Distribution and petrogenetic behaviour of trace elements in granitic pegmatite quartz from South Norway

The present study documents that the trace-element distribution in granitic quartz is highly sensitive to CAFC processes in granitic melts. Igneous quartz efficiently records both the origin and the evolution of the granitic pegmatites. Aluminium, P, Li, Ti, Ge and Na in that order of abundance, comprises >95% of the trace elements. Most samples feature >1 ppm of any of these elements. The remnant 5% includes K, Fe, Be, B, Ba and Sr whereas the other elements are present at concentrations lower than the detection limit. Potassium, Fe, Be and Ti are relatively compatible hence obtain the highest concentrations in early formed quartz. Phosphorous, Ge, Li and Al are relatively incompatible and generally obtain the highest concentrations in quartz that formed at lower temperatures from more evolved granitic melts. The Ge/Ti, the Ge/Be, the P/Ge and the P/Be ratios of quartz are strongly sensitive to the origin and evolution of the granitic melts and similarly the Rb/Sr and the Rb/K ratios of K-feldspars may be utilised in petrogenetic interpretations. However, the quartz trace element ratios are better at distinguishing similarities and differences in the origin and evolution of granitic melts. After evaluating the different trace element ratios, the Ge/Ti ratio appears to be most robust during subsolidus processes in the igneous systems, hence probably should be the preferred ratio for analysing and understanding petrogenetic processes in granitic igneous rocks.Editorial responsibility: J. Hoefs     

Relating millimeter-scale turbulence to meter-scale subtidal erosion and accretion across the fringe of a coastal mangrove forest

Within a wave-exposed mangrove forest, novel field observations are presented, comparing millimeter-scale turbulent water velocity fluctuations with contemporaneous subtidal bed elevation changes. High-resolution velocity and bed level measurements were collected from the unvegetated mudflat, at the mangrove forest fringe, and within the forest interior over multiple tidal cycles (flood–ebb) during a 2-week period. Measurements demonstrated that the spatial variability in vegetation density is a control on sediment transport at sub-meter scales. Scour around single and dense clusters of pneumatophores was predicted by a standard hydraulic engineering equation for wave-induced scour around regular cylinders, when the cylinder diameter in the equations was replaced with the representative diameter of the dense pneumatophore clusters. Waves were dissipated as they propagated into the forest, but dissipation at infragravity periods (> 30 s) was observed to be less than dissipation at shorter periods (< 30 s), consistent with the predictions of a simple model. Cross-wavelet analysis revealed that infragravity-frequency fluctuations in the bed level were occasionally coherent with velocity, possibly indicating scour upstream of dense pneumatophore patches when infragravity waves reinforced tidal currents. Consequently, infragravity waves were a likely driver of sediment transport within the mangrove forest. Near-bed turbulent kinetic energy, estimated from the turbulent dissipation rate, was also correlated with bed level changes. Specifically, within the mangrove forest and over the unvegetated mudflat, high-energy events were associated with erosion or near-zero bed level change, whereas low-energy events were associated with accretion. In contrast, no single relationship between bed level changes and mean current velocity was applicable across both vegetated and unvegetated regions. These observations support the theory that sediment mobilization scales with turbulent energy, rather than mean velocity, a distinction that becomes important when vegetation controls the development of turbulence.     

Growth and provenance of a Paleozoic subduction complex in the Broken River Province,Mossman Orogen: evidence from detrital zircon ages

