Oxygen Isotope Exchange Kinetics Research Articles

An experimental simulation of oxygen isotope exchange reaction between amorphous silicate dust and carbon monoxide gas in the early Solar System

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

Oxygen isotope exchange kinetics between CAI melt and carbon monoxide gas: Implication for CAI formation in the earliest Solar System

Coarse-grained igneous calcium-aluminum-rich inclusions (CAIs) are suggested to have experienced gas–melt isotope exchange of oxygen during the melting events of their precursors. Therefore, their oxygen isotope variation would preserve information about the high-temperature processes in the earliest Solar System. We experimentally determined oxygen isotope exchange kinetics between CAI analog melt and carbon monoxide (CO) gas at 1420 °C and 1460 °C under CO gas partial pressures of 0.1, 0.5, and 1 Pa to understand the role of CO gas on the oxygen isotope exchange. We observed oxygen isotope zoning profiles inside the reacted samples that formed through the oxygen isotope exchange reaction at the melt surface and oxygen diffusion in the melt. The zoning profiles were fitted using a three-dimensional spherical diffusion model with time-dependent surface concentration. The oxygen isotope exchange efficiency for colliding CO molecules is estimated to be ∼3.3 × 10–4, which is much smaller than that for H2O (0.28). The oxygen diffusion coefficient obtained in this study is similar to that obtained in the oxygen isotope exchange experiments between the CAI melt and H2O, suggesting that the diffusion species in the melt is O2–, despite the surrounding atmospheres.A comparison of the isotope exchange reaction kinetics between (1) CAI melt and CO gas, (2) CAI melt and H2O gas, and (3) CO and H2O gases shows that the reaction rate decreases in the order of (3), (2), and (1). The rapid isotope exchange of the reaction (1) indicates that the oxygen isotopic compositions of H2O and CO should have been equilibrated during the melting and crystallization processes of igneous CAIs. Both H2O and CO change the oxygen isotope compositions of molten CAI in the same direction, although reaction (2) controls the isotope exchange timescale between the CAI melt and surrounding gas. Our dataset demonstrates that type B CAIs having melilite with homogeneous oxygen isotope composition should have been heated for 2–3 days at PH2 > 100 Pa above the melilite liquidus (∼1400 °C) in the solar protoplanetary disk.

An experimental study on oxygen isotope exchange reaction between CAI melt and low-pressure water vapor under simulated Solar nebular conditions

Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and 18O-enriched water vapor (PH2O = 5 × 10−2 Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The 18O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10−11 m2 s−1, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at PH2O < ∼10−2 Pa and by self-diffusion of oxygen in the melt at PH2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing 16O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at PH2O > 10−2 Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H2O compared to that between H2O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h−1 during fassaite crystallization.

Oxygen isotope exchange between the gas-phase and the electrochemical cell O2, Pt | YSZ | Pt, O2 under conditions of applied potential difference

The kinetics of oxygen isotope exchange between gas-phase oxygen and the electrochemical cell O2, Pt | ZrO2 + 10 mol % Y2O3 (YSZ) | Pt, O2 with applied potential difference (ΔU = ±1.2 V) is studied in the temperature range of 600–800°С and the oxygen pressure interval of 3–13 kPa. An original design of a vacuum electrochemical cell with the separated gas space is put forward for studying how the potential difference on the electrochemical cell influences the kinetics of interaction of gas-phase oxygen with the gas electrode O2, Pt | YSZ in the electrochemical cell. It is shown that the oxygen interphase exchange rate is the higher the more negative the charge on the electrode studied; moreover, the mechanism of gas-phase oxygen exchange with the gas electrode O2, Pt | YSZ in the electrochemical cell depends fundamentally on the electrode charge sign. The possible reasons for the revealed differences are discussed; the corresponding models are proposed.

