Donnees experimentales sur la diffusion des elements majeurs entre verres ou liquides de compositions basaltique,rhyolitique et phonolitique,entre 900°C et 1300°C,a pression ordinaire

Diffusion coefficients for Si, Ti, Al, Na, K, Ca, Mg and Fe between pairs of glasses of basaltic, rhyolitic or phonolitic compositions have been determinated experimentally. This method involves the heating of coaxial cylinders of paired glasses under atmospheric conditions, over a range of temperatures from 900 to 1300°C, followed by microprobe analysis determination of the concentration gradients across the interface.The measured diffusivities are similar for all cations and range from 10^?13 cm²/s at 900°C to 5 × 10^?9 cm²/s at 1300°C. Depending mainly on the composition contrast, the diffusion is characterized by asymmetrical concentration profiles. This peculiar feature increases with temperature and chemical gradients across the contact surface of the glasses and leads to higher diffusion coefficients (D) in the more “basic” glass of a given pair. In the case of the rhyolite-basalt couple, this variation increases by a factor of about 10 at 1300°C. Diffusion dependence on temperature follows an Arrhenius equation which gives activation energies ranging from 65 to 85 kcal/mole. Assuming a constant and overall D for the two glasses we have attempted to apply our results to some geological examples such as exchanges between molten enclaves and liquids of contrasting composition.     

Rhythmic layering in the magma chamber of basic-ultrabasic intrusions due to diffusion and intermittent convection

The following facts have been established in this study. (1) Fine rhythmic layering can form due to diffusion. For example, the behavior of the elements Fe, Mg, Ca, and Al with a high diffusion coefficient (D ₁ ? 1.1 × 10^?5 cm²/s) can lead to the formation of fine rhythmic layering in rocks, whereas elements with low diffusion coefficients (D ₁ ? 2.7 × 10^?10 cm²/s) do not produce rhythmic layering and the distribution of their concentrations is monotonic. Equations derived to describe layering are used to calculate the distribution of magnesium (MgO) concentration in the Kivakka layered intrusion. Comparison of the theoretical distribution curve with in situ data showed their fairly good agreement, confirming the possible role of diffusion in the formation of fine rhythmic layering. (2) Conditions favorable for intermittent convection can occur in a magma chamber. The equation derived to describe the intermittent convection has a periodic solution. A distinctive feature of this type of convection is that zones of active mixing alternate with zones of relative stagnation. Conditions suitable for diffusion can arise in the latter, leading to the formation of fine (with layers up to 5 cm thick) rhythmic layering. An example of possible development of intermittent convection is the layered Akanvaara massif, where zones with a relatively smooth distribution of chemical elements alternate with zones of contrasting layering of rocks.     

Excess bottom222Rn profiles and their implications in the northwestern Pacific Ocean

Vertical profiles of excess bottom²²²Rn and potential temperature were measured at 23 stations in the northwestern Pacific Ocean. The Rn profiles were classified into the following three types: quasi-exponential (type E), benthic boundary layer (type B), and horizontally disturbed (type H). The ratio among types E, B and H was approximately 2 : 3 : 1.An apparent vertical eddy diffusivity (K) was calculated by applying a one-dimensional diffusion model to the Rn profiles of types E and B. Type E had K values ranging from 15 to 180 cm² s^?1 (average: 70 cm² s^?1). As to type B, K values for the benthic boundary layer (4.5–260 cm² s^?1, average: 120 cm² s^?1) were always more than an order of magnitude larger than those for the upper layer (0.2–35 cm² s^?1, average: 7 cm² s^?1), indicating more active vertical mixing in the benthic boundary layer than in the upper layer.Rn profiles were measured in regions where the bottom topography is known. It was verified that the occurrence of type H related closely with local bottom topographic features accompanied by lateral transient supply of Rn-rich or Rn-poor water.A couple of Rn profiles at the same location, measured at time intervals of several years, were compared with each other for three locations. The general characteristics of Rn profiles were shown to remain unaltered with time, while the fine structure of Rn profiles may have short-term variations caused by local bottom topography and fluctuations of bottom current as indicated in type H.     

