Barium isotopes in individual presolar silicon carbide grains from the Murchison meteorite

Barium isotopic compositions of single 2.3-5.3 μm presolar SiC grains from the Murchison meteorite were measured by resonant ionization mass spectrometry. Mainstream SiC grains are enriched in s-process barium and show a spread in isotopic composition from solar to dominantly s-process. In the relatively coarse grain size fraction analyzed, there are large grain-to-grain variations of barium isotopic composition. Comparison of single grain data with models of nucleosynthesis in asymptotic giant branch (AGB) stars indicates that the grains most likely come from low mass carbon-rich AGB stars (1.5 to 3 solar masses) of about solar metallicity and with approximately solar initial proportions of r- and s-process isotopes. Measurements of single grains imply a wide variety of neutron-to-seed ratios, in agreement with previous measurements of strontium, zirconium and molybdenum isotopic compositions of single presolar SiC grains.     

NanoSIMS isotopic analysis of small presolar grains: Search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si₃N₄ grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for ²⁶Al/²⁷Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 μm) and 153 SiC grains from KJG (diameters 1.8-3.7 μm), 154 SiC and 23 Si₃N₄ grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si₃N₄ grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si₃N₄ grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si₃N₄ grain, with ¹³C and ¹⁴N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5-7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred²⁶Al/²⁷Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low ¹²C/¹³C ratios of Z grains, but this process apparently does not lead to increased ²⁶Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high ²⁶Al/²⁷Al ratios. The low ^46,47,49Ti/⁴⁸Ti ratios found in Z grains and their correlation with low ²⁹Si/²⁸Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in ³⁰Si and ⁵⁰Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in ⁵⁰Ti (and ⁴⁹Ti) are much larger than those found. Even lowering the strength of the ¹³C pocket cannot solve this discrepancy in a consistent way.     

A NanoSIMS study of Si- and Ca-Ti-isotopic compositions of presolar silicon carbide grains from supernovae

We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in ²⁸Si (up to 5× solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal ⁴²Ca/⁴⁰Ca ratios. Seven X grains show enhanced ⁴⁴Ca/⁴⁰Ca ratios of up to 6× the solar ratio. Spatial distributions of ⁴⁴Ca excesses and Ti are positively correlated, giving strong support to the view that excesses in ⁴⁴Ca are due to the decay of radioactive ⁴⁴Ti. Inferred initial ⁴⁴Ti/⁴⁸Ti ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic ⁴⁴Ca is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic ⁴⁴Ca and ⁴⁸Ti as well as in ⁴⁴Ti/⁴⁸Ti, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with ³⁰Si/²⁹Si = 2.2× the solar ratio and C- and N-isotopic compositions as typically observed in X grains.     

Auger Nanoprobe analysis of presolar ferromagnesian silicate grains from primitive CR chondrites QUE 99177 and MET 00426

We have investigated the presolar grain inventories of two CR chondrites, QUE 99177 and MET 00426, which are less altered than most members of this meteorite group. Both meteorites contain high abundances of O-anomalous presolar grains, with concentrations of 220 ± 40 and 160 ± 30 ppm for QUE 99177 and MET 00426, respectively. The presolar grain inventories are dominated by ferromagnesian silicates with group 1 oxygen isotopic compositions, indicative of origins in low mass red giant or asymptotic giant branch stars. Grains with pyroxene-like compositions are somewhat more common than those with olivine-like compositions, but most grains are non-stoichiometric with compositions intermediate between these two phases, consistent with recent work suggesting that amorphous interstellar silicates have stoichiometries between olivine and pyroxene type silicates. Although structural data are not available, one grain contains only Si and O, and has a stoichiometry consistent with SiO₂.Our presolar grains are much more Fe-rich than predicted by astronomical observations. Although secondary alteration may play a role in enhancing the Fe contents of presolar grains, it seems unlikely that the large and ubiquitous Fe enrichments observed in the grains from this study can be due only to secondary processing, particularly given the highly primitive nature of these two meteorites. Grain condensation in the stellar outflows where these grains formed likely proceeded under rapidly changing kinetic conditions that may have enhanced the incorporation of Fe into the grains over that expected based on equilibrium condensation theory.Both QUE 99177 and MET 00426 appear to contain unusually low abundances of oxide grains and have higher silicate/oxide ratios than other primitive meteorites analyzed to date. We explore various possibilities for this discrepancy, but note that most scenarios are not likely to result in the preferential destruction of oxides relative to silicates. Thus, the highest silicate/oxide ratios, such as those observed in the CR chondrites, should reflect the true initial proportions of presolar silicate and oxide grains in the parent molecular cloud from which the solar nebula evolved.     