16O/18O oxygen isotope exchange kinetics in MnO 4 − as probed by 55Mn NMR

The effect of the 16O ↔ 18O substitution in the coordination sphere of permanganate anion MnO4− on the chemical shift of 55Mn nuclei have been studied by 17O and 55Mn NMR. Time constants τn,k of oxygen exchange in the water–permanganate anion system have been estimated. In nearly neutral solutions (pH ≈ 6.8–7.2), the oxygen exchange time is on the order of tens of hours. Bubbling gaseous HCl through this solution for a few seconds leads to the equilibrium distribution of oxygen isotopes in the manganese coordination sphere. The observed temperature dependences of isotope-induced 55Mn NMR shifts in Mn16 O44-n18On– (n = 0–4) have been treated as a result of rovibrational averaging of Mn–O bond lengths. The change in the Mn—O bond length in caused by the 16O → 18O isotope substitution is on the order of 10–4 A.

Isotope Exchange between &lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; Gas and Mechanoactivated Oxides of the Family Rare Earth – Manganese – Oxygen

The results of measurements of the bulk diffusion of tracer oxygen atoms in the oxides LnMnO3+δ (Ln = La, Nd, Sm) in the temperature range 400 – 750°С are presented. The measurements were carried out on micro-and nanopowders. Nanoscale powders were prepared by mechanical activation. A method based on the study of the kinetics of oxygen isotope exchange between the powder and gaseous oxygen enriched with 18O isotope was used to obtain data on the diffusion coefficients. The average concentration of 18O isotope in the powders was measured using NRA technique. The obtained diffusion coefficients lay in the range of 10-21 - 10-24 m2/s, the diffusion activation energy for all the oxides have been close to 1 eV. These results suggest that the migration of tracer oxygen in oxides LаMnO3+δ, NdMnO3+δ and SmMnO3+δ at low temperatures is realized via structural defects. As for the oxide LaMnO3+δ, its oxygen diffusion coefficients at low temperatures have been lower than the values extrapolated from high temperatures. Such behavior of diffusion properties has not been previously observed in other metal oxides. In this regard, the vacancy formation energy in the rare earth manganites has been supposed to increase with decreasing temperature.

The effect of carbonic anhydrase on the kinetics and equilibrium of the oxygen isotope exchange in the CO2–H2O system: Implications for δ18O vital effects in biogenic carbonates

Interpretations of the primary paleoceanographic information recorded in stable oxygen isotope values (δ18O) of biogenic CaCO3 can be obscured by disequilibrium effects. CaCO3 is often depleted in 18O relative to the δ18O values expected for precipitation in thermodynamic equilibrium with ambient seawater as a result of vital effects. Vital effects in δ18O have been explained in terms of the influence of fluid pH on the overall δ18O of the sum of dissolved inorganic carbon (DIC) species (often referred to as “pH model”) and in terms of 18O depletion as a result of the kinetic effects associated with CO2 hydration (CO2+H2O↔H2CO3↔HCO3−+H+) and CO2 hydroxylation (CO2+OH−↔HCO3−) in the calcification sites (so-called “kinetic model”). This study addresses the potential role of an enzyme, carbonic anhydrase (CA), that catalyzes inter-conversion of CO2 and HCO3− in relation to the underlying mechanism of vital effects. We performed quantitative inorganic carbonate precipitation experiments in order to examine the changes in 18O equilibration rate as a function of CA concentration. Experiments were performed at pH 8.3 and 8.9. These pH values are comparable to the average surface ocean pH and elevated pH levels observed in the calcification sites of some coral and foraminiferal species, respectively. The rate of uncatalyzed 18O exchange in the CO2–H2O system is governed by the pH-dependent DIC speciation and the kinetic rate constant for CO2 hydration and hydroxylation, which can be summarized by a simple mathematical expression. The results from control experiments (no CA addition) are in agreement with this expression. The results from control experiments also suggest that the most recently published kinetic rate constant for CO2 hydroxylation has been overestimated. When CA is present, the 18O equilibration process is greatly enhanced at both pH levels due to the catalysis of CO2 hydration by the enzyme. For example, the time required for 18O equilibrium is nearly halved by the presence of 3.7×10−9M of CA used for the experiments. Despite its significant influence on the oxygen isotope exchange kinetics, the equilibrium oxygen isotope fractionation between individual DIC species and H2O is unaffected by CA. Because many CaCO3-secreting organisms possess active CA, our findings imply that 18O equilibration of the CO2–H2O system is possible within realistic timescales of biogenic calcification.

