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Cosmo- conclusion emphasizes the importance of correlated mul- chim. Acta 68, — Bern C. Acta 74, mally delivered silicon that has been sequentially frac- — Average d30Si values and the lowest biological fractionation of Silicon isotopes in the Eastern d30Si values from Isua 3. Acta 72, — Carbonate petrography, kerogen distribution, and carbon and oxygen isotope variations in an early Proterozoic transition 4. Although quartz in Isua BIFs is dominated by low d30Si from limestone to iron-formation deposition, Transvaal Super- values of hydrothermal origin, higher d30Si values group, South Africa.
This suggests inverse segregation, and banded iron formations. Nature , that continental weathering was a locally important pro- — Nature , — Cardinal D. Cates N. Hess, sample prepara- rocks from the Nuvvuagittuq supracrustal belt, northern tion and polishing; J. We thank C. Johnson, B. Beard, S. Moorbath, David J.
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Klein and R. Dymek for the Isua samples; and B. Norsted for The Porpoise Cove supracrustal sequence, Inukjuak area: a assistance in curation of Hamersley, Transvaal and Biwabik sam- unique example of Paleoarchean crust ca. We thank Associate Editor Trevor R. Ireland, two anony- Superior Province. Johnson and H.
Xu for helpful comments on an ear- David J. Davis Craton, Canada. GSA Bull. This research utilized sponges and the reconstruction of the silicon isotope compo- NSF-supported shared facilities at the University of Wisconsin. Geology 31, — De La Rocha C. Acta 61, — Supplementary data associated with this article can be De La Rocha C. Acta 64, — Acta 73, — Princeton Univ. Press, Princeton, New Jersey, tion. Ding T. Jiang S. China Series B 36, — Yangtze River, China. Zhang F. Cosmo- Cosmochim. Jones L. Kaufmann A.
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Halliday A. Geology Lovley D. Anaerobic production of magnetite by a dissimilatory iron- Herrick M. Unlike Mg, enriched stable isotopes for many other elements can be manufactured using other production techniques, such as centrifugation, and are available through private companies.
For the present study, the minor isotope 25 Mg that was enriched to 0. This processing was carried out at the ORNL isotope processing facility. Modified Shewmon-Rhines capsule[ 44 ] and setup: a Mg capsule, b Setup showing the diffusion capsule inserted in a quartz tube and pushed into the heated copper block region using a thermocouple attached to the cap of the diffusion capsule.
With this new design, the Mg capsule could be quenched in liquid nitrogen, reducing cool-down times. Additionally, a thermocouple TC was incorporated in the Mg capsule lid for monitoring the real-time temperature during diffusion annealing experiments, and for corrections on account of heat-up and cool-down times Fig. The Mg capsule body was light and small with dimensions: 0. It also included fins on the outer walls for more rapid thermal equilibration.
The modified capsule was utilized both for pre-annealing Mg samples for grain growth and the tracer diffusion annealing experiments. A number of diffusion capsules were machined for the present work. For depth profile measurements, a precision of 0. Sputter-induced topography[ 28 , 50 ] can be a major source of error in SIMS diffusion depth profile measurements by causing a broadening of the depth profile, thus resulting in a higher tracer diffusivity. This effect is more pronounced in small grained polycrystals where the average grain size, e.
The use of a eucentric rotation stage has been shown to have some success in reducing sputter roughening in polycrystalline matrices;[ 51 ] however, this capability is not routinely available with most SIMS systems, including the one used in this study, and furthermore, it does not directly address the causes of sputter roughening in single crystals. Additionally, the use of a eucentric rotation stage can introduce complications with the interpretation of signal intensity measurements during depth profiling because of the averaging procedures used.
The primary ion angle measured relative to normal incidence and energy were varied by changing the primary and secondary accelerating voltages. The oxygen partial pressures were measured using the sample chamber ion gauge. The main benefit of the oxygen leak is a higher steady state oxygen concentration that leads to the formation of a thicker and adherent amorphous MgO layer during the depth profiling. Sputtering from such an amorphous oxide layer gives less roughness than that from a weakly oxidized magnesium polycrystalline layer.
