Mg-rich Olivine Research Articles

Ultra-primitive interplanetary dust particles from the comet 26P/Grigg–Skjellerup dust stream collection

Cometary material and pristine interplanetary dust particles (IDPs) best resemble the unaltered components from which our solar system was built because they have remained largely unaltered in a cold undisturbed environment since accretion in the outer protoplanetary disk. IDPs might supply more primitive assemblages for laboratory analysis than Stardust samples from comet 81P/Wild 2 but their individual provenances are typically unknown. We speculate that some IDPs collected by NASA in April 2003 may be associated with comet 26P/Grigg–Skjellerup because their particularly pristine character coincides with the collection period that was predicted to show an enhanced flux of particles from this Jupiter-family comet. Some IDPs from this collection contain the most primitive assembly of interstellar matter found to date including an unusually high abundance of presolar grains and very isotopically anomalous and disordered organic matter as well as fine-grained carbonates and an amphibole associated with a GEMS-like object (glass with embedded metals and sulfides) that potentially imply formation in a nebular rather than planetary environment. The two most primitive IDPs may contain assemblages of molecular cloud material at the percent level which is supported by the presence of four rare 17O-depleted presolar silicate grains possibly of supernova(e) origin within one ~ 70 μm 2-sized IDP and the close association of a Group 1 Mg-rich olivine from a low-mass red giant star with a carbonaceous nano-globule of potentially interstellar origin. Our study together with observations of comet 9P/Tempel 1 during the Deep Impact experiment and 81P/Wild 2 dust analyses reveal some compositional variations and many similarities among three Jupiter-family comets. Specifically carbonates and primitive organic matter or amorphous carbon were widespread in the comet-forming regions of the outer protoplanetary disk and not all comets contain as much inner solar system material as has been inferred for comet 81P/Wild 2. The bulk and hotspot hydrogen and nitrogen isotopic anomalies as well as the carbon Raman characteristics of the organic matter in IDPs and the most primitive meteorites are remarkably similar. This implies that the same mixture of molecular cloud material had been transported inward into the meteorite-forming regions of the solar system.

THE DUST CLOUD AROUND THE WHITE DWARF G 29-38. II. SPECTRUM FROM 5 TO 40 μm AND MID-INFRARED PHOTOMETRIC VARIABILITY

We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1-35 microns and combining a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals for G29-38 are amorphous carbon, amorphous and crystalline silicates, water ice, and metal sulfides. The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The best-fitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 degrees. A `moderately optically thick' torus model with mass 2x10^19 g fits the spectrum well. A physically thin (less than the white dwarf radius) and optically thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong silicate features. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to the G29-38 dust. Finally, we suggest the surviving core of a `hot jupiter' as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.

Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88–80) with relatively sodic plagioclase (An75–60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc.

Circumstellar Dust Created by Terrestrial Planet Formation in HD 113766

We present an analysis of the gas-poor circumstellar material in the HD 113766 binary system (F3/F5, 10 - 16 Myr), recently observed by the Spitzer Space Telescope. For our study we have used the infrared mineralogical model derived from observations of the Deep Impact experiment. We find the dust dominated by warm, fine (~1 um) particles, abundant in Mg-rich olivine, crystalline pyroxenes, amorphous silicates, Fe-rich sulfides, amorphous carbon, and colder water-ice. The warm dust material mix is akin to an inner main belt asteroid of S-type composition. The ~440 K effective temperature of the warm dust implies that the bulk of the observed material is in a narrow belt ~1.8 AU from the 4.4 L_solar central source, in the terrestrial planet-forming region and habitable zone of the system (equivalent to 0.9 AU in the solar system). The icy dust lies in 2 belts, located at 4-9 AU and at 30 - 80 AU. The lower bound of warm dust mass in 0.1 - 20 um, dn/da ~ a^-3.5 particles is very large, at least 3 x 10^20 kg, equivalent to a 320 km radius asteroid of 2.5 g cm^-3 density. Assuming 10m largest particles present, the lower bound of warm dust mass is at least 0.5 M_Mars The dust around HD 113766A originates from catastrophic disruption of terrestrial planet embryo(s) and subsequent grinding of the fragments, or from collisions in a young, extremely dense asteroid belt undergoing aggregation. The persistence of the strong IR excess over the last two decades argues for a mechanism to provide replenishment of the circumstellar material on yearly timescales.

