Modelling grain‐recycling zoning during metamorphism

Chemical zoning, recorded by grain growth during metamorphism, is a key source of information about P–T–t paths. Interpretation of these data must be carried out using appropriate models and recognizing their inherent assumptions. To assist with defining how zoned minerals form, a set of geometric criteria for three types of chemical zoning developed in minerals (diffusion, growth and grain recycling) is outlined. Re‐equilibration of minerals by lattice diffusion causes zoning if the re‐equilibration is incomplete. Growth of porphyroblasts is commonly considered in pelites, but in metagranitoids, large monophase domains undergo coarsening by recycling of material from one grain to another as grain boundaries migrate driven by surface energy. This type of grain size increase is termed here ‘grain recycling’. Zoning developed during grain recycling due to equilibration of the recycled material with grain‐boundary chemistry is termed ‘grain‐recycling zoning’. Furthermore, short lattice diffusion lengths relative to grain sizes cause metamorphic fractionation because material in the grain cores is not in communication thermodynamically with the rest of the rock. A new model is derived for this sort of grain size increase coupled with metamorphic reactions using Theriak–Domino. An example is given of plagioclase undergoing an increase in anorthite content as epidote breaks down during amphibolite facies metamorphism of a metagranitoid. Agreement between naturally occurring zoning profiles and those derived from modelled P–T–t paths shows that this model can be used to extract metamorphic conditions from rocks which are not accessible using conventional thermobarometry.     

Numerical Modeling of Growth Zoning at Nonstationary Crystallization of Solid Solutions: Metamorphic Garnets

In this paper we consider crystallization of solid solutions and formation of growth zoning in minerals. To ascertain the role of various mechanisms producing zoning we have constructed kinetic models of nonsteady solid solution crystal growth. The equations obtained describe the temporal evolution of the solute and crystal composition. Since these equations are not solvable analytically we have solved them numerically by a fourth-order Runge–Kutta method. On the basis of this solution we can compute the zoning profiles for different crystallization modes and conditions. The constructed models have been used for study of mechanisms of zoning formation in metamorphic garnets. We conclude that the main mechanism of production of growth zoning is fractionation. The role of change of distribution coefficient in equilibrium crystallization is negligible. The modelling of zoning profiles reveals that simple arc-shaped profiles originate from crystallization in a closed system while complicated nonmonotonic profiles appear with crystallization in open systems under fluid flow. The duration of metamorphic garnet crystallization is estimated.     

辽北太古宙变质岩中的振荡型矿物环带及意义

对辽北太古宙层状变质岩系中石榴石、黑云母的成分分析发现,其中存在明显的振荡型矿物环带。对其形成机理的研究表明,这是矿物在结晶过程中由于偏离理想状态而形成的一种非平衡态自组织现象,即耗散结构。矿物内部成分的振荡性并不是由于外界条件的周期变化所造成的,而是体系内部固有属性的反映。这种认识对研究变质作用的动力学过程具有重要的借鉴意义,文中还就这种振荡性矿物环带在变质作用研究中的意义作了进一步的阐述。     

The microstructures of microcline from some granitic rocks and pegmatites

Numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents. The simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased, and that the water content in the melt has little influence on this result. Comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees C can be rapidly imposed on plutonic phenocrysts. Large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals. The observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth; the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates. Even at large undercooling, there is no significant increase in temperature at the interface caused by the release of heat of crystallization.Simulations are presented to illustrate how disequilibrium growth processes due to undercooling can modify the normal zoning profiles expected from fractionation. Assuming that an undercooling is necessary to cause nucleation, normal zoning can result if crystal growth takes place at constant or increasing undercooling, but reverse zoning can occur at decreasing undercooling. Undercooling during growth is controlled by the relative rate of cooling and the rate at which the liquidus temperature is decreased by the accumulation of residual components and volatiles in the melt. Consequently, normal zoning should be promoted by rapid cooling, contemporaneous crystallization of other phases, and absence of volatiles, while reverse zoning should be expected in phenocrysts grown in slowly-cooled melts or in melts where volatiles are concentrated. The zoning patterns found in many plutonic plagioclase crystals suggest that their compositions are in significant disequilibrium with the melt; consequently, they are unsuitable for use in geothermometers.Approximate calculations suggest that the amount of water concentrated at the surface of growing phenocrysts in plutons can promote local convection. Comparison of simulated and observed oscillatory zoning profiles suggests that oscillatory zoning is not explained by a re-nucleationdiffusion model (Harloff 1927), but is readily explained by periodic local convection.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs equilibrium processes

Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ³⁸S versus Δ³⁴S diagrams are deciphered theoretically. Taking into account dissolved H₂S and SO₄^2- in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4-species closed system is suggested for describing the conservation of mass among all sulphur-bearing species on the δ-Δ diagrams. The covariation in the δ³⁴S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ³⁴S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H₂S and/or SO₄^2-. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.     

The significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization

The spatial disposition, compositional zoning profiles, and size distributions of garnet crystals in 11 specimens of pelitic schist from the Picuris Range of New Mexico (USA) demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks. An ordered disposition of garnet centers and a significant correlation between crystal radius and near-neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre-existing crystals. Compositional zoning profiles require diffusionally controlled growth, the rate of which increases exponentially as temperature increases with time; an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen. Crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process, but decline in the later stages because of suppression effects in the vicinity of earlier-formed nuclei. Simulations of porphyroblast crystallization, based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics, quantitatively replicate textural relations in the rocks. The simulations employ only two variable parameters, which are evaluated by fitting of crystal size distributions. Both have physical significance. The first is an acceleration factor for nucleation, with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock. The second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it. Crystal size distributions for the Picuris Range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals. The same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet, phlogopite, sphene, and pyroxene in rocks from both regional and contact metamorphic occurrences. These commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments.     

A thermal-tectonic model for high-pressure rocks in the Basal Gneiss Complex of western Norway

The Basal Gneiss Complex (BGC) of western Norway is a segment of continental crust that was subjected to eclogite facies metamorphism during the Caledonian Orogeny and then was overprinted by amphibolite facies conditions. Numerical methods have been used to construct a model for thermal evolution of the BGC. The calculated temperature-depth-time (Tzt) paths for the BGC are in good agreement with the sequence of mineral assemblages that occur in metamorphosed mafic and ultramafic rocks. However, the thermal model indicates that retrograde zoning of minerals in the garnet-bearing assemblages of eclogite and ultramafite may have developed metastably at pressures below 10 kbar. Numerical modeling of iron diffusion in garnet grains adjacent to olivine inclusions was used to calculate zoning profiles based upon the thermal model. The calculated zoning profiles have shorter decay distances than the observed zoning profiles, which may be due to uncertainties in the diffusion coefficients. Also, discrepancies occur between the Tzt paths and specific PT values calculated for eclogite and garnet ultramafite due either to the preservation of pre-Caledonian mineral assemblages or to utilization of geobarometers and thermometers which are based on different elements in different phases (such as A1 in orthopyroxene and Fe---Mg in olivine and garnet).     

Oscillatory zoning in a (Ba,Sr)SO4 solid solution: Macroscopic and cellular automata models

Many minerals exhibit oscillatory zoning (OZ), whereby their chemical composition varies more or less regularly along a crystal core-to-rim profile. A well-known example of oscillatory zoning was previously obtained in the (Ba,Sr)SO₄ solid solution system under controlled laboratory conditions. The OZ crystals were precipitated at room temperature from counter-diffusing aqueous solutions. In this contribution, we review a macroscopic model for the self-organized formation of oscillatory zoning in such binary solid solution. The model combines diffusive solute transport and an autocatalytic continuous crystal growth with a rate dependent on the mineral surface composition. Oscillatory solutions are obtained. It is also shown that fluctuations in the aqueous solution concentrations may contribute to the OZ formation by causing noise-induced transitions in the crystal growth regimes. We also present, for the first time, a cellular automata-based microscopic model, which considers diffusive motion and autocatalytic attachment kinetics of individual molecular units. OZ is obtained when the probability of attachment of a unit onto the crystal surface is calculated from the crystal composition averaged over the crystal surface (mean-field approach). However, OZ fails to develop when this probability is calculated from the local crystal composition at the attachment site in the absence of a lateral synchronization mechanism.     

