Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre,Gardar Province,South Greenland

The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt Sø Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 ± 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 ± 28 Ma and the other at 349 ± 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to ~900 Ma after the magmatism. However, the Pb–Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 ± 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at ~350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U–Pb zircon and the Pb–Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.     

Granitic magmatism,basement ages,and provenance indicators in the Malay Peninsula: Insights from detrital zircon U–Pb and Hf-isotope data

The Malay Peninsula lies on two continental blocks, Sibumasu and East Malaya, which are intruded by granitoids in two provinces: the Main Range and Eastern. Previous models propose that Permian–Triassic granitoids are subduction-related and syn-to post-collisional. We present 752 U–Pb analyses that were carried out on zircons from river sands in the Malay Peninsula; of these, 243 grains were selected for Hf-isotope analyses. Our data suggest a more complex Sibumasu–East Malaya collision history. ¹⁷⁶Hf/¹⁷⁷Hf_i ratios reveal that Permian–Triassic zircons were sourced from three magmatic suites: (a) Permian crustally-derived granitoids, (b) Early-Middle Triassic granitoids with mixed mantle–crust sources, and (c) Late Triassic crustally-derived granitoids. This suggests three Permian–Triassic episodes of magmatism in the Malay Peninsula, two of which occurred in the Eastern Province. Although the exact timing of the Sibumasu–East Malaya collision remains unresolved, current data suggest that it occurred before the Late Triassic, probably in Late Permian–Early Triassic. Our data also indicate that Sibumasu and East Malaya basements are chronologically heterogeneous, but predominantly of Proterozoic age. Some basement may be Neoarchaean but there is no evidence for basement older than 2.8 Ga. Finally, we show that Hf-isotope signatures of Triassic zircons can be used as provenance indicators.     

Uranium and Thorium Concentration and Isotopic Composition in Five Glass (BHVO‐2G,BCR‐2G,NKT‐1G,T1‐G,ATHO‐G) and Two Powder (BHVO‐2, BCR‐2) Reference Materials

Here we report uranium and thorium isotopic ratios and elemental concentrations measured in solid reference materials from the USGS (BHVO‐2G, BCR‐2G, NKT‐1G), as well as those from the MPI‐DING series (T1‐G, ATHO‐G). Specifically created for microanalysis, these naturally‐sourced glasses were fused from rock powders. They cover a range of compositions, elemental concentrations and expected isotopic ratios. The U‐Th isotopic ratios of two powdered source materials (BCR‐2, BHVO‐2) were also characterised. These new measurements via multi‐collector thermal ionisation mass spectrometry and multi‐collector inductively coupled plasma‐mass spectrometry can now be used to assess the relative performance of techniques and facilitate comparison of U‐Th data amongst laboratories in the geoscience community for in situ and bulk analyses.     

A Comparative Study of Five Reference Materials and the Lombard Meteorite for the Determination of the Platinum‐Group Elements and Gold by LA‐ICP‐MS

A range of independently characterised reference materials (RMs) for LA‐ICP‐MS, used for the determination of the platinum‐group elements (PGE) and Au in a sulfide matrix, were analysed and compared: 8b, PGE‐A, NiS‐3, Po727‐T1, Po724‐T and the Lombard meteorite. The newly developed RM NiS‐3 was used as the RM for the calibration of all LA‐ICP‐MS analyses and the measured concentrations of the other RMs compared against their published concentrations. This data were also used to assess the consistency of concentrations calibrated against the different RMs. It was found that Po727‐T1 and 8b produced results that were comparable, within uncertainty, for all elements. Po727‐T1 also produced consistent results with NiS‐3 for all elements. All other RMs showed differences for some elements, especially Ru in Po724‐T, and Os, Ir and Au in PGE‐A. The homogeneity of the PGE and Au in each RM was assessed, by comparing the precision of multiple LA‐ICP‐MS spot analyses with the average uncertainty of the signal. Po724‐T, Po727‐T1 and the Lombard meteorite were found to be homogeneous for all elements, but 8b, PGE‐A and NiS‐3 were heterogeneous for some elements. This is the first direct comparison between a range of independently characterised PGE and Au LA‐ICP‐MS RMs.     

Elevation adjustments of paired natural levees during flooding of the Saskatchewan River

Natural levees control the exchange of water between an alluvial channel and its floodplain, but little is known about the spatial distribution and evolution of levee heights. The summer 2005 flood of the Saskatchewan River (Cumberland Marshes, east‐central Saskatchewan) inundated large areas of floodplain for up to seven weeks, forming prominent new deposits on natural levees along main‐stem channels. Measurements of flood‐deposit thickness and crest heights of 61 levee pairs show that the thickest deposits occur on the lower pre‐flood levee in 80% of the sites, though no clear relationship exists between deposit thickness and magnitude of height difference. Only 16% of the pairs displayed thicker deposits on the higher levee, half of which occurred at sites where relatively clear floodbasin waters re‐entered turbid channels during general flooding. Difference in crest elevation (ΔE) between paired levees is approximately log‐normally distributed, both before and after the flood, though with different mean values. Supplemental observations from tank experiments indicate that during near‐bankfull flows, temporally and spatially variable deposition and erosion occur on levees due to backwater effects associated with nearby channel bars and irregular rises of the channel bed forced by channel extension. During floods, preferential deposition in lows tends to even out crest heights. Copyright © 2009 John Wiley & Sons, Ltd.     

