Trace Element Diffusion Research Articles

Trace element diffusion in rhyolitic melts: comparison between synchrotron radiation X-ray fluorescence microanalysis (-SRXRF) and secondary ion mass spectrometry (SIMS)

Two microbeam techniques, synchrotron radiation X-ray fluorescence micro-analysis (μ-SRXRF) and secondary ion mass spectrometry (SIMS) are compared for analyzing diffusion profiles of trace elements in two hydrous rhyolitic glasses (1.87 and 5.00wt% H 2 O). In order to verify the results, laser ablation coupled to inductively coupled plasma optical emission (LA-ICP-OES) has been used on one sample. Samples were produced by diffusion couple experiments performed in an internally heated gas pressure vessel at 1200 °C and 500MPa. One half of each couple was doped with 24 trace elements representing different geochemical groups: low field strength elements (Rb, Sr, Ba), transition metals (Cr, Co, Ni, Cu, Zn), rare earth elements (La, Ce, Nd, Sm, Eu, Gd, Er, Yb) + Y, high field strength elements (V, Zr, Nb, Hf, Ta) and main group elements (Ge, Sn). Several profiles were measured with both μ-SRXRF and SIMS on both samples. In principle, concentrations of all elements can be extracted simultaneously from a single SRXRF spectrum. However, some trace elements could not be reliably quantified with our analytical system: Ta and Pb (used for detector collimator material), Ti, V (low energy of K α ), Co (K α -peak overlapping with Fe K β -peak) and Cr, Ni, Cu, Zn (overlapping with 1-lines of REEs). In contrast, SIMS analyses measure each element sequentially. Hence, not all elements of the large total set of trace elements could be analyzed in a single run. Some elements requiring a high mass resolution (NaSi interfering with V, CaO interfering with Ni) or having low yields (Sn) were not profiled. Multiple diffusivities derived from μ-SRXRF and SIMS profiles are in very good agreement for most elements. In general, the trace element diffusivity decreases with increasing valence state, e.g. in sample D22 containing 1.87wt% H 2 O from log D=-10.80 for the monovalent Rb to log D=-13.34 for the tetravalent Zr (Din m 2 /s). By increasing the water content in sample D18 to 5.00wt%, diffusion coefficients increase approximately by one order of magnitude for all elements studied.

Application of laser ablation inductively coupled plasma (dynamic reaction cell) mass spectrometry for depth profiling analysis of high-tech industrial materials

A 193 nm ArF excimer-based laser ablation system coupled to an inductively coupled plasma mass spectrometer equipped with a dynamic reaction cell (LA-ICP-DRC-MS) was successfully used for the depth profiling analysis of several (multi-layered) industrial materials characterised by markedly different ablation behaviours and sample composition. To obtain the optimum depth resolution for each of the samples, crater geometry and parameters such as laser pulse energy were studied as a function of the sample composition and layer thickness. Layers with a thickness of 200 μm could be studied. Combining LA-ICP-MS data with results obtained by scanning electron microscopy (SEM) and profilometry allowed useful information about the crater geometry and the ablation depth per pulse to be obtained. In this way, it was possible to convert the original intensity-versus-time into intensity-versus-depth profiles and to evaluate the figures of merit of LA-ICP-MS for depth profiling studies. To illustrate the possibilities of the technique under investigation, depth profiling analysis of a thermographic material, a CsBr phosphor screen and thermal printing plates was carried out. In all cases, the spatial resolution of the laser ablation system was sufficient to differentiate between the separate layers (even for very thin layers of ±1 μm), which makes the technique suitable for studying trace element distributions and diffusion effects in several high-tech materials. Spectral interferences hampering the accurate determination of S and K in the printing plate were overcome by using chemical resolution in the DRC. The use of O2 as a reaction gas permitted the simultaneous determination of all elements of interest (S as SO+, K as K+ and Zr as ZrO+).

Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies

Large 226Ra excesses in a number of mafic arc magmas, and geophysical observations of earthquake hypocenters locations, indicate that mafic melts can get transferred from source to surface within days to ∼1 kyr. Decreasing 226Ra– 230Th disequilibria with increasing SiO 2 in co-magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems, or less in open systems. However, the rapid decrease of global 238U– 230Th disequilibria with increasing SiO 2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238U excesses. In the case of rapid magmatic evolution, open system processes are frequently involved in the production of andesites and dacites, and the data imply that the open system component is close to 238U– 230Th secular equilibrium. MELTS modeling of magma evolution at Santorini in the Aegean arc provides corroborating evidence from an individual well-studied arc volcano, that closed system fractional crystallization fails to produce the more evolved compositions, and increasing 87Sr/ 86Sr ratios with increasing SiO 2 from andesites to rhyolites suggests assimilation of old crust is involved. These observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust. Melt production in deep crustal hot zones provides an explanation for low 238U– 230Th disequilibria of the evolved magmatic component, as significant accumulation of evolved melts through partial re-melting of previously intruded basalts requires >100 kyr incubation time at typical magmatic flux rates. Thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems. Further insights into magmatic evolution can be gained from dating of minerals from mafic, intermediate and felsic arc magmas. (a) Many U–Th mineral isochrons in mafic arc magmas yield ages significantly older than those obtained from Ra–Th mineral isochrons from the same samples. A crystal size distribution case study from Soufrière, St. Vincent, indicates that this may result from the abundance of cumulate xenocrysts within more mafic arc magmas. (b) The majority of U–Th mineral isochrons in intermediate arc magmas yield ages within error of eruption age, indicating that other dating techniques that may be more suitable for deciphering the shorter timescales yielded by these compositions. In a case study of crystals of remobilized andesites from the Soufrière Hills volcano, Montserrat, intra-crystalline trace element diffusion modeling yields crystallization timescales of the order of 10–1000 years. Varying temperature-time histories of individual crystals can be interpreted to reflect multi-stage magma ascent and the development of ephemeral subvolcanic magma reservoirs. (c) U–Th mineral isochrons from felsic arc magmas yield very variable ages from >200 ka to shortly prior to eruption. A case study from the Taupo Volcanic Zone has shown that both remobilization of previous intrusives and melting of old country rock are occurring, giving rise to a this range in ages. Both intermediate and felsic magmas therefore can be shown to have multi-stage ascent histories. Future work will have to address the reasons for the apparent inter-volcano variability in time-averaged subvolcanic magma volumes within the arc crust. These include differences in magma supply rate; thickness, composition and thermal structure of the crust; and magma composition and volatile content, which may vary between different tectonic settings.

Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (ΔAn ≥ 10%) followed by gradual decrease in An-content (ΔAn ≤ 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.

Trace element and C–O–Sr–Nd isotope evidence for subduction-related carbonate–silicate melts in mantle xenoliths (Pannonian Basin, Hungary)

Carbonate globules have been found in amphibole-bearing glass veins of peridotite xenoliths from Szentbékálla (Balaton Highland, Central Hungary). Strong resorption of amphibole suggests multiple fluid-melt infiltration. Textural features of carbonate globules and the surrounding silicate glass indicate a magmatic melt origin for the carbonate and exclude the possibility of significant alteration. The application of the cpx structural barometer for clinopyroxenes formed in the silicate glass indicates high-pressure vein formation. Laser-ablation ICP-MS analyses revealed trace element zoning within the carbonate. Uniform profiles observed for various elements within the carbonate indicate that the zoning is related to non-diffusive mixing during carbonate–silicate interaction rather than to element diffusion. The trace element composition of the centers of the globules (e.g. low Ce/Pb and Nd/Pb ratios) suggests a crustal origin for the carbonate melt. The C, O, and Sr isotope compositions of the calcite globules ( δ 13C about −14‰; δ 18O about 22‰; 87Sr/ 86Sr=0.70533) also argue for a crustal origin of the carbonate melt. Trace element and C–O–Sr isotopic compositions of calcite globules suggest that the carbonate melt was derived from subducted material. Injection of the carbonate melt into the peridotite triggered amphibole melting. The carbonate and silicate melts intermingled, but their interaction was confined to the margins of carbonate globules. Preservation of trace element zoning and element diffusion calculations indicate that the vein-hosting peridotite was rapidly brought to the surface after the carbonate melt injection.

