U(Th)Pb systematics and ages of Himalayan leucogranites, South Tibet

The age and origin of five leucogranites from the High and Tethys Himalaya, and two country-rock gneisses were investigated by UPb dating of zircon fractions and single grains, and fractions of monazite. Additionally, ThU concentrations in whole rock powders and isotopic compositions of Pb in leached K-feldspars were determined. Monazites yield ages of 16.8 ± 0.6 m.y. for the Nialam migmatite-granite, 15.1 ± 0.5 m.y. for the Lhagoi Kangri granite, 14.3 ± 0.6 m.y. for a granite from Mt. Everest, and 9.8 ± 0.7 m.y. and 9.2 ± 0.9 m.y. for two varieties of the Maja granite. These data, together with monazite ages of 21.9 ± 0.2 and 24.0 ± 0.4 m.y., determined earlier on the Makalu granite [1], substantiate a period of intracontinental granite emplacements from 24 to 9 m.y. ago, i.e. from uppermost Oligocene to late Miocene times. Such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of India with Eurasia. Furthermore, the individual ages, together with structural relationships between granites and country rocks suggest that granite formation and tectono-metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m.y. Inherited lead components, present in all granite zircons point to large proportions of Precambrian material in the magma source regions, up to 2200 m.y. old.ThU systematics between monazite and country rocks indicate that U has been leached from most of the granites after crystallisation of monazite.Zircon dating of the Kangmar granite gneiss, which occurs in a window through the Tethys Himalayan sediments, shows that this pluton, transformed to a gneiss during the Alpine orogeny, crystallised in lowermost Palaeozoic times 562 ± 4 m.y. ago.     

The screw dynamo in a thick torus

The problem of magnetic field generation under screw motion in a toroidal channel is studied numerically. The screw dynamo in the cylinder with periodical boundary conditions was found to be a suitable approximation for generation of the magnetic field by a screw flow in a thin torus. For the thick torus, a principally new solution of the screw dynamo problem was obtained. In this case the growing global magnetic field mode has the scale of a maximal geometrical size of the torus and does not vanish on the axis of the torus (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diurnal fluctuations in glacier ice deformation: Haut Glacier d'Arolla,Switzerland

Measurements of surface velocity, ice deformation (at 42 and 89% ice depth) and proglacial stream discharge were made at Haut Glacier d'Arolla, Switzerland, to determine diurnal patterns of variation in each. Data are analysed in order to understand better the relationship between hydraulically induced basal motion and glacier ice deformation over short timescales. The data suggest that hydraulically induced localized basal ‘slippery’ spots are created over diurnal cycles, causing enhanced basal motion and spatially variable glacier speed‐up. Our data indicate that daily glacier speed‐up is associated with reduced internal deformation over areas previously identified as slippery spots and increased deformation in areas located adjacent to or down‐glacier from slippery spots. We interpret this pattern in terms of a transfer of mechanical support for basal shear stress away from slippery spots to adjacent sticky areas, where the resulting stronger ice–bed coupling causes increased ice deformation near the bed. These patterns indicate that basal ice is subjected to stress regimes that are variable at a high spatial and temporal resolution. Such variations may be central to the creation of anomalous vertical velocity profiles measured above and down‐glacier of basal slippery zones, which have shown evidence for ‘plug flow’ and extrusion flow over annual timescales. Copyright © 2007 John Wiley & Sons, Ltd.     

Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite

The Block IIF satellites feature a new generation of high-quality rubidium clocks for time and frequency keeping and are the first GPS satellites transmitting operational navigation signals on three distinct frequencies. We investigate apparent clock offset variations for the Block IIF-1 (SVN62) spacecraft that have been identified in L1/L2 clock solutions as well as the L1/L5-minus-L1/L2 clock difference. With peak-to-peak amplitudes of 10?C40?cm, these variations are of relevance for future precision point positioning applications and ionospheric analyses. A proper characterization and understanding is required to fully benefit from the quality of the new signals and clocks. The analysis covers a period of 8?months following the routine payload activation and is based on GPS orbit and clock products generated by the CODE analysis center of the International GNSS Service (IGS) as well as triple-frequency observations collected with the CONGO network. Based on a harmonic analysis, empirical models are presented that describe the sub-daily variation of the clock offset and the inter-frequency clock difference. These contribute to a better clock predictability at timescales of several hours and enable a consistent use of L1/L2 clock products in L1/L5-based positioning.     

