REE Diffusion Research Articles

Trace‐element zoning patterns in porphyroblastic garnets in low‐T eclogites: Parameter optimization of the diffusion‐limited REE‐uptake model

AbstractCompositional zoning patterns of the major elements and REEs in prograde‐zoned garnets whose Mg/(Mg + Fe) atomic ratios increase rimward have been widely used to understand the metamorphic P–T–t trajectories, and the diffusion‐limited REE‐uptake model is a promising way to interpret their growth rates and the REE diffusion kinetics in the low‐temperature eclogite. In order to elucidate their growth kinetics with Skora et al.'s (2006) diffusion‐limited REE uptake model for prograde‐zoned garnets, we examine the trace‐element zoning patterns of two prograde‐zoned porphyroblastic garnets (~6 mm in size) in low‐temperature eclogites from two different localities. Core‐to‐rim trace‐element profiles in a garnet (prp5–9alm61–67sps1–3grs24–30) of a glaucophane‐bearing epidote eclogite of Syros (Cyclades, Greece) are characterized by the presence of Y + HREE peaks in the mantle, which might be attributed to a continuous breakdown of the titanite to form rutile during the garnet growth. In contrast, those in a garnet (prp4–7alm61–68sps3–10grs23–24) extracted from a lawsonite‐eclogite of the South Motagua Mélange (SMM) (Guatemala) have prominent central peaks of Y + HREEs. Although the REE profiles of both the garnets can be explained by the diffusion‐limited uptake, their Mn profiles suggest that their growth‐rate laws are different: i.e., diffusion‐controlled (Syros) and interface‐controlled (SMM). Prior to the model application, we optimize the number of the parameters as the garnet grows with the interface‐controlled processes based on the growth Péclet number. In particular, we propose the ratio of the REE diffusivity in the eclogitic matrix to the garnet growth rate as the new parameter. Visualizing the values of the new parameters allows to readily understand the relationship between the REE profiles and the REE‐diffusion/garnet‐growth kinetics in low‐T eclogite. Our model refinement leads to the simple quantitative characterization of core‐to‐rim REE profiles in garnet in low‐temperature eclogites.

Multispecies Diffusion of Yttrium, Rare Earth Elements and Hafnium in Garnet

AbstractWe report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm pressure and oxygen fugacity corresponding to the quartz–fayalite–magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REE is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are logDREE(f)(m2 s−1)=−10·24(±0·21)−221057(±4284)2·303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm pressure, and logDREE(s)(m2 s−1)=−9·28(±0·65)−265200(±38540)+10800(±2600)×P(GPa)2·303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (>∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion–reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by logDHf(m2 s−1)=−8·85(±0·38)−299344(±15136)+12500(±900)×P(GPa)2·303RT(K)−0·52(±0·09)×log⁡10aSiO2. In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu–Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g. garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm–Nd system within garnet, thus increasing the separation between garnet Lu–Hf and Sm–Nd isochron dates, owing to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.

Nd and Sr Isotope Composition in the Tooth Enamel from Fe–Mn Nodules of the Cape Basin (Atlantic Ocean): Age and Sources

Abstract—The Nd and Sr isotope compositions were determined for the first time in biogenic apatite sampled throughout the tooth section (from the base to tip) of predatory fish in the nucleus of Fe–Mn nodules from the Cape Basin. The results showed that diagenetic recrystallization of apatite does not affect the 87Sr/86Sr ratio in the tooth enamel, but leads to the decrease of Sr content. The age of tooth was determined using Sr isotope stratigraphy at 5.2 ± 0.2 Ma for sample 2188/4 and 6.6 ± 0.3 Ma for sample 2188/5. The calculated growth rate of Fe and Mn oxyhydroxide layers varies within 0.4–2.8 mm per 1 Ma. The 143Nd/144Nd ratio in the tooth enamel varies within single station and depends on the local Nd sources in pore water. The value of eNd varies from –5.2 to –6.9 in the enamel of tooth 2188/4 and remains constant at –8.7 ± 0.1 in sample 2188/5. A change of Nd isotope composition in sample 2188/4 likely reflects temporal variations of Nd fraction from bottom and pore waters that penetrated inside the enamel during REE diffusion. The value of eNd in the oxyhydroxide layers of Fe–Mn nodule 2188/4 (from –7.8 to –7.9) is homogenous for the external and internal parts of the tooth. In order to use eNd in apatite enamel and authigenic Fe and Mn oxyhydroxides in sediments for paleoreconstructions of thermohaline water circulation, it is necessary to develop additional criteria for selecting diagenetically unaltered matter.