An assessment of local and regional isotopic equilibrium in the mantle

The assumption of local equilibrium during partial melting is fundamental to the interpretation of isotope and trace element data for mantle-derived rocks. If disequilibrium melting is significant, the scale of the chemical and isotopic heterogeneity in the mantle indicated by the data could be as small as the grain size of the mantle rock, and the isotope data themselves are then of doubtful value to the understanding of mantle processes. To assess the scale of isotopic heterogeneity in a partially molten asthenosphere we review the Sr isotopic data of volcanic rocks from oceanic regions and the available experimental data on diffusion kinetics in minerals and melts similar to those existing in the mantle. Although diffusion data are scarce and afflicted with uncertainties, most of the diffusion coefficients for cations in mantle minerals at temperatures of 1000–1200°C appear to be greater than 10^?13 cm² s^?1. Sr diffusion in liquid basalt is more rapid, with diffusion coefficients of D = 10^?7 to 10^?6cm²s^?1 near 1300°C. Simple model calculations show that, with these D values, a fluid-free mantle can maintain a state of disequilibrium on a centimeter scale for periods of 10⁸ to 10⁹ years. The state of disequilibrium found in many mantle-derived xenoliths is thus easily explained. A partially molten mantle, on the other hand, will tend to equilibrate locally in less than 10⁵ to 10⁶ years. The analytical data on natural rocks likewise indicate that the inhomogeneities are both old (>FX1.5 b.y.) and regional in character and that the consistent isotopic difference between ocean island and ocean floor volcanics cannot be explained by small-scale heterogeneity of the source rock.     

Ion microprobe analysis of a potassium self-diffusion experiment in biotite

Self-diffusion of potassium in biotite has been measured by inducing isotopic exchange between the potassium in the biotite and potassium enriched in ⁴¹K in a hydrothermal alkali chloride solution at 650°C and 2000 bars pressure, and then analyzing the resulting distribution of the K isotopes in the mica using an ion microprobe mass analyzer. The distribution of K isotopes in one mica flake was studied by three-dimensional mapping. The results show the diffusivity in the a or b directions to be greater than in the c direction by 2 to 4 orders of magnitude. In the c direction, the isotopic profile yields a diffusion coefficient of D = 1 × 10^?17cm²sec^?1 with an estimated error of a factor of ten. These results are in good agreement with bulk isotopic transport measurements on the same experiment reported by A.W. Hofmann and B.J. Giletti (1970). It is thus shown that diffusivities involving very slow net transport can be measured using an isotopic exchange technique and measuring the effect using an ion microprobe mass analyzer.     

Elasticity of single crystal pyrope and implications for garnet solid solution series

The elastic moduli of a synthetic single crystal of pyrope (Mg₃Al₂Si₃O₁₂) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)₃ (Al, Fe, Cr)₂ Si₃O₁₂. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe²⁺/Mg²⁺ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg²⁺ and Fe²⁺ in these different minerals.     

The relationship between vertical eddy diffusion and buoyancy gradient in the deep sea

Vertical eddy diffusivities (K_v's) have been estimated at fourteen widely separated locations from fourteen²²²Rn profiles and two²²⁸Ra profiles measured near the ocean floor as part of the Atlantic and Pacific GEOSECS programs. They show an inverse proportionality to the local buoyancy gradient [(g/?)(??_pot/?z)] calculated from hydrographic measurements. The negative of the constant of proportionality is the buoyancy flux [?K_v(g/?)(??_pot/?z)] which has a mean of ?4 × 10^?6 cm²/sec³. Our results suggest that the buoyancy flux varies very little near the ocean floor. K_v's for the interior of the deep Pacific calculated from the relationship K_v = (4 × 10^?6cm²/sec³)/[(g/?)(??_pot/?z)] agree well with published estimates. K_v's calculated for the pycnocline are one to two orders of magnitude smaller than upper limits estimated from tritium and⁷Be distributions.Heat fluxes calculated with the model K_v's obtained from the²²²Rn profiles average 31 μcal cm^?2 sec^?1 in the Atlantic Ocean and 8 μcal cm^?2 sec^?1 in the Pacific Ocean.     

Magnetic properties of synthetic titanomagnetites

Magnetic properties and crystal structure parameters of synthetic solid solutions Fe₃O₄Fe₃TiO₄, Fe₂O₄MgFe₂O₄ and Fe₃O₄Mg₂TiO₄ have been studied. Basic regularities in the behaviour of saturation magnetisation (I_s), Curie temperature (T_C) and cubic lattice parameter a during the substitution of Ti and Mg ions for Fe ions have been found. As the concentration of Ti ions increases, I_s reduces from 70 Gs·cm³ g^?1 to 0, T_C changes from 580 to 130°C and a from 8.391 to 8.520 Å. Growth of the Mg concentration leads to changes in I_s to 19.8 Gs·cm³, g^?1, T_C, to 440°C and a, to 8.360 Å. The full Fe ions substitution gives $\text{“a”=8.440}\text{A}\text{?}$.Chemical compositions of the samples, in which the valency variation of Fe ions at oxidation leads to an increase in susceptibility and T_C, have been determined.     