Limestone assimilation and the origin of CO 2 emissions at the Alban Hills (Central Italy): Constraints from experimental petrology

The Alban Hills volcanic region (20 km south of Rome, in the Roman Province) emitted a large volume of potassic magmas (> 280 km 3) during the Quaternary. Chemical interactions between ascending magmas and the ∼ 7000–8000-m-thick sedimentary carbonate basement are documented by abundant high temperature skarn xenoliths in the eruptive products and have been frequently corroborated by geochemical surveys. In this paper we characterize the effect of carbonate assimilation on phase relationships at 200 MPa and 1150–1050 °C by experimental petrology. Calcite and dolomite addition promotes the crystallization of Ca-rich pyroxene and Mg-rich olivine respectively, and addition of both carbonates results in the desilication of the melt. Furthermore, carbonate assimilation liberates a large quantity of CO 2-rich fluid. A comparison of experimental versus natural mineral, glass and bulk rock compositions suggests large variations in the degree of carbonate assimilation for the different Alban Hills eruptions. A maximum of 15 wt.% assimilation is suggested by some melt inclusion and clinopyroxene compositions; however, most of the natural data indicate assimilation of between 3 and 12 wt.% carbonate. Current high CO 2 emissions in this area most likely indicate that such an assimilation process still occurs at depth. We calculate that a magma intruding into the carbonate basement with a rate of ∼ 1 – 2 · 10 6 m 3/year, estimated by geophysical studies, and assimilating 3–12 wt.% of host rocks would release an amount of CO 2 matching the current yearly emissions at the Alban Hills. Our results strongly suggest that current CO 2 emissions in this region are the shallow manifestation of hot mafic magma intrusion in the carbonate-hosted reservoir at 5–6 km depth, with important consequences for the present-day volcanic hazard evaluation in this densely populated and historical area.

Testing space weathering models on A-type asteroid (1951) Lick

Aims. A-type asteroid (1951) Lick has a very red visible and near-infrared reflectance spectrum, and it is one of the reddest NearEarth objects. Its surface is probably affected by heavy space weathering processes. Here we investigate the possibility of a weathering saturation regime. Methods. We compare Lick’s spectrum with those of silicate olivine after laser ablation experiments, reported by Brunetto et al. (2006a, Icarus, 180, 546; 2007, Icarus, in press). We apply scattering and space weathering models developed by Hapke (1981, J. Geophys. Res., 86, 3039; 2001, J. Geophys. Res., 106, 10039). Results. We successfully fit the spectrum of Lick, in terms of slope, band profile, and albedo. Results confirm that Lick has olivine with Mg number higher than that of the Brachina meteorite. The use of metallic iron inclusions in the model yields a product of volume fraction of inclusions times effective optical path length of about 0.039 µm, similarly to what observed experimentally after UV laser ablation of Mg-rich olivine. Conclusions. The surface of Lick is extremely weathered, indicating a saturation regime similar to what observed in the laboratory, and suggesting that the asteroid surface is probably older than about 10 7 –10 8 years.

Survival times of anomalous melt inclusions from element diffusion in olivine and chromite