Sources and timing of pyroxenite formation in the sub-arc mantle: Case study of the Cabo Ortegal Complex, Spain

Pyroxenites exposed in ophiolites and orogenic peridotite massifs may record petrogenetic processes occurring in mantle domains generated and/or transferred in supra-subduction environments. However, the timing of their formation and the geochemical characteristics of their source region commonly are obscured by metamorphic and metasomatic overprints. This is especially critical in arc-related environments, where pyroxenites may be formed during the differentiation of primitive magmas. Our approach combines Sr- and Nd-isotope geochemistry and geochronology, and modelling of REE diffusion, to further constrain the origin of a well-characterized set of pyroxenites from the arc-related Cabo Ortegal Complex, Spain. In the light of petrological constraints, Sr- and Nd-isotope systematics consistently indicate that cpx and amphibole have acquired disequilibrium during two main episodes: (1) a magmatic/metasomatic episode that led to the formation of the pyroxenites, coeval with that of Cabo Ortegal granulites and corresponding to the incipient stage of a potential Cadomian arc (459–762 Ma; isochron and second-stage Nd model ages); (2) an episode of metamorphic amphibolitization upon the percolation of relatively unradiogenic and LREE-enriched hydrous fluids, subsequent to the delamination of the pyroxenites from their arc-root settings during Devonian subduction. Calculations of diffusional timescale for the re-equilibration of REE are consistent with this scenario but provide only poor additional constraints due to the sensitivity of this method to grain size and sub-solidus temperature. We thus emphasize the necessity to combine isochron ages and Nd model ages corrected for radiogenic ingrowth to put time constraints on the formation of subduction- and/or collision-related pyroxenites, along with petrological and geochemical constraints. Homogeneous age-corrected 143Nd/144Nd of 0.5121–0.5125 (εNd between 0 and +7.5) and 87Sr/86Sr of 0.7037–0.7048 provide information on the sources of the metasomatic agents involved (and potentially the parental melts) and notably indicate the contributions from enriched mantle components (EM I and/or II). This suggests the involvement of an old crustal component, which is consistent with the derivation of the pyroxenites and granulites from an ensialic island arc, potentially built on the northern margin of either Gondwana or a pre-Gondwanan continental block. This case study thus documents the role of melt–rock reactions as major pyroxenite-forming processes in the sub-arc mantle, providing further constraints on their sources and timing in the Cabo Ortegal Complex.

Effect of pressure on closure temperature of a trace element in cooling petrological systems

Closure temperature is important to many diffusion-related problems involving cooling. The classic model of Dodson and its modifications for cooling petrological systems are formulated at constant pressure. Many petrologic processes involve changes in both temperature and pressure. The effect of changing pressure on diffusional loss in cooling petrological systems has not been considered in Dodson’s model. During upwelling, the decompression rate is related to the cooling rate through the slope of the upwelling path. Simple analytical expressions for the average or mean closure temperature and closure pressure in cooling-upwelling mono-mineralic and bi-mineralic systems are obtained by noting that both temperature and pressure decrease as a function of time along the upwelling path. These pressure-adjusted equations are nearly identical to closure temperature equations for isobaric cases if one replaces the activation energy and pre-exponential factor for diffusion in the isobaric formulations by the path-dependent activation energy and pre-exponential factor. The latter also depend on the slope of the upwelling path. The competing effects between pressure and temperature on diffusion during upwelling result in reductions in the effective activation enthalpy for diffusion and exchange enthalpy for partitioning, which in turn leads to systematic deviations in closure temperatures from cases of constant pressure. For systems with large activation volume for diffusion, it may be possible to deduce upwelling path and upwelling rate from closure temperatures and closure pressures of selected elements. Examples of closure temperature and closure pressure for REE diffusion in garnet and clinopyroxene and in garnet–clinopyroxene aggregates are presented and discussed in the context of the minor’s rule and the REE-in-garnet–clinopyroxene thermobarometer. Closure temperatures for middle-to-heavy REE in garnet–clinopyroxene aggregates are controlled primarily by diffusion in clinopyroxene unless the modal abundance of garnet is very small or the effective grain size of clinopyroxene is considerably smaller than that of garnet.