Heuristic numerical and analytical models of the hydrologic controls over vertical solute transport in a domed peat bog,Jura Mountains,Switzerland

We report the results of numerical and analytical simulations to test the hypothesis that downward vertical flow of porewater from the crests of domed alpine and kettle bogs controls vertical porewater distributions of major solutes such as Ca and Mg. The domed Etang de la Gruère bog (EGr), Switzerland, characterized by a vertical downward gradient of 0·04 and stratified layers of peat, is chosen as a field site for the model calibration and evaluation. The middle 4‐m section of the 6·5 m thick bog peat is heavily humified and has a hydraulic conductivity of ～10^?5·6 cm s^?1. Above and below, peat is less humified with a hydraulic conductivity of ～10^?3 cm s^?1. Heuristic finite difference simulations, using Visual MODFLOW, of the bog hydraulics show that the higher conductivity peat at the bog base is critical to create the observed deep, local flow cells that substantively recharge porewater. Model results and Peclet number calculations show that before ～7000 ¹⁴C yr BP diffusion of solutes from underlying mineral soils controlled the vertical distribution of porewater chemistry. From 7000 to ～1250 ¹⁴C BP the porewater chemistry was probably controlled by both upward diffusion and downward advection, and after ～1250 ¹⁴C yr BP porewater chemistry was probably controlled by downward advection. Concentrations of conservative major solutes in the porewaters of alpine, ombrotrophic bogs are the net effect of both downward vertical porewater movement and upward vertical diffusion, the magnitudes of which are delicately poised to the configuration of the bog water table over time and subsurface peat stratigraphy. Copyright © 2002 John Wiley & Sons, Ltd.     

Ion microprobe studies of water in silicate melts: Temperature-dependent water diffusion in obsidian

The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and [P_T(H₂O] = 700 bars is evaluated from water concentration-distance profiles measured in glass with an ion microprobe. Diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10^?8 cm²/s at 650°C to about 10^?7 cm²/s at 950°C at 2 wt.% water. Water solubility also varies with temperature at a rate of ?0.14 wt. per 100°C increase. The avtivation energy (E_a) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H₂O. Comparison of these data with results for cation diffusion indicates that this value is a minimum E_a for diffusion of any species in a rhyolite melt.Compensation plots of log₁₀D₀ (the frequency factor) versus E_a indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D₀ increases for H₂O and Ca²⁺ [1] as E_a decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca²⁺ and H₂O differently than it does monovalent cation diffusion. The results imply that dramatic increases in cation diffusivities by hydration [1] may occur with additions of less than 1 wt.% H₂O.     

Angra dos Reis: Plutonium distribution and cooling history

The preatmospheric mass of Angra dos Reis is estimated to be about 80 kg, based on cosmic ray track densities of 1?3 × 10⁶ cm^?2 in feldspars, olivines, and pyroxenes. The²⁴⁴Pu-fission track densities in pyroxenes and adjacent olivine-feldspar track detectors, as well as in whitlockites and adjacent pyroxene-olivine detectors are about 20 and 280 × 10⁶ cm^?2 respectively. These values correspond exactly to the²⁴⁴Pu-fission xenon contents in the two minerals. This suggests an almost instantaneous cooling (>70 K per m.y.) of the Angra dos Reis material 4.55 b.y. ago. This fast cooling is the reason for a quantitative⁴He retention since that time. If the⁴He excess found in Angra dos Reis is a real effect, the source of this excess remains unknown.     

