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Rare silica-rich glass inclusions (69 < SiO 2 < 82 wt.%) are described within magnesian olivines of porphyritic Type IA chondrules. These glass inclusion compositions are clearly out of equilibrium with their host Mg-olivines and their presence within the olivines is generally attributed to an unclear secondary process such as a late interaction with nebular gases. We performed dynamic crystallisation experiments that demonstrate that these Si-rich glass inclusions are actually magmatic in origin and were trapped inside olivines that crystallized slowly from a magma with a CI, i.e. solar, composition. Their silica-rich compositions are the consequence of the small volumes of inclusions, which inhibit the nucleation of secondary crystalline phase (Ca-poor pyroxene) but allow olivine to continue to crystallize metastably on the walls of the inclusions. We suggest that Si-rich glass inclusions could be the only reliable relicts of what were the first magmas of the solar system, exhibiting a CI, i.e. non-fractionated, composition. [ABSTRACT FROM AUTHOR]

Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal–silicate partition coefficients (D met/sil) vary as a function of P , T , and oxygen fugacity (f O 2). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their D met/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of D met/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at f O 2 slightly below the iron–wüstite Fe–FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (∼ IW − 5.5), to the most oxidized angrite parent body (IW − 1.5 to IW + 1). In this study, we performed and analyzed 38 metal–silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW − 6.4 to IW − 1.9) to expand the available Ni and Co D met/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co D met/sil values as a function of T (1573–5700 K), P (1 bar to 100 GPa), and f O 2 (IW − 6.4 to IW + 0.2). We also modeled the evolution of Ni and Co D met/sil values along the liquidus of a chondritic mantle at various P and f O 2 conditions to investigate the thermodynamic conditions of various planetary bodies' magma oceans. The P and f O 2 conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body. [ABSTRACT FROM AUTHOR]

Knowledge of N solubility in silicate melts is key for understanding the origin of terrestrial N and the distribution and exchanges of N between the atmosphere, the silicate magma ocean, and the core forming metal. To place constraints on the incorporation mechanism(s) of N in silicate melts, we investigated the effect of the oxygen fugacity (f O 2) and melt composition on the N solubility through N equilibration experiments at atmospheric pressure and high temperature (1425 °C). Oxygen fugacity (expressed in log units relative to the iron-wüstite buffer, IW) was varied from IW –8 to IW +4.1, and melt compositions covered a wide range of polymerization degrees, defined by the NBO/T ratio (the number of non-bridging oxygen atoms per tetrahedrally coordinated cations). The N contents of the quenched run products (silicate glasses) were analyzed by in-situ secondary ion mass spectrometry and bulk CO 2 laser extraction static mass spectrometry, yielding results that are in excellent agreement even for N concentrations at the (sub-)ppm level. The data obtained here highlight the fundamental control of f O 2 and the degree of polymerization of the silicate melt on N solubility. Under highly reduced conditions (f O 2 = IW –8), the N solubility increased with increasing NBO/T from 17.4 ± 0.4 ppm·atm−1/2 in highly polymerized melts (NBO/T = 0) to 6710 ± 102 ppm·atm−1/2 in depolymerized melts (NBO/T ∼ 2.0). In contrast, under less reducing conditions (f O 2 > IW –3.4), N solubility is very low (≤2 ppm·atm−1/2), irrespective of the NBO/T value. Our results provide constraints on N solubility in enstatite chondrite melts and in the shallow part of a planetary magma ocean. The nitrogen storage capacity of an enstatite chondrite melt, which may approximate that of planetesimals that accreted and melted early in the inner Solar System, varies between ∼60 and ∼6000 ppm at IW –5.1 and IW –8, respectively. In contrast, a mafic to ultra-mafic magma ocean could have incorporated ∼0.3 ppm to ∼35 ppm N under the f O 2 conditions inferred for the young Earth (i.e., IW –5 to IW). The N storage capacity of a reduced magma ocean (i.e., IW –3.4 to IW) in equilibrium with a N-rich atmosphere is ≤1 ppm, comparable to the N content of the present-day mantle. However under more reducing conditions (i.e., IW –5 to IW –4), the N storage capacity is significantly higher (∼35 ppm); in this case, Earth would have lost N to the atmosphere and/or N would have been transported into and stored within its deep interior (i.e., deep mantle, core). [ABSTRACT FROM AUTHOR]

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a m n -type law, where n ≥ 1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk. [ABSTRACT FROM AUTHOR]