Simple models for disequilibrium fractional melting and batch melting with application to REE fractionation in abyssal peridotites

Disequilibrium melting arises when the kinetics of chemical exchange between a residual mineral and partial melt is sluggish compare to the rate of melting. To better understand the role of a finite crystal–melt exchange rate on trace element fractionation during mantle melting, we have developed a disequilibrium melting model for partial melting in an upwelling steady-state column. We use linear kinetics to approximate crystal–melt mass exchange rate and obtain simple analytical solutions for cases of perfect fractional melting and batch melting. A key parameter determining the extent of chemical disequilibrium during partial melting is an element specific dimensionless ratio (ε) defined as the melting rate relative to the solid–melt chemical exchange rate for the trace element of interest. In the case of diffusion in mineral limited chemical exchange, ε is inversely proportional to diffusivity of the element of interest. Disequilibrium melting is important for the trace element when ε is comparable to or greater than the bulk solid–melt partition coefficient for the trace element (k). The disequilibrium fractional melting model is reduced to the equilibrium perfect fractional melting model when ε is much smaller than k. Hence highly incompatible trace elements with smaller mobilities in minerals are more susceptible to disequilibrium melting than moderately incompatible and compatible trace elements. Effect of chemical disequilibrium is to hinder the extent of fractionation between residual solid and partial melt, making the residual solid less depleted and the accumulated melt more depleted in incompatible trace element abundances relative the case of equilibrium melting.Application of the disequilibrium fractional melting model to REE and Y abundances in clinopyroxene in abyssal peridotites from the Central Indian Ridge and the Vema Lithospheric Section, Mid-Atlantic Ridge revealed a positive correlation between the disequilibrium parameter ε and the degree of melting, which can be explained by an increase in melting rate and a decrease in REE diffusion rate in the upper part of the melting column. Small extent of disequilibrium melting for LREE and equilibrium melting for HREE in the upper part of the melting column can explain the elevated LREE abundances or spoon-shaped REE patterns in clinopyroxene in more refractory abyssal peridotites. The latter has often been attributed to melt refertilization.

A simple model for closure temperature of a trace element in cooling bi-mineralic systems

Closure temperature is defined as the lower temperature limit at which the element of interest effectively ceases diffusive exchange with its surrounding medium during cooling. Here we generalize the classic equation of Dodson (1973) for cooling mono-mineralic systems to cooling bi-mineralic aggregates by considering diffusive exchange of a trace element between the two minerals in a closed system. We present a simple analytical model that includes key parameters affecting the closure temperature of a trace element in cooling bi-mineralic systems: cooling rate, temperature-dependent diffusion coefficients for the trace element in the two minerals, temperature-dependent partition coefficient of the trace element between the two minerals, effective grain sizes of the two minerals, and volume proportions of the minerals in the system. We show that closure temperatures of a trace element in cooling bi-mineralic systems are bounded by the closure temperatures of the trace element in the two mono-mineralic systems and that our generalized model reduces to Dodson’s equation when one of the mineral serves as “an effective infinite” reservoir to the other mineral. Application to closure temperatures of REE in orthopyroxene and clinopyroxene bi-mineralic systems highlights the importance of REE diffusion and partitioning in the pyroxenes as well as clinopyroxene modal abundance and grain size in the systems. Closure temperatures for REE in two-pyroxene bearing equigranular rocks are controlled primarily by diffusion in orthopyroxene unless the modal abundance of clinopyroxene is very small. This has important bearings on the interpretation of temperatures derived from the REE-in-two-pyroxene thermometer.