A Paleozoic subduction complex dominates the Mossman Orogen developed at the northern extremity of the Tasmanides, eastern Australia. Its southern part, displayed in the Broken River Province, is characterised by dismembered ocean-plate stratigraphy in which turbidite-dominated packages and widespread tectonic mélange development are characteristic. The Broken River complex is characterised by formations with quartzose sandstone alternating with those largely formed of sandstone of more labile character. The two compositional groups are considered to reflect separate, age-significant sedimentary regimes, but their ages have hitherto been poorly constrained. With the use of 1082 concordant detrital zircon ages from 13 samples we provide age control for the complex and track its sedimentary provenance. Of quartzose units, the Tribute Hills Arenite and Pelican Range Formation are late Cambrian–Early Ordovician, and the Wairuna Formation is Middle to Late Ordovician, in age. The more labile units (Greenvale, Perry Creek and Kangaroo Hills formations) are collectively of late Silurian–mid-Devonian age. Development of the complex spanned some 130 Myr. Continent-derived sediment involved in accretion of much the complex, from mid-Ordovician to mid-Devonian, was largely sourced from a nearby magmatic arc of late Cambrian–Devonian age, now represented by granitoid plutons of the Macrossan and Pama igneous associations. An older far-field Pacific-Gondwana sediment source is characteristic of early-phase (late Cambrian–Early Ordovician) accretion, in common with sedimentary units of this age generally developed in the Tasmanides. We consider the complex to have grown largely by underplating that positioned younger components beneath those that are older, with out-of-sequence thrust interleaving of these components occurring late in the accretionary history. A Late Devonian contractional folding and cleavage development (Tabberabberan orogenesis) is uniformly expressed across the entire complex and reflects an abrupt change in plate engagement with imposition of a compressional stress regime.     

Photonic dating of prehistoric irrigation canals at Phoenix,Arizona, U.S.A.*

A number of archeological features, including in‐filled irrigation canals of uncertain prehistoric age, occur within the Holocene floodplain of the Salt River at Phoenix, Arizona. In the first attempt to date irrigation‐canal sediments using luminescence methods, we obtained age estimates of 1640 ± 190 yr B.P. (1σ) (multi‐aliquot or MA) and 1621 ± 95 yr B.P. (post‐IR single‐aliquot‐regenerative‐dose or SAR) for a single sample from the base of the oldest canal‐infilling deposits (all IR‐PSL ages reported in this article are in calendar years before A.D. 2001). For the remaining canal samples, weighted mean luminescence ages of 819 ± 45 yr (MA) and 826 ± 32 yr (post‐IR SAR) were obtained. Thus from photonic dating we can resolve the first and last phases of canal use at this Phoenix site: initiation at ca. 1600 years ago and final use at ca. 800 years ago. These results demonstrate the power of SAR luminescence sediment dating to enhance our understanding of prehistoric irrigation‐canal development and usage here and elsewhere in the world. © 2004 Wiley Periodicals, Inc.     

Potential Impact of Production Chemicals on the Toxicity of Produced Water Discharges from North Sea Oil Platforms

Production chemicals are used on offshore oil production platforms to prevent corrosion and scale formation, and to assist oil–water separation. A proportion of these chemicals may enter the marine environment via the produced water discharge. This study investigated the potential impact of 11 oilfield production chemicals on the toxicity of the produced water discharge. The Microtox® system was used for toxicity assessment of the chemicals, both directly in aqueous preparations and following their partitioning between oil (crude and low toxicity mineral base oil) and North Sea brine. For the majority of the chemicals tested, the toxicity of the aqueous phase to the test organism following partitioning against crude oil, was not significantly altered by the presence of process chemicals when used in their normal field dosage concentrations. However, there was evidence that certain chemicals could increase the partitioning of oil components into the aqueous phase by an order of magnitude, when applied at high dosage concentrations.     

Overhead Detection of Underground Nuclear Explosions by Multi-Spectral and Infrared Imaging

The Comprehensive Nuclear Test Ban Treaty allows for Multi-Spectral and Infrared Imaging from an aircraft and on the ground to help reduce the search area for an underground nuclear explosion from the initial 1,000 km². Satellite data, primarily from Landsat, have been used as a surrogate for aircraft data to investigate whether there are any multi-spectral features associated with the nuclear tests in Pakistan, India or North Korea. It is shown that there are multi-spectral observables on the ground that can be associated with the nominal surface ground zero for at least some of these explosions, and that these are likely to be found by measurements allowed by the treaty.     

The Niassa Gold Belt,northern Mozambique – A segment of a continental-scale Pan-African gold-bearing structure?

