This paper presents new hafnium isotope data for the Bure volcanic rocks from the northwestern Ethiopian plateau to constrain the nature of mantle sources within the broad mantle upwelling. The ¹⁷⁶Hf/¹⁷⁷Hf values range from 0.282901 (εHf = 4.79) to 0.283206 (εHf = 15.71) for the Oligocene transitional tholeiite and 0.282915 (εHf = 5.45) to 0.283069 (εHf = 10.85) for the alkaline basalts; these ratios are distinct from MORB but typical to OIB sources. The εHf values correlate positively with εNd. The scattered variation between εHf and contamination indices (such as Nb/La and Ce/Pb) for the transitional tholeiites implies that most tholeiitic basalts are affected by crustal contamination processes. Components 1 (C1) and 2 (C2), which are proposed to be the end-member components for the Afar mantle plume, have a Hf isotopic signature of εHf = 15-17 and 5-9, respectively. The diversity of these compositions relies on the proportion of recycled materials in the deep source, its formation age, and proportion of sediments relative to oceanic crust in the recycled materials.

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Rodingite containing vesuvianite with the world's highest Cr₂O₃ and TiO₂ contents was found to occur in the ultramafic body of the Early to Middle Devonian Darbut-Sartuohai ophiolite zone belonging to the Sartuohai alpine-type ophiolitic melange, west Junggar district, Xinjiang, China. Many blocks of massive and vein rodingite were located at the Sartuohai chromium deposit in the serpentinized ultramafic body along the suture zone near Karamay. The rodingite is mainly composed of vesuvianite, grossular-andradite series garnet, diopside, and chlorite. The Cr₂O₃ content of the rodingite masses and veins is up to 0.2 wt% and 0.1 wt%, respectively. While the Cr₂O₃ content (wt%) of each mineral species in rodingite is up to 0.04 for vesuvianite, 0.4 for garnet and 0.4 for diopside, respectively. Tiny sporadic inclusions of vesuvianite and garnet of 5 μm to 35 μm in diameter also disseminate along the cleavage or in the grain of diopside of the massive rodingite. The vesuvianite inclusions are characterized by a high concentration in chromium of Cr₂O₃ = 6.21 wt% and titanium of TiO₂ = 7.43 wt% of all the reported data, in contrast to the low concentration in the host diopside. The garnet inclusions belong to the grandite series. The morimotoite and uvarovite components are 18% and 11% at most, respectively. The rodingite was considered to have been formed by two stages of metasomatism between gabbroic rock and a calcium-rich solution, which was probably produced in the process of serpentinization of peridotite under Cr- and Ti-rich, and Cr- and Ti-free environments.

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An economically important rare-metal mineralization is recorded in the pegmatite bodies of Gabal Ras Baroud younger granitic pluton, Central Eastern Desert of Egypt. These pegmatite bodies are of variable size and are compositionally zoned. Radiometric measurements of some anomalous pegmatite samples show that their equivalent uranium (eU) content is 219-328 ppm, whereas their equivalent thorium (eTh) content is 783-1101 ppm. On the other hand, the analysis of several separated mineral grains of some pegmatite samples using a scanning electron microscope and X-ray diffraction revealed the presence of several economic minerals. These minerals include zircon, thorite, phlogopite mica, and columbite, in addition to the samarskite-Y mineral. Thorite was found as numerous inclusions of variable size and pattern in zircon. Electron microprobe analysis confirmed the presence of samarskite-Y whose composition corresponds to the empirical formula [(Y_0.49, REE_0.41, Th_0.06, Si_0.05, Ca_0.03, U_0.02, Fe_0.01, Zr_0.00)_Σ1.05(Nb_0.75, Ta_0.17, Ti_0.01)_Σ0.94O₄]. Accordingly, the mineralized Ras Baroud pegmatite can be considered as a promising target ore for its rare-metal mineralization that includes mainly Nb, Ta, Y, U, and REE together with Zr and Th.

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Structural changes of ground SiO₂ glass were investigated using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. SiO₂ glass samples were ground in a ball mill for up to 500 h. Reduction in the particle size of ground SiO₂ glass samples was observed up to ∼ 80 h of milling. After 80 h of milling, the particle size of ground SiO₂ glass remained almost constant at approximately 3 μm. The first sharp diffraction peak (FSDP) position (q₁) of the XRD pattern for ground SiO₂ glass shifted from 1.500 (original) to 1.544 (500 h of milling) Å^-1. The variation may be attributed to a change of the intermediate-range structure. The FSDP position of the 500 h ground SiO₂ glass returned to the original value after annealing at 1000 °C. This annealing behavior includes two components. The variation in q₁ for annealing up to ∼ 500 °C is small (0.0087 Å^-1) and may be attributed to an increase of the Si-O-Si angle. However, for annealing at 500-1000 °C, a large variation was observed (0.0350 Å^-1) and it may be attributed to a decrease of 3-membered rings. IR spectra of samples milled up to ∼ 80 h revealed sharpening of the ν = 1100 cm^-1 band (Si-O stretching mode) and a decrease in the absorption of the ν = 800 cm^-1 band (Si-O-Si bending mode). The IR analyses indicate the shrinkage of Si-O-Si linkages by milling. For over 80 h of milling, these variations are small. The particle size remains almost constant for over 80 h of milling, and therefore, only structural changes in a particle would be responsible for the changes in the IR spectra. The Raman spectra exhibited an increase in the luminescence intensity for milling up to ∼ 40 h. The luminescence may result from surface defect structures on SiO₂ glass grains, and the increase in the luminescence intensity is consistent with particle size reduction by milling.