The nature and distribution of sulfides within 17 porphyritic chondrules of the Sahara 97096 EH3 enstatite chondrite have been studied by backscattered electron microscopy and electron microprobe in order to investigate the role of gas–melt interactions in the chondrule sulfide formation. Troilite (FeS) is systematically present and is the most abundant sulfide within the EH3 chondrite chondrules. It is found either poikilitically enclosed in low-Ca pyroxenes or scattered within the glassy mesostasis. Oldhamite (CaS) and niningerite [(Mg,Fe,Mn)S] are present in ≈60% of the chondrules studied. While oldhamite is preferentially present in the mesostasis, niningerite associated with silica is generally observed in contact with troilite and low-Ca pyroxene. The Sahara 97096 chondrule mesostases contain high abundances of alkali and volatile elements (average Na 2 O = 8.7 wt.%, K 2 O = 0.8 wt.%, Cl = 7100 ppm and S = 3700 ppm) as well as silica (average SiO 2 = 62.8 wt.%). Our data suggest that most of the sulfides found in EH3 chondrite chondrules are magmatic minerals that formed after the dissolution of S from a volatile-rich gaseous environment into the molten chondrules. Troilite formation occurred via sulfur solubility within Fe-poor chondrule melts followed by sulfide saturation, which causes an immiscible iron sulfide liquid to separate from the silicate melt. The FeS saturation started at the same time as or prior to the crystallization of low-Ca pyroxene during the high temperature chondrule forming event(s). Protracted gas–melt interactions under high partial pressures of S and SiO led to the formation of niningerite-silica associations via destabilization of the previously formed FeS and low-Ca pyroxene. We also propose that formation of the oldhamite occurred via the sulfide saturation of Fe-poor chondrule melts at moderate S concentration due to the high degree of polymerization and the high Na-content of the chondrule melts, which allowed the activity of CaO in the melt to be enhanced. Gas–melt interactions thus appear to be a key process that may control the mineralogy of chondrules in the different classes of chondrite. [ABSTRACT FROM AUTHOR]

Noble gases are ideal probes to study the structure of silicate glasses and melts as the modifications of the silicate network induced by the incorporation of noble gases are negligible. In addition, there are systematic variations in noble gas atomic radii and several noble gas isotopes with which the influence of the network itself on diffusion may be investigated. Noble gases are therefore ideally suited to constrain the time scales of magma degassing and cooling. In order to document noble gas diffusion behavior in silicate glass, we measured the diffusivities of three noble gases (4He, 20Ne and 40Ar) and the isotopic diffusivities of two Ar isotopes (36Ar and 40Ar) in two synthetic basaltic glasses (G1 and G2; 20Ne and 36Ar were only measured in sample G1). These new diffusion results are used to re-interpret time scales of the acquisition of fractionated atmospheric noble gas signatures in pumices. The noble gas bearing glasses were synthesized by exposing the liquids to high noble gas partial pressures at high temperature and pressure (1750-1770 K and 1.2 GPa) in a piston-cylinder apparatus. Diffusivities were measured by step heating the glasses between 423 and 1198 K and measuring the fraction of gas released at each temperature step by noble gas mass spectrometry. In addition we measured the viscosity of G1 between 996 and 1072 K in order to determine the precise glass transition temperature and to estimate network relaxation time scales. The results indicate that, to a first order, that the smaller the size of the diffusing atom, the greater its diffusivity at a given temperature: D(He) > D(Ne) > D(Ar) at constant T. Significantly, the diffusivities of the noble gases in the glasses investigated do not display simple Arrhenian behavior: there are well-defined departures from Arrhenian behavior which occur at lower temperatures for He than for Ne or Ar. We propose that the non-Arrhenian behavior of noble gases can be explained by structural modifications of the silicate network itself as the glass transition temperature is approached: as the available free volume (available site for diffusive jumps) is modified, noble gas diffusion is no longer solely temperature-activated but also becomes sensitive to the kinetics of network rearrangements. The non-Arrhenian behavior of noble gas diffusion close to Tg is well described by a modified Vogel-Tammann-Fulcher (VTF) equation: D/a² = A1/a² * exp (-B1/R(T -- T2 - C/RT) where D is the diffusion coefficient, a the diffusion domain size (taken to be the size of the sample), A1 and C are respectively equivalent to the pre-exponential factor and to the activation energy (Ea in J mol-1) of the classical Arrhenius equation, B1 can be interpreted as a "pseudo-activation energy" that reflects the influence of the silicate network relaxation, T2 is the temperature where the diffusion regime switches from Arrhenian to non-Arrhenian, and R is the gas constant (=8.314 J K-1 mol-1). Finally, our step heating diffusion experiments suggest that at T close to Tg, noble gas isotopes may suffer kinetic fractionation at a degree larger than that predicted by Graham's law. In the case of 40Ar and 36Ar, the traditional assumption based on Graham's law is that the ratio D 40Ar/D 36Ar should be equal to 0.95 (the square root of the ratio of the mass of 36Ar over the mass of 40Ar). In our experiment with glass G1, D40Ar/D36Ar rapidly decreased with decreasing temperature, from near unity (0.98 ± 0.14) at T > 1040 K to 0.76 when close to Tg (T = 1003 K). Replicate experiments are needed to confirm the strong kinetic fractionation of heavy noble gases close to the transition temperature. [ABSTRACT FROM AUTHOR]