The chemical composition of basaltic magma erupted at the Earth's surface is the end product of a complex series of processes, beginning with partial melting and melt extraction from a mantle source and ending with fractional crystallization and crustal assimilation at lower pressures. It has been proposed that studying inclusions of melt trapped in early crystallizing phenocrysts such as Mg-rich olivine and chromite may help petrologists to see beyond the later-stage processes and back to the origin of the partial melts in the mantle. Melt inclusion suites often span a much greater compositional range than associated erupted lavas, and a significant minority of inclusions carry distinct compositions that have been claimed to sample melts from earlier stages of melt production, preserving separate contributions from mantle heterogeneities. This hypothesis is underpinned by the assumption that melt inclusions, once trapped, remain chemically isolated from the external magma for all elements except those that are compatible in the host minerals. Here we show that the fluxes of rare-earth elements through olivine and chromite by lattice diffusion are sufficiently rapid at magmatic temperatures to re-equilibrate completely the rare-earth-element patterns of trapped melt inclusions in times that are short compared to those estimated for the production and ascent of mantle-derived magma or for magma residence in the crust. Phenocryst-hosted melt inclusions with anomalous trace-element signatures must therefore form shortly before magma eruption and cooling. We conclude that the assumption of chemical isolation of incompatible elements in olivine- and chromite-hosted melt inclusions is not valid, and we call for re-evaluation of the popular interpretation that anomalous melt inclusions represent preserved samples of unmodified mantle melts.

Evidence for kinks in structural and thermodynamic properties across the forsterite–fayalite binary from thin-film IR absorption spectra

Infrared absorbance spectra over ∼100 to 1,800 cm−1 were collected from optically thin films of 21 samples with compositions spanning the forsterite–fayalite binary. Polarization information from previous specular reflectance data on end-members was used in tracing the peaks across the entire binary. Peak positions (νi) were constrained by Lorentzian decompositions. Fitting also constrained widths for singlet peaks but for doublets and triplets, variation in νi with composition among the constituent polarizations alters widths from intrinsic. Because film thicknesses of 0.6–1.4 μm were estimated, our band strengths are approximate; however, relative intensities should be correct. Only for a few peaks does νi vary smoothly across the entire binary; instead, distinct linear trends exist for Fe- and Mg-rich olivines. Discontinuities and kinks in νi(X) occur at X = Mg/(Fe + Mg) = 0.7 and are accompanied by a change in intensity patterns. This interesting behavior was not revealed in previous spectra of powder dispersions. The contrasting character of IR vibrations for Fe- and Mg-rich olivines is inferred to arise from structural variations because (1) frequency is related to bond length, (2) other factors affecting frequency (cation mass and probably bonding type) vary linearly across the binary, and (3) available data on unit cell parameters are consistent with distinct trends for forsterites and fayalites. Vibrational components of heat capacity (C V) and enthalpy (H) calculated from νi, were found to be slightly more negative than linear interpolations between values for forsterite and fayalite. Our computations give smaller negative excesses from ideality in H than do previous calorimetric measurements, but are equal within experimental uncertainties.

Relict olivine, chondrule recycling, and the evolution of nebular oxygen reservoirs

Chondrules often contain relict olivine grains that did not crystallize in situ, providing opportunities to decipher how chondrule components evolved. We studied olivine in the Sahara-97210 (LL3.2), Wells (LL3.3) and Chainpur (LL3.4) chondrites using SEM, EMPA, and SIMS techniques. Oxygen isotopes were analyzed in 16 objects from all three meteorites, and trace elements were analyzed in Sahara-97210 and Chainpur. Two groups of olivine are identified based on oxygen isotope compositions. One group is enriched in 16O (Δ 17O ∼ − 8 to − 4‰) and falls close to the CCAM mixing line; it includes forsterite and Mg-rich olivine present as relict grains in Type II (ferrous) chondrules and the forsteritic cores of some isolated grains. These low-Δ 17O grains are poor in MnO (< 0.2 wt.%) and are usually enriched in CaO (∼ 0.3–0.65 wt.%). The other group is less enriched in 16O (Δ 17O ∼ − 3 to + 4 ‰); it includes normal (non-relict) ferrous olivine in type II chondrules, normal (non-relict) Mg-rich olivine in Type I (magnesian) chondrules, dusty olivine relict grains in Type I chondrules, and Mg-rich olivine relicts in a Type II and a Type I chondrule. These high-Δ 17O grains have variable CaO (0–0.95 wt.%) and MnO (∼ 0–0.45 wt.%) contents, with the more calcic and Mn-poor compositions associated with forsteritic olivine. Trace-element data show that forsteritic olivine grains in both oxygen groups are similarly enriched in refractory elements (Al, Sc, Y, Ca, Ti, V) and depleted in volatile elements (Cr, Mn, P, Rb, sometimes K and Na) compared to normal ferrous olivine, suggesting that variations in chemical composition reflect the extent of thermal processing (greater for magnesian olivine). The data are consistent with a model in which nebular reservoirs became less enriched in 16O with time. An earlier episode of chondrule formation produced Type I chondrules and isolated forsterites in carbonaceous chondrites, and forsteritic grains that were incorporated into ordinary chondrites as relict grains in chondrules and as isolated grains. Later episodes of chondrule formation produced both Type I and Type II chondrules in ordinary chondrites under a variety of thermal and redox conditions.