SRIM[ 53 ] calculations have shown that the interaction depth with the optimal beam conditions is about one half of that under typical conditions that use much larger impact energies at higher primary ion angles with no oxygen leak. Composition specifications in weight percent of a typical pure Mg lot provided by Alfa Aesar. While this polishing recipe utilized de-ionized DI water as the primary solvent medium, there are other polishing recipes for Mg that have utilized non-aqueous solvents throughout with reasonable success.
The polished Mg samples were then coated with a thin isotopic film of 25 Mg using the UHV sputter deposition system that was previously discussed 3. Precise knowledge of the coating thickness was not essential, since a non-linear fit of the SIMS depth profile data to the thin-film solution provided the thickness as a fitting parameter.
Measured tracer diffusion annealing temperatures and effective times computed after correcting for heat-up and cool-down times from the temperature-time profiles. Normal: diffusion direction along rod axis; Ortho: diffusion direction perpendicular or orthogonal to rod axis. Set 1: Measurement of temperature-time profile on identical control sample no tracer ; Set 2: Direct measurement.
For other samples annealed at lower temperatures and much longer times, the corrections were usually lower but depended upon when the time counter was initiated and terminated. The diffusion coefficients at a specified temperature were determined at two to four locations on a single specimen with the SIMS technique, while they were determined in specimens with the radiotracer technique.
SIMS depth profile of the excess 25 Mg tracer concentration abundance after tracer diffusion annealing at The non-linear fit using the Gaussian thin film solution and the residuals are also shown. Note that the data from the first micron of the measured depth profile was not used for the fit. Arrhenius parameters for the Mg-self diffusion coefficients determined using the SIMS this work and radiotracer techniques[ 44 ]. The Arrhenius parameters for the combined data are also shown. Purity levels of Mg are provided in weight percent.
Arrhenius fit of Mg self-diffusion data using both SIMS this work and radiotracer data[ 44 ] in polycrystalline Mg normal samples. An Arrhenius fit to the polycrystalline data from both types of measurements Fig. This appears to suggest a single diffusion mechanism in pure Mg, although a more conclusive analysis would require single crystal diffusion measurements over a broad range of temperatures.
Extending the temperature range of measurements on such samples would be desirable for a better fit, and for exploring whether the differences in the self-diffusivities between the normal and ortho samples are significant. This may be indicative of a larger variation in the orientation-dependent diffusivity at lower temperatures, or a grain boundary diffusion field that affects SIMS profiles at locations that are less remote from grain boundaries.
A one-to-one mapping of the SIMS diffusivities as a function of the local orientation and position relative to grain boundaries at various temperatures would shed further light on this issue. Hence a more laborious undertaking involving measurements on the same sample using independent instruments for SIMS and EBSD mapping would be needed for such studies. The polycrystalline self-diffusivities in Mg measured using either the SIMS this work or radiotracer methods[ 44 ] were higher than the experimental single crystal diffusivities reported[ 19 , 45 ], which in turn are higher than the theoretical predictions.
A SIMS study as a function of grain size and grain orientation in isotropic and anisotropic materials at various temperatures would be helpful in assessing the role of grain boundary diffusion and orientation-dependent diffusivity on bulk diffusivity measurements in the initial Harrison B region. Data from such measurements could then be compared with corresponding single crystal measurements using both SIMS and radiotracer methods.
Even though SIMS-based tracer diffusion studies are more convenient than radiotracer diffusion studies, there is still a need to develop more efficient procedures for gathering SIMS data in order to facilitate the development of tracer diffusion databases that permit the use of the complete Onsager diffusion formalism. One such method based on an interdiffusion, isotopic analysis in a diffusion couple was recently discussed by Belova et al. Since some naturally occurring elements in the periodic table are monoisotopic e. The recent availability of the rare and expensive 26 Al radioactive isotope[ 48 ] would be a good test case for more efficient tracer diffusion measurements, noting that Al tracer diffusion data in many commercial Al-alloy systems are not easily available.
An extension of this study to SIMS-based tracer diffusion measurements in Mg alloys is expected in a future publication. Special experimental techniques developed during the course of this work included a UHV deposition system for low-oxygen isotopic thin-film deposition, and a modified Shewmon-Rhines annealing capsule for accurate temperature-time profile measurements of Mg tracer diffusion samples in a protected environment. Optimum conditions for minimizing sputter-roughening during SIMS depth profiling were established and utilized for the tracer diffusion measurements.