The Niassa Gold Belt, in northernmost Mozambique, is hosted in the Txitonga Group, a Neoproterozoic rift sequence overlying Paleoproterozoic crust of the Congo–Tanzania Craton and deformed during the Pan-African Orogeny. The Txitonga Group is made up of greenschist-facies greywacke and schist and is characterized by bimodal, mainly mafic, magmatism. A zircon U–Pb age for a felsic volcanite dates deposition of the sequence at 714 ± 17 Ma. Gold is mined artisanally from alluvial deposits and primary chalcopyrite-pyrite-bearing quartz veins containing up to 19 ppm Au have been analyzed. In the Cagurué and M’Papa gold fields, dominantly N–S trending quartz veins, hosted in metagabbro and schist, are regarded as tension gashes related to regional strike-slip NE–SW-trending Pan-African shear zones. These gold deposits have been classified as mesozonal and metamorphic in origin. Re–Os isotopic data on sulfides suggest two periods of gold deposition for the Cagurué Gold Field. A coarse-crystalline pyrite–chalcopyrite assemblage yields an imprecise Pan-African age of 483 ± 72 Ma, dating deposition of the quartz veins. Remobilization of early-formed sulfides, particularly chalcopyrite, took place at 112 ± 14 Ma, during Lower Cretaceous Gondwana dispersal. The ～483 Ma assemblage yields a chondritic initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.123 ± 0.058. This implies a juvenile source for the ore fluids, possibly involving the hosting Neoproterozoic metagabbro. The Niassa Gold Belt is situated at the eastern end of a SW–NE trending continental-scale lineament defined by the Mwembeshi Shear Zone and the southern end of a NW–SE trending lineament defined by the Rukwa Shear Zone. We offer a review of gold deposits in Zambia and Tanzania associated with these polyphase lineaments and speculate on their interrelation.     

Conodont biostratigraphy across the Permian–Triassic boundary at the Dawen section, Great Bank of Guizhou, Guizhou Province, South China: Implications for the Late Permian extinction and correlation with Meishan

Several continuous Permian–Triassic boundary (PTB) sections are well exposed in the interior of the Great Bank of Guizhou (GBG) on the east limb of the Bianyang syncline, Luodian County, Guizhou Province, South China. Fourteen conodont taxa are identified, including Clarkina kazi, Clarkina lehrmanni n. sp., Clarkina taylorae, Clarkina zhejiangensis, Hindeodus eurypyge, Hindeodus inflatus, Hindeodus sxlatidentatus, Hindeodus parvus erectus, Hindeodus parvus parvus, Hindeodus postparvus, Hindeodus praeparvus, Hindeodus typicalis, Isarcicella staeschei, and Merrillina ultima based on a detailed study of the Permian-Triassic interval at the Dawen section. The first occurrence (FO) of H. parvus parvus in the lower Daye Formation, at about 7.45 m above the contact surface between the Upper Permian skeletal packstone and a calcimicrobial framestone unit, is correlated with the Permian–Triassic boundary; the occurrence of H. eurypyge, H. praeparvus and M. ultima immediately below and H. postparvus above the interval supports this interpretation. A morphometric analysis of 31 Hindeodus specimens helped distinguish H. parvus erectus from H. parvus parvus and H. postparvus. Correlation with the Meishan section (PTB GSSP) using both conodont biostratigraphy and carbon isotopes, indicates that the major extinction at the two localities is simultaneous and coincides with the top of the skeletal packstone at Dawen. The contact between the skeletal packstone and the calcimicrobialite is very irregular and has previously been interpreted as a dissolution surface and correlated with a surface in the lower part of bed 27 at Meishan. Our results confirm this interpretation and reveal that the dissolution event postdated the extinction.     

A poincaré model for the earth's fluid core

Abstract

The superconducting-gravimeter data of Melchior and Ducarme (1986) has been interpreted as internal motion in the Earth's core by Aldridge and Lumb (1987) using a Poincaré model. Several low-order modes with periods of 13–16 hours have been tentatively identified in the core which is taken to be an incompressible, homogeneous fluid within a rigid, rotating container. The identification is based on asymptotic values of the frequencies which change slowly with time while the modes decay with an e-folding time of about 280 days. The slow change in frequency with time implies a small temporal variation in the rotation rate of the core. This mean flow is a nonlinear effect often observed in laboratory experiments designed to excite Poincaré modes. Interaction among modes during free ringdown is also observed in those experiments and apparently in the data of Melchior et al. (1988) as well. Laboratory work thus provides the link to extend the Poincaré model to include viscous and nonlinear effects in order to interpret the gravimetric observations as core modes.