Trace element diffusion and element partitioning between garnet and andesite melt using synchrotron X-ray fluorescence microanalysis (-SRXRF)

Synchrotron radiation X-ray fluorescence microanalysis (μ-SRXRF) was applied to products of experimental geochemistry to determine (1) trace element diffusivities in andesite melts and (2) trace element partitioning behaviour between garnet and melt. To achieve sufficient spatial resolution, non-focusing and focusing glass capillaries reduced the incoming synchrotron beam down to sizes of 20 and 2.7 μm, respectively. (1) Diffusion couples of trace element-doped and undoped andesite melts were prepared in internally heated pressure vessels. A special sample setup allowed the pencil-shaped synchrotron beam to irradiate volume elements showing identical diffusion behaviour. Eighteen trace elements were measured simultaneously and quickly, resulting in diffusion profiles well suited for evaluating diffusion coefficients. (2) Garnet and andesitic melt were synthesized and equilibrated in a piston cylinder apparatus. The garnets were exceptionally large due to specially designed dehydration-melting experiments with monazite as a trace element source. Coexisting garnets and melt were analyzed with μ-SRXRF, and new distribution coefficients for Sr (0.126), Y (5.27), Zr (0.533), La (0.014), Ce (0.020), Nd (0.245), Sm (1.21), Eu (1.18), Gd (5.29), Yb (52.5), and Lu (76) were determined. However, the general use of μ-SRXRF for experimental partitioning studies is limited due to the relatively poor spatial resolution caused by the penetrating character of the synchrotron beam and due to limited count rates at high energies.

Diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase

Residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals. We present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects. In particular, we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements (e.g., Mg), the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension. We show that contrary to intuition, diffusive fluxes of trace elements may be coupled to major element concentration gradients, and ignoring such coupling can lead to incorrect estimates of timescales. We illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a Holocene dacitic lava flow of Volcán San Pedro (Chilean Andes, 36°S). The inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 (30 to 148) yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods.

Potential of cathodoluminescence (CL) microscopy and spectroscopy for the analysis of minerals and materials.

The present study summarizes results of cathodoluminescence (CL) microscopy and spectroscopy applied to minerals and materials. CL can be used both in a purely descriptive way to detect and distinguish different minerals or mineral generations by their variable CL colours or as an effective method for spatially resolved analysis of point defects in solids by spectral CL measurements. The cathodoluminescence emission is in all cases either related to lattice defects (e.g. electron defects on broken bonds, vacancies or radiation induced defects) and/or to trace activator ions such as REE(2+/3+), Fe(3+), Cr(3+), Al(3+), Mn(2+), Pb(2+), Cu(2+), Sn(2+) or uranyl groups. CL spectroscopy is an outstanding method to characterize the degree of purity of materials or to detect trace elements in natural and synthetic minerals. In this way, alterations, diffusion of trace elements or formation of new phases are successfully detectable even in the case of materials with heterogeneous texture and high contents of non-crystalline phases.

Consequences of diffusive reequilibration for the interpretation of melt inclusions

Trace element abundances in melt inclusions are commonly used to interpret melting and melt extraction processes. These interpretations, however, often assume that the chemical compositions of melt inclusions are identical to the liquid from which the host crystal grew, even though driving forces for postentrapment diffusion and modification are demonstrable. This paper begins to quantify the effects of diffusion on melt inclusions using a numerical model. The model calculates the compositional evolution of a spherical inclusion which initially is in equilibrium with a crystal host out to some distance rjump and out of equilibrium beyond. In particular we consider the end‐member scenario, whereby the trapped melt is initially out of equlibrium with the neighboring crystal as this sets the minimum time for reequilibration. A package of numerical codes is provided that allows the user to explore other initial conditions. The model calculates the change in inclusion composition and also the structure of diffusion halos that grow around the inclusion as it reequilibrates with the surrounding crystal. The detection of these profiles in naturally occurring inclusions may allow the time since entrapment and the initial inclusion concentration to be estimated. The extent of reequilibration is most strongly influenced by the partition coefficient, diffusivity, and the inclusion radius. Fast‐diffusing elements with high mineral/melt partition coefficients are modified rapidly, particularly in small inclusions. Because minerals have very different Dmineral/melt for the various elements, the effects of diffusive reequilibration differ substantially from one mineral to another. For example, the higher partition coefficients of the heavy rare earth elements (HREE) in olivine make HREE concentrations easier to modify than light rare earth elements (LREE) concentrations. In contrast, Sr, Eu, and Ba in plagioclase hosted inclusions equilibrate more rapidly than the other trace elements. Examination of published trace element concentrations of olivine hosted inclusions show little evidence for reequilibration, at least for the light REE and other highly incompatible elements. It is difficult, however, to provide firm constraints due to the uncertainties in olivine diffusivities and the initial condition. In contrast, trace element diffusivities in plagioclase have been determined experimentally [Cherniak, 2001], and the trace element concentrations of published plagioclase hosted inclusions show evidence for extensive diffusive exchange with the host in a manner consistent with model predictions. Postentrapment modification therefore is likely an important factor in the interpretation of some melt inclusion data.