Ontogenetic analysis of individual olivine grains in ultramafic rocks

The morphology and internal structure of individual olivine grains from ultramafic rocks in the Guli and Gal’moenan dunite massifs differing in origin are considered. To restore the ontogeny of mineral aggregates, traces of elastic deformation retained in mineral grains have been used. Comparison of anatomy of olivine grains from these two massifs showed that the mechanism of accommodation of rocks to changing geological settings is expressed as the response of the mineral aggregate structure and variation in the anatomy of individual mineral grains. At the level of individual grains, this is annihilation of older defects and origination of younger dislocations; refinement of the crystal lattice; exsolution; formation and transformation of new mineral phases; and creep and migration of subboundaries within grains. At the aggregate level, this is rotation and migration creep of the internal boundaries of rock; formation of new boundaries of mineral intergrowths; reorientation of boundaries; and variation in their extent, density, and grain dimensions. The prehistory of massifs controls the manifestation and abundance of various elastic deformations and related types of recrystallization of olivine grain boundaries and subboundaries in aggregates. New conditions and accommodation of mineral aggregates to these conditions have instigated specific schemes of recrystallization, which bear information on the history of rocks and their massifs.     

Pristine presolar silicon carbide

We report the results of a study of 81 micrometer-sized presolar SiC grains in the size range 0.5-2.6 μm from the Murchison (CM2) carbonaceous chondrite. We describe a simple, nondestructive physical disaggregation technique used to isolate the grains while preserving them in their pristine state, as well as the scanning electron microscopy energy-dispersive X-ray mapping procedure used to locate them.Nine-tenths of the pristine SiCs are bounded by one or more planar surfaces consistent with cubic (3C polytype) crystal faces based on manifest symmetry elements. In addition, multiple polygonal depressions (generally <100 nm deep) are observed in more than half of these crystal faces, and these possess symmetries consistent with the structure of the 3C polytype of SiC. By comparison of these features with the surface features present on heavily etched presolar SiC grains from Murchison separate KJG, we show that the polygonal depressions on pristine grains are likely primary growth features. The etched SiCs have high densities of surface pits, in addition to polygonal depressions. If these pits are etched linear defects in the SiC, then defect densities are quite high (as much as 10⁸ -10⁹/cm²), about 10³-10⁴ times higher than in typical synthetic SiCs. The polygonal depressions on crystal faces of pristine grains, as well as the high defect densities, indicate rapid formation of presolar SiC.No other primary minerals are observed to be intergrown with or overgrown on the pristine SiCs, so the presence of overgrowths of other minerals cannot be invoked to account for the survival of presolar SiC in the solar nebula. We take the absence of other primary condensates to indicate that further growth or back-reaction with the gas became kinetically inhibited as the gas-phase densities in the expanding asymptotic giant branch (AGB) stellar atmospheres (in which most of the grains condensed) became too low. However, we did observe an oxygen peak in the X-ray spectra of most pristine grains, implying silica coatings of as much as several tens of nm thickness, perhaps due to oxidation of the SiC in the solar nebula.We see little or no evidence on the pristine grains of the surface sputtering or cratering that are predicted theoretically to occur in the interstellar medium (ISM) due to supernova shocks. A possible implication is that the grains may have been protected during their residence in the ISM by surface coatings, including simple ices. Residues of such coatings may indeed be present on some pristine SiCs, because many (60%) are coated with an apparently amorphous, possibly organic phase. However, at present we do not have sufficient data on the coatings to draw secure inferences as to their nature or origin.A few irregular pristine SiCs, either fragments produced by regolith gardening on the Murchison parent body or by grain-grain collisions in the ISM, were also observed.     

