The effect of initial230Th disequilibrium on young UPb ages: the Makalu case,Himalaya

Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic²⁰⁶Pb which originate from an excess or deficit of²³⁰Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess²⁰⁶Pb due to excess²³⁰Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of²⁰⁶Pb due to deficiency of²³⁰Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of²⁰⁶Pb were, with reference to the amount of radiogenic²⁰⁶Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic²⁰⁷Pb originating from the decay of initial excess²³¹Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected²³⁸U²⁰⁶Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.     

The Palung granite (Himalaya); high-resolution UPb systematics in zircon and monazite

UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.     

New interferometric techniques for measuring horizontal earth crustal movements

An interferometer using modern interferometric techniques has been developed at ZIPE, which allows the measurement of distances of more than 1 km. The interferometric equipment was designed for comparing distances of different lengths and directions and is appropriate for testing horizontal earth crustal movements. The new interferometer is based on Michelson’s interferometer using collimated light. The interference fringes are received by means of photoelectric detectors and can be observed visually by an oscilloscope. The duration of an interference signal need be only 1 ms and the corresponding path difference is registered by printing. In order to get such interferograms, the path difference of the two beams is periodically changed within an effective range of 8 mm, whilst the total difference between the beams can reach an amount of 1 m. The interferometer has been tested on the Potsdam International Standard Base Line up to a distance of 2.7 km.     

Detection of Entrainment Influences on Surface-Layer Measurements and Extension of Monin–Obukhov Similarity Theory

We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution.     

On the Mátern covariance family: a proposal for modeling temporal correlations based on turbulence theory

Current variance models for GPS carrier phases that take correlation due to tropospheric turbulence into account are mathematically difficult to handle due to numerical integrations. In this paper, a new model for temporal correlations of GPS phase measurements based on turbulence theory is proposed that overcomes this issue. Moreover, we show that the obtained model belongs to the Mátern covariance family with a smoothness of 5/6 as well as a correlation time between 125–175 s. For this purpose, the concept of separation distance between two lines-of-sight introduced by Schön and Brunner (J Geod 1:47–57, 2008a) is extended. The approximations made are highlighted as well as the turbulence parameters that should be taken into account in our modeling. Subsequently, fully populated covariance matrices are easily computed and integrated in the weighted least-squares model. Batch solutions of coordinates are derived to show the impact of fully populated covariance matrices on the least-squares adjustments as well as to study the influence of the smoothness and correlation time. Results for a specially designed network with weak multipath are presented by means of the coordinate scatter and the a posteriori coordinate precision. It is shown that the known overestimation of the coordinate precision is significantly reduced and the coordinate scatter slightly improved in the sub-millimeter level compared to solutions obtained with diagonal, elevation-dependent covariance matrices. Even if the variations are small, turbulence-based values for the smoothness and correlation time yield best results for the coordinate scatter.     

The TropGrid2 standard tropospheric correction model

TropGrid2 is a new version of a tropospheric model that is based on climatology and provides tropospheric propagation delay corrections for standard positioning users without temperature, pressure and humidity measurements. Zenith hydrostatic and wet delays are modeled as special harmonic functions taking seasonal and diurnal variations into consideration. The grid-point values are height-reduced and can be interpolated horizontally to the user position. The database used to derive this model consists of more than 9 years of 3D numerical weather fields of the NOAA NCEP GDAS weather model. We validated this standard model using 10 years of GPS-derived zenith path delays at 290 International GPS service reference stations. The gridded version is accurate at a level of 3.8 cm (root mean square in zenith direction) on global average; the average long-term bias is ?0.3 cm. The standard deviations computed by the model turn out to be slightly too pessimistic for almost all stations under investigation, in contrast to the site-specific version, which is only marginally (1 mm) more accurate on global scale.     

Precise position determination using a Galileo E5 single-frequency receiver

The European Galileo system offers one dedicated signal that is superior to all other signals currently available in space, namely the broadband signal E5. This signal has a bandwidth of at least 51 MHz using an AltBOC modulation. It features a code range noise at centimeter level. Additionally, the impact of multipath effects on this signal is significantly lower compared to all other available GNSS signals. These unique features of Galileo E5 drastically improve the precision of code range measurements and hence enable precise single-frequency positioning. Certain scientific and non-scientific applications in the positioning domain could likely benefit from the exploitation of E5 measurements. A positioning approach based on an additive combination of code range and carrier phase measurements (CPC—“code-plus-carrier”) to eliminate the ionospheric delay could be used to perform precise positioning over long distances. Unfortunately, this derived observable contains the ambiguity term as an additional unknown what normally requires longer observation windows in order to allow sufficient convergence of the ambiguity parameters. For this reason, a rapid convergence algorithm based on Kalman filtering was implemented in addition to the conventional CPC approach that is also discussed. The CPC-based results yield a positioning precision of 2–5 cm after a convergence time of about 3 h. The rapid convergence filter allows fixing the ambiguity terms within a few minutes, and the obtained position results are at the sub-decimeter level. Regarding one selected test, real data from Galileo experimental satellite GIOVE A were used in order to confirm our assumptions. However, since the current Galileo constellation is not sufficient for real-world positioning trials yet, all major results are based on simulated data.     

Suitability of Existing Numerical Model Codes and Thermodynamic Databases for the Prognosis of Calcite Dissolution Processes in Near-Surface Sediments Due to a CO2 Leakage Investigated by Column Experiments

Among the risks of CO₂ storage is the potential of CO₂ leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO₂ can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO₂ storage site is essential for assessing the impact of CO₂ leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO₂ partial pressures are rare. Here, the first study for column experiments is presented applying CO₂ partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N₂-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi:10.2475/ajs.278.2.179, 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO₂ leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO₂ leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m.