Magnetic signature of indoor air pollution: Household dust study

The combination of magnetic and geochemical methods was used to determine the mineralogy, grain size and domain structure of magnetic particles in indoor dust collected in 195 sites in Warsaw, Poland. Data show an asymmetric distribution of magnetic susceptibility (χ) in the wide range of 20–1514 × 10^?8 m³ kg^?1. Comparison of magnetic parameters shows that the internal dust contains outside pollution characteristic for air and soil. More than 90% of indoor dust samples were characterized by roughly uniform magnetic mineralogy, typical for fine grained magnetite (diameter of 0.2–5 μm), and grain size between pseudo-single-domain and small multi-domain with small contribution of superpara-magnetic particles (～10%). Samples with χ larger than 220 × 10^?8 m³ kg^?1 contain mainly magnetite and an anthropogenic metallic Fe with T _C > 700°C. The indoor dust contains, characteristic for the urban areas, spherical magnetic particles originated from fossil fuel combustion processes and mixture of irregular angular iron-oxides grains containing other elements, including Na, Ca, Al, Si, K, S, Mn, Cl, and Mg.     

Apatite/liquid partition coefficients for the rare earth elements and strontium

Sixteen sets of apatite/liquid partition coefficients (D^ap/liq) for the rare earth elements (REE; La, Sm, Dy, Lu) and six values for Sr were experimentally determined in natural systems ranging from basanite to granite. The apatite + melt (glass) assemblages were obtained from starting glasses artificially enriched in REE, Sr and fluorapatite components; these were run under dry and hydrous conditions of 7.5–20 kbar and 950–1120°C in a solid-media, piston-cylinder apparatus. An SEM-equipped electron microprobe was used for subsequent measurement of REE and Sr concentrations in coexisting apatites and quenched glasses. The resulting partition coefficient patterns resemble previously determined apatite phenocryst/groundmass concentration ratios in the following respects: (1) the rare earth patterns are uniformly concave downward (i.e., the middle REE are more compatible in apatite than the light and heavy REE); (2) D_REE^ap/liq is much higher for silicic melts than for basic ones; and (3) strontium (and therefore Eu²⁺) is less concentrated by apatite than are the trivalent REE. The effects of both temperature and melt composition on D_REE^ap/liq are systematic and pronounced. At 950°C, for example, a change in melt SiO₂ content from 50 to 68 wt.% causes the average REE partition coefficient to increase from ～7 to ～30. A 130°C increase in temperature, on the other hand, results in a two-fold decrease in D_REE^ap/liq. Partitioning of Sr is insenstitive to changes in melt composition and temperature, and neither the Sr nor the REE partition coefficients appear to be affected by variations in pressure or H₂O content of the melt.The experimentally determined partition coefficients can be used not only in trace element modelling, but also to distinguish apatite phenocrysts from xenocrysts in rocks. Reported apatite megacryst/host basalt REE concentration ratios [12], for example, are considerably higher than the equilibrium partition coefficients, which suggest that in this particular case the apatite is actually xenocrystic.A reversal experiment incorporated in our study yielded diffusion profiles of REE in apatite, from which we extracted a REEαCa interdiffusion coefficient of 2–4×10^?14 cm²/s at 1120°C. Extrapolated downward to crustal temperatures, this low value suggests that complete REE equilibrium between felsic partial melts and residual apatite is rarely established.     

Magnesium isotopes constraints on the origin of Mg-rich olivines from the Allende chondrite: Nebular versus planetary?

High precision Mg isotope measurements by multi-collector ion microprobe show that refractory olivines from the Allende chondrite, either olivines isolated in the matrix (2 samples studied) or olivines in type I chondrules (6 samples studied), have variable δ²⁶Mg* enrichments and deficits (calculated in permil as the ²⁶Mg deviation from the instrumental mass fractionation line) relative to the Earth. Most average δ²⁶Mg* (noted δ²⁶Mg*_av) values (between 10 and 20 analyses per chondrule) are negative but the total range is from ?0.029 (± 0.010) ‰ (2 sigma errors) to + 0.011 (± 0.011) ‰ with an exception of one olivine at + 0.043 (± 0.023) ‰. These variations in δ²⁶Mg*_av reflect the formation of the olivines from reservoirs enriched in various amounts of ²⁶Mg by the decay of short-lived ²⁶Al (T_1/2 = 0.73 Ma). Similarly, 30 analyses of olivines from the Eagle Station pallasite show a δ²⁶Mg*_av value of ?0.033 ± 0.008‰, as negative as some olivines from Allende chondrules and the Solar system initial δ²⁶Mg* value of ?0.038 ± 0.004‰ (defined at the time of formation of type B Ca–Al-rich inclusions – CAIs – when ²⁶Al/²⁷Al = 5.23 × 10^?5, Jacobsen et al., 2008).Because olivines are Al-poor and because their Mg isotopic compositions are not reset during the chondrule forming events, their δ²⁶Mg*_av can be used to calculate model crystallization ages relative to various theoretical Mg isotope growth curves. The two end-member scenarios considered are (i) a “nebular” growth in which the Al/Mg ratio remains chondritic and (ii) a “planetary” growth in which a significant increase of the Al/Mg ratio can be due to, for instance, olivine magmatic fractionation. The low δ²⁶Mg*_av value of olivines from the Eagle Station pallasite demonstrate that metal-silicate differentiation occurred as early as ~ 0. 15_- 0. 23^+ 0. 29 Ma after CAIs in either of the growth scenarios. Similarly the variable δ²⁶Mg*_av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the Eagle Station parent body. Accretion of these planetesimals must have been coeval to the formation of CAIs and their disruption could explain why their fragments (Mg-rich olivines) were distributed in the chondrule forming regions of the disk.     