Time scales of diffusive re-equilibration in bi-mineralic systems with and without a fluid or melt phase

Diffusion is a fundamental mass transfer process in magmatic and metamorphic systems. Simple recipes for evaluating time scales for diffusive re-equilibration of a trace element in bi-mineralic aggregates with and without the presence of intergranular fluid or melt are obtained through approximate solutions to diffusion equations. Diffusive re-equilibration times for the trace element depend on average grain sizes of the two minerals, diffusion coefficients and partition coefficients for the element in the two minerals, and volume fractions or volume fluxes of the minerals and fluid or melt in the system. There are two time scales for diffusive re-equilibration in fluid-bearing bi-mineralic systems. The fast time scale is proportional to fluid fraction and dominated by the mineral that has a faster diffusion rate. The slow time scale is bounded by the time scales of diffusion in the two minerals and determines the overall time scale of diffusive re-equilibration for the system. Presence of small amount of fluid or melt does not lead to a reduction in the time scale of diffusive re-equilibration for the bi-mineralic aggregate. Percolation of fluid or melt drives the system further away from equilibrium and increases the length and time scales for diffusive re-equilibration in the system. The time scales for subsolidus re-equilibration in bi-mineralic aggregates are determined by the minor’s rule: the mineral that contains a lesser amount of the trace element of interest in the system contributes more to the overall diffusive time.Applications to REE redistribution between clinopyroxene and orthopyroxene and between plagioclase and orthopyroxene highlight the importance of REE diffusion and partitioning in pyroxene and plagioclase. The time scales for subsolidus re-equilibration of REE in noritic rocks are dominated by light REE diffusion in orthopyroxene and by middle and heavy REE diffusion in plagioclase. The time scales for subsolidus re-equilibration of REE in two-pyroxene bearing rocks are controlled primarily by REE diffusion in orthopyroxene. The time scales for diffusive re-equilibration of REE in melt-bearing two-pyroxene aggregates are determined by diffusion in clinopyroxene for light REE and by diffusion in orthopyroxene for middle to heavy REE. Presence of melt flow further expands the role of clinopyroxene. This contrasting behavior in diffusive time scale may provide a simple explanation for the ubiquitous chemical disequilibrium in light REE between clinopyroxene and orthopyroxene observed in mantle peridotites from a wide range of tectonic settings.

Sm-Nd chronology of porphyroblastic garnets from granulite facies metabasic rocks in Calabria (Southern Italy): inferences for preserved isotopic memory and resetting

Metabasic rocks related to pre-Cambrian protoliths from the lower portion of the deep crust of the Serre (Calabria, southern Italy) contain porphyroblastic garnet up to 5–6 cm in diameter. Garnet forms coronas around the inclusions of clinopyroxene and is in contact with various matrix minerals. Both inner and outer coronas formed under granulite facies conditions after the thermal peak during the Hercynian reworking. Six porphyroblastic garnets (≥1 cm in diameter) from four samples have been dated with the Sm-Nd method to constrain the distinct metamorphic stages and, possibly, to investigate the diffusion of Sm and Nd in garnet. They show in the core major element flat profiles whereas one of these, analyzed for REEs, preserves only a feeble zoning. This suggests that the diffusion rates of REEs are effective at the crystal scale. The apparent Nd ages range from 354 to 88 Ma, without any reproducibility in each and in all rock samples. The oldest age of 354 Ma is interpreted as the primary isotopic signatures linked to prograde metamorphism. The interpretation of younger ages (309, 272, 215, 143 and 88 Ma) requires a detailed discussion about: (i) possible modification of chemical and isotopic composition of the rocks during and after garnet growth, (ii) possible contamination by inclusions in garnet, (iii) inherited isotopic disequilibrium, (iv) new growth or recrystallization of garnet and (v) possible isotopic resetting of large crystals which, in principle, is hampered by the slow diffusion of REE’s in garnet. Some of the Nd ages are similar to U-Pb ages of zircon from the metabasic rocks of deep crustal rocks of the Serre (350, 300 and 280 Ma). This convergence of apparent ages can hardly be considered as simply fortuitous. Thus, since: (i) corona formation was fluid-assisted and (ii) all porphyroblasts were broken up into several fragmented subgrains by sets of fractures resulting in smaller volumes, the volume diffusion and the possible role of high-T fluids on the resetting of Sm-Nd ages are discussed. The calculated ages of 354, 309 and 272 Ma are considered as geologically meaningful and related to the thermal peak and subsequent decompression and cooling stage of the Variscan metamorphism.

