A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (T_C) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including T_C and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite ( 34%) and apatite ( 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a T_C-Z scale “calibrated” with field-based T_C data for titanite (≥ 680 ± 20°C) and apatite ( 500°C). Extrapolation of this scale yields T_C estimates for the following minerals: staurolite (T_C ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.     

Ab initio 2-D periodic Hartree–Fock study of Fe-substituted lizardite 1T– a simplified cronstedtite model

 A 2-D periodic ab initio Hartree–Fock LCAO study was performed on Fe-substituted 1:1 sheet silicate, lizardite. The atomic orbitals were described by large-core pseudopotentials (Fe atoms) and a sp basis set (the other atoms). Calculated atomic charges and the results of bond population analysis indicate that the Fe-O bond was as ionic as the Mg-O. This hypothesis was supported by calculated density of states and electron density maps. Increasing Fe for Mg substitution changed the shape of projected density of states (PDOS) of O(p) orbitals. Received: 27 January 2000 / Accepted: 21 May 2000     

Formation of solid bituminous matter in pegmatites: Constraints from experimentally formed organic matter on microporous silicate minerals

The formation of solid bituminous matter (SBM) on surfaces of microporous silicates was experimentally studied at pressure and temperature conditions typical of late-stage magmatic and hydrothermal processes. Aliquots of microporous silicate minerals (zorite and kuzmenkoite-Mn, Lovozero Alkaline Massif, Kola Peninsula, Russia) were exposed to solid or liquid organic carbon sources (natural brown coal and liquid 1-hexene for synthesis purposes) in a 0.1 M NaCl-solution for 7 days, at constant pressure (50 MPa), and at three individual temperatures (200, 275, and 300 °C). No thermal decomposition of the solid organic sources happened at 200 °C and only a thin film of brown coal derivatives on the silicates’ surfaces and no formation of SBM were observed at 275 °C and 300 °C. But solid bituminous matter on the surfaces of both microporous silicates were detected in experiments with liquid 1-hexene as organic carbon source and at temperatures of 275 °C and 300 °C with a more pronounced formation of SBM at 300 °C compared to 275 °C. The aromatic and aliphatic hydrocarbons, as well as alcoholic compounds of the experimentally produced SBM are similar, if not even partly identical, with natural SBM occurrences of the Khibiny and Lovozero Massifs, Kola Peninsula, Russia, and from the Viitaniemi granitic pegmatite, Finland, as shown by FT-IR and ¹H NMR spectroscopy. This strengthens the hypothesis of formation of natural solid bituminous matter by catalytic reactions between microporous Ti-, Nb- and Zr-silicates and hydrocarbons at postmagmatic hydrothermal conditions.     

Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO₄ + 9.5 wt% H₂O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si + 12.35 wt% H₂O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7 wt% H₂O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500 °C and 23 GPa) in a hydrous Martian mantle.     

Equations of state of magnesium silicates anhydrous B and superhydrous B

High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V ₀?=?838.86?±?0.04?ų, K_T,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V ₀?=?624.71?± 0.03?ų, K_T,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (K_c?=?137?±?3?GPa), the b-axis is least compressible (K_b?=?175?±?4?GPa), and the a-axis is intermediate (K_a?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (K_a?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (K_b?=?146?±?3?GPa; K_c?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO₆ and SiO₆ octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ～22% in Anhy-B to ～9% in Shy-B.     

激光熔蚀微量氧同位素分析方法及其地质应用

研究小尺度或微区的同位素组成变化已成为地球化学分析技术发展的重要方向之一。在研制CO2激光熔蚀氧同位素制样装置的基础上,通过该装置与MAT253质谱计联机实现了硅酸盐和氧化物矿物的氧同位素在线分析。所测样品包括NBS28石英标样、石英玻璃,以及天然的石榴子石、锆石、橄榄石等一些难熔矿物。该装置分析的颗粒样品氧同位素分析精度为±022‰,最低样品量为8μmol的O2;石英玻璃原位分析的氧同位素分析精度为±035‰,最低样品量为7μmol的O2。这种装置和分析方法具备速度快,熔样温度高的特点,尤其适于难熔矿物。该装置可采用脉冲激光实现石英玻璃的微区原位分析,可应用于单个矿物生成环带的氧同位素研究。     

Thermal Infrared Spectra of Comet Hale-Bopp at Heliocentric Distances of 4 and 2.9 AU

We present 10 and 20 μm spectra of comet Hale-Bopp taken at UKIRT on 1996 June 20 and 1996 September 29.The 10 μm spectra clearly show a strong silicate feature with peaks at 10.0 and 11.2 μm. The 20 μm spectrum on September 29 has strong excess flux relative to a blackbody and a peak near 19μm, in good agreement with the ISO SWS spectra obtained a week later. However, the 20 μm spectrum on June 20 has significantly lower flux than would be expected based on a blackbody extrapolation from the flux at 12.5 μm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Computational study of tetrahedral Al–Si ordering in muscovite

