This is a review of a series of discussions on the development of snowball structures in garnet porphyroblasts. The discussion promises future progress in the kinematic interpretation of porphyroblasts in metamorphic tectonites.
Grain boundaries in metamorphic rocks were obsereved by high-resolution transmission electron microscopy. Most of the boundaries have no distinct secondary phases at a nanometer scale. Not only between the same kind of minerals but also between different minerals, mineral grains are directly connected at their boundaries, showing that the structual width of the grain boundaries is less than 0.5nm. Grain boundaries with spheroidal voids are observed in the boundaries. It can be concluded that this type of grain boundary is formed by the healing of microcracks. Low-index plane boundaries are well-developed, showing that this type of boundary has low grain boundary energy. Local structural modification, indicating the segregation of impurities, in grain boundaries is observed. The present results show that the actual width of the grain boundary in rocks is too small for the grain boundary diffusion to be as the dominant transport mechanism.
The relative intensities of Fe Lα1, 2 and Lβ1 X-ray emission peaks differ significantly with valance state and bond association. These X-ray emission peaks cannot be distinguished from each other using an energy-dispersive spectrometer (EDS), because the difference in energy between these peaks is smaller than the spectral resolution of EDS (approximately 100-150 eV). However, these emission peaks can be distinguished from each other using a wavelength-dispersive spectrometer (WDS). A crystal whose 2d value is about 60 A is considered to be suitable for analyzing the first order of Fe Lα1, 2 and L β1 X-ray emission peaks. The second order of these peaks can also be analyzed using lead stearate (STE : 2d=100.4 Å) crystal. Although it is difficult to use the ' peak seek' routine for a mapping analysis, the effects of chemical shifts can be suppressed by configuring the spectrometer. The interferences of high orders of Si and Na emission peaks can be suppressed by a pulse height analysis. Applying a mapping technique to measuring Fe Lα 1, 2 and Lβ1 emission peaks, Fe2+/Fe3+ mapping can be obtained using an electron probe microanalyzer.
This paper reviews recent advances in the analysis of diffusion in garnet in relation to efforts to evaluate the time scale of metamorphism. First, the basic concept of diffusion in crystals (volume diffusion) is presented to clarify the cumbersome terminology in the study of diffusion, the usage of which is sometimes confused. Then some physical and mathematical treatments for analyzing multi-component diffusion, such as the mean-field theory and the eigen vector analysis are discussed in detail. Finally, efforts to evaluate the time scale of metamorphism by Ganguly et al. (1996) using a natural garnet-garnet diffusion couple is critically discussed.
The concept of composition space of J.B. Thompson, Jr. is introduced to Japanese readers. It is emphasized that composition space is useful as it is systematic, but it may be too abstract in some cases. It is also emphasized that the concept of the chemical potential of exchange component is particularly useful as it simplifies the expressions of simultaneous chemical equations.
Isotopic geothermometry is an important tool to clarify the thermal history of the metamorphic events because of independence of pressure. When applying the thermometer, it is necessary that the temperature dependence of the fractionation factors among the rock forming minerals be well calibrated. Carbon isotopic geothermometry between calcite and graphite is the most suitable thermometer to record peak metamorphic temperatures in crystalline limestone, because of the inert property of graphite in an isotopic exchange reaction during retrograde metamorphism. Various authors and approaches have reported fractionation factors in the calcitegraphite system. There is, however, still inconsistency among experimental, theoretical, and empirical results. In particular, the slope of the fractionation curve for temperature dependence still has a discrepancy between theoretical-experimental and empirical data. The causes of this discrepancy may be due to uncertainty in individual approaches, e.g., in experimental studies, uncertainty as to the reaction mechanism due to the inert property of crystalline graphite for isotopic exchange experiments ; in theoretical studies, uncertainty caused by a deficiency of reliable data on the vibrational frequency of calcite and graphite crystals; and, in empirical studies, incomplete crystallinity of carbonaceous matter. According to empirical data on carbon isotopic systematics, in crystalline limestone that metamorphosed at temperatures higher than 400°C, isotopic equilibria have been mostly attained in many cases. At temperatures lower than 400°C, data sets show an increased disequilibrium. Because the carbon isotopic fractionation between calcite-graphite is more than 3‰ at about 700°C, this thermometer has great potential as a tool for determining ultra-high temperature metamorphism. Furthermore, because of the inert property of graphite in an isotopic exchange reaction during retrograde metamorphism, graphite-calcite assemblage is the most probable candidate for preserving the highest temperature condition. Again, carbon isotopic zonation in graphite crystal could provide another tool to clarify the thermal history of polymetamorphic terrains. Oxygen isotopic thermometry was long been widely used to determine the temperatures of metamorphic rocks. It has long been recognized that oxygen isotope thermometers often record discordant temperatures during slowly cooling retrograde metamorphism, because of a relatively rapid diffusion rate during retrograde metamorphism. Revised correction techniques for re-equilibration during a retrograde isotopic exchange reaction under the such high temperatures help in using oxygen isotopic thermometry in deducing thermal history.
SrO contents of epidote group minerals and lawsonite are 1-4 order magnitude higher than those of coexisting major constituent minerals in eclogitic rocks. Epidote group minerals contain >70 % of the whole-rock SrO. CaAl-hydrous silicates are regarded as the most important Sr reservoirs in subducted slab and are capable of transporting Sr deep into the mantle. Their breakdown would contribute significantly to the Sr budget of the mantle and to its Sr isotopic evolution.