| 1. |
- Högdahl, Karin, et al.
(författare)
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Low P-T Caledonian resetting of uranium-rich Palaeoproterozoic zircons, central Sweden
- 2001
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Ingår i: American Mineralogist. - 0003-004X .- 1945-3027. ; 86:4, s. 534-546
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Tidskriftsartikel (refereegranskat)abstract
- Uranium-rich zircons from a Paleoproterozoic, high-grade deformation zone in the Fennoscandian Shield, central Sweden, show an almost complete resetting of the U-Pb system in early Phanerozoic time. A mylonitic gneiss in the deformation zone contains two types of highly discordant (>70%), U-rich zircons: large, brown, cloudy prisms, and small milky-white irregularly shaped grains. The gneiss also contains mostly clear prismatic zircon of lower U content with mildly discordant to concordant U-Pb ages. Laser Raman spectroscopy reveals that the dark cathodoluminescent areas in brown zircons have a highly metamict crystal structure, whereas the structures of both the dark cathodoluminescent milky-white grains and the bright cathodoluminescent clear prisms have higher degrees of crystallinity.Age dates obtained by U-Pb SIMS analysis of 40 zircons of the three types described above range continuously from concordant at 1871 ± 11 Ma to 98% discordant at 384 ± 15 Ma. The strongly discordant zircons clearly have suffered severe disturbance at about the time of the Caledonian orogeny. However, Caledonian metamorphic temperatures and pressures in this region did not exceed 150–200 °C and 1–3 kbar, too low to strongly disturb the U-Pb systematics in non-metamict zircon by thermal means alone. Independent evidence indicates that saline fluids were circulating in the Paleoproterozoic basement rocks at this time, possibly driven by hydrological gradients generated in front of the encroaching Caledonian orogenic wedge. These low-temperature saline fluids are inferred to be responsible for causing both strong Pb loss in the mostly metamict brown zircons via a diffusive process, and the formation of small milky-white zircon via a low-temperature recrystallization or dissolution/re-precipitation process.
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| 2. |
- Dinnebier, RE, et al.
(författare)
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Bulk moduli and high-pressure crystal structures of minium, Pb3O4, determined by X-ray powder diffraction
- 2003
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Ingår i: American Mineralogist. - 0003-004X. ; 88:7, s. 996-1002
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Tidskriftsartikel (refereegranskat)abstract
- We report the pressure dependence of the crystal structure of lead tetroxide (P less than or equal to 41.05 GPa, T 298 K) using high-resolution angle-dispersive X-ray powder diffraction. Pb3O4 shows two reversible phase transitions in the measured pressure range. The crystal structures of the modifications identified have in common frameworks of Pb+4O6 octahedra and irregular Pb+2O4+1 respectively Pb2+O6+1 polyhedra. At ambient conditions, Pb3O4 crystallizes in space group P4(2)/mbc (phase 1). Between 0.11 and 0.3 GPa it exhibits a displacive second order phase transition to a structure with space group Pbam (phase II). A second displacive phase transition occurs between 5.54 and 6.6 GPa to another structure with space group Pbam (phase 111) but halved c dimension. A non-linear compression behavior over the entire pressure range is observed, which can be described by two Vinet relations in the ranges from 0.28 to 5.54 GPa and from 6.6 to 41.05 GPa. The extrapolated bulk moduli of the high-pressure phases were determined to be K-0 = 21(2) GPa for phase 11 and K-0 = 91(3) GPa for phase III. The crystal structures of all phases were refined from X-ray diffraction powder data collected at several pressures between 0.06 and 41.05 GPa. Except for their cell dimensions, phases I and 11 were found to be isostructural to the corresponding phases at low temperatures, whereas phase III can be derived from the Sr2PbO4 aristotype. With increasing pressure, the lone pair which is localized at Pb2+ adopts increasingly pure s-character, which is reflected by the similar coordination polyhedra of Ph2+ in Pb3O4 (phase 111) and of Sr2+ in Sr2PbO4.
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| 3. |
- Dubrovinsky, LS, et al.
(författare)
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Gruneisen parameter of epsilon-iron up to 300 GPa from in-situ X-ray study
- 2000
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Ingår i: AMERICAN MINERALOGIST. - : MINERALOGICAL SOC AMER. - 0003-004X. ; 85:2, s. 386-389
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Tidskriftsartikel (refereegranskat)abstract
- We show that high-quality powder X-ray diffraction data, collected in diamond anvil cells, provide sufficient information for Rietveld refinement and determination of temperature factors. For the first time using a new method based on combination of therm
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| 4. |
- Saxena, SK, et al.
(författare)
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Iron phases at high pressures and temperatures: Phase transition and melting
- 2000
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Ingår i: AMERICAN MINERALOGIST. - : MINERALOGICAL SOC AMER. - 0003-004X. ; 85:2, s. 372-375
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Tidskriftsartikel (refereegranskat)abstract
- Based on an in situ X-lay study of Fe and a review of the available data, we propose the following triple-points in the Fe phase diagram-the hexagonal closest packed (HCP), face-centered cubic (FCC), and the beta phase: P = 40 (4) GPa at T = 1550 (100) K,
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| 5. |
- Skogby, H, et al.
(författare)
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An assessment of nuclear microprobe analyses of B in silicate minerals
- 2003
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Ingår i: American Mineralogist. - 0003-004X. ; 88:10, s. 1601-1604
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Tidskriftsartikel (refereegranskat)abstract
- With recent findings of substantial amounts of B in rock-forming minerals, it has become evident a need exists for microprobe techniques allowing quantitative B determinations in minerals. A microprobe method based on the nuclear reaction B-11(p,alpha)2alpha has the potential to become a powerful tool for analyses of minerals with B contents spanning a wide concentration range. In this paper, we present nuclear microprobe analyses of B in a set of silicate minerals containing stoichiometric concentrations of B in the range 1.9-8.8 wt%. Our results strongly indicate that nuclear reaction analyses of silicates utilizing the B-11(p,alpha)2alpha nuclear reaction are virtually matrix-insensitive, and saturation effects are negligible within the studied range of B concentrations. This finding suggests that it is adequate to use a single B standard for calibration purposes for analyses covering wide ranges of B concentrations in silicates. In addition, we demonstrate how B-concentration maps of high contrast and resolution may be produced by scanning the proton beam over the samples.
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