Apatite, Monazite, and Xenotime in Metamorphic Rocks

This chapter focuses on phosphates that are significant in metamorphic rocks. A quick survey of phosphate mineral descriptions at a commercial mineral web-site revealed over 500 phosphate mineral names. Remarkably, only three are common in metamorphic rocks: apatite, monazite, and xenotime, and this chapter is restricted to discussion of these minerals. Apatite, monazite, and, to a lesser extent, xenotime, have enjoyed intensive study during the previous half-century. Following World War II, considerable study was made of minerals that contain fissionable materials, and the sometimes large concentrations of U and Th in monazite culminated in a number of seminal papers on the occurrence of that mineral (e.g., Overstreet 1967). During the 1970s and early 1980s, the study of apatite and monazite turned towards their role as a sink for REEs and other trace elements in rocks (e.g. Watson 1980, Watson and Capobianco 1981). Throughout this period, monazite (and to some extent xenotime) enjoyed attention as a geochronometer (e.g., Parrish 1990), and through today this attention has continued to grow. Accessory minerals such as monazite, xenotime, and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons. First, it has recently been recognized that trace elements in metamorphic rocks, and especially trace element zoning in garnet and other major phases, contain considerable detailed information about the reaction history a rock has experienced. Coupled with the fact that diffusion of many trace elements in metamorphic minerals is, in general, considerably slower than diffusion of major elements in those same minerals, the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable. Inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements, attention has naturally turned to their paragenesis. It has also recently been discovered that …

Trace element diffusion in andesitic melts: an application of synchrotron X-ray fluorescence analysis

—We have investigated the diffusivity of trace elements in hydrous iron-free andesitic melts containing 4.5 to 5.2 wt.% water at a pressure of 500 MPa and at temperatures between 1100 and 1400°C using the diffusion couple technique. The studied elements can be combined in several groups of particular geochemical interest: low field strength elements (LFSE: Rb, Sr, Ba), transition elements (Cr, Fe, Ni, Zn), rare earth elements (REE: La, Nd, Sm, Eu, Gd, Er, Yb, Y), and high field strength elements (HFSE: Zr, Nb, Hf). The diffusion profiles of the trace elements were measured using the synchrotron X-ray fluorescence (SYXRF) microprobe. H 2O distribution in the samples was analyzed by IR microspectroscopy. Diffusion profiles are excellently reproduced, assuming concentration-independent diffusion coefficients. For all trace elements, the temperature dependence of diffusion in the hydrous melt can be described by a simple Arrhenius law. In general, the diffusivity decreases from the LFSE, to transition elements, to REE, and to the HFSE, a trend that can be correlated to the increase of charge in the same order. The activation energy shows a similar trend, increasing from 129 kJ/mol for Rb to 189 kJ/mol for Zr. For the transition elements Cr and Fe, the activation energy is relatively high (228 and 193 kJ/mol, respectively), which can be explained by increasing contributions of divalent cations to the diffusion flux with increasing temperature. Higher diffusivity of Eu compared to its neighbor elements also is attributed to contributions of divalent cations. Modeling Eu-diffusivity using data of Sr as representative for Eu 2+, and of Sm and Gd as representative for Eu 3+, shows that at all temperatures Eu 3+ is clearly dominating in the hydrous melt. To quantify the effect of water, an additional experiment was performed at 1400°C using a nominally anhydrous melt. The obtained diffusion coefficients are (for most of the elements) by one and a half orders of magnitude lower than for a melt containing 4.5 wt.% H 2O. Chemical diffusion coefficients D η, which were calculated from the viscosity data of Richet et al. (1996) using the Eyring equation, and which assumed a jump distance of 3 Å, are in excellent agreement with the diffusivity of the HFSE for both dry and hydrous melt. Most of the investigated elements show a linear relation between log diffusivity and log viscosity, enabling the prediction of diffusivities in hydrous andesite systems at various conditions. Provided viscosity data are available, we suggest that this relation can be a useful tool to estimate trace element diffusivities for silicate melts with different compositions. The new diffusion data show that water strongly enhances diffusivity of trace elements in andesitic melts. After 10,000 yr at 1200°C, diffusion produces in the dry melt relatively short profiles with lengths (defined as x=( Dt) 1/2) between 0.8 and 0.07 m (for Sr and Zr, respectively), whereas in hydrous melts (5 wt.% water), profiles are much longer with lengths between 3.9 and 0.92 m (for Sr and Zr, respectively).