Ti diffusion in natural pyroxene

Diffusion of Ti has been characterized in natural orthopyroxene and clinopyroxene under buffered conditions (IW, NNO, and QFM) and in air. In all pyroxene compositions studied, titanium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions. For Ti diffusion in a natural enstatite and a Kilbourne Hole orthopyroxene (KBH opx), we obtain the following Arrhenius relations for diffusion over the temperature range 950–1150°C,DEn=1.45×10-11exp(-270±34kJ mol-1/RT)m2s-1,DKBH=3.70×10-11exp(-285±34kJ mol-1/RT)m2s-1.For Ti diffusion in a natural diopside and a chromian diopside at temperatures 900–1250°C we obtain the following Arrhenius relations, respectively,DDi=8.01×10-11exp(-282±20kJ mol-1/RT)m2s-1,DCrDi=2.97×10-11exp(-282±40kJ mol-1/RT)m2s-1.Diffusion rates of Ti in the Cr-diopside and KBH opx are essentially the same, whereas diffusion rates of Ti in the enstatite and diopside are only 1.3–1.8 times and 2.7–2.9 times that in the KBH opx, respectively. Titanium diffusion appears relatively insensitive to pyroxene composition, in contrast to findings for diffusion of some other cations. At magmatic temperatures, the rates of Ti diffusion in pyroxene are comparable to the rates of REE diffusion in opx and the rates of Al and MREE diffusion in diopside, although activation energies of Ti diffusion are considerably lower than those for REE diffusion in the pyroxenes. Finally, diffusion rates of Ti in pyroxenes are 1–3 orders of magnitude smaller than those of Mg and Fe in pyroxenes.Diffusivities of Ti in pyroxenes obtained from this study can be used to understand a range of geochemical mass transfer problems involving diopside and orthopyroxene. The closure temperatures for Ti in orthopyroxene and diopside are very similar to each other for comparable pyroxene grain size. Subsolidus redistribution of Ti between orthopyroxene and diopside results in characteristic reverse Ti zoning in orthopyroxene and normal Ti zoning in diopside. Such zoning patterns in coexisting orthopyroxene and diopside may be used to infer cooling rate of two-pyroxene bearing ultramafic and mafic rocks.

Influence of plastic deformations in zircon on its chemical composition: Evidence from gabbroids of the spreading zone of the Mid-Atlantic Ridge, Markov trough, 6° N

Zircon is considered to be one of the most stableminerals of rocks and is the main mineral used for rockdating by the U–Pb method. Interest in study of thismineral has increased significantly during the past fewyears in the context of the possibilities discovered fordating individual grains on ion microprobes. Theproblem of interpretation of the dating resultsobtained has become more important.Determination of zircon genesis is based on thecrystal habit, zonation type, and geochemical peculiarities. It is not considered that the idiomorphicshape of the crystal is not always primary, but may beformed as a result of recrystallization. Study of themorphology and internal structure of zircon usingcathode luminescence (CL) demonstrated that thediversity of zircon morphology was controlled byrepeated structural metamorphic transformations ofthe parental magmatic rocks [1, 2].The chemical stability of zircon in the course ofstructural metamorphic transformations of magmaticrocks was considered in an example of this mineralfrom olivine gabbro from the slowly spreading IndianRidge. A link between the increase of the REE diffusion rate and plastic deformation of zircon crystals wasdemonstrated [3, 4]. However morphogenetic andgeochemical alterations of zircons resulting from deformations superposed on host rocks, i.e., zircon evolutionin oceanic floor rocks, have not been studied yet.Because of this, we performed a study of zirconfrom gabbroids of the axial part of the MidAtlanticRidge (MAR) previously dated by two methods:SHRIMPII [5] and laser ablation [6]. Zircon grainswere extracted from gabbroids of the lower oceaniccrust dredged from walls of the Markov deep trough(6° N) in the axial part of the MAR, in the 10th cruiseof R/V

