The Higo metamorphic terrane in west–central Kyushu, southwest Japan, consists of a sequence of greenschist– to granulite–facies metamorphic rocks. This study reports the U–Pb Zrn ages and rare earth elements (REE) characteristics of Zrns and Grts from pelitic gneisses in the Higo metamorphic terrane to constrain its timing of high–temperature metamorphism. Based on the morphology, internal textures, occurrences, Th/U ratios, REE patterns and ages of Zrn, and the heavy REE (HREE) partition coefficient between Zrn and Grt, the results indicate that the prograde metamorphism occurred at ~ 120 Ma immediately after precursor deposition at 120–130 Ma and peak metamorphic conditions were attained during the period between 110 and 120 Ma. The ~ 110 Ma age might be indicative of the onset of cooling and exhumation after the peak metamorphic stage. Retrograde metamorphism continued at least to 105 Ma as obtained from Zrns in the granitic intrusion.
Ultrahigh–temperature (UHT) granulites characterized by the parageneses of sapphirine + quartz and orthopyroxene + sillimanite + garnet + quartz are found in Rundvågshetta, Lützow–Holm Complex (LHC), East Antarctica. In this study, we report previously undocumented features of the UHT pelitic granulites, including the presence of felsite–nanogranite inclusions (FNIs), which are interpreted to have formed by the supercooling of melt inclusions in garnet, and the presence of three Al2SiO5 polymorphs within the same garnet porphyroblast. Whereas sillimanite is present as inclusions in garnet and in the matrix, kyanite occurs as inclusions in garnet and andalusite appears in FNIs. Porphyroblastic garnet is compositionally zoned with Mg–rich cores surrounded by Fe– and Mn–rich rims, indicating homogenization and later modification of Fe, Mg, and Mn. Ca contents oscillate from core to rim, which may be a remnant of growth zoning and suggests compression before high–temperature decompression. Integration of these new data with those previously documented leads to the inference of a rapid change in pressure–temperature conditions during regional UHT metamorphism in the late Neoproterozoic to early Paleozoic Gondwana–forming collisional orogen. Locally occurring intimate intergrowths of kyanite/sillimanite + sapphirine + garnet ± quartz in garnet may be a breakdown product of Mg–rich staurolite and thus may be a link between high–pressure and UHT metamorphism.
We performed triaxial deformation experiments on cylindrical specimens of mafic and ultramafic rocks to quantify their dilatant behaviors. Experiments were performed using an intra–vessel deformation and fluid flow apparatus at room temperature, a constant strain rate of ~ 10−6 s−1, and a confining pressure of 20 MPa. Axial and radial strains were monitored using strain gauges glued to the specimens. The onset of dilatancy of mafic rocks ranged from 41 to 64% of the maximum differential stress, and the inelastic volumetric strain reached ~7 × 10−3 at the maximum differential stress. These results are generally typical of crystalline rocks. Microstructural observation of recovered samples after deformation indicates that the dilatancy of mafic rocks were caused by opening of axial microcracks. In contrast, the onset of dilatancy of ultramafic rocks occurred at >77% of the maximum differential stress, and the inelastic volumetric strain at the maximum differential stress was less than 2.7 × 10−3. These contrasting behaviors of dilatancy could be related to different crack modes developed during deformation, where shear microcracks along grain boundary were dominated in ultramafic rocks as is evident from a nearly random crack orientation along grain boundary. As dilatancy is related to the opening of microcracks, these contrasting dilatant behaviors of mafic and ultramafic rocks can lead to different modification of the physical and transport properties during brittle deformation.
Electrical conductivities of three granulite samples (main minerals: plagioclase, quartz, and biotite) with different chemical compositions (WB, the weight percent of Fe2O3 = 5.49, 8.75, and 14.79 wt%) were researched using a complex impedance spectroscopic technique at 1.0–3.0 GPa and 623–1073 K from 10−1 to 106 Hz. The experimental results indicated that the granulite conductivities markedly increased with temperature and slightly decreased with increasing pressure. Under the experimental conditions, the temperature dependence of the conductivities of granulite followed an Arrhenius relationship at certain temperatures. The electrical conductivities of granulite significantly increased with increasing biotite content and WB. According to the activation enthalpies and previous studies, the conduction mechanism of the granulite samples with WB = 8.75 and 14.79 wt% was small polaron conduction under the experimental conditions, and the conduction mechanisms of the granulite sample with WB = 5.49 wt% were small polaron conduction at high temperatures and impurity conduction at low temperatures. The high conductivity anomalies under the ductile shear zones in southern India can be interpreted by the conductivities of granulite with interconnected biotite and a high iron content (>14.79 wt%).
The Inner Zone of Southwest Japan is dominated by voluminous Cretaceous to Paleogene igneous rocks. The central part of Yamaguchi Prefecture, in the Susuma–Nagaho area, displays a dominance of the Susuma–Nagaho Plutonic Complex that consists of granodiorite and gabbro. The gabbro can be subdivided into fine–grained gabbro and coarse–grained cumulus gabbro. The fine–grained gabbro has a high Mg–number similar to primitive basalt with negative εNd values corrected to 92 Ma although Cr and Ni contents are relatively low, which indicates that the basaltic magma may have been derived from enriched mantle. In consideration of the geochemical signatures including the Sr and Nd isotopic compositions, the enriched mantle characterized by negative εNd isotopic values would have existed underneath the Inner Zone of Southwest Japan during the Cretaceous.
Amorphous magnesium carbonate (AMC) is an important phase in the early formation stage of magnesium carbonate. In this study, precipitation experiments were conducted to clarify the formation and transformation process of AMC in aqueous solution. Fine AMC particles precipitated, immediately after mixing of Na2CO3 and MgCl2 solutions. Chemical composition of the AMC was determined to be approximately MgCO3·2H2O although two hydration states were expected to exist for AMCs. Subsequently, the AMC transformed in aqueous solutions into needle–like crystals of nesquehonite (MgCO3·3H2O), eventually to tiny polycrystalline particles of dypingite [Mg5(CO3)4(OH)2·5H2O] via a solvent–mediated processes.