Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them. 相似文献
Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CBb chondrite Hammadah al Hamra 237 and none were observed in the CBa chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always 16O-rich (δ17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly 16O-rich or 16O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either 16O-rich (δ17,18O∼−40‰) or 16O-poor (δ17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in 16O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in 16O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with 16O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in 16O-poor gaseous reservoirs or coated by 16O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids. 相似文献
The effect of crystal structure relaxation in oxygen-based Cr3+-containing minerals on the crystal field stabilization energy (CFSE) is considered. It is shown that the dependence of
\textCFSE\textCr 3+ {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} , which is found from optical absorption spectra, on the average interatomic distances is described by the power function
with a negative exponent
c \mathord
/
\vphantom c [`(R)]n [`(R)]n {c \mathord{\left/ {\vphantom {c {\bar{R}^{n} }}} \right. \kern-\nulldelimiterspace} {\bar{R}^{n} }} , where n approaches 5, as predicted theoretically, for pure Cr3+ compounds, but decreases to 1.0–1.5 for Cr3+-containing oxide and silicate solid solutions. The deviation of the experimental dependence for solid solutions from the
theoretical curve is due to structure relaxation, which tends to bring the local structure of Cr3+ ions closer to the structure in the pure Cr compound, thus producing changes in interatomic distances between the nearest
neighbors with respect to those in the average structure determined by X-ray diffraction. As a consequence, the mixing enthalpy
of Cr3+-bearing solid solutions can be represented by the sum of contributions from lattice strain and CFSE. The latter contribution
is most often negative in sign and, therefore, brings the Al–Cr solid solutions close to an ideal solid solution. It is supposed
that the increased Cr content in minerals from deep-seated mantle xenoliths and mineral inclusions in diamonds results from
the effect of
\textCFSE\textCr 3+ {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} enhanced by high pressure. 相似文献
Analysis of E||X-polarized optical absorption spectra of natural olivines of various origin in the range of electronic spin-allowed dd-transitions of Fe2+ evidences that in some crystals, there is a weak ordering of Fe2+ as in M1, as in M2 structural sites. The samples of three different depth facies seem showing a vague tendency of lowering of kD-values from spinel-pyroxene (Sp-Px) through graphite-pyrope (G-Py) to diamond-pyrope (D-Py) facies, but the statistics are too poor (24 samples only) to be certain of it. Weak relations between Mg, Fe2+-distribution and iron content were found for the samples of Sp-Px- and G-Py-facies, while there is practically no one for those of the deepest D-Py facie. 相似文献
Calc-alkaline magmatism in the south-west Ukraine occurred between 13.8 and 9.1 Ma and formed an integral part of the Neogene subduction-related post-collisional Carpathian volcanic arc. Eruptions occurred contemporaneously in two parallel arcs (here termed Outer Arc and Inner Arc) in the Ukrainian part of the Carpathians. Outer Arc rocks, mainly andesites, are characterized by LILE enrichment (e.g. K and Pb), Nb depletion, low compatible trace element abundances, high 87Sr/86Sr, high δ18O and low 143Nd/144Nd isotopic ratios (0.7085–0.7095, 7.01–8.53, 0.51230–0.51245, respectively). Inner Arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas. They also show low compatible element abundances but have lower 87Sr/86Sr, δ18O and higher 143Nd/144Nd ratios (0.7060–0.7085, 6.15–6.64, 0.5125–0.5126, respectively) than Outer Arc rocks. Both high-Nb and low-Nb lithologies are present in the Inner Arc. Based on the LILE enrichment (especially Pb), a higher fluid flux is suggested for the Outer Arc magmas compared with those of the Inner Arc.