Volatilization kinetics of silicon carbide in reducing gases: an experimental study with applications to the survival of presolar grains in the solar nebula

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO₂ (cristobalite or glass) were determined in H₂-CO₂, CO-CO₂, and H₂-CO-CO₂ gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wüstite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin (<1 μm thick), continuous layer of cristobalite. SiC exposed to IW-6.0 lacks such a layer (or its thickness is <0.01 μm), although some SiO₂ was found within pits and along incised grain boundaries.In H₂-CO₂ gas mixtures above ∼IW-3, the similarity of the SiC volatilization rate and of its dependence on temperature and fO₂ to that for SiO₂ suggests that SiC volatilization is controlled by volatilization of a SiO₂ layer that forms on the surface of the SiC. With decreasing log fO₂ from ∼IW-3 to ∼IW-6, the SiC volatilization rate is constant at constant temperature, whereas that for SiO₂ increases. The independence of the SiC volatilization rate from the gas composition under these conditions suggests that the rate-controlling step is a solid-solid reaction at the internal SiC/SiO₂ interface. For gas compositions more reducing than ∼IW-6, the SiC volatilization rate increases with decreasing fO₂, with both bare SiC surfaces and perhaps silica residing in pits and along incised grain boundaries contributing to the overall reaction rate.If the volatilization mechanism and reaction rate in the solar nebula were the same as in our H₂-CO₂ experiments at IW-6.0, then estimated lifetimes of 1-μm-diameter presolar SiC grains range from several thousand years at ∼900°C, to ∼1 yr at 1100°C, ∼1 d at 1300°C, and ∼1 h at 1400°C. The corresponding lifetimes for 10-μm SiC grains would be an order of magnitude longer. If the supply of oxidants to surfaces of presolar SiC grains were rate limiting—for example, at T > 1100°C for P^tot= 10⁻⁶ atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.     

The deuterium enrichment of individual amino acids in carbonaceous meteorites: A case for the presolar distribution of biomolecule precursors

The δD values of over 40 amino acids and two pyridine carboxylic acids of the Murchison and Murray meteorites have been obtained by compound-specific isotopic analyses. For compounds with no known terrestrial distribution, these values range from approximately +330‰ (for cyclic leucine) to +3600‰ (for 2-amino-2,3-dimethylbutyric acid). The latter value is the highest ever recorded for a soluble organic compound in meteorites and nears deuterium to hydrogen ratios observed remotely in interstellar molecules. Deuterium content varies significantly between molecular species and is markedly higher for amino acids having a branched alkyl chain. The δD value of Murray l-isovaline, with an enantiomeric excess of ∼ 6% in the meteorite, was within experimental error of that determined for the combined dl-isovaline enantiomers. Overall, the hydrogen isotope composition of meteoritic amino acids is relatively simple and their δD values appear to vary more with the structure of their carbon chains than with the number and relative distribution of their functionalities or ¹³C content. The magnitude and extent of deuterium enrichment shared by many and varied amino acids in meteorites indicate that cosmic regimes such as those found in the interstellar medium were capable of producing, if not all the amino acids directly, at least a suite of their direct precursors that was abundant, varied, and considerably saturated.     

Analyzing the individual and social rights condition of climate refugees from the international environmental law perspective

In evaluating international treaties, in particular, the United Nations Framework on Climate Change Convention, this paper shows that necessary preparations have not been made to protect the rights of individuals who are refugees or have been forced to migrate from their homeland. Immigrants are deprived of the most essential human rights, such as language, culture, and having a right to a healthy living environment. The current legal administration has not made the necessary contingencies for responding to the environmental consequences of immigration and is extremely inefficient in expanding this phenomenon. This paper seeks to address the question of to what extent existing forms of legal and operational protection can apply in climate change-related displacement in general and cross-border displacement in particular. In this work some strategies have been proposed for dealing with these existing gaps. It is expected that analyzing these gaps and determining the international community??s duties and commitments (governments and international organizations) can result in efficient management of this crisis and prevent the outbreak of chaos across the globe.     