Li diffusion in zircon

Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0–1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE+3 that might provide charge balance. Diffusion experiments performed in the presence of H2O–CO2 fluid were run in a piston–cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce H2O–CO2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7Li(p,γ)8Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703–1.151°C at 1 atm for experiments run with the spodumene source: $$ D_{\text{Li}} = 7.17 \times 10^{ - 7} { \exp }( - 275 \pm 11\,{\text{kJmol}}^{ - 1} /{\text{RT}}){\text{m}}^{2} {\text{s}}^{ - 1}.$$ Diffusivities are similar for transport parallel to the c-axis, indicating little anisotropy for Li diffusion in zircon. Similar Li diffusivities were also found for experiments run under fluid-present conditions and for the experiment run with the Dy-bearing source. Li diffusion is considerably faster than diffusion of other cations in zircon, with a smaller activation energy for diffusion. Although Li diffusion in zircon is comparatively rapid, zircons will be moderately retentive of Li signatures at mid-crustal metamorphic temperatures, but they are unlikely to retain this information for geologically significant times under high-grade metamorphism.

Rare earth element diffusion in natural enstatite

Chemical diffusion coefficients of La, Nd, Eu, Gd, and Yb in natural enstatite have been measured at 850–1250 °C and 1 atm. Anhydrous diffusion experiments were run in Pt capsules in air, or in sealed silica glass capsules under an iron–wüstite (IW) solid buffer. The sources of diffusant were pre-reacted mixtures of synthetic enstatite powder and microcrystalline rare-earth aluminate garnet. Rutherford Backscattering Spectrometry (RBS) was used to measure diffusion profiles. For Gd diffusion in air over the temperature range 1000–1250 °C, the following Arrhenius relation is found for diffusion normal to (210): D Gd,air = 2.55 × 10 - 9 exp ( 321 ± 85 kJ mol - 1 / RT ) m 2 s - 1 . There is no significant difference between Gd diffusion in air and under IW-buffered conditions. Behavior similar to Gd is also noted for Nd. The Arrhenius relationship for Eu diffusion in enstatite, normal to (210) and at 850–1150 °C and IW-buffered conditions, is D Eu,IW = 6.93 × 10 - 6 exp ( - 384 ± 29 kJ mol - 1 / RT ) m 2 s - 1 . For Eu diffusion in air over the temperature range 1000–1200 °C for the same orientation, the following Arrhenius relation is found: D Eu,air = 1.70 × 10 - 8 exp ( - 350 ± 42 kJ mol - 1 / RT ) m 2 s - 1 . For Eu diffusion under IW-buffered conditions and for experiments run in air, diffusivities normal to (001) are similar to those for diffusion normal to (210). Eu diffusion under IW-buffered conditions is more than an order of magnitude faster than Eu diffusion in air. It is likely that majority of Eu is in the divalent state for diffusion under IW-buffered conditions, but Eu is in the trivalent state for diffusion in air. In the case of Nd and Gd, where valence state does not change under the investigated fO 2 conditions, diffusivities measured for experiments run both in air and under IW-buffered conditions are comparable to those obtained for trivalent Eu. Further, measurements of La, Nd, Eu +3, Gd, and Yb diffusion suggest that diffusion of trivalent REE in enstatite is not sensitive to ionic size, in contrast to that observed for REE diffusion in diopside [Van Orman, J.A., Grove, T.L., Shimizu, N., 2001. Rare earth element diffusion in diopside; influence of temperature, pressure, and ionic radius, and an elastic model for diffusion in silicates. Contrib. Mineral. Petrol. 141, 687–703]. These differences in diffusive behavior of REE between diopside and enstatite, as well as Eu 2+ and Eu 3+ in enstatite, can result in significant REE fractionation between coexisting pyroxenes during partial melting, melt migration, and subsolidus reequilibration processes in the Earth’s mantle and that of the Moon.

Monazite behaviour and age significance in poly-metamorphic high-grade terrains: A case study from the western Musgrave Block, central Australia

Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at ∼1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at ∼1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on ∼1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at ∼1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages.