Plagioclase residence times at two island arc volcanoes (Kameni Islands, Santorini, and Soufriere, St. Vincent) determined by Sr diffusion systematics

The diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers. Initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase-melt partition coefficient. Trace element diffusion will subsequently act to modify this initial profile and – given enough time – produce a profile that is in equilibrium with the anorthite variations within the crystal. We argue that the trace element partition coefficient Da/b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal-liquid partition coefficients, and that the equilibrium profile of the crystal can be calculated. The time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase. Strontium plagioclase-melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions. A one-dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir. The model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the Kameni Island dacites, Santorini, and from the 1979 Soufriere andesite, St. Vincent. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase. Maximum crystal residence times range from 100 to 450 years. This is in good agreement with estimates from crystal size distribution and from Ra-Th disequilibrium studies for the Kameni Islands. For Soufriere, however, such short residence times are incompatible with U-Th mineral errorchron data that suggest residence times of >40 ka in a thermally buffered magma reservoir. To reconcile these apparently different ages, we invoke a more complicated magmatic history for Soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption.

Complex zoning in apatite from the Idaho Batholith; a record of magma mixing and intracrystalline trace element diffusion

Apatite crystals in a sample of biotite granodiorite from the Idaho batholith display complex chemical zoning, characterized by abrupt changes in REE+Y+Si content, more subtle variations in S, Na, La/Yb, and Mn contents, and petrographic evidence for multiple episodes of partial resorption. The zoning is attributed to changes in melt composition, resulting from magma mixing and differentiation, although the possibility that some rounded cores may be inherited cannot be disproved. High S contents in apatite cores and the presence of an included anhydrite grain indicate crystallization from an oxidized host magma. Divalent cations that occupy the apatite Ca sites (Sr, Mn, Fe) show evidence of having been redistributed between zones by intracrystalline diffusion, whereas cations that participate in coupled substitutions involving the tetrahedral site (Si, Y, REE, Na, S) were not readily redistributed. The main REE substitution in this case is REE3+ + Si4+ ↔ Ca2+ + P5+, and REE diffusion is rate-limited by slow Si diffusion. However, exchange of LREE (e.g., LaSm-1) on the Ca sites does not involve Si and proceeds more rapidly, resulting in homogenization of La/Sm between zones within individual crystals. Relative diffusion rates inferred from zoning profiles in this study are: Mn, Sr, Fe, and LaSm-1 are faster than Na, S, and LaYb-1, which are faster than Si. These data imply that REE patterns can be decoupled from REE abundances during diffusion, and that even apatite zones or cores that appear sharply bounded in backscattered electron images may not retain their original chemical or Sr-Nd isotopic traits.

Measuring the diffusion and sorption of the trace elements and radionuclides in soil, clay and bentonite

Diffusion is an important migration mechanism of trace elements and radionuclides in the biosphere, soils, clays, river sediments. Naturally, in the last few years there has increased also the relevance to diffusion of trace elements and radionuclides and methods of its determination in bentonite. Bentonite is proposed as sealing and backfill material in the near-field of deep repositories of spent nuclear fuel and of radioactive wastes. This paper presents capillary modification of measuring diffusivity, some of its results in porous natural materials and barrier materials of repositories for wastes.