A preliminary in situ Raman spectroscopic study of the oxygen isotope exchange kinetics between H 2O and (PO 4) aq

Studies on the mechanism and kinetics of the oxygen isotope exchange reaction between H2O and (PO4)aq are essential for the interpretation of natural variations in the oxygen isotope composition of biogenic and inorganic phosphate and utilization of the oxygen isotope composition of phosphates as a geochemical indicator. To gain a deeper understanding of the exchange kinetics we have started an in situ Raman spectroscopy study using an in-house-made Teflon-based fluid cell. Raman spectroscopy allows monitoring the exchange reaction because the substitution of O in aqueous phosphate species by O should cause a shift of those vibrational modes to lower frequencies which involve the motion of oxygen due to the different masses of both isotopes. First in situ exchange experiments have been carried out with a 2 M (NH4)2HPO4 solution (room temperature pH of 8) at 150(±1) °C and 175(±1) °C for 112 and 45 h, respectively. Speciation calculations show that under these conditions HPO4 is the main aqueous phosphate species. The phosphate solution was prepared with H2O enriched with 97 at.% O. At the beginning of the reaction at the experimental temperatures the Raman spectrum of the HPO4 phosphate solution is characterised by distinct bands near 310, 321, 366, 384, 398, 533, 867, 983, and 1076 cm. The latter three bands are the most intense bands and can be assigned to the ν(P–OH), νs(PO3), and the νas(PO3) vibration of the HPO4 molecule (based on a C3v symmetry), respectively. All bands involving oxygen motions are

Oxygen isotope exchange kinetics between H 2O and H 4SiO 4 from ab initio calculations

Oxygen isotope exchange between H 2O and H 4SiO 4 was modeled with ab initio calculations on H 4SiO 4 + 7H 2O. Constrained optimizations were performed with the B3LYP/6–31+G(d,p) method to determine reactants, transition states, and intermediates. Long-range solvation was accounted for using self-consistent reaction field calculations. The mechanism for exchange involves two steps, a concerted proton transfer from H 4SiO 4 forming a 5-coordinated Si followed by a concerted proton transfer from the 5-coordinated Si forming another H 4SiO 4. The 5-coordinated Si intermediate is C2 symmetric. At 298K and with implicit solvation included, the Gibbs free energy of activation from transition state theory is 66 kJ/mol and the predicted rate constant is 16 s −1. Equilibrium calculations between 298K and 673K yield α H4SiO4-H2O that are uniformly less than, but similar to, α qtz-H2O, and therefore α qtz-H4SiO4 is expected to be relatively small in this temperature range.

Experimentally-controlled carbon and oxygen isotope exchange between bioapatites and water under inorganic and microbially-mediated conditions