 The nature of Al–Si ordering across the tetrahedral sites in muscovite, K₂Al₄(Si₆Al₂O₂₀)(OH)₄, was investigated using various computational techniques. Values of the atomic exchange interaction parameters J _l were obtained. From these parameters, a two-dimensional Al–Si ordering scheme was deduced. The transition temperature T _c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions, based on different stacking sequences of ordered sheets of tetrahedral sites. Monte Carlo simulations of both two-dimensional and three-dimensional ordering were performed, but in the three-dimensional simulation only the two-dimensional ordering is seen, implying that three-dimensional ordering is too slow to be attained during the timescale of the simulation. The effect of the three-dimensional interactions is to raise the two-dimensional ordering temperature to 2140 K. From the three-dimensional Monte Carlo simulation, the frequency of occurrence of 4Si0Al, 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by ²⁹Si MAS–NMR measurements reasonably well. In fact, the match suggests that the cation ordering seen in experiments corresponds to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature just above an ordering phase transition. Received: 28 August 2000 / Accepted: 12 March 2001     

High-temperature single-crystal neutron diffraction study of natural chondrodite

The H-atom environment in a Tilly Foster chondrodite was analyzed using single-crystal neutron-diffraction data collected at 500, 700 and 900 K and previously published low temperature data collected at 10, 100 and 300 K on the same crystal (Mg_4.64Fe_0.28Mn_0.014Ti_0.023(Si_1.01O₄)₂F_1.16(OH)_0.84; Friedrich et al. in Am Mineral 86:981–989, 2001). The full mean square displacement matrix Σ of the O–H pair was determined from the temperature dependence of the anisotropic displacement parameters, enabling a proper correction of the O–H bond for thermal vibration without assumptions about the correlation of O and H movements. The results show that the perpendicular O–H motions in chondrodite are intermediate between the riding and the independent motion models. The corrected O–H bond lengths do not change with temperature whereas the corrected H···F distances show an increase of ~0.02 Å with temperature, as do the Mg–O distances. This result shows that spectroscopic observations on the strength of the covalent O–H bond cannot be interpreted unambiguously in terms of a corresponding behaviour of the associated H···O/F hydrogen bond.     

Seasonal variations of physical and chemical erosion: A three-year survey of the Rhone River (France)

Numerous studies of weathering fluxes have been carried out on major world rivers during the last decade, to estimate CO₂ consumption rates, landscape evolution and global erosion rates. For obvious logistical reasons, most of these studies were based on large scale investigations carried out on short timescales. By comparison, much less effort has been devoted to long term monitoring, as a means to verify the temporal variability of the average characteristics, their trends, and the representativeness of short-term investigations. Here we report the results of a three-year survey (November 2000 to December 2003) of the major and trace element composition of dissolved and suspended matter in the lower Rhone River (France), the largest river of the Mediterranean area. Subsurface water samples were collected in Arles, about 48 km upstream of the estuary, twice a month routinely, and at higher frequency during flood events.During each flood event, the suspended particulate matter (SPM) show the usual trend of clockwise hysteresis with higher SPM concentrations on the rising limb of the flood than at the same discharge on the falling limb. We show that the annual average SPM flux of the Rhone River to the Mediterranean Sea (7.3 ± 0.6 × 10⁶ tons yr⁻¹) was largely controlled by the flood events (83% of the solid discharge occurred in less than 12% of the time), and that the precision on the total output flux depends strongly on the precise monitoring of SPM variations during the floods.The chemical composition of water and SPM are characterized by the predominance of Ca²⁺ due to the abundance of carbonate rocks in the Rhone watershed. Chemical budgets have been calculated to derive the contributions of atmospheric deposition, carbonate, silicate and evaporite weathering, and anthropogenic inputs. The chemical weathering rate of carbonates is estimated to be 89 ± 5 t km⁻² yr⁻¹ compared to 14.4 ± 3 t km⁻² yr⁻¹ from silicates. By contrast, the physical erosion rate of silicates is about 51 t km⁻² yr⁻¹ against 19 t km⁻² yr⁻¹ for carbonates.The steady-state model of Gaillardet et al. (1995) has been applied to the chemical composition of dissolved and solid products. The results show that the Rhone River currently exports much less material than produced at steady-state by weathering in its watershed. The sediment flux inferred from the steady-state calculation (21-56 × 10⁶ t yr⁻¹) is on the same order as that estimated in literature for the 19th and the beginning of the 20th centuries. This imbalance may suggest that the Rhone is under a transient erosion regime following climate change (i.e. significant decrease of the flooding frequency since the beginning of the 19th century). On the other hand, the imbalance may also be due to the trapping of alluvion by the numerous dams on the river and its tributaries.Our data corroborate with previous studies that suggest a strong coupling between chemical and physical erosion fluxes, during the hydrological seasonal cycle of the Rhone River. The correlation between physical and chemical transport rates is, however, clearly different from that reported for global annual averages in large world rivers.