Trace element diffusion in jadeite and diopside melts at high pressures and its geochemical implication

Diffusivities of geochemically important trace elements (eleven rare earth elements (REE), Rb, Sr, Ba, and Y) in jadeite and diopside melts and those of Zr, Nb, Th, and U in jadeite melt have been measured at pressures between 7.5 and 20 kbar and at temperatures 50–200° above the liquidus, using diffusion couples and ion microprobe analysis. The concentrations of these elements in the experimental charges are close to those in natural igneous rocks. In the jadeite melt which is nearly fully polymerized (NBO/T ∼ 0), (1) diffusivities of REE increase with increasing ionic radii, (2) diffusivities of tri- and tetravalent elements increase with increasing pressure at constant temperature, whereas those of mono- and divalent elements do not change significantly with pressure, and (3) diffusivities of these elements decrease with increasing their ionic charge at constant pressure and temperature. In the diopside melt which is considerably less polymerized (NBO/T ∼ 2), (1) diffusivities of these trace elements depend mainly on their ionic radii rather than ionic charges; the diffusivities of mono- and divalent ions decrease with increasing ionic radii at constant pressure and temperature, and (2) diffusivities of REE are nearly the same as that of Ca and decrease with increasing pressure at constant temperature. The behavior of these trace elements is correlated with that of major elements; in the jadeite melt, tri- and tetravalent elements behave similarly to network-forming cations Al and Si, whereas mono- and divalent elements behave as network-modifying cations similarly to Na. In the diopside melt, the diffusion behavior of all these trace elements is similar to that of network-modifying cations Ca and Mg. The results of the present experiments suggest that the abundance of some trace elements in igneous rocks may have been affected by diffusion process at the magmatic stage. In the case of REE, for example, if two different magmas with high and low REE concentrations become in contact one another by multiple intrusion, and a zoned magma chamber is formed, diffusion begins to take place between them, and near the interface, the REE-enriched magma will become more light REE-depleted, whereas REE-depleted magma will become more light REE-enriched, and in addition, if magmas are reduced, the former will show a negative Eu anomaly, whereas the latter will show a positive Eu anomaly.