Modern bone and enamel powders have reacted at 301 K with 13C- and 18O-labelled waters under inorganic and microbial conditions. The aim of the study is to investigate the resistance of stable isotope compositions of bioapatite carbonate (δ 13C, δ 18Oc) and phosphate (δ 18Op) to isotopic alteration during early diagenesis. Rapid and significant carbon and oxygen isotope changes were observed in the carbonate and phosphate fractions of bone apatite before any detectable change occurred in the crystallinity or organic matter content. These observations indicate that chemical alterations of bone apatite are likely to start within days of death. Enamel crystallites are much more resistant than bone crystallites, but are not exempt of alteration. Non removable carbon and oxygen isotope enrichments were measured in the carbonate phase of bone (50–90%) and enamel (40%) after the acetic acid treatment. This result indicates that a significant part of 13C and 18O-labelled coming from the aqueous fluid has been durably incorporated into the apatite structure, probably through isotopic exchange or secondary carbonate apatite precipitation. As a result, acetic acid pre-treatments that are currently used to remove exogenous material by selective dissolution, are not adequate to restore pristine δ 13C and δ 18Oc values of fossil apatites. Under inorganic conditions, kinetics of oxygen isotope exchange are 10 times faster in carbonate than in phosphate. On the opposite, during biologically-mediated reactions, the kinetics of oxygen isotope exchange between phosphate and water is, at least, from 2 to 15 times faster than between carbonate and water. Enamel is a more suitable material than bone for paleoenvironmental or paleoclimatical reconstructions, but interpretations of δ 18Op or δ 13C values must be restricted to specimens for which no or very limited trace of microbial activity can be detected.

Oxygen diffusion in the superconductors of the YBaCuO family: isotope exchange measurements and models

Abstract The thermodynamic and kinetic aspects of the oxygen exchange in the solid oxide-oxygen phase system are discussed for the YBaCuO compounds. For both YBa 2 Cu 3 O 7− x (Y123) and Y 2 Ba 4 Cu 7 O 15− x (Y247) a vacancy model can successfully be used to describe the phase equilibrium. The dependence of some properties (lattice constants, Raman scattering, magnetic properties, and local structure EXAFS) of the nearly-equilibrium Y123 samples on the oxygen-stoichiometry is briefly presented. Studies of the carbon dioxide content in the YBaCuO family compounds show that apart of YBa 2 Cu 4 O 8 (Y124), a complete removal of the carbon without decomposition of the compounds is not possible. In the case of Y247 the influence of the CO 2 content on T C was found to be large. The oxygen-isotope exchange kinetics (tracer diffusion) for all three YBaCuO compounds is discussed in detail. In the structure of Y123 and Y247 the CuO chains are the ‘fastest paths’ for the 18 O diffusion, from which further exchange with the apex and planar sites takes place. A three-dimensional diffusion model for the oxygen exchange is presented. The model and the experimentally determined values of the diffusion coefficients allow the calculation of the isotope distribution for all three oxygen sites as a function of temperature, time and grain size. The kinetics of the low temperature ( 2 Cu 3 O 7− x and Y 0.7 Pr 0.3 Ba 2 Cu 3 O 6.98 . The total OIE in underdoped oxygen depleted Y123 increases with decreasing T C . It has been shown that the OIE in site-selectively substituted samples is mainly due to the oxygen in CuO 2 sites.

Modelling of the oxygen isotope evolution of seawater: implications for the climate interpretation of the δ 18O of marine sediments

It is often argued that the δ 18O value of oceanic water was maintained close to 0‰ for hundreds of millions of years, as a consequence of oxygen isotope exchange between oceanic crust and seawater. However, for several decades, the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because, with increasing age, a general trend of decreasing δ 18O values (about 6‰) is observed in most carbonates, cherts and phosphates, especially for the Paleozoic and early Mesozoic. We developed a dynamical model of seawater-crust interaction that computes the δ 18O value in these two reservoirs as function of time. This model takes into account the continuous production of crust at oceanic ridges, its expansion rate, the permeability profile with space and time, the mineralogical mode of the crust, and the kinetics of oxygen isotope exchange between rock-forming minerals and seawater. The model indicates that the δ 18O value of seawater may vary by ±2‰ with a time response ranging from 5 to 50 Ma for expansion rates of 1 to 10 cm.a −1. The variation of ±2‰ is fixed by both integrated water-rock ratio and closure time of the seawater-crust system by sediments. Variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low δ 18O values of pre-Jurassic marine sediments. According to our model, marine paleotemperatures could be up to 10°C lower than those expected when applying the classical hypothesis of an ice-free ocean with a δ 18O value of −1‰.