A numerical procedure was employed to correct for heat-up and cool-down time corrections from the temperature-time profiles. A least-squares method based on the Gaussian thin-film solution was successfully used to obtain the diffusion coefficients along with the background tracer concentrations and the initial tracer film thicknesses from the measured SIMS depth profiles.
The authors are grateful for the support provided by the U. The authors thank the ORNL isotope processing facility, including Scott Aaron and Lee Zevenbergen for providing the enriched 25 Mg isotopic foils used in this work and for helpful discussions. The assistance of Jay Tuggle and the students of one of the authors, J.
Hunter, at Virginia Tech during the course of this project are appreciated. The authors thank Sarah Brennan and the graduate students of one of the authors, Y. Skip to main content Skip to sections. Advertisement Hide. Download PDF. Article First Online: 23 September Introduction Diffusion in multicomponent systems has been best described using the Onsager flux equations of irreversible processes that are based on linear relations between the fluxes and driving forces in the lattice frame of reference.
The relations between intrinsic, interdiffusion and phenomenological diffusion coefficients in multicomponent systems are more complex[ 9 , 10 ] and are not considered here, but simplifications are possible if the off-diagonal terms in the L ij matrix can be ignored i.
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The well-known thin-film method and solution[ 22 , 23 , 24 ] used in radioactive tracer diffusion measurements is also applicable for such measurements carried out with the SIMS technique where enriched stable isotopes serve as tracers. Physical vapor deposition PVD techniques such as sputtering or evaporation are the common ones utilized though other techniques such as electroplating can also be used. The initial tracer film is considered to be a thin-film if the film thickness is much smaller than the diffusion length, i.
Mg Stable Isotopes Mg consists of three stable isotopes whose natural abundances are: 24 Mg—0. Open image in new window. To address this, Shewmon and Rhines[ 44 ] designed a special Mg container with Mg turnings placed in close proximity of the samples to be annealed Fig.
The Mg capsule was in turn placed in a pyrex glass container and sealed Fig. The Mg turnings not only provided the necessary equilibrium vapor pressure over the diffusion samples and prevented evaporative loss, but also protected against oxidation. By placing two active containing tracer Mg surfaces facing each other, the loss of the tracer due to evaporation was further minimized. This elegant design was further improved in this work with the use of a permanent quartz tube for containing the Mg capsule vapors Fig.
This resulted in higher throughput and lower glass processing costs. The use of a permanent quartz tube also reduced heat-up times of the Mg-capsule containing the Mg samples, since the quartz tube was already at the selected annealing temperature prior to insertion of the Mg capsule by pre-placement in a copper Cu heater block at the desired annealing temperature. The pre-heated Cu heater block had a Additional factors that can have a more pronounced impact on sputter roughening during SIMS depth profiling in polycrystalline samples are the grain size and grain size distribution.
Depth profiling measurements on the pre-annealed large-grained samples using typical and optimized conditions clearly showed a sharpening of the depth profile in the latter case,[ 52 ] thus demonstrating the dual benefits of depth profiling on large grained samples with optimized SIMS conditions. Samples used in this study were transferred between ORNL sample preparation, annealing , Virginia Tech SIMS, polishing and University of Central Florida isotope deposition using special containers that protected the samples from movement and minimized exposure to the environment.
Pure Mg The small average grain size and the broad grain size distribution in the as-received Mg rod samples Fig. Samples that were sectioned in a direction parallel to the rod axis would have a tracer diffusion direction that was nearly parallel to the c-axis of the hcp crystal. Since the initial version of the capsule did not have a thermocouple in the lid, the temperature-time profiles for this first set of samples were obtained using similar size control samples equipped with a thermocouple and subjected to the identical annealing conditions.
Nominal temperature Nominal time o C K h s , 4 14, 1 3, 0. All samples were quenched in liquid nitrogen on completion of the diffusion anneals. The intensities for all the three Mg isotopes 24, 25, 26 Mg were measured as a function of time. The sputter rate was calculated from the total measurement time and the final crater depth that was measured usinga profilometer.