Order and anti-order in olivine III: Variation of the cation distribution in the Fe,Mg olivine solid solution series with temperature and composition

The structures of four Fe,Mg olivine crystals Fa11.6, Fa22.3, Fa27.6 and Fa27.8 have been investigated by ex situ and in situ experiments using single-crystal X-ray diffractometry. For the ex situ experiments, the crystals were quenched after equilibration at temperatures between 500 °C and 900 °C. Resetting effects were only observed for samples quenched from 900 °C. The in situ experiments were performed at temperatures up to 750 °C. With increasing equilibration temperature, Fe 2+ was found to progressively segregate into the octahedral M1 site, i.e. the degree of anti-order increases with rising temperature. Thus, contrary to the results of Rinaldi et al. (2000) and Redfern et al. (2000), no indication of an ordering reversal at high temperatures is found. Incorporation of the results of Heinemann et al. (2006) on olivine Fa47.9 into the present study allows for the first time to systematically investigate the compositional variation of the temperature dependence of the Fe 2+ ,Mg site occupancies. The formulation of Thompson (1969, 1970) for solid solutions undergoing non-convergent disordering processes, (1) \[{-} RTln K_{D} = [{\Delta}H_{exch}^{0} {-} (L_{M1}^{H} {-} L_{M2}^{H})X] {-} T[{\Delta}S_{exch}^{0} {-} (L_{M1}^{S} {-} L_{M2}^{S})X],\] yielded \begin{eqnarray*}&&{\Delta}H_{exch}^{0} = 1153 ({\pm}67) J/mol{ }{ }{\Delta}S_{exch}^{0} = 3.743({\pm}0.072) J/molK\\&&L_{M1}^{H} {-} L_{M2}^{H} = 973 ({\pm}211) J/mol{ }{ }L_{M1}^{S} {-} L_{M2}^{S} = {-} 0.89 ({\pm}0.24) J/molK.\end{eqnarray*} ΔH exch 0 and ΔS exch 0 are the exchange enthalpy and entropy, respectively, related to the Fe 2+ ,Mg site exchange, L M1,M2 H and L M1,M2 S denote enthalpic and entropic intrasite interaction parameters. The compositional parameter X varies between −1 for forsterite (Fa0) and +1 for fayalite (Fa100). The magnitudes and signs of the four refined quantities can be rationalized in terms of thermodynamic and crystal-chemical considerations. (L M1 H − L M2 H ) > 0 indicates that the interactions between the M1 sites are stronger than those between the M2 sites, conforming with the M1-M1 distances being shorter than the M2-M2 distances. (L M1 S − L M2 S ) exch 0 increases towards the Mg endmember while ΔS exch 0 decreases. Since ΔH exch 0 > 0 stabilizes the ordered state whereas ΔS exch 0 >0 stabilizes the anti-ordered state, it follows that the ordered state is progressively favoured with increasing Mg content. Consequently, ordered site distributions should occur only in slowly cooled Mg-rich olivines, whereas olivines with Fa > 25 mol% should be found in their anti-ordered states frozen during cooling. This conclusion is supported by structure refinements and ordering path simulations for a metamorphic olivine Fa12.4 as well as two volcanic olivines Fa25.6 and Fa27.8.