Oxygen site exchange kinetics observed with solid state NMR in a natural zeolite

17O solid state nuclear magnetic resonance (NMR) was applied to study the oxygen exchange kinetics between framework oxygens and oxygens in the channel waters in the zeolite mineral stilbite. We have shown that framework oxygens in Al-O-Si sites react faster with channel water than oxygens in Si-O-Si sites and that water self-diffusion in stilbite is fast and thus the oxygen site exchange reaction is the rate limiting step in bulk exchange, at least for temperatures from 150 to 200°C and for exchange times up to 40 h. The observed total amount exchanged agreed well with bulk isotope measurements. The observed oxygen isotope exchange kinetics for Al-O-Si and Si-O-Si sites are not simple exponential. However, they could be approximated with two first-order rate reactions. The mechanism for the oxygen exchange reactions and possible explanations for the kinetics are discussed.

Oxygen isotope exchange kinetics between coexistent minerals and water in the Ertaibei granite pluton, northern Xinjiang

Oxygen isotope exchange kinetics between coexistent minerals and water in the Ertaibei granite pluton, northern Xinjiang

An on-line experimental/analytical method for measuring the kinetics of oxygen isotope exchange between CO 2 and saline/hypersaline salt solutions at low (25–50°C) temperatures

An on-line experimental/analytical method for measuring the kinetics of oxygen isotope exchange between CO 2 and saline/hypersaline salt solutions at low (25–50°C) temperatures has been tested in the system CO 2H 2OCa 2+Cl −. The rate constants can be expressed by: ln k=−6.39−180mT -1 where m=molality (mol kg −1); and T=temperature (K) for CaCl 2 solutions from 0 to 4.0 m between 25° and 50°C. The effect of the shaking frequency on equilibration time is significant at frequencies below 2.0 Hz. Equilibration times are increased because the isotopic exchange is increasingly limited by transport of gas through the liquid. Above 2.0 Hz the rate constants are independent of the shaking frequency. The rate constants do not depend on p CO 2 over the range used in this study (300–600 mbar).

Oxygen isotope exchange kinetics and site-selective oxygen substitution in YBa2Cu318O7−χ

Studies of the oxygen isotope exchange kinetics in 18O substituted YBa2Cu3O7 (123) have been performed. As a weight decrease is observed during the 18O→16O exchange, high resolution thermogravimetric measurements (both isothermal and non-isothermal) were used to investigate the kinetics. The exchange process takes place at relatively low temperatures, 225–430°C, and was found to be non-isokinetic in this temperature range. Two ranges with different activation energies have been found: (1) 1.88 eV both in the ranges 433-398°C (total exchange of 18O in the planes+apex+chains) and 353-307°C (exchange mainly in apex+chains) but with different pre-exponential factors and (2) 0.71 eV at t<290°C (exchange mainly in the chains). Based on these investigations, it was possible to determine the thermodynamic and kinetic conditions for the site-selective (O(1) or O(1), O(4)) oxygen exchange.

Oxygen isotope exchange kinetics of mineral pairs in closed and open systems: Applications to problems of hydrothermal alteration of igneous rocks and Precambrian iron formations