Automated isotopic measurements of micron-sized dust: application to meteoritic presolar silicon carbide

We report the development of a new analytical system allowing the fully automated measurement of isotopic ratios in micrometer-sized particles by secondary ion mass spectrometry (SIMS) in a Cameca ims-6f ion microprobe. Scanning ion images and image processing algorithms are used to locate individual particles dispersed on sample substrates. The primary ion beam is electrostatically deflected to and focused onto each particle in turn, followed by a peak-jumping isotopic measurement. Automatic measurements of terrestrial standards indicate similar analytical uncertainties to traditional manual particle analyses (e.g., ∼3‰/amu for Si isotopic ratios). We also present an initial application of the measurement system to obtain Si and C isotopic ratios for ∼3300 presolar SiC grains from the Murchison CM2 carbonaceous chondrite. Three rare presolar Si₃N₄ grains were also identified and analyzed. Most of the analyzed grains were extracted from the host meteorite using a new chemical dissolution procedure. The isotopic data are broadly consistent with previous observations of presolar SiC in the same size range (∼0.5-4 μm). Members of the previously identified SiC AB, X, Y, and Z subgroups were identified, as was a highly unusual grain with an extreme ³⁰Si enrichment, a modest ²⁹Si enrichment, and isotopically light C. The stellar source responsible for this grain is likely to have been a supernova. Minor differences in isotopic distributions between the present work and prior data can be partially explained by terrestrial contamination and grain aggregation on sample mounts, though some of the differences are probably intrinsic to the samples. We use the large new SiC database to explore the relationships between three previously identified isotopic subgroups—mainstream, Y, and Z grains—all believed to originate in asymptotic giant branch stars. The isotopic data for Z grains suggest that their parent stars experienced strong CNO-cycle nucleosynthesis during the early asymptotic giant branch phase, consistent with either cool bottom processing in low-mass (M < 2.3M_⊙) parent stars or hot-bottom burning in intermediate-mass stars (M > 4M_⊙). The data provide evidence for a sharp threshold in metallicity, above which SiC grains form with much higher ¹²C/¹³C ratios than below. Above this threshold, the fraction of grains with relatively high ¹²C/¹³C decreases exponentially with increasing ²⁹Si/²⁸Si ratio. This result indicates a sharp increase in the maximum mass of SiC parent stars with decreasing metallicity, in contrast to expectations from Galactic chemical evolution theory.     

Allende inclusions: volatile-element distribution and evidence for incomplete volatilization of presolar solids

Neutron activation data on 14 trace elements in Allende bulk samples and in fractions of spheroidal Ca-Al-rich inclusions show several distinct distribution patterns. Refractories Ir and Sc have high inclusion/bulk ratios and show little variation with depth. Manganese, Fe, Co, Ni, Ga, Cd and In have low inclusion/bulk ratios and decrease with increasing depth; their presence in the inclusions reflects matrix contamination. Sodium and other alkalies have high inclusion/bulk ratios (near 0.5) and decrease with increasing depth; their high concentration despite moderate volatility seems related to condensation reactions in which refractory Al-bearing minerals are reactants. Chromium, Zn, Ge and Au show patterns similar to those of the alkalies; this seems to indicate that refractory minerals are reactants in their condensation reactions, but thermodynamic support for this hypothesis has not been found.We propose that the large size of Allende spheroidal inclusions indicates an origin by incomplete vaporization of presolar solid matter followed by recondensation of refractories on a limited number of condensation nuclei. The low abundance of large refractory inclusions in ordinary and enstatite chondrites reflects complete vaporization of presolar solids at their formation locations; constraints on homogeneous nucleation resulted in the simultaneous condensation of refractories and olivine at these locations.Quadruplicate analyses of the Orgueil chondrite are in good agreement with previous determinations with the exception of small systematic differences in Au and Ir.     