Abstract: The coarse-grained, igneous, anorthite-rich (Type C) CAIs from Allende studied (100, 160, 6-1-72, 3529-40, CG5, ABC, TS26, and 93) have diverse textures and mineralogies, suggesting complex nebular and asteroidal formation histories. CAIs 100, 160, 6-1-72, and 3529-40 consist of Al,Ti-diopside (fassaite; 13–23 wt% Al2O3, 2–14 wt% TiO2), Na-bearing åkermanitic melilite (0.1–0.4 wt% Na2O; Åk30–75), spinel, and fine-grained (∼5–10μm) anorthite groundmass. Most of the fassaite and melilite grains have “lacy” textures characterized by the presence of abundant rounded and prismatic inclusions of anorthite ∼5–10μm in size. Lacy melilite is pseudomorphed to varying degrees by grossular, monticellite, and pure forsterite or wollastonite. CAI 6-1-72 contains a relict Type B CAI-like portion composed of polycrystalline gehlenitic melilite (Åk10–40), fassaite, spinel, perovskite, and platinum-group element nuggets; the Type B-like material is overgrown by lacy melilite and fassaite. Some melilite and fassaite grains in CAIs 100 and 160 are texturally similar to those in the Type B portion of 6-1-72. CAIs ABC and TS26 contain relict chondrule fragments composed of forsteritic olivine and low-Ca pyroxene; CAI 93 is overgrown by a coarse-grained igneous rim of pigeonite, augite, and anorthitic plagioclase. These three CAIs contain very sodium-rich åkermanitic melilite (0.4–0.6 wt% Na2O; Åk63–74) and Cr-bearing Al,Ti-diopside (up to 1.6 wt% Cr2O3, 1–23 wt% Al2O, 0.5–7 wt% TiO2). Melilite and anorthite in the Allende Type C CAI peripheries are replaced by nepheline and sodalite, which are crosscut by andradite-bearing veins; spinel is enriched in FeO. The CAI fragment CG5 is texturally and mineralogically distinct from other Allende Type Cs. It is anorthite-poor and very rich in spinel poikilitically enclosed by Na-free gehlenitic melilite (Åk20–30), fassaite, and anorthite; neither melilite nor pyroxene have lacy textures; secondary minerals are absent. The Al-rich chondrules 3655b-2 and 3510-7 contain aluminum-rich and ferromagnesian portions. The Al-rich portions consist of anorthitic plagioclase, Al-rich low-Ca pyroxene, and Cr-bearing spinel; the ferromagnesium portions consist of fosteritic olivine, low-Ca pyroxene, and opaque nodules. We conclude that Type C CAIs 100, 160, 6-1-72, and 3529-40 formed by melting of coarse-grained Type B-like CAIs which experienced either extensive replacement of melilite and spinel mainly by anorthite and diopside (traces of secondary Na-bearing minerals, e.g., nepheline or sodalite, might have formed as well), or addition of silica and sodium during the melting event. CG5 could have formed by melting of fine-grained spinel-melilite CAI with melilite and spinel partially replaced anorthite and diopside. CAIs ABC, 93, and TS-26 experienced melting in the chondrule-forming regions with addition of chondrule-like material, such as forsteritic olivine, low-Ca pyroxene, and high-Ca pyroxene. Anorthite-rich chondrules formed by melting of the Al-rich (Type C CAI-like) precursors mixed with ferromagnesian, Type I chondrule-like precursors. The Allende Type C CAIs and Al-rich chondrules experienced fluid-assisted thermal metamorphism, which resulted in pseudomorphic replacement of melilite and anorthite by grossular, monticellite, and forsterite (100, 160, 6-1-72, 3592-40) or by grossular, monticellite, and wollastonite (ABC, 93, TS-26). The pseudomorphic replacement was followed or accompanied by iron–alkali metasomatic alteration resulting in replacement of melilite and anorthite by nepheline and sodalite, enrichment of spinel in FeO, and precipitation of salite–hedenbergite pyroxenes, wollastonite, and andradite in fractures and pores in and around CAIs. [Copyright &y& Elsevier]