A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation

New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earth's oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone. (C) 2003 Elsevier Science B.V. All rights reserved.

Inheritance of early Archaean Pb-isotope variability from long-lived Hadean protocrust

Comparison of initial Pb-isotope signatures of several early Archaean (3.65-3.82 Ga) lithologies (orthogneisses and metasediments) and minerals (feldspar and galena) documents the existence of substantial isotopic heterogeneity in the early Archaean, particularly in the Pb-207/Pb-204 ratio. The magnitude of isotopic variability at 3.82-3.65 Ga requires source separation between 4.3 and 4.1 Ga, depending on the extent of U/Pb fractionation possible in the early Earth. The isotopic heterogeneity could reflect the coexistence of enriched and depleted mantle domains or the separation of a terrestrial protocrust with a U-238/Pb-204 (mu) that was ca. 20-30% higher than coeval mantle. We prefer this latter explanation because the high-p signature is most evident in metasediments (that formed at the Earth's surface). This interpretation is strengthened by the fact that no straightforward mantle model can be constructed for these high-mu lithologies without violating bulk silicate Earth constraints. The Pb-isotope evidence for a long-lived protocrust complements similar Hf-isotope data from the Earth's oldest zircons, which also require an origin from an enriched (low Lu/Hf) environment. A model is developed in which greater than or equal to3.8-Ga tonalite and monzodiorite gneiss precursors (for one of which we provide zircon U-Pb data) are not mantle-derived but formed by remelting or differentiation of ancient (ca. 4.3 Ga) basaltic crust which had evolved with a higher U/Pb ratio than coeval mantle in the absence of the subduction process. With the initiation of terrestrial subduction at, we propose, ca. 3.75 Ga, most of the greater than or equal to3.8-Ga basaltic shell (and its differentiation products) was recycled into the mantle, because of the lack of a stabilising mantle lithosphere. We argue that the key event for preservation of all greater than or equal to3.8-Ga terrestrial crust was the intrusion of voluminous granitoids immediately after establishment of global subduction because of complementary creation of a lithospheric keel. Furthermore, we argue that preservation of !3.8-Ga material (in situ rocks and zircons) globally is restricted to cratons with a high U/Pb source character (North Atlantic, Slave, Zimbabwe, Yilgarn, and Wyoming), and that the Pb-isotope systematics of these provinces are ultimately explained by reworking of material that was derived from ca. 4.3 Ga (i.e. Hadean) basaltic crust.

The source of the Great Dyke, Zimbabwe, and its tectonic significance: Evidence from Re-Os isotopes

Re-Os data for chromite separates from 10 massive chromitite seams sampled along the 550-km length of the 2.58-Ga Great Dyke layered igneous complex, Zimbabwe, record initial 187Os/188Os ratios in the relatively narrow range between 0.1106 and 0.1126. This range of initial 187Os/188Os values is only slightly higher than the value for the coeval primitive upper mantle (0.1107) as modeled from the Re-Os evolution of chondrites and data of modern mantle melts and mantle derived xenoliths. Analyses of Archean granitoid and gneiss samples from the Zimbabwe Craton show extremely low Os concentrations (3-9 ppt) with surprisingly unradiogenic present-day 187Os/188Os signatures between 0.167 and 0.297. Only one sample yields an elevated 187Os/188Os ratio of 1.008. Using these data, the range of crustal contamination of the Great Dyke magma would be minimally 0%-33% if the magma source was the primitive upper mantle, whereas the range estimated from Nd and Pb isotope systematics is 5%-25%. If it is assumed that the primary Great Dyke magma derived from an enriched deep mantle reservoir (via a plume), a better agreement can be obtained. A significant contribution from a long-lived subcontinental lithospheric mantle (SCLM) reservoir with subchondritic Re/Os to the Great Dyke melts cannot be reconciled with the Os isotope results at all. However, Os isotope data on pre-Great Dyke ultramafic complexes of the Zimbabwe Craton and thermal modeling show that such an SCLM existed below the Zimbabwe Craton at the time of the Great Dyke intrusion. It is therefore concluded that large melt volumes such as that giving rise to the Great Dyke were able to pass lithospheric mantle keels without significant contamination in the late Archean. Because the ultramafic-mafic melts forming the Great Dyke must have originated below the SCLM (which extends to at least a 200-km depth ), the absence of an SCLM signature precludes a subduction-related magma-generation process.