Effect of composition on adsorption of water on perfect olivine surfaces

The origin of water in the terrestrial planets is controversial. Both comets and asteroids have isotopic properties inconsistent with Earth’s water. We are investigating if adsorption of water onto mineral surfaces in the accretion disk could be a viable source of water in the terrestrial planets. Gases coexist with dust in the accretion disk for long periods. The equilibrium H2O/H2 ratio in the accretion disk was 5 · 10 , which corresponds to a pH2O of 10 8 bar. Note that the equilibrium partial pressure is probably a lower limit. Astronomical observations show that dust clouds consist of Mg-rich olivine, pyroxenes, and other refractory minerals with radii 100 kJ/mol). In environments with low pH2O such highenergy sites are most likely the first sites to interact with water molecules. The calculations of (Stimpfl et al.; de Leeuw et al., 2006) using pure forsterite suggest that the inner disk could sustain adsorption of water, both associative and dissociative, onto perfect olivine surfaces (de Leeuw et al., 2000). To gain a better understanding of the mechanism of adsorption, we are performing minimum energy simulations with olivine with Fa up 14%. We are also conducting molecular dynamic simulations to gain an understanding of the kinetics of desorption. In this talk we will present the results from these latest simulations and quantitatively assess the contribution of this mechanism to the water budgets of the terrestrial planets.

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630– 1690 � C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Agecorrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1. 3t oþ4.0]. The higher eNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have oceanisland-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-eNd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.

High-resolution valence-band XPS spectra of the nonconductors quartz and olivine

High-resolution core-level and valence-band x-ray photoelectron spectra (XPS) of the nonconductor silicates, quartz, and two olivines (${[\mathrm{Mg},\mathrm{Fe}]}_{2}\mathrm{Si}{\mathrm{O}}_{4}$, Mg rich and Fe rich) have been obtained with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. Observed total linewidths are about $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for several of the peaks in these spectra, much narrower than previously obtained. The quartz and olivine valence-band spectra are very different, even though the dominant contributions to both come from molecular orbitals of $\mathrm{Si}{\mathrm{O}}_{4}$ groups. High-quality calculations of the band structure using pseudopotential density functional theory with generalized gradient approximation (GGA) for quartz yield a theoretical XPS valence-band spectrum in the Gelius approximation that is in excellent agreement with the experimental spectrum. $\mathrm{GGA}+U$ (where $U$ is the on-site Coulomb energy) calculations on Mg-rich and Fe-rich olivines yield theoretical XPS spectra in good qualitative agreement with experiment. The valence-band spectral contributions are readily assigned using the calculated partial density of states. For example, the Fe $3d$ ${t}_{2g}$ and ${e}_{g}$ orbitals in $M1$ and $M2$ sites of the olivine are located at the top of the olivine valence band and are readily resolved in both observed spectra and theoretical calculations. The valence-band spectrum of quartz is about $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ more dispersed than the valence bands of the olivines and considerably greater than the $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ difference calculated previously. The difference in dispersion arises mainly from two effects. The dispersion of the quartz valence band is primarily a response to the nature of metal atoms in the second coordination sphere of the central Si atom of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. For example, the $\mathrm{Si}\ensuremath{-}\mathrm{O}$ moiety is more negatively charged in olivine than in quartz because of the transfer of electrons from Mg to $\mathrm{O}\ensuremath{-}\mathrm{Si}$. The calculations indicate very small Mg $2s$ contributions to the forsterite $({\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4})$ valence band. Hence all Mg $2s$ electrons effectively reside on O atoms (O $2p$ orbitals) and the $\mathrm{Mg}\ensuremath{-}\mathrm{O}$ bond is ionic. Polymerization (network formation) also contributes to dispersion in that it results in large splittings in the Si $3s$ and Si $3p$ partial density of states. Fe $3d$ electrons of olivines are of both ``bonding'' and ``nonbonding'' according to the calculations. In the Mg-rich olivine, the great majority of Fe $3d$ electrons are nonbonding and give rise to separate Fe $3d$ ${t}_{2g}$ and ${e}_{g}$ peaks at the top of the valence band. The same no-bonding contributions are observed in Fe-rich olivine, but O $2p$ contributions near the top of the valence band indicate appreciable $\mathrm{Fe}\ensuremath{-}\mathrm{O}$ bonding.