Laser microprobe for the study of noble gases and nitrogen in single grains: A case study of individual chondrules from the Dhajala meteorite

A laser microprobe capable of analysing nitrogen and noble gases in individual grains with masses less than a milligram is described. It can be used in both continuous wave (CW) mode, useful for stepwise heating of an individual grain, as well as in pulsed mode, useful for ablating material from a small selected area of a sample, for gas extraction. We could achieve low blanks (in ccSTP units) for 4He(4.8 x 10{-12}),²²Ne(1.0 x 10{-12}),³⁶Ar(1.0 x10 ^-13),⁸⁴Kr(2.9 x 10{-14}),¹³² Xe(2.6 x 10{-14}), and N (87 pg), using this system. Preliminary data for individual chondrules from the Dhajala meteorite show that noble gases and nitrogen from grains as small as 170 microgram can be analysed using the present laser microprobe setup. The amount of trapped neon in Dhajala chondrules is very small, and nitrogen in the chondrules is isotopically heavier as compared to the bulk meteorite.     

不同粒径大理岩样声学特性的研究

基于不同粒径大理岩样进行了超声波测试,研究了浸水时间和宏观裂隙对波速的影响规律,以及岩样声学参数的离散程度。结果表明,浸水时间对岩样的纵波速度有很大的影响,但对横波速度影响甚微;宏观裂隙的存在降低了岩样的纵波速度,波幅降低程度与岩样破坏程度有关;随着岩石平均粒径的增大,岩石的声学参数先减小后增大,并非单调关系,而且动态泊松比的变化幅度明显高于其他声学参数,不能作为材料的力学参数表征岩石的变形特性。     

Carbon-isotopic analysis of individual pollen grains from C3 and C4 grasses using a spooling-wire microcombustion interface

Pollen grains from grasses using the C₃ and C₄ photosynthetic pathways have distinct ranges of δ¹³C values that may be used to estimate their relative abundance in paleorecords. We evaluated a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer (SWiM-IRMS) for δ¹³C analysis of individual grass-pollen grains. Pollen from four C₃ and four C₄ grass species was isolated through micromanipulation and analyzed as single grains suspended in water. A carbon yield greater than the 2σ range of the carbon content of blanks containing only water was used to distinguish samples containing pollen (“pollen present”) from those not containing pollen. This criterion resulted in the exclusion of ∼45% of the 946 samples applied to the wire. The average δ¹³C values (±1σ) of the remaining samples were −26.9‰ (±6.3‰) and −11.5‰ (±9.6‰) for C₃ grasses and C₄ grasses, respectively, after blank-correcting the δ¹³C data. These results suggest that the SWiM-IRMS system can be used to distinguish C₃ from C₄ grass pollen. The high variability in measured δ¹³C values is likely caused by a combination of factors. These include natural isotopic variability among individual pollen grains; the relatively poor precision that can be obtained when determining δ¹³C values of such small samples; and the uncertainty in the magnitude, isotopic composition, and stability of the analytical blank. Nonetheless, high percentages of individual pollen grains were correctly classified as being of either C₃ or C₄ origin. On average, 90% (range = 78-100%) of pollen grains from C₃ grasses had δ¹³C values more negative than the cutoff threshold of −19.2‰; while 84% (range = 77-90%) of pollen grains from C₄ grasses had δ¹³C values more positive than −19.2‰. Compared with analysis using an elemental analyzer interfaced with an IRMS (EA-IRMS), the number of pollen grains required for δ¹³C-based evaluation of C₃/C₄ grass composition is many times lower with the SWiM-IRMS. Additionally, δ¹³C data from the SWiM-IRMS does not need to be incorporated into a mixing model to derive estimates of the abundance of C₃ and C₄ grass pollen. Carbon-isotopic analysis of individual grass-pollen grains using the SWiM-IRMS system may help improve our understanding of the evolutionary and ecological significance of grass taxa in the paleorecord.     