Combined trace element and Sr–Nd–O isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex. Compositional differences between the Inner and Outer Arcs were produced by introduction of variable proportions of slab-derived sediments and fluids into a heterogeneous mantle wedge, and by different extents of upper crustal contamination. Degrees of magmatic fractionation also differed between the two arcs. The most primitive magmas belong to the Inner Arc. Isotopic modelling shows that they can be produced by adding 3–8% subducted terrigenous flysch sediments to the local mantle wedge source. Up to 5% upper crustal contamination has been modelled for fractionated products of the Inner Arc. The geochemical features of Outer Arc rocks suggest that they were generated from mantle wedge melts similar to the Inner Arc primitive magmas, but were strongly affected by both source enrichment and upper crustal contamination. Assimilation of 10–20% bulk upper crust is required in the AFC modelling, assuming an Inner Arc parental magma. We suggest that magmagenesis is closely related to the complex geotectonic evolution of the Carpathian area. Several tectonic and kinematic factors are significant: (1) hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes; (2) thermal disturbance caused by ascent of hot asthenospheric mantle during the back-arc opening of the Pannonian Basin; (3) clockwise translational movements of the Intracarpathian terranes, which facilitated eruption of the magmas. 相似文献
This study investigates the convective boundary layer (CBL) that develops over anon-homogeneous surface under different thermal and dynamic conditions. Analysesare based on data obtained from a Russian research aircraft equipped with turbulentsensors during the GAME-Siberia experiment over Yakutsk in Siberia, from April to June 2000.Mesoscale thermal internal boundary layers (MTIBLs) that radically modified CBLdevelopment were observed under unstable atmospheric conditions. It was found thatMTIBLs strongly influenced the vertical and horizontal structures of virtual potentialtemperature, specific humidity and, most notably, the vertical sensible and latent heatfluxes. MTIBLs in the vicinity of the Lena River lowlands were confirmed by clouddistributions in satellite pictures.MTIBLs spread through the entire CBL and radically modify its structure if the CBL isunstable, and strong thermal features on the underlying surface have horizontal scalesexceeding 10 km. MTIBL detection is facilitated through the use of special parameterslinking shear stress and convective motion.The turbulent structure of the CBL with and without MTIBLs was scaled usingthe mosaic or flux aggregate approach. A non-dimensional parameterLRau/Lhetero (where LRau is Raupach's length and Lhetero is the horizontal scale of the surface heterogeneity)estimates the application limit of similarity and local similarity scaling models forthe mosaic parts over the surface. Normalized vertical profiles of wind speed, airtemperature, turbulent sensible and latent heat fluxes for the mosaic parts withLRauLhetero < 1 could be estimated by typical scalingcurves for the homogeneous CBL. Traditional similarity scaling models for the CBLcould not be applied for the mosaic parts with LRau/Lhetero > 1.For some horizontally non-homogeneous CBLs, horizontal sensible heat fluxes werecomparable with the vertical fluxes. The largest horizontal sensible heat fluxes occurred at the top of the surface layer and below the top of the CBL.Formerly affiliated to the Frontier Observational Research System for Global ChangeFormerly affiliated to the Frontier Observational Research System for Global Change 相似文献
Abstract— Natural calcium monoaluminate, CaAl2O4, has been found in a grossite‐rich calcium‐aluminum‐rich inclusion (CAI) from the CH chondrite Northwest Africa 470. The calcium monoaluminate occurs as colorless ～10 μm subhedral grains intergrown with grossite, perovskite, and melilite. Nebular condensation is the most likely origin for the precursor materials of this CAI, but calculations suggest that dust/gas ratios substantially enhanced over solar are required to stabilize CaAl2O4. 相似文献
We investigated the khatyrkite–cupalite holotype sample, 1.2 × 0.5 mm across. It consists of khatyrkite (Cu,Zn)Al2, cupalite (Cu,Zn)Al, and interstitial material with approximate composition (Zn,Cu)Al3. All mineral phases of the holotype sample contain Zn and lack Fe that distinguishes them from khatyrkite and cupalite in the Khatyrka meteorite particles (Bindi et al. 2009 , 2011 , 2012 , 2015 ; MacPherson et al. 2013 ; Hollister et al. 2014 ). Neither highly fractionated natural systems nor geo‐ or cosmochemical processes capable of forming the holotype sample are known so far. The bulk chemistry and thermal history of khatyrkite–cupalite assemblage in the holotype sample hint for its possible industrial origin. Likewise, the aluminides in the Khatyrka meteorite particles may also be derived from industrial materials and mixed with extraterrestrial matter during gold prospecting in the Listvenitovy Stream valley. 相似文献
Differently colored natural Brazilian andalusite crystals heat-treated under reducing and oxidizing conditions were analyzed
by optical spectroscopy. The intensity of a broad intense band at around 20,500 cm−1 in the optical absorption spectra of all color zones of the sample is proportional to the product of Ti- and Fe-concentrations
and herewith proves its attribution to electronic Fe2+/Ti4+ IVCT transition. The band is strictly E||c-polarized, causing an intense red coloration of the samples in this polarization. The polarization of the Fe2+/Ti4+ IVCT band in andalusite, E||c, shows that the electronic charge-transfer process takes place in Al–O octahedral groups that share edges with neighbors
on either side, forming chains parallel to the c-axis of the andalusite structure. Under thermal treatments in air, the first noticeable change is some intensification of
the band at 800°C. However, at higher temperatures its intensity decreases until it vanishes at 1,000°C in lightly colored
zones and 1,100°C in darkly colored ones. Under annealing in reducing conditions at 700 and 800°C, the band also slightly
increases and maintains its intensity at treatments at higher temperatures up to 1,000°C. These results demonstrate that weakening
and disappearance of the Fe2+/Ti4+ IVCT band in spectra of andalusite under annealing in air is caused by oxidization of Fe2+ to Fe3+ in IVCT Fe2+/Ti4+-pairs. Some intensification of the band at 800°C is, most probably, due to thermally induced diffusion of Fe2+ and Ti4+ in the structure that leads to aggregation of “isolated” Ti4+ and Fe2+ ions into Fe2+–Ti4+-pairs. At higher temperatures, the competing process of Fe2+ → Fe3+ oxidation overcomes such “coupling” and the band continues to decrease. The different thermal stability of the band in lightly
and darkly colored zones of the samples evidence some self-stabilization over an interaction of Fe2+/Ti4+-pairs involved in IVCT process. 相似文献