Ion microprobe studies of water in silicate melts. Concentration-dependent water diffusion in obsidian

The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H₂O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H₂O standards and unknowns.A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 barsP_t(H₂O) [1]. Transient diffusion into a semi-infinite medium is described by the equation:?(φ/2)?¸/?φ=?(D_w?¸/?φ)/?φ #x003B8;=1, φ=0, θ→ 0, θ→∞, wherex =distance from the cylinder edge,t =time,C₀ =initial concentration,C_s =concentration at the edge,C =concentration at x,θ = C ? C₀/C_s ? C₀,φ = x/t^1/2, andD_w =diffusivity of water. An iterative technique has been used to calculate solutions to the diffusion equation as a function ofD_w [2]. Comparison of these solutions with the ion probe data indicate that, for0.2wt.% ≤ C ≤ 3.7wt.%H₂O,D_w can be described by an exponential function of θ, of the formD_w = D₀exp(bθ), withD₀ (i.e.,D_w at 0.2%) = (0.8?2.2) × 10^?8 cm²/s and2 ≤ b ≤ 4.     

Petrology of clinopyroxene-amphibole inclusions from the roque nublo volcanics,gran canaria,canary islands

Inclusions consisting of clinopyroxene, amphibole, Fe-Ti oxides and apatitc are abundant in the Roque Nublo volcanics, a unit of Late Tertiary age that is widespread on Gran Canaria Island. The unit includes alkalic basalts and breccias. Mafic minerals in several inclusions and in one basalt host have been analysed with the electron microprobe. Although the inclusions vary in size, texture and mineralogy, they show certain common teatures. The pyroxenes analyzed are all salites-augites and their position in the Ca-Fe Mg quadrilateral suggests that they are early formed representatives of the pyroxene crystallization trend characteristic of alkaliolivine basalt. The amphibole is invariably kaersutite. A common variety of inclusion is composed largely of kaersutite and titaniferous clinopyroxene. The kaersutite (TiO₂ 5.27%, K₂O 1.58%) is homogeneous, except for slight iron enrichment in the margins of crystals. The clinopyroxene is an hourglass-zoned, brownish titansalite, Ca 50 Mg 35 Fe 15, TiO₂ 3.08%, with a green core of Ca 49 Mg 38 Fe 13, TiO₂ 2.15%. Compositions of coexisting titanilerous magnetite and ilmenite, Usp 44 Mt 56 and Ilm 85 Hem 15, respectively, indicate they formed at approximately 975°C and pO, 10^?10.5 atm. In another type of inclusion and its host basalt, pyroxene relations are more complex. Inclusion pyroxene is markedly but diffusely zoned. Predominant is a green salite, Ca 47 Mg 38 Fe 15, TiO₂ 1.11%, which has small, patchy core zones of brownish. Ti-rich salite. Ca 48 Mg 35 Fe 17, TiO₂ 1.94%. Cores of crystals in the host basalt are Ca 47 Mg 41 Fe 12, TiO₂ 2.23%; rims are pale green, Cr-rich diopsidic augite, Ca 44 Mg 45 Fe 11, TiO₂ 1.32%, Cr₂O₃ 0.48%. This «reverse» Fe-Mg zoning is attributed to increasing partial pressure of oxygen as crystallization proceeded. Kaersutite similar to that mentioned above occurs in both the inclusion and its host, in which it is highly resorbed. The available field and analytical evidence strongly suggests that the inclusions and the associated basalts are genetically related. Resorption of the kaersutite at depth may have given rise to the alkalic basalts of the Roque Nublo series.     

