Evidence from zircon dating for existence of approximately 2.1 Ga old crystalline basement in southern Bohemia,Czech Republic

Zircon ages are reported for three Moldanubian amphibolite grade orthogneisses from the southern Bohemian Massif obtained by conventional U/Pb analyses. For two of these orthogneisses, conventional U/Pb data are supported by ion microprobe single zircon ages or single grain evaporation data. The amphibolite grade orthogneisses, occurring in three small tectonic lenses within the Varied Group close to the South Bohemian Main Thrust, are of tonalitic, granodioritic or quartz dioritic composition.Conventional bulk size fraction and ion microprobe analyses of nearly euhedral zircons from a metatonalite, erroneously interpreted as a metagreywacke in a previous study, yielded an upper Concordia intercept age of 2048 ± 12 Ma. The well preserved euhedral grain shapes of the zircons suggest crystallization from a magmatic phase, and the upper Concordia intercept age is now interpreted as reflecting a magmatic event at that time. The age of this rock is compatible with the conventional zircon data and the (²⁰⁷Pb/²⁰⁶Pb)^* single grain evaporation result from two further orthogneisses providing intrusion ages of 2 060 ± 12, 2 104 ± 1 and 2 061 ± 6 Ma, respectively. For one sample a concordant U/Pb age for sphene of 355 ± 2 Ma defines the age of amphibolite facies metamorphism. The upper Concordia intercept ages of three orthogneisses constitute the first direct evidence for the presence of early Proterozoic crust under the supracrustal cover in the southern part of the Bohemian Massif. Correspondence to: J. I. Wendt     

Authigenic titanite in weathered basalts: Implications for paleoatmospheric reconstructions

The stability of titanite is sensitive to temperature and partial pressure of CO₂. The finding of authigenic titanite grains in weathering regolith formed on Paraná basalts, Brazil, under tropical climatic conditions, reveals the thermodynamically-driven conversion from calcite to titanite at elevated ambient temperatures. Being unusual nowadays, this phase transition provides important implications for the understanding of silicate weathering in earlier geological epochs.Two types of secondary titanites were identified in the weathering profile of the study area. The tiny grains of 10 ​μm are forming in the microscopic voids in the rock. Also, large fractures filled with Fe-rich clay minerals contain bigger specimens of up to 170 ​μm. The titanites of second type often coexist with chalcedony and barite. No carbonate minerals were found in the weathering profile. Weathering sphene can be discriminated from other titanite types by its strong positive Eu anomaly, increased Al₂O₃ content and low content of trace elements. Its specific chemical composition and reactive transport modeling link this secondary mineral with dissolution of plagioclase. The titanite precipitation is controlled by slow diffusion in poorly-aerated, highly-alkaline pore fluids.The subaerial weathering of basaltic rocks provides a significant reservoir for atmospheric CO₂. However, the deposition of carbonate minerals is thermodynamically avoided at the stability field of titanite. We demonstrate a complex feedback between CO₂ and soil carbonates. The rise in pCO₂ triggers the precipitation of calcite in the weathering regolith, but the greenhouse effect increasing the temperature can cease carbonate deposition. Secondary titanites were found in several paleosols and at least a part of them can be of weathering origin.     

Low T heat capacity measurements and new entropy data for titanite (sphene): implications for thermobarometry of high-pressure rocks

The accepted standard state entropy of titanite (sphene) has been questioned in several recent studies, which suggested a revision from the literature value 129.3 ± 0.8 J/mol K to values in the range of 110–120 J/mol K. The heat capacity of titanite was therefore re-measured with a PPMS in the range 5 to 300 K and the standard entropy of titanite was calculated as 127.2 ± 0.2 J/mol K, much closer to the original data than the suggested revisions. Volume parameters for a modified Murgnahan equation of state: V _P,T  = V ₂₉₈° × [1 + a°(T − 298) − 20a°(T − 298)] × [1 – 4P/(K ₂₉₈ × (1 – 1.5 × 10⁻⁴ [T − 298]) + 4P)]^1/4 were fit to recent unit cell determinations at elevated pressures and temperatures, yielding the constants V ₂₉₈° = 5.568 J/bar, a° = 3.1 × 10⁻⁵ K⁻¹, and K = 1,100 kbar. The standard Gibbs free energy of formation of titanite, −2456.2 kJ/mol (∆H°_f = −2598.4 kJ/mol) was calculated from the new entropy and volume data combined with data from experimental reversals on the reaction, titanite + kyanite = anorthite + rutile. This value is 4–11 kJ/mol less negative than that obtained from experimental determinations of the enthalpy of formation, and it is slightly more negative than values given in internally consistent databases. The displacement of most calculated phase equilibria involving titanite is not large except for reactions with small ∆S. Re-calculated baric estimates for several metamorphic suites yield pressure differences on the order of 2 kbar in eclogites and 10 kbar for ultra-high pressure titanite-bearing assemblages.     

A thermobarometer for sphene (titanite)

Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree. In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content (ppm by weight), pressure (GPa) and reciprocal absolute temperature: The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Structure and phase transition of CaGe<Subscript>2</Subscript>O<Subscript>5</Subscript> revisited

The structure of CaGe₂O₅ between room temperature and 923 K has been determined by X-ray powder diffraction. A continuous phase transition from triclinic C1¯ to monoclinic C2/c symmetry at T_c=714±3 K is observed. The transition is accompanied by a weak heat capacity anomaly. This anomaly and the strain analysis based on the measured lattice parameters indicate a classical second-order phase transition. The order parameter, as measured by the strain component e₂₃, is associated with the displacement of the Ca cation. Electronic structure optimization by density functional methods is used to verify the centric space group of the low-temperature structure of CaGe₂O₅.