Truncation of the distribution of ground-motion residuals

Recent studies to assess very long-term seismic hazard in the USA and in Europe have highlighted the importance of the upper tail of the ground-motion distribution at the very low annual frequencies of exceedance required by these projects. In particular, the use of an unbounded lognormal distribution to represent the aleatory variability of ground motions leads to very high and potentially unphysical estimates of the expected level of shaking. Current practice in seismic hazard analysis consists of truncating the ground-motion distribution at a fixed number (ε _max) of standard deviations (σ). However, there is a general lack of consensus regarding the truncation level to adopt. This paper investigates whether a physical basis for choosing ε _max can be found, by examining records with large positive residuals from the dataset used to derive one of the ground-motion models of the Next Generation Attenuation (NGA) project. In particular, interpretations of the selected records in terms of causative physical mechanisms are reviewed. This leads to the conclusion that even in well-documented cases, it is not possible to establish a robust correlation between specific physical mechanisms and large values of the residuals, and thus obtain direct physical constraints on ε _max. Alternative approaches based on absolute levels of ground motion and numerical simulations are discussed. However, the choice of ε _max is likely to remain a matter of judgment for the foreseeable future, in view of the large epistemic uncertainties associated with these alternatives. Additional issues arise from the coupling between ε _max and σ, which causes the truncation level in terms of absolute ground motion to be dependent on the predictive equation used. Furthermore, the absolute truncation level implied by ε _max will also be affected if σ is reduced significantly. These factors contribute to rendering a truncation scheme based on a single ε _max value impractical.     

An Analytic Model For A Mantle Plume Fed By A Boundary Layer

Summary. An approximate analytical solution for flow in a mantle plume of constant radius, viscosity, and density contrast is obtained in cylindrical coordinates. the differential equations for vertical velocity of the mantle surrounding the plume and for topography are homologous to the equation for flexure of an elastic plate. Although the model is too simple to be fully applicable to the Earth, one can conclude that the vertical velocity in the mantle changes significantly away from plumes, that the viscosity of the plume is important for controlling flow rate, and that the long-wavelength geoid anomalies are sensitive to the viscosity of the surrounding mantle. the first induced upwelling away from a plume is quite weak and unlikely to control the spacing of plumes.     

Quantifying chemical changes in hydrothermally altered volcanic sequences — silica enrichment as a guide to the crandon massive sulfide deposit,Wisconsin, U.S.A.

A multi-element geochemical study of the wallrocks which host the large Precambrian Crandon deposit (≈ 70 million tons of zinc, copper ore) in Wisconsin, U.S.A. was carried out to identify pathfinder elements which significantly enlarge the size of the deposit as an exploration target. The Crandon deposit occurs in what appears to be a “normal”, subalkaline, intermediate to felsic volcanic sequence. The silica concentration of the wallrocks shows neither a systematic relationship to volcanic stratigraphy nor a distinctive pattern around ore.To determine if silica has been added to the host rocks, a regression analysis was performed on published data from 496 unaltered, subalkaline volcanic rocks from the Abitibi Subprovince of Canada, with silica as the dependent variable and the ratio of “immobile” elements Zr/TiO₂ as the independent variable. Based on the second degree regression curve and the associated standard error of estimate, predicted and residual silica values were calculated and plotted for the Crandon host rocks.The results show that massive sulfide ore was deposited in andesitic rocks with an original SiO₂ content of 52–62%, and that more than 7.4% SiO₂ was added to the rocks stratigraphically beneath the deposit and on strike with the deposit. Silica was added to the hanging wall rocks also, which confirms that hydrothermal activity at Crandon continued after cessation of massive sulfide ore deposition. In contrast to the pattern of silica enrichment at Crandon which significantly enlarges the size of the exploration target, two noneconomic prospects within ten miles of Crandon contain low to moderate levels of silica enrichment and corresponding low base metal values.Silica enrichment occurs in some systems unassociated with mineralization and is therefore not a specific signature of ore-forming processes. However, from the data presented on the Crandon deposit, if silica enrichment does exist in an exploration search area, the chances of finding a massive sulfide ore deposit are improved.     

The mass of the star formed in a cloud core

The forming star grows by mass inflow from the parent cloud core, mainly through the accretion disk. However, the core matter which has not yet contracted much is seriously disturbed by the activities of the forming star. We consider mass outflow and emission of ultraviolet radiation as such activities and determine the stellar mass as a function of the physical quantities of the parent cloud core.