Mode II delamination testing in uniaxially oriented PVC pipes

The mode II fracture toughness of an oriented PVC pipe was measured using an End Notched Flexure test geometry. A relatively low value of G(IIC) was found of 1.07 kJ m(-2) and this indicates that it is energetically more favorable for a crack to propagate in the tangential direction rather than radially through the wall of the pipe. Examination of the mechanism of crack advanced showed that although the crack was propagating globally in mode II, micro-cracks were opening ahead of the crack in mode I or in mixed mode. Growth of the crack occurred by linking up of these micro-cracks. This is similar to the mechanism found for mode II cracking in carbon fibre epoxy composites. (C) 2004 Kluwer Academic Publishers.

Petrology and geochemistry of early cretaceous bimodal continental flood volcanism of the NW Etendeka, Namibia. Part 1: Introduction, mafic lavas and re-evaluation of mantle source components

The bimodal NW Etendeka province is located at the continental end of the Tristan plume trace in coastal Namibia. It comprises a high-Ti (Khumib type) and three low-Ti basalt (Tafelberg, Kuidas and Esmeralda types) suites, with, at stratigraphically higher level, interstratified high-Ti latites (three units) and quartz latites (five units), and one low-Ti quartz latite. Khumib basalts are enriched in high field strength elements and light rare earth elements relative to low-Ti types and exhibit trace element affinities with Tristan da Cunha lavas. The unradiogenic Pb-206/Pb-204 ratios of Khumib basalts are distinctive, most plotting to the left of the 132 Ma Geochron, together with elevated Pb-207/Pb-204 ratios, and Sr-Nd isotopic compositions plotting in the lower Nd-143/Nd-144 part of mantle array (EM1-like). The low-Ti basalts have less coherent trace element patterns and variable, radiogenic initial Sr (similar to0.707-0.717) and Pb isotope compositions, implying crustal contamination. Four samples, however, have less radiogenic Pb and Sr that we suggest approximate their uncontaminated source. All basalt types, but particularly the low-Ti types, contain samples with trace element characteristics (e.g. Nb/Nb-*) suggesting metasediment input, considered source-related. Radiogenic isotope compositions of these samples require long-term isolation of the source in the mantle and depletions (relative to unmodified sediment) in certain elements (e.g. Cs, Pb, U), which are possibly subduction-related. A geodynamic model is proposed in which the emerging Tristan plume entrained subducted material in the Transition Zone region, and further entrained asthenosphere during plume head expansion. Mixing calculations suggest that the main features of the Etendeka basalt types can be explained without sub-continental lithospheric mantle input. Crustal contamination is evident in most low-Ti basalts, but is distinct from the incorporation of a metasedimentary source component at mantle depths.

Scabies: more than just an irritation

Human scabies, caused by skin infestation with the arthropod mite, Sarcoptes scabiei, typically results in a papular, intensely pruritic eruption involving the interdigital spaces, and flexure creases. Recent research has led to a reassessment of the morbidity attributable to this parasite in endemic communities, particularly resulting from secondary skin sepsis and postinfective complications including glomerulonephritis. This has led to studies of the benefits of community based control programmes, and to concerns regarding the emergence of drug resistance when such strategies are employed. The renewed research interest into the biology of this infection has resulted in the application of molecular tools. This has established that canine and human scabies populations are genetically distinct, a finding with major implications for the formulation of public health control policies. Further research is needed to increase understanding of drug resistance, and to identify new drug targets and potential vaccine candidates.

Theoretical and numerical analysis of large-scale heat transfer problems with temperature-dependent pore-fluid densities

Purpose - In many scientific and engineering fields, large-scale heat transfer problems with temperature-dependent pore-fluid densities are commonly encountered. For example, heat transfer from the mantle into the upper crust of the Earth is a typical problem of them. The main purpose of this paper is to develop and present a new combined methodology to solve large-scale heat transfer problems with temperature-dependent pore-fluid densities in the lithosphere and crust scales. Design/methodology/approach - The theoretical approach is used to determine the thickness and the related thermal boundary conditions of the continental crust on the lithospheric scale, so that some important information can be provided accurately for establishing a numerical model of the crustal scale. The numerical approach is then used to simulate the detailed structures and complicated geometries of the continental crust on the crustal scale. The main advantage in using the proposed combination method of the theoretical and numerical approaches is that if the thermal distribution in the crust is of the primary interest, the use of a reasonable numerical model on the crustal scale can result in a significant reduction in computer efforts. Findings - From the ore body formation and mineralization points of view, the present analytical and numerical solutions have demonstrated that the conductive-and-advective lithosphere with variable pore-fluid density is the most favorite lithosphere because it may result in the thinnest lithosphere so that the temperature at the near surface of the crust can be hot enough to generate the shallow ore deposits there. The upward throughflow (i.e. mantle mass flux) can have a significant effect on the thermal structure within the lithosphere. In addition, the emplacement of hot materials from the mantle may further reduce the thickness of the lithosphere. Originality/value - The present analytical solutions can be used to: validate numerical methods for solving large-scale heat transfer problems; provide correct thermal boundary conditions for numerically solving ore body formation and mineralization problems on the crustal scale; and investigate the fundamental issues related to thermal distributions within the lithosphere. The proposed finite element analysis can be effectively used to consider the geometrical and material complexities of large-scale heat transfer problems with temperature-dependent fluid densities.

