Pb-Pb age of sedimentary phosphorite reworking in Lower Riphean carbonate sediments,the Satka Formation of Southern Urals

The mineral composition and U-Pb and Rb-Sr systematics of phosphorites from the Satka Formation of Lower Riphean carbonates, the Burzyan Group of Southern Urals, are studied. Phosphorites occurring as small lenses between stromatolite layers are composed largely of fluorapatite with admixture of detrital quartz, feldspars, illite, and chlorite. Phosphorite samples have been subjected to stepwise dissolution in 1 N (fraction L-1) and 2 N (fraction L-2) HCl. As is established, the maximum apatite content is characteristic of fraction L-1, while fraction L-2 is enriched in products of dolomite and sulfide dissolution and in elements leached from siliciclastic components. The Sr content in the Satka apatites (280–560 ppm) is substantially lower as compared with that in unaltered marine apatite. The ⁸⁷Sr/⁸⁶Sr “initial ratio in the phosphorites studied (0.71705–0.72484) and host dolomites from the lower part of the Satka Formation is significantly higher than in the Early Riphean seawater that indicates a reset of the Rb-Sr original systems in sediments. The Pb-Pb age of 1340 ± 30 Ma (MSWD = 6.4) estimated based on 7 data points characterizing fractions L-1 and L-2 is younger than the formation time of overlying Burzyan sediments, being consistent, within the error range, with date of the Mashak rifting event recorded at the Early-Middle Riphean boundary. The comparative U-Pb characteristics of two soluble fractions (L-1 and L-2) and silicate residue of phosphorites show that epigenetic redistribution of Pb and U was characteristic of the phosphorite horizon only. The initial Pb isotope composition and μ (²³⁸U/²⁰⁴Pb) estimated according to model by Stacey and Kramers for the early diagenetic fluids in carbonate and phosphate sediments of the Satka Formation suggest that they were in isotopic equilibrium with erosion products of the Taratash crystalline complex.     

Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite

The phase boundary between MnTiO₃ I (ilmenite structure) and MnTiO₃ II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO₃ I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔH^o=5300±1000 J·mol⁻¹, ΔS^o = 1.98 ±1J·K⁻¹· mol⁻¹. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol⁻¹. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO₃ is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.     

Tensile strength of rock by the double-punch method

Summary This paper presents an experimental study of the newly developed double-punch test method for determination of the tensile strength of rocks. The method for preparation of test specimens and test procedure is reported and discussed. Comparisons of tensile strength determined from double-punch and split-cylinder tests are also presented.With 5 Figures     

Haines Structure: a probable Eocene complex impact structure in the offshore Canning Basin,Western Australia

A seismic structure imaged in a single 2D seismic profile from the offshore Canning Basin, Western Australia, is interpreted to be a possible complex impact crater on the basis of its seismic character. The feature, herein referred to as the Haines Structure, is symmetrical in two dimensions over ～2.5 km and comprises a centrally uplifted basal surface, a depressed upper surface, a highly deformed intervening package and an overlying horizon that is ‘sagging’ over the depression. The possible impact structure lies within carbonate units of Eocene or Early Oligocene age as determined from seismic correlation to petroleum exploration wells. The structure has not been drilled; therefore distinguishing characteristics used to define an impact origin, such as shock metamorphism, are not available. Comparison with other features in the Neogene succession of Australia's North West Shelf that have previously been interpreted as impact structures highlights the presence of key elements in the Haines Structure that characterise known complex impact craters, and the absence of seismic features related to alternative processes, such as a link to deeper structures that would be expected beneath a volcanic pipe.     

The lacustrine microbial carbonate factory of the successive Lake Bonneville and Great Salt Lake,Utah, USA

The Bonneville Basin is a continental lacustrine system accommodating extensive microbial carbonate deposits corresponding to two distinct phases: the deep Lake Bonneville (30 000 to 11 500 ¹⁴C bp ) and the shallow Great Salt Lake (since 11 500 ¹⁴C bp ). A characterization of these microbial deposits and their associated sediments provides insights into their spatio‐temporal distribution patterns. The Bonneville phase preferentially displays vertical distribution of the microbial deposits resulting from high‐amplitude lake level variations. Due to the basin physiography, the microbial deposits were restricted to a narrow shoreline belt following Bonneville lake level variations. Carbonate production was more efficient during intervals of relative lake level stability as recorded by the formation of successive terraces. In contrast, the Great Salt Lake microbial deposits showed a great lateral distribution, linked to the modern flat bottom configuration. A low vertical distribution of the microbial deposits was the result of the shallow water depth combined with a low amplitude of lake level fluctuations. These younger microbial deposits display a higher diversity of fabrics and sizes. They are distributed along an extensive ‘shore to lake’ transect on a flat platform in relation to local and progressive accommodation space changes. Microbial deposits are temporally discontinuous throughout the lake history showing longer hiatuses during the Bonneville phase. The main parameters controlling the rate of carbonate production are related to the interaction between physical (kinetics of the mineral precipitation, lake water temperature and runoff), chemical (Ca²⁺, Mg²⁺ and HCO₃^? concentrations, Mg/Ca ratio, dilution and depletion) and/or biological (trophic) factors. The contrast in evolution of Lake Bonneville and Great Salt Lake microbial deposits during their lacustrine history leads to discussions on major chemical and climatic changes during this interval as well as the role of physiography. Furthermore, it provides novel insights into the composition, structure and formation of microbialite‐rich carbonate deposits under freshwater and hypersaline conditions.     