Re-Os Systematics in Meteorites

Extensive results have been obtained on the Re-Os system in iron meteorites, stony irons, and in chondrites. This has required the development of reliable techniques, with the single-most critical analytical consideration being the complete isotope equilibrium of tracer and sample. The work on iron meteorites has concentrated on whole rock measurements. Whole rock samples for irons of groups lAB, IIA, liD, IliA, IVA and IVB have yielded a well defined isochron with deviations from the isochron for any single sample at the level of 2-3%o. The data yield a slope of 0.07863 __ 0.0003, corresponding to T = 4.615+_0.018 AE [L(lS7Re) = 1.64 x 10 -11 a -1] and init ial (187Os/lS8Os)o = 0.09560_+ 0.0002. There is a suggestion that group IVA irons are slightly older. Since the uncertainty in the aSTRe decay constant is ~3%, the isochron age can be renormalized to T/4.55 AE, corresponding to 9~(187Re) = 1.664 x 10-11 al, in order to obtain agreement with U-Pb, Sm-Nd, and Rb-Sr ages on silicates. The welldefined whole rock isochron depends on substantial observed variations in Re/Os, which reflect chemical fractionation during initial fractional crystallization of FeNi masses, including as pods and as cores of early formed planetesimals. For iron meteorites, 'whole rock' samples measured are typically cubes of metal, 0.25 to lg, several mm on the side. Given that the crystal size for metal phases in iron meteorites is similar, the close adherence of the data to a single isochron requires very limited fractionation of Re-Os after initial FeNi crystallization and during the later exsolution of the low-Ni phase (kamacite) from the initial taenite, at lower temperatures. Furthermore, since the whole rock samples do not show a hint of a Re-Os secondary isochron at times younger than the age of the primary isochron, then any melting of the FeNi masses at younger times must have also been characterized by limited Re/Os fractionation and thus very limited further fractional crystallization. We have also determined Re-Os in FeS and in massive schreibersites in irons. Apparent, single-stage distribution coefficients are D(FeNi/Sulphide) = (2-4) x 102 for Re and 1.5 x 103 for Os. Similarly, D(FeNi/ Schreibersite) = 7-14 for Re and 30-54 for Os. The distribution coefficients for sulphide are sufficiently large that the segregation of immiscible sulphide would not affect the Re-Os systematics in the FeNi whole rocks. Similarly, based also on the abundance of P and, consequently, the limited abundance of schreibersite, the formation of the schreibersite would also be of minor importance for the whole-rock Re and Os mass balance. Measurements of PGE adjacent to massive schreibersites show gradients in Re and in Os abundances (up to 25%) and in Re/Os (up to 10%) for distances 1-2 mm away from the schreibersite. The schreibersites themselves show low Re and Os abundances but large enrichments in Re/Os, highly radiogenic 187Os/lSSOs and a range of model ages from 4.5 AE down to 3.5 AE. These data clearly indicate that the minor phases in iron meteorites show an extensive evolution, including slow cooling of the metal and diffusion of minor and trace elements for extended times, after the time of primary crystallization of the FeNi recorded by the whole rock isochron. Given the distribution coefficients and the abundances of sulphides and schreibersites, it is clear that these processes are not reflected in the Re-Os systematics of whole rock iron meteorite samples. The Re-Os narrow time evolution for irons is consistent with the systematics of the short lived chronometers Pd-Ag and Hf-W. We have determined Re-Os systematics also in pallasites. The data for whole-rock FeNi samples show a wide range in Re/Os and permit the determination of a metal whole rock isochron which is consistent with the measurements on iron meteorites. The data include measurements on Eagle Station, a meteorite with silicates preserving a unique oxygen isotope composition. While pallasites have been viewed as originating in core-mantle boundaries, in differentiated planetesimals, the Re-Os data on the FeNi phases are consistent with early formation and fast cooling by fractional crystallization, within 3%0 in age, or 14 Ma. The data are not consistent with slow cooling of the initial molten FeNi. Observed, slow metallographic cooling rates

Chemical characteristics of migmatites: accessory phase distribution and evidence for fast melt segregation rates

Equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration (armouring, slow dissolution). When segregation occurs by channelised migration along high-porosity pathways, melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates. Evidence for channelised flow and deformation-enhanced melt segregation into boudin necks, fractures and micro-shears at low melt fractions is present in the Moine Kirtomy Migmatitie Suite (KMS) in Sutherland, Scotland. Melt migration distances are on a metre to tens of metres scale. Concordant leucosomes in stromatic migmatities in the KMS have low Zr contents, low LREE (light rare-earth element) and H (heavy) REE contents and positive Eu anomalies. REE patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of LREE- and HREE-bearing accessory phases in water-undersaturated melts. Melting in the KMS, however, occurred at or near the wet granite solidus, leaving biotite as a residual phase. Detailed back-scattered electron imaging shows that REE-bearing accessory phases remained as residual phases, and were concentrated in the melanosome and at the melanosome-leucosome boundary. Irregularly shaped patches of diatexite contain a small proportion of excess Zr, consistent with entrainment of melanosome-schlieren enriched in zircon. These data indicate that deformation-enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time-scale less than that required to saturate the melt in Zr. Leucosomes were thus prevented from equilibrating with accessory phases before extraction.