The systematics of stable-isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass-balance considerations, both in closed and in open systems. On mineral-pair δ 18O plots, samples from terranes that have exchanged with large amounts of fluid typically map out steep positively-sloped non-equilibrium arrays. Analytical models are derived to explain these effects; these models allow for different exchange rates between the various minerals and the external fluids, as well as different fluid fluxes. The steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories. These isochrons have initially-steep near-vertical positive slopes that rotate toward a 45° equilibrium slope as the exchange process proceeds to completion. The actual data-point array is thus analogous to the hand of an “isotopic clock” that measures the duration of the hydrothermal episode. The dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter ( u k ) determines the shape of each individual exchange trajectory. In a fluid-buffered system ( u k ⪢ 1 ), the solutions to the equations: (1) are independent of the mole fractions of the solid phases; (2) correspond to Taylor's open-system water/rock equation; and (3) yield straight-line isochrons that have slopes that approach 1 f , where f is the fraction reacted of the more sluggishly exchanging mineral. The isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models, thereby simplifying the application of theory to actual hydrothermal systems in nature. In all of the models an order of magnitude of time (in units of kt) separates steep non-equilibrium arrays (e.g., slope ≈ 10) from arrays approaching an equilibrium slope of unity on a δ-δ diagram. Because we know the approximate lifetimes of many hydrothermal systems from geologic and heat-balance constraints, we can utilize the 18 O 16 O data on natural mineral assemblages to calculate the kinetic rate constants ( k's) and the effective diffusion constants ( D's) for mineral-H 2O exchange: these calculated values ( k qtz ≈ 10 −14, k feld ≈ 10 −13–10 −12) agree with experimental determinations of such constants. In nature, once the driving force or energy source for the external infiltrating fluid phase is removed, the disequilibrium mineral-pair arrays will either: (1) remain “frozen” in their existing state, if the temperatures are low enough, or (2) re-equilibrate along specific closed-system exchange vectors determined solely by the temperature path and the mineral modal proportions. Thus, modal mineralogical information is a particularly important parameter in both the open- and closed-system scenarios, and should in general always be reported in stable-isotopic studies of mineral assemblages. These concepts are applied to an analysis of 18 O 16 O systematics of gabbros (Plagioclase-clinopyroxene and plagioclase-amphibole pairs), granitic plutons (quartz-feldspar pairs), and Precambrian siliceous iron formations (quartz-magnetite pairs). In all these examples, striking regularities are observed on δ-δ and δ-Δ plots, but we point out that δ-δ plots have many advantages over their equivalent δ-Δ diagrams, as the latter are more susceptible to misinterpretation. Using the equations developed in this study, these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation. In particular, we present a new interpretation indicating that Precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide (magnetite precursor?) has exchanged much faster with low-temperature (< 400°C) fluids than has the relatively inert quartz.

On the mechanisms and kinetics of oxygen isotope exchange in quartz and feldspars at elevated temperatures and pressures

Stereological and scanning electron microscope (SEM) analyses of quartz-water and plagioclase feldspar–water interactions have been combined with 18O/17O and 17O/16O measurements of the associated isotope exchange, in an attempt to develop an understanding of mineral-water interactions at elevated temperatures and pressures. The quartz-water exchange (T = 250 to 600 °C; Ph2o = 1 to 22 kbar) is dominated by an “Ostwald ripening” recrystallization mechanism, in which reprecipitated quartz develops as faceted overgrowths on original grains that have been rounded by initial solution processes. Correspondingly, there is a linear decrease in specific surface area (S) as a function of fractional extent of exchange (f). The unrecrystallized quartz cores exchange through diffusional mechanisms that also become dominant in later stages when recrystallization becomes retarded and increasingly ineffective as a mechanism of isotopic exchange. This transition from solution-precipitation to diffusional mechanisms accounts for a “plateau” pressure effect on exchange rate, observed in later stages of exchange. Substantial recrystallization does not occur in feldspar-water exchange at 500 and 600 °C, Ph2o = 2 to 15 kbar; the feldspar grains rapidly degrade into smaller fragments and become incoherently coated by a small amount of fine precipitated crystals. Subsequently, isotopic exchange occurs without further textural change and at constant S. These observations, the preservation of Al/Si order in albite throughout exchange, the high calculated values of oxygen-diffusion coefficients and possible dependence of these coefficients on (Ph2o)½ indicate inhibition of feldspar recrystallization and are consistent with diffusional mechanisms for penetration of water into the feldspar and exchange with shared oxygen atoms in alumino-silica tetrahedra. Equilibrium 18O/16O fractionation factors for both quartz-water and albite-water agree well with those measured previously in this laboratory.