Mineralogy of Antarctic ureilites and a working hypothesis for their origin and evolution

Mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for Antarctic ureilites Y74123, Y74130, Y74659, Y790981, ALHA77257, ALHA78019 and ALH82106 extend the chemical and textural variations among ureilites beyond those known for non-Antarctic ureilites. Low-Ca pyroxenes, possibly orthopyroxene, poikilitically enclosed in olivine and coexisting with Mg-rich pigeonites, have been identified in Y74659 and ALHA77257. Coexisting low-Ca pyroxene, pigeonite, and augite, together with the very Fe-rich olivine in ureilites were observed in Y74130. Almost Fe-free pigeonite and diopside coexist in ALH82106. No systematic differentiation trend has been reconstructed from these data, but the pyroxene pairs yield formation temperatures above 1200°C. A weak anticorrelation between MnO and FeO delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations. A working hypothesis for ureilite formation involves high-temperature episoded and a planetesimal-scale collision, which might have facilitated compaction and recrystallization of mafic silicates. During the episodes, FeNiS eutectic melt and a partial melt rich in Ca, Al and Fe might have been lost from a carbonaceous-chondrite-like source material.     

Author index volume 40

Tracer diffusion coefficients for Li in glasses of albite, orthoclase, and obsidian composition have been determined by a method involving deposition of a thin source on polished glass wafers, anneal under controlled temperature conditions (300–900°C), and ion-microprobe determination of the concentration profile. All results conform to an Arrhenius-type relationship,D = D₀exp(?Q/RT), whereQ is 23, 17, and 22 kcal mol^?1;D₀ is 0.2, 0.003, and 0.03 cm²s^?1 for albite and orthoclase glasses, and obsidian respectively. Lithium is thus a fast diffusing ion and behaves similarly to sodium in the same glasses. A mechanism involving jumps of the diffusing ions through oxygen hexagonal rings is suggested by consideration of ionic radii ratio of alkali (H, Li, Na, K, Rb, and Cs) ions to the oxygen anions.     

A combined chemical-petrological study of separated chondrules from the Richardton meteorite

Individual chondrules have been separated from the H5 chondrite Richardton and subjected to a detailed chemical-petrological study. A portion of each chondrule has been examined petrographically and phase chemistry determined by electron microprobe analysis. Of the remaining portion an aliquot was taken for measurement of major element abundances by microprobe using a microfusion technique. Rb, Sr,⁸⁷Sr/⁸⁶Sr and REE were determined by mass spectrometric isotope dilution.The chondrules define a Rb-Sr isochron age of 4.39 ± 0.03Ga(λ = 1.42 × 10^?11 a^?1) and an initial ratio of 0.7003 ± 7. The age is interpreted as a metamorphic age and indicates that Sr isotope equilibration occurred in the Richardton parent body for some 100 Ma or more after condensation of the solar system. Metamorphism had little effect on chondrule textures but effected Fe/Mg exchange to produce highly uniform olivine and pyroxene compositions, and may have caused some redistribution of REE.The major element compositions of Richardton chondrules are mostly constant and close to reported averages for Tieschitz, Bishunpur and Chainpur. They contain high-temperature condensate elements in close to cosmic proportions, but are deficient in Fe. Theories of chondrule origin are briefly reviewed, and while it is difficult to distinguish between direct condensation and dust fusion by impacting, it is postulated that iron was fractionated from silicate prior to or during chondrule formation.     

Grain boundary migration in olivine at atmospheric pressure

Annealing experiments in order to study grain boundary migration (GBM) were carried out at temperatures of 1513–1773 K from 10 min to 100 hours at atmospheric pressure. Grain growth due to GBM is observed in the formation of margins of neoblastic grains which display very different structures of dislocations from that of consumed porphyroclastic and initial neoblastic grains.The velocity of GBM obtained here is approximated to be $\text{c=}\text{gh}$ in which k is 1.15×10^?9 cm³ s^?1, Q is the activation energy for GBM in olivine at 210±20 kJ mol^?1 and ρ is the dislocation density of consumed olivine (cm^?2.Inasmuch as GBM in static annealing reduces stored strain energy in olivines, it is one of the softening processes counteracting work-hardening by dislocation multiplication and tangling as well as dislocation annihilation. GBM-softening is dominant in low temperature annealing but in high temperatures dislocation recovery predominantly takes place.