Effective bulk composition changes due to cooling: a model predicting complexities in retrograde reaction textures

Diffusive processes are a strong function of temperature. Thus, during cooling of rocks, mineral grains may develop zoning profiles as successively larger parts of the grain “close” to the diffusive exchange with the rock. One of the consequences of this process is that, during cooling, successively larger parts of zoned minerals (depending on grain size) are effectively removed from the reacting part of the rock volume. Thus, the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size. Because the sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition, this process may have the consequence that two rocks of identical overall bulk composition, but of different grain size, may experience a different sequence of reactions. Qualitatively identical peak paragenesis may therefore react to form qualitatively different retrograde reaction textures. The model is applied to examples in the pelitic system. There, garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition. It is shown that, during cooling of pelitic rocks from amphibolite facies conditions, typical aluminous peak parageneses of garnet-muscovite-kyanite ± biotite may react to form either staurolite, chlorite or muscovite (or different combinations thereof), depending on grain size. During cooling from the granulite facies, aluminous peak parageneses of garnet-cordierite-sillimanite may form biotite, either on the expense of cordierite or garnet, also depending on grain size. The two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature. Received: 12 March 1996 / Accepted: 7 April 1997     

Trace element zoning in garnet from the Kwoiek Area,British Columbia: disequilibrium partitioning during garnet growth?

Trace element zoning in garnets from two contact-metamorphosed rocks from the Kwoiek area, British Columbia (Hollister 1969a), was measured with an ion microprobe. Zoning profiles have three distinct parts with chemical breaks defined by co-variation of major and trace elements. Important features of the trace element zoning profiles are: (1) roughly bell-shaped zoning profiles for Y and the HREEs, (2) an abrupt increase in Ti at a point midway through each garnet with inflections in the zoning profiles of other elements (Li, Na, Cr, V, Y, Zr, and the HREE), and (3) irregular Cr and V profiles. Unlike Mn zoning, the zoning profiles of most other trace elements cannot be easily modeled using simple Rayleigh fractionation models. Ti activity in the two samples is buffered by phase relations with ilmenite. Garnets from a continuously heated contact metamorphic environment should display continuous Ti zoning profiles if equilibrium was maintained and provided the Ti buffering assemblage did not change during garnet growth. The irregular Ti profiles suggest disequilibrium behavior. Several elements (Cr, V) may indicate breakdown of a phase enriched in trace elements during metamorphism. The source for the mass excess of these elements is probably the refractory cores of ilmenite grains. Either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point. The preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth.     

Metamorphic interpretation of high-pressure-temperature metapelites with preserved growth zoning in garnet, eastern Grenville Province, Canadian Shield

Abstract High-pressure-temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the Gagnon terrane (Parautochthonous Belt, eastern Grenville Province) were investigated in order to constrain depths of burial and P-T paths. Mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800° C. However, growth zoning is apparently well preserved in garnets and only narrow rims (width = 100–500 μm) are obviously affected by diffusional retrograde resetting. Despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth, it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 MPa, and that thermal relaxation did not occur during the initial stages of unloading. On the other hand, calculated retrograde P-T conditions are consistent with steep decompression paths. The inferred 'hair-pin'-shaped P-T path is consistent with independent evidence of rapid, tectonically driven exhumation, resulting in the preservation of growth zoning in garnets from such a high-temperature regime.     

Weathering-pedogenesis of Carbonate Rocks and Its Environmental Effects in Subtropical Region

