EFFECTIVE DISCHARGE CALCULATION GUIDE

1 mTRonvcnox: moareS Aun CoxcmSAJluvial rivers have the pOtenhal to adjust their shaPe and dimensions to all flows that tranSPOrtsediment, but Inglis (l94l) suggested that, for rivers that are in regime, a single steady flow could beidenhfied which would Produce the same bankfll dimensions as the natural sequence of events. Hereferrd to this now as the dondnan discharge.Wolman and Mller (l960) idenhfied that the flow doing most bed material transPort over a period ofyears may be taken tO…     

Micro-zircon: origin and evolution during metamorphism

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2–3.0 μm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 × 10⁶ micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.     

Systematic palaeontology and biostratigraphy of two Early Cretaceous condensed sections from the Barents Sea

Bivalve, brachiopod and cirripede faunas from the latest Jurassic and Early Cretaceous Barents Sea boreholes 7320/3-U-l and 7425/9-U-l are systematically described and illustrated. Microfossils have also been studied and the cores arc dated on the basis of the fossil recovery. The bivalve Buchia whose zonal sequence has been used for correlation of boreal marine sections is the most important biostratigraphic marker group in the condensed Boreal Berriasian-Hauterivian intervals of these cores. A new species of cirripede Zeugmatolepas? borealis Collins sp. nov. and dinoflagellate cyst Muderongia aequicornus Århus sp. nov. are described.
The Late Jurassic fine-grained elastics of core 7320/3-U-l are overlain by about 3 m of grey dolomitic limestone of Valanginian and Hauterivian age. The lowermost part of 7425/9-U-l is represented by a latest Volgian-earliest Berriasian fossiliferous greyish green marl. It is followed by a reddish brown fossilifcrous claystone of Berriasian and perhaps partly Valanginian age. Core 7425/9-U-l also contains a mainly Valanginian greyish green marly limestone which changes into a dark grey to black limestone of Early Barremian age in its upper part. The sedimentological change from condensation to dark grey clay deposition took place in the middle Barremian H. rude-fissicostatum ammonite Zone in 7425/9-U-l and probably slightly earlier in 7320/3-U-l. This was commenced at about the same time as deposition of the inaccurately and only indirectly dated fluvio-deltaic Festningen Sandstone Member on Spitsbergen. The dark claystone may thus be a distal equivalent to this sandstone unit.     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Melt Generation by Plumes: A Study of Hawaiian Volcanism

The mantle plume underlying the Hawaiian Swell has been modellednumerically using a stationary steady axisymmetric plume undera solid conducting lid. A method of calculating the rate ofmelt production from the plume has been developed, and the totalmelt production rate, the residual depth anomaly and the geoidanomaly have been used to constrain the model. The plume hasa central potential temperature of 1558 ?C and the mechanicalboundary layer is 72 km thick. An average of 6?6% melting occursin a melt-producing region which has a vertical extent of 55km and a radial extent of 130 km to produce 0?16 km³/y of melt.A parameterization of melt composition has been developed thatis consistent with laboratory experiments, with models of MORBgeneration, and with primitive Hawaiian tholeiites containing 16% MgO. There is no evidence that the major and minor elementconcentrations in the source region of Hawaiian tholeiites differfrom those in the source region of MORB. The model is consistentwith the REE contents of Kilauean tholeiites if the source regionhas primitive REE contents. The viscosity of the low-viscositylayer is constrained to be 10¹⁶m²/s.     

Chemical Characteristics of Carbonaceous Aerosols During Dust Storms over Xi'an in China

Characterization of carbonaceous aerosols including CC （carbonate carbon）, OC （organic carbon）, and EC （elemental carbon） were investigated at Xi＇an, China, near Asian dust source regions in spring 2002. OC varied between 8.2 and 63.7μgm^- 3, while EC ranged between 2.4 and 17.2 μ m^-3 during the observation period. OC variations followed a similar pattern to EC and the correlation coefficient between OC and EC is 0.89 （n=31）. The average percentage of total carbon （TC, sum of CC, OC, and EC） in PM2.5 during dust storm （DS） events was 13.6%, which is lower than that during non-dust storm （NDS） periods （22.7%）. CC, OC, and EC accounted for 12.9%, 70.7%, and 16.4% of TC during DS events, respectively. The average ratio of OC/EC was 5.0 in DS events and 3.3 in NDS periods. The OC-EC correlation （R^2=0.76, n=6） was good in DS events, while it was stronger （R^2=0.90, n=25） in NDS periods. The percentage of watersoluble OC （WSOC） in TC accounted for 15.7%, and varied between 13.3% and 22.3% during DS events. The distribution of eight carbon fractions indicated that local emissions such as motor vehicle exhaust were the dominant contributors to carbonaceous particles. During DS events, soil dust dominated the chemical composition, contributing 69% to the PM2.5 mass, followed by organic matter （12.8%）, sulfate （4%）, EC （2.2%）, and chloride （1.6%）. Consequently, CC was mainly entrained by Asian dust. However, even in the atmosphere near Asian dust source regions, OC and EC in atmospheric dust were controlled by local emission rather titan the transport of Asian dust.     