Estimation of satellite position,clock and phase bias corrections

Precise point positioning with integer ambiguity resolution requires precise knowledge of satellite position, clock and phase bias corrections. In this paper, a method for the estimation of these parameters with a global network of reference stations is presented. The method processes uncombined and undifferenced measurements of an arbitrary number of frequencies such that the obtained satellite position, clock and bias corrections can be used for any type of differenced and/or combined measurements. We perform a clustering of reference stations. The clustering enables a common satellite visibility within each cluster and an efficient fixing of the double difference ambiguities within each cluster. Additionally, the double difference ambiguities between the reference stations of different clusters are fixed. We use an integer decorrelation for ambiguity fixing in dense global networks. The performance of the proposed method is analysed with both simulated Galileo measurements on E1 and E5a and real GPS measurements of the IGS network. We defined 16 clusters and obtained satellite position, clock and phase bias corrections with a precision of better than 2 cm.     

Quality assessment of FORMOSAT-3/COSMIC and GRACE GPS observables: analysis of multipath, ionospheric delay and phase residual in orbit determination

The precise orbit determination antennas of F3/C and GRACE-A satellites are from the same manufacturer, but are installed in different configurations. The current orbit accuracy of F3/C is 3 cm at arcs with good GPS data, compared to 1 cm of GRACE, which has a larger ratio of usable GPS data. This paper compares the qualities of GPS observables from F3/C and GRACE. Using selected satellites and time spans, the following average values for the satellite F3/C and satellite A of GRACE are obtained: multipath effect on the pseudorange P1, 0.78 and 0.38 m; multipath effect on the pseudorange P2, 1.03 and 0.69 m; occurrence frequency of cycle slip, 1/29 and 1/84; standard error of unit weight, 4 and 1 cm; dynamic–kinematic orbit difference, 10 and 2 cm. For gravity determination using F3/C GPS data, a careful selection of GPS data is critical. With six satellites in orbit, F3/C’s large amount of GPS data will make up the deficiency in data quality.     

Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps)

U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess ²⁰⁶Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism. The age of growth of the new monazite crystals is approached by their ²⁰⁷Pb/²³⁵U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism. Received: 7 July 1998 / Accepted: 4 November 1998     

Gabbro and related rock emplacement beneath rifting continental crust: UPb geochronological and geochemical constraints for the Galicia passive margin (Spain)

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites (‘peridotite ridge’) lying about 300 km off the coast in the North Atlantic ocean. From this ridge, a gabbro and a chlorite rock were studied in an attempt to substantiate rift-related subcontinental magmatism, occurring prior to sea-floor spreading. U-Pb dating of 13 different zircon fractions yields a precise age of 122.1 ± 0.3 Ma (2σ) for the emplacement of the chlorite rock protolith, from which more than 50% of Si and alkali-calc-alkali elements were lost during greenschist facies tectonometamorphism. Sr and Nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in LILE, relative to a chondritic reservoir. No evidence for the presence of continental material in the magma source regions can be observed. From the new zircon age of 122.1 ± 0.3 Ma, and earlier determined³⁹Ar⁴⁰Ar age of 122.0 ± 0.6 Ma for amphibole from the same locality, it can be documented that magma formation, solidification and unroofing of the mantle rocks occurred during a short period of time of about 3.4 Ma, which means that the peridotite ridge detached from the continent and rose to the surface immediately after, or even coevally with mantle melting.