ISO Observations of Comets

The Infrared Space Observatory (ISO) offered us the opportunity to observe celestial bodies from 2.4 to 196 microns. A wealth of new results were obtained for comets. Spectroscopic investigations have shown band fluorescence emissions for H2O, CO and CO2, and rotational line emissions for H2O. High resolution spectra of water vapor around 2.7 microns in comets C/1995 01 (Hale-Bopp) and 103P/Hartley 2 have permitted measurement of the water rotational temperature and ortho-to-para ratio. The thermal region of the spectra showed the signatures of crystalline, Mg-rich olivine and emission features attributed to water ice. Broad band photometry of the coma allowed to investigate several properties of cometary dust, such has color temperature, size distribution and production rate. For several comets, it was possible to separate the nuclear and coma contribution in infrared images obtained from the ISO camera. Inferences concerning the nucleus size and albedo were obtained.

Minimum XPS core-level line widths of insulators, including silicate minerals

A monochromatic AlKα source and magnetic confinement charge compensation yield an As 3d line width of 0.52 eV for the bulk insulator As 2 S 3 (orpiment) which is as narrow as the As 3d line width obtained for the semiconductor FeAsS (arsenopyrite). These results, and the XPS results for silicates, demonstrate that charge broadening of XPS spectra has been overcome for most insulators provided care is taken with sample preparation. XPS core level spectral investigations of these minerals now may be conducted at the same level of detail as have been conducted on semi-conductors and metals. The Si 2p spectrum of a non-conducting orthosilicate (Mg-rich olivine) displays a total line width of 1.36 eV compared to very high resolution synchrotron XPS line widths of 1.1 eV for an Si 2p spectrum of gaseous Si(OCH 3 ) 4 , and 1.42 eV for the Si 2p peak of non-charging SiO 2 thin-films grown on Si metal. These results demonstrate that Si 2p peak broadening of these silicates does not result from differential charging. Comparison of the Si 2p spectrum of Si in olivine which contains non-polymerized, independent SiO 4 tetrahedra, with that of a gaseous analogue, Si(OCH 3 ) 4 , also composed of non-polymerized Si atoms tetrahedrally bonded to O, provides insight into the cause of Si 2p peak broadening for olivine (and probably for all other silicates). The vibrational contributions to the Si 2p spectrum of Si(OCH 3 ) 4 were used to fit the Si 2p peak of olivine, with each vibrational contribution broadened to reflect the minimum line width of the XPS instrument. Although the Si 2p envelope resulting from this procedure yields a reasonable fit to the olivine Si 2p spectrum, additional minor vibrational contributions may be required to reproduce accurately the Si 2p spectrum of olivine. Continued development of the technique should allow determination of transition metal oxidation states and other chemical state properties of non-conductor Al-silicates

Oxygen isotopic distribution in an amoeboid olivine aggregate from the Allende CV chondrite: Primary and secondary processes

The oxygen isotopic distribution in an amoeboid olivine aggregate (AOA), TTA1-02, from the Allende CV3 chondrite has been determined by secondary ion mass spectrometry. The irregular shaped TTA1- 02 (5×3mm) consists mostly of olivine grains of ca. 5μm in diameter. Olivine grains of Mg-rich (Fo 95) and Fe-rich (Fo 60) composition are in direct contact with each other, with a sharp compositional boundary. Oxygen isotopic compositions of Fe-rich olivine grains are 16O-poor (Δ 17O ≅ −5‰), whereas Mg-rich olivine is 16O-rich (Δ 17O ≅ −25‰). Several Al-rich inclusions (<ca. 500 μm in diameter) are enclosed by olivine grains in the AOA. Oxygen isotopic compositions of spinel and fassaite in Al-rich inclusions are 16O-rich (Δ 17O ≅ −20‰), whereas those of anorthite, nepheline and phyllosilicate are 16O-poor (Δ 17O ≅ −5‰). We propose the following sequence of events during the formation of AOAs in the Allende meteorite: 1) Formation of Al-rich inclusions with 16O-rich oxygen isotopic composition; 2) Accretion of Mg-rich olivine grains with 16O-rich oxygen isotopic composition around Al-rich inclusions; 3) Accretion into parent body; and 4) Aqueous alteration in the parent body, which led to crystallization of 16O-poor minerals, Fe-rich olivine, anorthite, nepheline, and phyllosilicate. This is reflecting reactions among primary 16O-rich AOA minerals and aqueous fluid having 16O-poor oxygen isotopic composition. Fe-rich olivine grains precipitated from aqueous fluids, which partially dissolved pre-existing Mg-rich olivine grains. Sintering and Mg-Fe diffusion occurred during thermal metamorphism. Anorthite, nepheline and phyllosilicate in Al-rich inclusions replaced primary anorthite or melilite during the aqueous alteration stage.