Geochronology of the Pengjiakuang and Rushan gold deposits, Eastern Jiaodong Gold Province, Northeastern China: Implications for regional mineralization and geodynamic setting

The Jiaodong gold province, the largest gold-producing district in China, is located in the jiaodong peninsula at the eastern margin of the North China craton and bounded by the continental scale Tan-Lu fault, 40 kin to the west. Previous geochronological studies suggest that pervasive gold deposition took place in the western part of the province between 122 and 119 Ma. Here we report high-quality Ar-40/Ar-39 ages of the Pengjiakuang and Rushan deposits from the eastern part of the jiaodong gold province, placing additional chronological constraints on the timing of regional mineralization. Seven sericite grains extracted from auriferous alteration assemblages at the Pengiiakuang deposit yielded well-defined plateau ages between 120.9 +/- 0.4 and 119.1 +/- 0.2 Ma (2 sigma). Three separates of igneous biotite from a sample of the Queshan gneissic granite, adjacent to the Pengjiakuang deposit, gave reproducible plateau ages of 124.6 +/- 0.6 to 123.9 +/- 0.4 Ma (2 sigma). Six sericite separates front two samples in the Rushan deposit yielded Ar-40/Ar-39 plateau ages at 109.3 +/- 0.3 to 107.7 +/- 0.5 Ma (2 sigma), whereas biotite from the Kunyushan monzogranite that hosts the Rushan deposit had plateau ages ranging from 129.0 +/- 0.6 to 126.9 +/- 0.6 Ma (3 separates front one sample). The apparent age gap between hydrothermal sericite and magmtic biotite from both deposits, together with the similar argon closure temperatures for these mica minerals, suggest that gold mineralization had no direct relationship to the granitoid magmatism. Instead, gold deposition coincided with the emplacement of mafic to intermediate dikes widespread in the jiaodong gold province, which have been dated at ca. 122 to 119 Ma and, less commonly, at 110 to 102 Ma. The new Ar-40/Ar-39 ages from the eastern jiaodong peninsula, when combined with published data from the western part suggest that gold mineralization was broadly contemporaneous throughout the district. The Early Cretaceous gold mineralization also is widely developed in four other major gold districts along the Tan-Lu fault. The temporal and spatial correlation of these gold deposits with mafic to intermediate dikes commonly found in most mineralized areas, the presence of well-documented metamorphic core complexes and half-graben basins along the Tan-Lu fault, and voluminous basalts therein, suggest that the Early Cretaceous was an important period of lithospheric extension, possibly caused by the late Mesozoic lithospheric thinning beneath the eastern block of the North China craton. Lithospheric thinning and extension could have resulted in abnormally high heat and fluid fluxes necessary for large-scaled gold mineralization.

From point defects to plate tectonic faults

Understanding and explaining emergent constitutive laws in the multi-scale evolution from point defects, dislocations and two-dimensional defects to plate tectonic scales is an arduous challenge in condensed matter physics. The Earth appears to be the only planet known to have developed stable plate tectonics as a means to get rid of its heat. The emergence of plate tectonics out of mantle convection appears to rely intrinsically on the capacity to form extremely weak faults in the top 100 km of the planet. These faults have a memory of at least several hundred millions of years, yet they appear to rely on the effects of water on line defects. This important phenomenon was first discovered in laboratory and dubbed ``hydrolytic weakening''. At the large scale it explains cycles of co-located resurgence of plate generation and consumption (the Wilson cycle), but the exact physics underlying the process itself and the enormous spanning of scales still remains unclear. We present an attempt to use the multi-scale non-equilibrium thermodynamic energy evolution inside the deforming lithosphere to move phenomenological laws to laws derived from basic scaling quantities, develop self-consistent weakening laws at lithospheric scale and give a fully coupled deformation-weakening constitutive framework. At meso- to plate scale we encounter in a stepwise manner three basic domains governed by the diffusion/reaction time scales of grain growth, thermal diffusion and finally water mobility through point defects in the crystalline lattice. The latter process governs the planetary scale and controls the stability of its heat transfer mode.