Implications of garnet resorption for the Lu–Hf garnet geochronometer: an example from the contact aureole of the Makhavinekh Lake Pluton,Labrador

In the contact aureole of the Makhavinekh Lake Pluton (MLP), Labrador, garnet resorption caused redistribution of Lu and loss of Hf, creating spuriously young Lu–Hf garnet ages. Garnet grew during granulite facies regional metamorphism at 1860–1850 Ma. At 1322 Ma, garnet rims were replaced by coronas of cordierite and orthopyroxene during contact metamorphism. Garnet–ilmenite Lu–Hf geochronology using bulk‐garnet separates yields apparent ages that young from 1876 ± 21 Ma at 4025 m from the contact to 1396 ± 8 Ma at 450 m from the contact. Toward the contact, garnet crystals are progressively more resorbed. Concentrations of Lu measured by LA‐ICP‐MS along radial traverses on central sections through relict garnet decrease gently away from the cores but rise steeply within 50–200 μm of the edges of the relicts. Enrichments of Lu in rims of relict garnet demonstrates strong partitioning of Lu into garnet during resorption and modest intracrystalline diffusion. Hafnium distributions could not be measured, but considering the strong incompatibility of Hf with garnet, it is likely that nearly all Hf in resorbed portions of the garnet was lost from the crystals. Lu–Hf ages in the aureole are thus controlled predominantly by this retention of Lu and loss of Hf during garnet resorption. This deduction was tested with a simple numerical model in which the partial retention of Lu and loss of Hf is tracked as a population of garnet is resorbed. Assuming a spherical geometry for garnet porphyroblasts, Rayleigh fractionation is used to approximate initial Lu zoning profiles ranging from flat to steeply decreasing toward garnet rims. The model simulates: (i) Lu–Hf decay for a specified period before resorption; (ii) instantaneous resorption with retention of Lu and loss of Hf from the resorbed portion of the crystal and (iii) Lu–Hf decay during a specified period after resorption. Several parameters influence the modelled age, but garnet resorption and Lu retention are the primary factors. When all other parameters are held constant, larger amounts of resorption and higher degrees of Lu retention produce younger apparent ages (false ages). Similarly, flatter initial Lu profiles yield younger apparent ages as a consequence of the larger proportion of Lu and Hf that resides in the outer portions of the porphyroblast. The difference between the apparent and actual ages is greater if the duration of the pre‐resorption decay period is large relative to the post‐resorption decay period. Larger crystals in a Gaussian crystal‐size distribution (CSD) generally dominate the Lu–Hf budget and produce an older apparent age relative to the age of the mean crystal size. Compared to a symmetrical Gaussian CSD, positively skewed CSDs result in reduced resorption of large crystals and produce an older apparent age. Application of the model to the MLP aureole, positing growth at 1850 Ma and resorption at 1320 Ma, yields model ages that young from 1850 to 1374 Ma toward the contact, in good agreement with the apparent ages determined from geochronology.     

Rare Earth Element Inversions and Percolation Models for Hawaii

A detailed study has been made of the evolution with time ofthe mantle source and degree of melting required to producea typical Hawaiian volcano. The preshield stage involves a changefrom a depleted to a primitive mantle source and an increasein the melt fraction. Important intra- and inter-volcano variationsoccur during the shield stage which may be related to melt transportprocesses. With time, Mauna Loa's lavas show depletion in increasinglycompatible elements, a result which is consistent with percolationprocesses. Postshield magmas are generated by very small degreesof melting of a depleted mantle source and may be importantin the development of posterosional volcanism. Posterosionallavas can be generated by variable degrees of melting of anamphibole-bearing source that has been enriched by postshieldmagma. A simple percolation model is consistent with the isotopic evolutionof Hawaiian volcanoes. All of the observed isotopic variationseen between Hawaiian volcanoes may be produced from only twomantle sources by small variations in the percolation processes. ^* Present address: Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK.     

The Eemian and Weichselian stratigraphy of the Langelandselv area, Jameson Land, East Greenland

Coastal Jameson Land is characterized by thick Quaternary deposits from the last interglacial/glacial cycle. The successions at the mouth of Langelandselv exhibit a key stratigraphy where sediments from the Langelandselv interglaciation (Eemian) are overlain by three till units interbedded with glacimarine and deltaic interstadial successions. Immediately after the retreat of glaciers after the extensive Scoresby Sund glaciation (Saalian). advection of warm Atlantic surface water surpassed what is known from the Holocene. The two lowermost Weichselian tills, deposited during the Aucellaelv and Jyllandselv stades (Early Weichselian), reflect short-lasting readvances of fjord glaciers. Luminescence dates and correlation with adjacent areas suggest ages of 110–80 ka and 70–60 ka for the Hugin Sø and the Møselv interstades, respectively.