 3 contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe 2+ , Mn 2+ , and Cr 3+ ). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.

Carbon in glass inclusions of Allende, Vigarano, Bali, and Kaba (CV3) olivines

Carbon contents and distribution have been measured in glass inclusions in olivines of CV3 carbonaceous chondrites by using the 12C(d,p) 13C nuclear reaction. All olivines from the four studied meteorites had low carbon contents (<70 ppm). Conversely, glasses of glass inclusions in the same olivines had highly variable carbon contents, all above 100 ppm. Glass inclusions in olivines from meteorites of the oxidized group (Allende, Kaba, and Bali) had carbon contents that varied from 100 to about 2000 ppm, whereas those from the reduced group member Vigarano were surprisingly poor in C (averaging 300 ppm). These relative abundances of carbon in these glasses of reduced and oxidized CV3 meteorites are in contrast to the abundances of interstellar SiC in these meteorites. This indicates that glass inclusions in olivines could have behaved, with respect to carbon, as closed systems that have escaped elemental exchange processes. The carbon content of the glasses is, therefore, likely to be primary and reflect the physico– chemical conditions during the formation of the host olivine and glass inclusions. The redox conditions prevailing during secondary processing of the olivines (e.g., metasomatic Fe–Mg exchange, Ca–Na exchange) appear not to have influenced the carbon distribution. Carbon could have been trapped initially as a refractory carbon species (e.g., carbide) by clear glass inclusions. Despite the fact that the nature of neither the primary nor that of the secondary C species was established, the commonly heterogeneous distribution of C in glass inclusions in olivines suggests entrapment of a solid C-bearing precursor.

Silicic glasses in hydrous and anhydrous mantle xenoliths from Western Victoria, Australia: at least two different sources

Abstract Glasses in a hydrous wehrlite and in anhydrous lherzolites from Western Victoria, Australia, are present as interstitial glasses and secondary glass inclusions. Interconnections between each other, generally observed as thin necks, are still preserved. These petrographic characteristics are suitable for establishing a space–time relationship. Glasses in hydrous and anhydrous xenoliths show continuous chemical trends apparently governed by different processes. Glass patches in the hydrous wehrlite are interpreted as the product of decompressional breakdown of hydrous phases like amphibole and phlogopite. However, abundances of some elements suggest mixing and the involvement of an additional source. After precipitation of secondary phases (e.g., olivine, clinopyroxene and spinel), the brown microlite-free melt migrated and reacted with primary clinopyroxene and in rare cases was trapped as glass inclusions. The observed chemical trend can be explained by crystallisation of secondary phases of the amphibole breakdown melt with addition of an alkali-volatile-rich phase. In the anhydrous lherzolites, petrographic and chemical evidences suggest the existence of two glasses: a silica-rich glass (glass A) and a very silica-rich glass (glass B). The silica-rich glass A (SiO 2 : 60–65 wt.%) is interpreted as an initial silicic melt, possibly generated at mantle depths, with a continuous chemical trend toward low-silica glasses (SiO 2 : 52 wt.%). This evolution is possibly governed by increasing melt fractions and dissolution of original apatite. Glass inclusions formed by this melt are rich in CO 2 and characterised by a feldspar–diopside–olivine normative composition. Furthermore, in the proximity to orthopyroxene and, due to a later event possibly related to the ascent of the xenolith, the silica-rich glass acquired a very silicic composition (glass B) by reaction with orthopyroxene and crystallisation of microlites [Zinngrebe, E., Foley, S.F., 1995. Metasomatism in mantle xenoliths from Gees, West Eifel, Germany: evidences for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment. Contrib. Mineral. Petrol., 122, 76–96]. The rare glass inclusions formed by this melt are CO 2 -free and have a quartz-feldspar normative composition.

Nitrogen microanalysis of glass inclusions in chondritic olivines by nuclear reaction

Measured were the abundance and distribution of nitrogen in glasses of glass inclusions in olivines of CV3, CO3, CR, C4, CH3, and LL chondritic meteorites by means of the 14 N(d, p) 15 N nuclear reaction. Similar to what was observed with carbon, nitrogen is present in low concentrations (20 ppm) in the structure of olivines but can by stored in variable amounts in glasses of glass inclusions. These primitive glasses, characterized by a Si-Al-Ca-rich composition, have highly variable nitrogen contents (30 to 1500 ppm) and highly inhomogeneous nitrogen distribution. Nitrogen contents are independent of the chemical composition of the glasses. The heterogeneous distribution is a common feature of all studied inclusions, as is evidenced by the variable contents of nitrogen in glass inclusions occurring in the same olivine grain. Nitrogen heterogeneity is suggestive of trapping of solid nitrogen carrier phases during formation of the constituents of chondrules. However, part of the originally trapped nitrogen appears to have been lost, possibly, by ulterior oxidation and subsequent transformation into volatile species. Copyright

PIXE and light element analysis (C,N) in glass inclusions trapped in meteorites with the nuclear microprobe

Abstract Proton-induced X-ray emission (PIXE) and light element analysis have been performed with the nuclear microprobe at the Laboratoire Pierre Sue (Saclay-France) in glass inclusions of the carbonaceous chondrites: Allende, Kaba and Renazzo, and in the achondrite meteorite: Chassigny. Carbon contents in olivine of chondrules are below the nuclear reactions analysis (NRA) detection limit, however, glasses from glass inclusions hosted by these grains, contain an appreciable and highly variable quantities of carbon (200–1600 ppm). This could indicate variable amounts of C trapped during glass inclusion formation. On the other hand, nitrogen is present in highly variable amounts in glasses of both, chondrites and achondrites minerals. Its abundance, correlated with depth from the section surface which suggests loss of N during analyses and therefore the possible existence of a very mobile (volatile?) species. A chondritic Rb/Sr and K/Rb ratio obtained by PIXE analyses in the glass-bearing inclusions of the Chassigny meteorite points towards a primitive source for the glass precursor of Chassigny inclusions.

Druse Pyroxenes in D’Orbigny: A Mössbauer Spectroscopy Study

D’Orbigny meteorite, the largest angrite known, is peculiar in its abundance of hollow shells, abundant presence of glasses, and open druses with perfectly crystallized augites of prismatic habit. We report on the distribution of Fe2+ among the nonequivalent sites in the druse pyroxenes of D’Orbigny using Mossbauer spectroscopy measurements on 200 mg of euhedral augites (70–400 mm). Electron microprobe analyses showed augites chemically homogeneous with a slight compositional change at the outermost ∼5 μ (rim). Cation distribution over nonequivalent lattice sites in minerals can give information on its pT history. When pyroxene crystals cool slowly to temperatures below 500°C, the Fe2+ ions populate primarily the M2 position, whereas the Mg2+ ions occupy predominantly the M1 position. Crystals cooling rapidly have a more disordered Mg,Fe distribution over the M1 sites. Mossbauer spectra of our sample obtained at RT consist of an intense inner doublet due to Fe2+ at the M2 sites and a less intense outer do...