Composition, Structure, and Size Distribution of Dust in the Local Interstellar Cloud

The average properties of interstellar dust have previously been inferred from remote astronomical observations of interstellar extinction and from gas depletion measurements. In addition to recent high-resolution observations of gas absorption spectra, in situ measurements of dust in the solar system provide an alternative approach to deducing the properties of interstellar dust, in particular, those of dust in the Local Interstellar Cloud (LIC), in which the Sun resides. We constrain the composition and structure of dust in the LIC by the dust-phase elemental abundances derived from gas absorption measurements and by the dynamical behavior inferred from dust impact measurements. The elemental abundances of the LIC dust are consistent with core-mantle grains consisting of Mg-rich pyroxene and Mg-rich olivine with inclusions of troilite, Fe-rich kamacite, and corundum in the core and organic refractory compounds of C, N, and O in the mantle. The mass of the organic refractory mantle is comparable to the mass of the silicate core that is abundant in pyroxene compared to olivine. Taking into account these results, the dynamical behavior of the LIC dust in the solar system indicates that bare silicates and bare carbonaceous materials may be present as grains smaller than 10-17 kg. The LIC grains with mass exceeding 10-17 kg are most likely aggregates of submicron-sized silicate core, organic mantle grains. The mass distribution of dust in the LIC can be well explained by coagulation growth of core-mantle grains but is scarcely explained by severe destruction of grains in interstellar shocks.

Geochemistry of metasomatised spinel peridotite xenoliths from the Dariganga Plateau, South-eastern Mongolia

One fresh (green), one altered (black) and one composite (green/black) peridotite xenolith from the Neogene-Quaternary basalts of the Dariganga Plateau, SE Mongolia, were studied by electron microprobe, X-ray fluorescence, wet chemical and instrumental neutron activation analysis. The history of the upper mantle underneath the Dariganga Plateau has been complex and is characterised by elemental depletions and enrichments processes. The rocks investigated appear to have been processed in several steps, have been moderately depleted (relative to the primitive upper mantle composition) in incompatible elements and subsequently metasomatically enriched in alkalis, Fe, Ca, LREE, Th and U. As a result, most peridotites are moderately depleted in Si, Cr, Ti, HREE and Hf, are slightly enriched in LREE and have elevated Th and U abundances. The minerals in all rocks are out of chemical equilibrium. In the green peridotites disequilibrium is modest but it is severe in the blackened lherzolites. The latter have experienced strong Fe metasomatism accompanied by strong oxidation. As a result, Mg-rich olivines formed by oxidation and precipitation of Fe oxides in the primary olivines (blackening) and Fe-rich olivines formed in the Fe metasomatic event. The latter could only have taken place after the oxidising event, otherwise the Fe-rich olivines would also have been affected by it. Three of the four rocks show negative anomalies (relative to the Ce abundance) of Hf and Ti, one is enriched in these elements, which is considered an indication of the action of carbonatitic melts/fluids in the upper mantle. Enrichment of U over Th in some of our samples seems also to indicate the presence of water in the fluid phase, however, the lack of (OH)-bearing minerals in the Dariganga xenoliths suggests a low activity of water in these fluids. The latest of the metasomatic events probably took place shortly before entrapment of the rocks by the basaltic lava that carried them to the earth’s surface. The composite sample consisting of a green harzburgite and a black lherzolite suggests that blackening took place at the original location of the rock rather than in the basaltic tuff because the latter should have altered the whole xenolith. It also demonstrates that metasomatic processes in the upper mantle can be confined to rather restricted locations with sharp boundaries towards the wall rocks. Blackening as well as the metasomatic events apparently took place because of a better permeability in one part of the rock as compared to the other, probably the result of tectonisation.