Uranium and manganese diffusion in apatite

Diffusion of manganese and uranium has been characterized by Rutherford Backscattering Spectrometry (RBS) in fluorapatite. The following Arrhenius relation is obtained for Mn diffusion in natural Durango fluorapatite, for diffusion parallel to c: D Mn = 5.4 × 10 − 7 exp ( − 288 kJ mol − 1 / R ⁢ T ) m 2 s − 1 . Mn diffusion normal to c appears to be similar to diffusion parallel to c, and diffusivities in natural and synthetic (free of significant amounts of REE or other minor constituents) apatites are the same within experimental uncertainties. Experiments run in air, where Mn is in a more oxidized state, yield diffusivities similar to those for experiments run in sealed silica capsules, which the above Arrhenius relation represents. Uranium diffusion experiments over the temperature range 1000–1300 °C yield the following Arrhenius relation for diffusion parallel to c: D U = 1.3 × 10 − 6 exp ( − 394 kJ mol − 1 / R ⁢ T ) m 2 s − 1 . Diffusion coefficients for transport normal to c are quite similar to those parallel to c. U diffusion experiments run in air, where U is most likely hexavalent, yield diffusivities about an order of magnitude slower than the Ni–NiO buffered experiments. Diffusivities of Mn are comparable to those established previously for Sr and are slightly slower than Pb in apatite. The similar diffusion rates for Sr and Mn, despite their significant differences in cationic radii, suggest that cation size does not exert strong influence on diffusion of divalent cations in apatite, a finding consistent with that previously observed for the trivalent REE. In contrast, cation charge does seem to influence diffusivities strongly in apatite (although Mn appears perhaps something of an exception). U diffusion (buffered at NNO) is about 4 orders of magnitude slower than Mn diffusion, and about 2 orders of magnitude slower than REE diffusion. The differences between U diffusivities in experiments run with the NNO buffer, when U is more likely in the tetravalent state, and in air, where U is hexavalent, are also consistent with this observation.

Rare earth element diffusion in a natural pyrope single crystal at 2.8 GPa

Volume diffusion rates of Ce, Sm, Dy, and Yb have been measured in a natural pyrope-rich garnet single crystal (Py71Alm16Gr13) at a pressure of 2.8 GPa and temperatures of 1,200–1,450 °C. Pieces of a single gem-quality pyrope megacryst were polished, coated with a thin layer of polycrystalline REE oxide, then annealed in a piston cylinder device for times between 2.6 and 90 h. Diffusion profiles in the annealed samples were measured by SIMS depth profiling. The dependence of diffusion rates on temperature can be described by the following Arrhenius equations (diffusion coefficients in m2/s): \(\)\(\matrix{ {\log _{10} D_{{\rm Yb}} = ( - 7.73 \pm 0.97) - \left( {343 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Dy}} = ( - 9.04 \pm 0.97) - \left( {302 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Sm}} = ( - 9.21 \pm 0.97) - \left( {300 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Ce}} = ( - 9.74 \pm 2.84) - \left( {284 \pm 91\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr } \) . There is no significant influence of ionic radius on diffusion rates; at each temperature the diffusion coefficients for Ce, Sm, Dy, and Yb are indistinguishable from each other within the measurement uncertainty. However, comparison with other diffusion data suggests that there is a strong influence of ionic charge on diffusion rates in garnet, with REE3+ diffusion rates more than two orders of magnitude slower than divalent cation diffusion rates. This implies that the Sm–Nd isotopic chronometer may close at significantly higher temperatures than thermometers based on divalent cation exchange, such as the garnet–biotite thermometer. REE diffusion rates in pyrope are similar to Yb and Dy diffusion rates in diopside at temperatures near the solidus of garnet lherzolite (~1,450 °C at 2.8 GPa), and are an order of magnitude faster than Nd, Ce, and La in high-Ca pyroxene at these conditions. At lower temperatures relevant to the lithospheric mantle and crust, REE diffusion rates in garnet are much faster than in high-Ca pyroxene, and closure temperatures for Nd isotopes in slowly-cooled garnets are ~200 °C lower than in high-Ca pyroxene.

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.

