This paper presents new Sr–Nd–Pb–Hf isotopic compositions of tholeiitic basalts dredged along the Aden Ridge of the central Gulf of Aden (45.5°E–49°E). Two groups of basalts are identified based on their contrasting spatial distributions and geochemical signatures. Basalts dredged from east of 46.2°E (Group 1) are LREE–depleted but have relatively wide variations in 87Sr/86Sr (0.70278–0.70304) and 206Pb/204Pb (18.21–19.03) and limited 143Nd/144Nd (0.51301–0.51309) and 176Hf/177Hf ranges (0.283224–0.283276; εHf = 15.98–17.83), analogous to the geochemical signature of enriched to depleted normal mid–oceanic ridge basalts (N–MORB). In contrast, the basalts dredged between 45.6°E and 46.2°E (Group 2) are more enriched in incompatible elements [ocean island basalt (OIB)–like] but have little variation in 87Sr/86Sr (0.70323–0.70341), 206Pb/204Pb (19.33–19.49), and 143Nd/144Nd (0.51285–0.51292) and a wide variation in 176Hf/177Hf (0.283020–0.283155; εHf = 8.77–13.54). Isotopic and trace element variations can be attributed to the mixing of three types of mantle source: depleted MORB–type mantle (Component i), matrix mantle of the Afar plume (Component ii), and recycled oceanic crust (Component iii) occurring as blobs, streaks, or ribbons within the plume matrix. Mixing between Components i and ii would have produced Group 1 basalts, and Components ii and iii produced Group 2 basalts. Our data suggest that the Afar plume components were involved in magma genesis westward from 48°E in the Aden Ridge.
In reconstructions of the Gondwana supercontinent, correlations of Archean domains between Madagascar and India remain debated. In this paper, we aim to establish correlations among these Archean domains using whole–rock geochemistry and U–Pb zircon geochronology of meta–granitoids from the Masora and the Antananarivo domains, central–eastern Madagascar. A meta–granitoid from the central part of Masora domain is dated at 3277 Ma and shows a typical Archean tonalite–trondhjemite–granodiorite composition, whereas a tonalitic gneiss from the southeastern part of the Antananarivo domain gives an age of 2744 Ma. The geochemical signature of this tonalitic gneiss differs from that of the ~ 2500 Ma granitoids of the northwestern part of Antananarivo domain. In addition, the geochemical composition of the ~ 760 Ma granitic gneisses is consistent with a volcanic–arc origin for the protolith. Based on the geochemical and geochronological results, along with existing data, we identified three episodes of granitic magmatism in central–eastern Madagascar at ~ 3300, 2700, and 2500 Ma. These three magmatic events are consistent with those reported for the Dharwar Craton in India, suggesting that the Archean Masora and Antananarivo domains in Madagascar were part of the Greater Dharwar Craton during the period of 3300–2500 Ma. The 700–800 Ma volcanic arc granites identified in eastern Madagascar have not been reported in India. Therefore, the subduction of the oceanic plate that led to the formation of these granites likely took place at the western margin of the Greater Dharwar Craton, which included part of eastern Madagascar.
The sorption characteristics of cesium (Cs) ions into weathered biotite with biotite–vermiculite interstratification collected from weathered granodiorite in Fukushima Prefecture, Japan has been investigated. Both single crystals and crushed powder forms of the weathered biotite were experimentally reacted with 20–2000 ppm CsCl aqueous solutions, and analyzed by powder X–ray diffraction (XRD), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) to examine the distribution of Cs inside the crystals. From the XRD pattern, the proportion of vermiculite unit layers in the weathered biotite was estimated at ~ 12%, with a tendency for segregation, and the whole XRD pattern was explained by the coexistence of biotite and vermiculite packets as well as the interstratified regions. Powder XRD of Cs–sorbed specimens showed that the 14.9 Å peak of the vermiculite packets was weakened at a low Cs concentration in the solution. Single crystals of the weathered biotite with a polished edge–surface were immersed in the CsCl solutions and examined using SEM and high–angular annular dark field (HAADF) imaging in STEM. Cs was not only incorporated in the vicinity of the exposed surface but also penetrated deeply inside the crystals. These analyses and observations revealed the Cs–sorption process in weathered biotite. At first, Cs preferentially replaced specific vermiculite interlayers in the vermiculite packets. With a higher Cs concentration in the solution, the Cs–substituted vermiculite interlayers increased in the vermiculite packets, and vermiculite layers interstratified in biotite also incorporated Cs.