We investigated the weathering-pedogenesis of carbonate rocks and its environmental effects in subtropical regions of China. The investigation demonstrated that the weathering- pedogenesis of carbonate rocks is the process of a joint action of corrosion and illuviation and metasomatism in subtropical region. It is characterized by multi-stage, multi-path and multi-style. With the persisting development of weathering-pedogenesis of carbonate rocks, metasomatic pedogenesis progressively became the main process of the weathering-pedogenesis and the dominant style of formation of minerals. And it proceeds through the whole process of evolution of the weathering-pedogenesis of carbonate rocks. The stage evolution of weathering-pedogenesis of carbonate rocks and the fractionation evolution of newly produced minerals are characterized by obvious vertically zoning structures and the rules of gradation of elements geochemical characteristics in the carbonate rocks weathering profiles. The geochemical process of weathering-pedogenesis of carbonate rocks can be divided into three geochemical evolution stages, i.e., the Ca, Mg-depletion and Si, Al-enrichment stage; the Fe, Mn enrichment stage and the Si-depletion and Al-enrichment stage in the subtropical regions. Consistent with the three geochemical evolution stages, the sequence of formation and evolution of minerals can be divided into the clay mineral stage; the Fe, Mn oxide and the gibbsite stage. The influence of weathering-pedogenesis of carbonate rocks on the chemical forms of heavy elements is mainly affected via newly produced components and minerals in the process of weathering-pedogenesis, e.g., iron oxide minerals and organic matters. The important mechanism for the mobilization, transport and pollution of F and As is affected the selective adsorption and desorption of F and As on the surface of iron oxide minerals in the subtropical karst zones, i.e., the selective adsorption and desorption on mineral surfaces of newly produced minerals in the process of weath     

Temperature and its control of isotope fractionation by a sulfate-reducing bacterium

A synthesis of previous results, which we dub the “standard model,” provides a prediction as to how isotope fractionation during sulfate reduction should respond to physiological variables such as specific rate of sulfate reduction and environmental variables such as substrate availability and temperature. The standard model suggests that isotope fractionation should decrease with increasing specific rates of sulfate reduction (rate per cell). Furthermore, the standard model predicts that low fractionations should be found at both high and low temperatures whereas the highest fractionations should be found in the intermediate temperature range. These fractionation trends are controlled, as a function of temperature, by the balance between the transfer rates of sulfate into and out of the cell and the exchange between the sulfur pools internal to the organism. We test this standard model by conducting experiments on the growth physiology and isotope fractionation, as a function of temperature, by the sulfate-reducing bacterium Desulfovibrio desulfuricans (DSMZ 642). Our results contrast with the “standard model” by showing a positive correlation between specific rates of sulfate reduction and fractionation. Also by contrast with the standard model, we found the highest fractionations at low and high temperatures and the lowest fractionations in the intermediate temperature range. We develop a fractionation model which can be used to explain both our results as well as the results of the “standard model.” Differences in fractionation with temperature relate to differences in the specific temperature response of internal enzyme kinetics as well as the exchange rates of sulfate in and out of the cell. It is expected that the kinetics of these processes will show strain-specific differences.     

胶北荆山群富铝岩系石榴石扩散环带特征及其成因指示意义

通过详细的微区成分测定，发现胶北荆山群富铝岩系中石榴石普遍发育扩散环带，但扩散环带的发育程度及样式很不均匀，明显受与其相邻矿物的控制。与黑云母接触时，石榴石晶体边部的镁含量最低，环带最为发育，与堇青石接触时次之，与长英质矿物接触时则环带发育较弱或不发育。这种特征的石榴石扩散环带样式与传统认识有很大差异，反映降温过程中石榴石与黑云母等镁铁矿物之间的Fe-Mg交换作用主要是通过彼此接触的界面来实现，粒间流体对组分的传输作用有限。但是当岩石中黑云母大量存在而石榴石含量又较低时，由于体系水活度增高，粒间流体也会传输一定的Fe、Mg组分，导致与长英质矿物相邻的石榴石晶体边部发育微弱的扩散环带。通过分析，确定粒径大于1500斗m的石榴石晶体核部可以保存变质峰期的平衡成分，基质中远离石榴石等镁铁矿物处于长英质矿物之间的大颗粒黑云母颗粒核部也基本可以保存变质峰期的平衡成分。     