Eclogitic inclusions in diamonds: Evidence of complex mantle processes over time

The first ion-probe trace element analyses of clinopyroxene-garnet pairs both included within diamonds and from the eclogite host xenoliths are reported; these diamondiferous eclogites are from the Udachnaya and Mir kimberlite pipes, Yakutia, Russia. The major and trace element analyses of these diamond-inclusion and host-rock pairs are compared in order to determine the relative ages of the diamonds, confirm or deny genetic relationships between the diamonds and the eclogites, evaluate models of eclogite petrogenesis, and model igneous processes in the mantle before, during, and after diamond formation. The most striking aspect of the chemical compositions of the diamond inclusions is the diversity of relationships with their eclogite hosts. No single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples. Garnet and clinopyroxene inclusions in the diamonds from two samples (U-65/3 and U-66/3) have lower Mg#s, lower Mg, and higher Fe contents, and lower LREE than those in the host eclogite. We interpret such variations as due to metasomatism of the host eclogite after diamond formation. One sample, U-41/3 shows enrichment in diamond-inclusion MREE enrichment relative to the eclogite host and may indicate a metasomatic event prior to, or during, diamond formation. Bulanova [2] found striking differences between inclusions taken from within different portions of the very same diamond. Clinopyroxene inclusions taken from the central (early) portions of Yakutian diamonds were lower in Mg# and Mg contents (by up to 25%) than those later inclusions at the rims of diamonds. These trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and/or P-T conditions in the mantle. Garnet trace element compositions are similar in relative proportions, but variable in abundances, between diamond inclusions and host eclogites. This is probably due to the rapid diffusion of trace elements in garnet under mantle temperatures and consequent alteration of the garnet, and not due to juvenile diamonds ‘locking in’ source heterogeneities (c.f., [3]). Trace element compositions of clinopyroxenes included in diamonds are generally similar to those in the host eclogite. However, one host clinopyroxene does show enrichment in the LREE compared to that in the inclusion and may be attributed to mantle metasomatism, not related to kimberlite transport. In another eclogite, M-46, the host clinopyroxene is depleted in the LREE and Fe, and enriched in the HREE and Mg, relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation. Sm Nd ratios in clinopyroxenes appear to be little affected by these processes for most samples, allowing SmNd isotopic studies to yield important information about ancient protoliths. Eclogitic mineral inclusions in Yakutian diamonds appear consanguineous with the diamonds, a contention supported by the observations of Bulanova [2]. Therefore, ReOs whole-rock and Sm Nd clinopyroxene age determinations of the Udachnaya eclogites also yield the time of diamond formation, approximately 2.9 Ga [32,33].

The use of ion beam techniques to characterize lead diffusion in minerals

The characterization of diffusion of lead and other trace elements in certain mineral phases has great significance in reconstructing the thermal history of the earth's crust. Such studies permit better interpretation of ages'' obtained from isotopic dating. This study uses ion implantation to introduce lead into the minerals apatite and zircon. Diffusion profiles are obtained using Rutherford backscattering (RBS) and the results fit with a model to determine D. Results for apatite over the temperature range 600--900{degree}C show agreement with earlier results obtained traditional geologic techniques. This suggests that radiation damage induced by ion implantation has little effect on diffusion in this case. This approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times. 17 refs., 5 figs.

Element zoning trends in olivine phenocrysts from a supposed primary high-magnesian andesite: an electron- and ion-microprobe study

Electron and ion-probe microanalysis have been used to obtain zoning profiles for major and trace elements in olivine phenocrysts from a high-magnesian andesite from Shodo-Shima island, southwest Japan. This rock was previously thought to represent undifferentiated, primary magma. Some crystals have unzoned cores, while others show cores which are reversely zoned with respect to Mg/ (Mg+Fe), Ni, Mn and Cr. In addition, some Ni profiles show a normally zoned “hump” at the most central portions of the reversely zoned crystals. All crystals show normally zoned rims. The Li concentrations are constant throughout the cores of all crystals studied, but rise sharply, by a factor of up to at least six, in the rims. The Ca and Co concentrations are essentially constant throughout all the crystals. Mechanisms for producing the observed zoning profiles are discussed, and it is concluded that the reverse zoning was produced by the introduction of crystals into a less differentiated magma than that in which they grew. The reversely zoned crystals could therefore represent xenocrysts which were introduced into an undifferentiated magma, or phenocrysts introduced into a more primitive magma by a magma mixing process. The Ni profiles are used to estimate the residence time of these crystals in the more primitive magma. The following trace element partition co-efficients have been estimated for the olivine-groundmass system in this rock: DNi=16; DMn=1.1; DCo=4.2; DCa =0.02; DTi=0.005; DV=0.05; DSc=0.2; DNa=0.0002. Studies of trace element zoning will become increasingly important as the new generation of trace element microprobes become available but a larger database of experimentally determined values for trace element partition coefficients and diffusion coefficients in crystals and magmas, and a better understanding of other disequilibrium processes are required to fully exploit the new data.