Petrological study of Nasu-Chausudake Volcano (ca. 16 ka to Present), northeastern Japan

Chausudake Volcano is representative of the active volcanoes in northeastern Japan, and has a record of many historical eruptions. Because its 16-ky eruptive history is well documented, Chausudake is well-suited for examining the temporal change of magma chamber processes and for assessing potential hazards. The activity of the Chausudake Volcano can be divided into six magmatic units (CH1–CH6). Most of its products have similar characteristics, but those from unit CH1 show wider variation. Most rocks are andesite and have plagioclase, clinopyroxene, orthopyroxene, and Fe–Ti oxides as phenocrysts, with or without olivine or quartz. Mafic inclusions, which are observed in most products, are basaltic andesites that have various combinations of the same phenocryst species. Petrographic features observed in host rocks and mafic inclusions, such as disequilibrium phenocrysts and resorbed textures, suggest magma mixing/co-mingling. Whole rock compositions of both host rocks and mafic inclusions show linear trends in variation diagrams, which suggest that the rocks are derived from the mixing/co-mingling between mafic and felsic end members. Bulk silica content of the mafic end-member magma is estimated to be ca. 52%, and contains Mg-rich olivine and An-rich plagioclase. The temperature of this end member is estimated to have been higher than 1,100 °C. Bulk silica content of the felsic end-member magma is estimated to be ~66%, and contains Mg-poor pyroxenes, An-poor plagioclase, and quartz phenocrysts, with a temperature of between 800 and 900 °C. Trace element compositions show that the end members have different origins, but have changed little over the entire 16-ky of activity. The mafic end-member magmas might come from a lower-crustal homogeneous, large magma chamber, whereas the felsic end-member magmas may be partial melts of crustal materials produced by the heat of the mafic end member. Felsic end-member magma may have accumulated in the middle crust before CH1 activity. The mixing ratio of the felsic to mafic end members was 0.5:0.5 to 0.4:0.6 for the CH1 unit, and ca. 0.4:0.6 for the other units. Considering that ca. 75% of the total volume of the eruptive products form the first unit, its wider compositional variation is attributed to more heterogeneous mixing ratios.

Supercooled rocks: development and significance of varioles, spherulites, dendrites and spinifex in Archaean volcanic rocks, Abitibi Greenstone belt, Canada

Many intriguing crystallization textures of Archaean volcanic rocks are ‘supercool’ in the sense that they are visually striking, and because they owe their origin to substantial undercooling of the silicate liquids from which they grew. Under conditions of high undercooling kinetic effects arise such that spherulitic, acicular or platy mineral habits are stabilized. The term variole refers to globular and spherical centimetre-scale, generally leucocratic masses visible on the weathered surfaces of mafic rock. At many localities the varioles can be shown to be plagioclase spherulites that grew near the quench margins of aphyric mafic lavas. Elsewhere, the varioles are droplets of felsic magma frozen into mafic magma as a result of magma mingling. Platy olivine spinifex in komatiites arises from a coupled process of hydrothermal cooling and constrained crystallization. Shrinkage of the komatiite crust during submarine cooling causes fracturing and sea water ingress such that a self-propagating vertically directed cooling/cracking-front develops. Within the magma below this front, forsterite crystals transport heat from the melt upwards to the hydrothermally cooled crust. Because of their higher thermal conductivity and greater transparency to near infrared thermal radiation, Mg-rich olivine crystals transfer heat more efficiently than the surrounding relatively Fe-rich liquid. The olivine forms vertically oriented platy crystals that cool the liquid directly in front of their tips and are thus self-propagating.