Origin of chondritic forsterite grains

Iron-poor and refractory lithophile element (RLE) rich forsterite grains occur in all major types of unequilibrated chondrites. In our laser ablation inductively coupled mass spectrometry (LA-ICPMS) minor and trace element study we show that refractory forsterites (RF) from carbonaceous (CC), unequilibrated ordinary (UOC) and a Rumuruti chondrite (RC) have similar chemical compositions with high RLE concentrations and low concentrations of Mn, Fe, Co and Ni. Fractionation of RLEs and rare earth elements (REEs) is in agreement with formation by crystallization from a RLE rich silicate melt. Low concentrations and the fractionation of moderately siderophile elements (Fe, Co, Ni) in RFs suggests formation at low oxygen fugacity, possibly in equilibrium with primitive Fe,Ni metal condensates in a gas of solar composition. Anomalously high Ti in the parental melt can be explained by Ti³⁺/Ti⁴⁺ ∼1.5, supporting formation of RF in highly reducing conditions. Low Mn concentrations indicate formation at high temperatures (>∼1160 K). The model of formation of RFs and the accompanying physico-chemical conditions during their formation as well as their relation to non refractory olivine are discussed.     

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.     

On wind velocity profile measurements taken in wind tunnels with saltating grains

This paper reviews some aspects of wind tunnel experiments on sand-transporting winds. It follows previous papers that have discussed the influence of the outer region of the boundary layer on wind velocity measurements. This influence was quantified with the use Coles’Wake function. In this paper this correction is applied to six previously described wind velocity profiles. An attempt is made to calculate the profile parameter II from these measurements. The values found were not consistent with the expected II, which was determined by Coles for clean air flow. This value (II=0.55) was assumed to be valid in previous analyses for sand-transporting winds. Evidence for a mutual dependency of friction velocity and profile parameter is presented and the difficulty in determining u. is pointed out. It is suggested that the constant stress region of the boundary layer should be kept large enough for measurements when Coles’Wake function is not to be used in the data analysis.     

Fatigue effects in quartz sand grains

Particles of quartz sand from a stream draining granite were found to exhibit noteworthy effects of fatigue. Both repeated static loading and collective movement in water weakened the particles markedly. This weakening must have significance in nature, where repeated stressing occurs in many environments, particularly in soils and during bed-load motion in both air and water. Tectonic movements may also create fatigue effects in sand grains. Fatigue effects appear also to be sufficiently pronounced to be significant in some industrial processes involving the use of sand.     

On the origins of GEMS grains

From their birth as condensates in the outflows of oxygen-rich evolved stars, processing in interstellar space, and incorporation into disks around new stars, amorphous silicates predominate in most astrophysical environments. Amorphous silicates were a major building block of our Solar System and are prominent in infrared spectra of comets. Anhydrous interplanetary dust particles (IDPs) thought to derive from comets contain abundant amorphous silicates known as GEMS (glass with embedded metal and sulfides) grains. GEMS grains have been proposed to be isotopically and chemically homogenized interstellar amorphous silicate dust. We evaluated this hypothesis through coordinated chemical and isotopic analyses of GEMS grains in a suite of IDPs to constrain their origins. GEMS grains show order of magnitude variations in Mg, Fe, Ca, and S abundances. GEMS grains do not match the average element abundances inferred for ISM dust containing on average, too little Mg, Fe, and Ca, and too much S. GEMS grains have complementary compositions to the crystalline components in IDPs suggesting that they formed from the same reservoir. We did not observe any unequivocal microstructural or chemical evidence that GEMS grains experienced prolonged exposure to radiation.We identified four GEMS grains having O isotopic compositions that point to origins in red giant branch or asymptotic giant branch stars and supernovae. Based on their O isotopic compositions, we estimate that 1-6% of GEMS grains are surviving circumstellar grains. The remaining 94-99% of GEMS grains have O isotopic compositions that are indistinguishable from terrestrial materials and carbonaceous chondrites. These isotopically solar GEMS grains either formed in the Solar System or were completely homogenized in the interstellar medium (ISM). However, the chemical compositions of GEMS grains are extremely heterogeneous and seem to rule out this possibility. Based on their solar isotopic compositions and their non-solar elemental compositions we propose that most GEMS grains formed in the nebula as late-stage non-equilibrium condensates.