Reconstructing hydrological variability from testate amoebae analysis in Carpathian peatlands

Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content, confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania, Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP, 3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow which affected eastern Europe as well as western and central Europe.     

Triaxial stress state deep in orogenic belts: an example from Turkey

A rare metachert pebble containing amphibole grains with microboudin structures in a wide range of orientations provides an opportunity to perform stress analysis in two orthogonal orientations on the foliation surface. The sample was analysed by the microboudin method to infer the triaxial stress state during microboudinage. Stress parameters proportional to the far-field differential stress were determined for sodic amphibole grains in the two orientations. The ratio of the stresses in the two orthogonal orientations (σ₁−σ₂)/(σ₁−σ₃) was calculated to be 0.64, indicating that σ₂ lies closer to the midpoint between σ₁ and σ₃ than to σ₃.     

Multidimensional upstream weighting for multiphase transport on general grids

The governing equations for multiphase flow in porous media have a mixed character, with both nearly elliptic and nearly hyperbolic variables. The flux for each phase can be decomposed into two parts: (1) a geometry- and rock-dependent term that resembles a single-phase flux; and (2) a mobility term representing fluid properties and rock–fluid interactions. The first term is commonly discretized by two- or multipoint flux approximations (TPFA and MPFA, respectively). The mobility is usually treated with single-point upstream weighting (SPU), also known as dimensional or donor cell upstream weighting. It is well known that when simulating processes with adverse mobility ratios, SPU suffers from grid orientation effects. An important example of this, which will be considered in this work, is the displacement of a heavy oil by water. For these adverse mobility ratio flows, the governing equations are unstable at the modeling scale, rendering a challenging numerical problem. These challenges must be addressed in order to avoid systematic biasing of simulation results. In this work, we present a framework for multidimensional upstream weighting for multiphase flow with buoyancy on general two-dimensional grids. The methodology is based on a dual grid, and the resulting transport methods are provably monotone. The multidimensional transport methods are coupled with MPFA methods to solve the pressure equation. Both explicit and fully implicit approaches are considered for time integration of the transport equations. The results show considerable reduction of grid orientation effects compared to SPU, and the explicit multidimensional approach allows larger time steps. For the implicit method, the total number of non-linear iterations is also reduced when multidimensional upstream weighting is used.     

Oligo-Miocene granitic magmatism in central Vietnam and implications for continental deformation in Indochina

Two granitoid intrusions within the Bu Khang extensional complex in central Vietnam have been dated by U–Pb and Rb–Sr geochronology. A monazite U–Pb age of 26.0 ± 0.2 (2σ) Myr was obtained for the Bu Khang pluton and 23.7 + 1.6/–1.7 Myr for monazite, allanite and zircon from the Dai Loc intrusion. These ages date crystallization of magmas previously assigned Precambrian to Devonian. Rb–Sr analyses of K-feldspar and biotite fractions from the samples yield ages of 19.8 ± 0.6 (2σ) Myr and 19.6 ± 0.5 Myr, respectively. The thermal history recorded by the different geochronometers implies an average exhumation rate of ∼2 mm yr⁻¹ corresponding to ∼9 km of unroofing. Magmatism was either (i) induced passively by lithospheric thinning driven by changes in regional tectonic stresses, or (ii) triggered actively by an ascending plume. Tertiary exhumation and magmatism documented elsewhere in Indochina (e.g. Ailao Shan-Red River and Wang Chao shear zones) favours a regional tectonic cause for extension and granitoid magmatism in the Bu Khang complex. On the other hand, the presence of an upwelling thermal anomaly since at least 35 Ma, causing mantle melting below Indochina, is supported by shear-wave velocity variations in the mantle, and source geochemistry of both the Bu Khang plutons and the Red River belt intrusions. In either case, Tertiary exhumation of the Bu Khang complex can account for previously undocumented NE–SW-directed extension, which is required in northern Vietnam to account for structural changes related to the opening of the South China Sea.