Mieite–(Y), ideally Y4Ti(SiO4)2O[F,(OH)]6, was found in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan. It occurs as an amber yellow mass with adamantine luster, approximate size of 1 cm and white streak. The mineral is associated with quartz, albite, K–feldspar, muscovite, allanite–(Ce), gadolinite–(Y), and magnesiorowlandite–(Y). Cleavage is not observed and fracture is uneven. The Mohs hardness is 6. The calculated density is 4.61 g/cm3. It is biaxial and refractive indices are α = 1.694(2) and γ = 1.715(5) with non–pleochroism. The mineral displays anomalous blue interference colors. The empirical formula is (Y3.13Dy0.20Gd0.17Yb0.08Nd0.08Sm0.07Er0.07Th0.05Tb0.03Ho0.03Lu0.03Ce0.02Tm0.02U0.02)Σ4.00(Ti0.52Al0.44Fe0.01)Σ0.97(Si1.92P0.12)Σ2.04O9[F3.83(OH)1.91]∑5.74 on the basis of 7 cations and 9 oxygen atoms pfu after electron microprobe (WDS), FT/IR and crystal structure analyses by means of single crystal XRD data. The raw material is significantly metamictized to give extremely weak diffraction peaks. The unit cell parameters refined from powder XRD pattern of recrystallized material are; a = 14.979(6), b = 10.548(5), c = 6.964(3) Å, V = 1100.3(8) Å3 and Z = 4. The 7 strongest lines in the powder XRD pattern [d(Å) (I/I0) hkl] are; 2.68 (100) 331, 3.76 (85) 400, 3.54 (83) 002, 3.48 (82) 130, 2.16 (78) 023, 4.26 (68) 021, 5.46 (58)111. The crystal structure was refined in space group Cmcm to R1 = 0.0825 and 0.0735 for 491 and 581 reflections with I > 2σ(I) single crystal XRD data of raw and recrystallized materials, respectively. The crystal structure of mieite–(Y) consists of infinite columns of corner–sharing TiO6 octahedra decorated by SiO4 tetrahedra. The columns are linked by two independent Y polyhedra with different coordination, YO2F5 and YO5F3. A coupled substitution of Ti4+ + F− = Al3+ + □ (vacancy) was suggested for mieite–(Y). Mieite–(Y) is isostructural with the ‘yftisite’, a discredited species. Mieite–(Y) can be classified in the Dana class 126.96.36.199 and Strunz class 9.AG.25, nesosilicates with additional anions.
The Abukuma plutonic rocks are one of the major Cretaceous granitic suites in the Japan Arc. In the central to eastern Abukuma Plateau, intrusive ages of 115–97 Ma were obtained using zircon U–Pb method. According to the compilation of all existing U–Pb ages, seven intrusive stages were detected between 121 and 96 Ma. Especially, differences of the intrusive ages and geochemical variations were identified between the eastern and western leucocratic granitoids, which was likely due to the heterogeneity of their source rock compositions and/or difference in the depth of magma geneses.
Yukonite and wallkilldellite–(Fe) were discovered at the Uriya deposit, Kiura mine, Ume, Saiki City, Oita, Japan, as first occurrences in Japan. Both minerals partly cover the surface of quartz and arsenopyrite masses and are closely associated, forming layered radiating aggregates. The layers are divided roughly into yellowish–brown and reddish–brown layers, where the former is yukonite and the latter is wallkilldellite–(Fe). The X–ray diffraction (XRD) pattern of Kiura yukonite has sharp peaks compared with that from the type locality, indicating that it is well crystalline. The empirical formula of the yukonite from electron probe microanalyzer (EPMA) analysis on the basis of As = 3 is Ca2.31Fe3+4.20 (AsO4)3.00(OH)8.22·12.07H2O. The empirical formula of the wallkilldellite–(Fe) on the basis of O = 20 is Ca3.51Fe2+5.13(AsO4)4.55(OH)3.64·12.46H2O. The wallkilldellite–(Fe) from the Kiura mine is very weak against heat, and the XRD pattern changes at 50 °C. On the other hand, the XRD pattern of the yukonite changes at 150 °C, a temperature higher than those of yukonite at other localities, indicating that it is well crystalline. We also found the signature of a structural relationship between the yukonite and wallkilldellite–(Fe) based on XRD and transmission electron microscope (TEM) observations.
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