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The crystal chemistry of piemontite from the Kamisugai mine in the Sambagawa metamorphic belt, Shikoku, Japan was studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction methods. The chemical formula derived from EMPA is (Ca_1.30Sr_0.19La_0.03Ce_0.05Nd_0.02Mn²⁺_0.41)_Σ2.00(Al_1.70Mn³⁺_0.77Fe³⁺_0.48Mg_0.01)_Σ2.96Si_3.04O_12.13(OH)_0.87 (Z = 2). This piemontite is characterized by high Sr (2.2-6.2 SrO wt%), Mn (16.2-18.8 Mn₂O₃ wt%) and REE (0.7-4.0 REE₂O₃ wt%) contents and it is mainly composed of clinozoisite Ca₂Al₃Si₃O₁₂(OH), piemontite-(Sr) CaSrAl₂Mn³⁺Si₃O₁₂(OH), epidote Ca₂Al₂Fe³⁺Si₃O₁₂(OH) and Mn²⁺CaAl₂Mn³⁺Si₃O₁₂(OH) components. The crystal structure of piemontite [a = 8.8673(2), b = 5.6747(1), c = 10.1594(3) Å, β = 114.71(1)°, space group P2₁/m] was refined using 1484 unique reflections to R₁ = 4.4%. The site occupancies at A1, A2, M1, M2 and M3 are Ca_0.59Mn_0.41, Ca_0.71Sr_0.19La_0.03Ce_0.05Nd_0.02, Al_0.72Mn_0.17Fe_0.11, Al_1.0, and Al_0.11Mn_0.55Fe_{0 .34}, respectively. The distribution coefficient K_D (= [(Mn + Fe)/Al]^M1/[(Mn + Fe)/Al]^M3) is 0.048. Although it is known that high Sr contents at A2 seem to promote incorporation of Mn³⁺ (+ Fe³⁺) at M1, the K_D value does not suggest a stronger transition metal preference at M1 than that of Ca end-member piemontite and epidote. High content of Mn²⁺ at A1 modifies the arrangement of the coordinating O atoms of A1O₉ polyhedra. Thus, the A1 coordination may be described as 6-coordinated. <A2-O> increases with increasing Sr content, and the lengthening of A2-Oi is anisotropic.

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Experiments were performed at high temperature and pressure to determine the effective viscosity of a crystal-bearing andesite using the falling sphere method. Because viscosity experiments with partly crystallized samples are difficult to realize (i.e., due to high sensitivity of phase equilibria to P, T and water content), we have added zircon crystals to adjust precisely the volume fraction and the size of crystals in a magma. Using this approach, the anhydrous melt composition does not vary significantly with temperature and water content of the melt and, hence, the effects of crystals on effective viscosity can be worked out accurately. The investigated systems (magma analogue) were composed of an andesitic melt containing 0.5 wt% to 4.1 wt% H₂O and 15 vol% to 40 vol% of zircon crystals with grain size <100 μm. Most experiments were performed at 300 MPa and 1523 K. The ZrO₂ content dissolved in the melt under these conditions is 1.61 wt% ± 0.16 wt% (0.5-4.0 wt% H₂O in melt) with no significant dependence on water content, which is about twice the value predicted by the model of Watson and Harrison (1983).
     The falling velocity of large platinum spheres was measured in the experimental magmas. The radius of the spheres (between 130 μm and 510 μm) was always much larger than the crystal sizes and the inter-grain distances, implying that the falling velocity of the spheres can be used to calculate the effective viscosity of the magmas. For magmas containing 15 vol% zircon, the measured viscosity was 0.7 log to 1.6 log units higher than the melt viscosity. The effective magma viscosity is higher than expected from literature models. The spheres did not move in systems containing 30 vol% to 40 vol% of crystals, even after 16 h at 1523 K. We attribute this observation to the presence of yield strength of more than 100 Pa, i.e., a threshold of accelerating force needs to be passed before the sphere can move in the magma.

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Edited by the Abstracting Committee, Japan Association of Mineralogical Sciences

Ore Minerals and Economic Geology
      Massive Sulfide Deposits 105026—105029;
      REE Deposits 105030—105031;
      Various Mineralizations 105032—105037

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