内蒙东南部早前寒武纪孔兹岩系中斜长石的成分环带

内蒙东南部早前寒武纪孔兹岩系中斜长石的Ca、Na、si、Al“等阳离子存在明显的成分环带。依据其成因机制可划分为生长环带和扩散环带。对不同成因的斜长石环带的成分,采用Ga—Pl—Al_2SiO_5(Sill)—Q压力计,配合Ga—Bi矿物对温度压力计估算,峰期前温度为570～620℃,压力1.0～1.2 GPa;峰期温度为750～850℃,压力0.75～0.85 GPa,峰期后温度为600～650℃,压力为0.5～0.62 GPa。可以确定本区早前寒武纪孔兹岩系变质作用演化为顺时针碰撞造山带型式的P—T—t轨迹。     

The importance of adsorption in igneous partitioning of trace elements

For accurate mathematical modeling of trace-element partitioning during igneous fractionation, adsorption should be considered. Because of adsorption, the partitioning of elements between liquid and a surface layer of a crystal is often not the same as the partitioning between liquid and the solid crystal at true equilibrium. In some minerals e.g. high-calcium pyroxene, the effect of adsorption during crystal growth may be very important; this is suggested by the frequent occurrence of sector zoning in augite, and the wide range in measured partition coefficients for such elements as rare earths. The ions which are enriched by adsorption are usually those which are favored substituents according to Goldschmidt's rules. In other minerals, uptake of trace elements may be closer to equilibrium partitioning, rather than being determined by kinetic factors. For example, the relative partitioning of REE, U, Th and Pb into feldspars is qualitatively predicted by Pauling's rules for complex ionic crystals, rather than by Goldschmidt's rules.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Lithium and boron isotopes in illite-smectite: The importance of crystal size

Clay minerals record chemical data about the past, acting like natural computer memory chips. To retrieve the data we must understand how they are stored. To achieve this we have examined the isotopic information revealed by two trace elements, lithium and boron, that are incorporated into the common clay minerals illite-smectite (I-S) during diagenesis. We used hydrothermal experiments at 300°C, 100 MPa, to speed up the reaction of smectite to illite that normally occurs during slow (10-100 Ma) sediment burial. During illitization, Li substitutes into the octahedral sites and B enters the tetrahedral sites of the silicate framework. Both Li and B are also adsorbed in the interlayer of smectite, but Li is preferred over B in the exchange sites. To determine the equilibrium isotope fractionation of the two trace elements it is important to remove these adsorbed interlayer species. By measuring the isotopic composition of Li and B in the silicate framework during reaction, we can address the relative timing of element exchange in the different crystallographic sites. Furthermore, because illitization of smectite is a crystal growth process (not an isomorphous replacement) we have examined the effect of crystal size on the isotope fractionation.The results show that Li and B approach an isotopic steady state when R1 ordering occurs, long before oxygen isotopes equilibrate with the fluid. The isotopic fractionation (α_{mineral-water}) for Li (0.989) is similar to that for B (0.984) at 300°C. However, when separated into <0.2, 0.2-2.0, and >2.0 μm fractions, there are significant differences in measured isotope ratios by as much as 9‰. Crystal growth mechanisms and surface energy effects of nanoscale crystals may explain the observed isotopic differences. The fact that different crystals equilibrate at different rates (based on size) may be applied to natural samples to reveal the changing paleofluid history, provided we understand the conditions of equilibrium. This has very important implications for the interpretation of diagenetic environments, fluid flow, and surficial geochemical cycling.     

Determination of reaction kinetics using grain size: An application for metamorphic zircon growth

The reaction kinetics of metamorphic minerals can be subdivided into interface‐ and diffusion‐controlled kinetics. The discrimination of reaction kinetics is crucial for estimating reaction rates. Here, we propose a new and simple method for discriminating reaction kinetics. This method requires measuring only the initial and final grain sizes during growth. The reaction kinetics is inferred from different plotted arrays of initial vs. final grain sizes after the mineral growth. Using metamorphic zircon, we take detrital core sizes as the initial sizes and post‐metamorphic grain sizes as the final sizes. The application of the method to the subduction‐related high‐pressure Nagasaki metamorphic complex in Japan shows that this metamorphic zircon grew under interface‐controlled kinetics even at the relatively low temperature of 440°C. This method is potentially applicable to other minerals that have time‐markers, such as chemical zoning or internal structures that are captured at a given point in time during growth.