Evaluation of olivine-spinel geothermometry as an indicator of thermal history for peridotites

Olivine and spinel in peridotites from the Miyamori ultramafic complex and the Ichinogemata crater of Northeast Japan show a systematic variation in the Mg/ (Mg+Fe) ratio which is correlated mainly with the grain size of spinel. This correlation can be explained by a diffusion model assuming a semi-infinite composite sphere under cooling or heating conditions. In order to obtain absolute temperatures of thermal events, the olivine-spinel geothermometer is applied to pairs of spinel core and olivine core (average composition). The calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel. In the Miyamori complex, the calculated temperatures decrease monotonically with decrease in grain size of spinel, whereas in the Ichinomegata lherzolite nodule those of spinel smaller than 0.2 mm increase as the grain size decreases and those of spinel larger than 0.2 mm remain constant regardless of further increase in grain size. These observations, in the light of the diffusion model, suggest that the Miyamori complex may have cooled from higher than 800° C to lower than 600° C and that the lherzolite nodule from the Ichinomegata crater may have been in equilibrium at 900° C before it was heated above 1,100° C for less than a few days. These two examples indicate that olivine-spinel pairs of peridotites do not always indicate an appropriate equilibration temperature. We cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity.     

Boron isotopic composition of fumarolic condensates and sassolites from Satsuma Iwo-jima,Japan

¹¹B/¹⁰B ratios of the high temperature fumarolic gases (>465°C) of this island were found to be constant within the limits of experimental error (¹¹B/¹⁰B = 4.066). This value may represent the ¹¹B/¹⁰B ratio of boron in the andesite magma. ¹¹B/¹⁰B ratios of the low temperature fumarolic gases (<235°C) were found to vary from 4.053 to 4.077. ¹¹B/¹⁰B ratios of some sassolites were approximately equal to that of the fumarolic condensates and the other ones were slightly enriched in ¹⁰B compared to the fumarolic condensates.     

High‐resolution national land use scenarios under a shrinking population in Japan

In sharp contrast with the global trend in population growth, certain developed countries are expected to experience rapid national population declines. Considering future land use scenarios that include depopulation is necessary to evaluate changes in ecosystem services that affect human well‐being and to facilitate comprehensive strategies for balancing rural and urban development. In this study, we applied a population‐projection‐assimilated predictive land use modeling (PPAP‐LM) approach, in which a spatially explicit population projection was incorporated as a predictor in a land use model. To analyze the effects of future population distributions on land use, we developed models for five land use types and generated projections for two scenarios (centralization and decentralization) under a shrinking population in Japan during 2015–2050. Our results suggested that population centralization promotes the compaction of built‐up areas and the expansion of forest and wastelands, while population decentralization contributes to the maintenance of a mixture of forest and cultivated land.     

Topotaxic relationships between spinel and pyroxene in kelyphite after garnet in mantle-derived peridotites and their implications to reaction mechanism and kinetics

Kelyphite is a reaction product between garnet and olivine, which was formed by subsolidus reactions upon decompression during the ascent of mantle peridotite. We studied crystallographic relationships among constituent (product) phases of kelyphite ?C orthopyroxene, clinopyroxene, spinel and reactant phases, garnet and olivine, using EBSD and found that, for a relatively high temperature sample (from Czech Moldanubian), spinel and pyroxenes are in a topotaxic relationship in such a way that spinel {111} coincides with pyroxene (100) and spinel {110} coincides with pyroxene (010); while the topotaxy is incomplete or non for a lower-temperature sample (from western Norway). On the basis of the observed microstructural and crystallographic relationships, we propose a hypothesis that the topotaxic relationship may be established at nucleation stages of the onset of the kelyphitization and that the degree of topotaxy may be related to the transformation temperature and the degree of supersaturation of the reaction. The lower the temperature, the higher the supersaturation and, therefore, more rapid the nucleation becomes, resulting in a more disordered state in topotaxic relationship.