Rare earth element and gallium diffusion in yttrium aluminum garnet

Diffusion of four rare-earth elements and gallium has been measured in yttrium aluminum garnet (YAG). Sources of diffusant were mixtures of alumina and rare-earth element oxides for REE diffusion, and mixtures of gallium and yttrium oxides for Ga diffusion. Diffusion profiles were measured with Rutherford backscattering spectrometry (RBS). For the rare-earth elements investigated, the following Arrhenius relations were obtained: DLa=6.87×10–1 exp (–582±21 kJ mol–1 /RT) m2s–1 DNd=1.63×10–1 exp (–567±15 kJ mol–1 /RT) m2s–1 DDy=2.70×100 exp (–603±35 kJ mol–1 /RT) m2s–1 DYb=1.50×10–2 exp (–540±26 kJ mol–1 /RT) m2s–1 Diffusion rates for the rare earths are quite similar, in contrast with trends noted for zircon. It is likely that these differences are a consequence of the relative ionic radii of the REE and the cations for which they substitute in the mineral lattice. For gallium, the following Arrhenius relation was determined: DGa=9.96×10–6 exp (–404±19 kJ mol–1 /RT) m2s–1 Gallium diffuses faster than the REE in YAG and has a smaller activation energy for diffusion. These data mirror relative trends in diffusion rates for YIG, in which trivalent cations occupying tetrahedral and octahedral sites (i.e., Al, Ga, Fe) diffuse faster than trivalent cations occupying dodecahedral sites (i.e., Y and the REE), and suggest that the rate-limiting process in the diffusion-controlled regime of solid-state creep of YAG is the diffusion of yttrium.

Progressive hybridisation between Late Devonian mafic-intermediate and felsic magmas in the Meguma Zone of Nova Scotia, Canada

Two felsic plutons of Late Devonian (385–370 Ma) age in the Meguma Zone of southwestern Nova Scotia contain three circa 376 Ma synplutonic mafic-intermediate intrusions that collectively record progressive stages of in situ hybridisation. A 5 m wide spessartite dyke in the Port Mouton Pluton probably underwent rapid cooling and crystallisation immediately after intrusion, which heated and coarsened the adjacent tonalite. An 85 m long sheet of pillowed kersantite (also in tonalite of the Port Mouton Pluton) presumably contained residual magma after quenching and obtained K, P, Ba, Rb, more radiogenic Sr, Zr, Nb, and light REE from the tonalite during magma mingling. The third synplutonic body, a >100 m wide diorite sheet, intrudes granodiorite of the adjacent Shelburne Pluton and has a circa 45 m wide gradational contact of metaluminous hornblende-tonalite. This tonalite dominantly records magma mixing by the transfer of Ti, Mg, Fe, Ca, and V in hornblende, biotite, plagioclase, and (at least in part) apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite, probably in response to convection. Scattered data points, unusual “concave-down” variation trends for Al2O3, P2O5, and Sr, and non-hyperbolic Sr-Nd isotopic characteristics in the tonalite, apparently reflect syn- or post-mixing fractionation and accumulation of xenocrysts from residual magma. Phosphorus may have assisted diffusion of Sr, Zr, Nb, and light REE, and caused premature quenching of the hybrids at Mcleods Cove and Birchtown, during magma mingling and mixing.

U - Th - REE Mobility and Diffusion in Granitic Environments During Alteration of Accessory Minerals and U - Ores: A Geochemical Analogue to Radioactive Waste Disposal

ABSTRACTU, Th and REE concentrations and distributions have been studied in granitic rocks, using a multidisciplinary approach involving fission-track micromapping of cracks in oriented samples, together with mineralogical and geochemical studies of the different U-Th-REE bearing phases. The behaviour of U, Th and Nd, considered as chemical analogue elements of the radiotoxic nuclides, was investigated either in the vicinity of microsites (accessory mineral environment) or along plurimetric sections around U-ore bodies. The different granite minerals, especially the accessory minerals (uraninite, monazite, thorite, apatite, xenotime), as well as U-ores, present different initial concentrations of U, Th and REE. Limitations to the analogy between these U-Th-REE concentrations and the radwastes is discussed as a function of their mineralogical features, chemical composition, size and solubilities. These primary concentrations present different behaviour when subjected to hydrothermal alteration, such as propylitization, phyllite type alteration, or clay alteration. Results show that in reduced media, in the temperature range 80–200°C, the rate of mobilization of U, Th, REE is relatively moderate. However, fluids enriched in fluorides, phosphates or carbonates may significantly solubilize and transport U and REE under specific conditions. In addition, the degree of opening of the microcracks and faults, as well as the oxidation-reduction processes, are critical parameters for the efficiency of the granitic geological barrier.