The compressed geomagnetic field as a function of dipole tilt

The currents on the precisely calculated magnetosphere surface previously reported have been integrated with the same accuracy as used in the surface calculation to give the magnetic field at 688 points within the magnetosphere for each of eight surfaces for the tilt of the Earth's dipole in steps of 5° between 0° and 35°. The magnetic fields have then been used to fit the coefficients of a 35 term spherical harmonic expansion of the scalar magnetic potential representing the field by the method of least squares fit. The coefficients for each of the eight surfaces were then represented as a power series in dipole tilt angle, λ so that the complete field can be given very conveniently for any λ. For λ = 0, the first two coefficients g₁₀ and g₂₁ are the dominant terms and the azimuthally dependent term g₂₁ is 30 per cent less than that calculated by Mead and Midgley from older less accurate surface and field calculations.     

The near earth magnetic field of the magnetotail current

In the forward part of the magnetosphere the distant tail current system approximates a magnetic quadrupole composed of two distorted adjacent solenoids. The current in the neutral sheet at this distance is accurately approximated as an infinitesimally thin current sheet. We have calculated the magnetic field near the Earth by integrating over the entire tail current system assuming the magnetotail is a cylinder of constant radius and that the tail current decreases with distance into the tail as $\text{|x|}{}^{\mathrm{<mtext>?1</mtext><mtext>3</mtext>}}$. The field is then represented by a scalar potential expanded into spherical harmonics which may be conveniently added to the spherical harmonic expansion of the scalar potential representing the magnetopause current system.     

Absolute Calibration of an Ashtech Z12-T GPS Receiver

Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated. We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate uncertainty of order 1 ns in the absolute calibration seems to be attainable. ? 2001 John Wiley & Sons, Inc.     

Iterative approach to groundwater flow modeling of the Martinsville Alternative Site,under consideration for low-level radioactive waste storage in Clark County,Illinois, U.S.A.

The regulatory requirements for characterization of the Martinsville Alternative Site (MAS) were fulfilled by applying an iterative approach to the groundwater flow modeling of the site and surrounding area. The approach consisted of field data collection and development of an initial conceptual model. The numerical model was then constructed to be consistent with the data and conceptual model. Next, the calibration results were evaluated statistically, and visually by a groundwater modeling review committee, to determine if the model accurately represented groundwater flow at the site. Initial results failed acceptance criteria because the values of numerical model input parameters had to be varied beyond observed data ranges to calibrate the results, and therefore the model was inconsistent with the initial conceptual model. This led to additional field data collection in areas where the numerical model deviated most from field-determined data. The new data provided sufficient information to revise the conceptual model and calibrate the numerical model successfully. Model calibration was followed by validation. Validation of the numerical model provided additional assurance that the model correctly simulated the observed system. No additional data were found to be necessary during validation of the MAS numerical model. The iterative approach proved to be successful for calibrating and validating this groundwater flow model and should be implemented from the onset of characterization planning in other applications.     

Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in ⁵⁶Fe/⁵⁴Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the ⁵⁶Fe/⁵⁴Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ⁵⁶Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ⁵⁶Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ⁵⁶Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ⁵⁶Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ⁵⁶Fe values; these seem best explained through production of high ⁵⁶Fe/⁵⁴Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ⁵⁶Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ⁵⁶Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.     

The Origins of Yakutian Eclogite Xenoliths

Owing to the association with diamonds, eclogite xenoliths havereceived disproportionate attention given their low abundancein kimberlites. Several hypotheses have been advanced for theorigin of eclogite xenoliths, from the subduction and high-pressuremelting of oceanic crust, to cumulates and liquids derived fromthe upper mantle. We have amassed a comprehensive data set,including major- and trace-element mineral chemistry, carbonisotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os,and oxygen isotopes in ultrapure mineral and whole-rock splitsfrom eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia.Furthermore, eclogites from two other Yakutian kimberlite pipes,Mir and Obnazhennaya, have been studied in detail and offercontrasting images of eclogite protoliths. Relative to eclogitesfrom southern Africa and other Yakutian localities, Udachnayaeclogites are notable in the absence of chemical zoning in mineralgrains, as well as the degree of light rare earth element (LREE)depletion and unradiogenic Sr; lack of significant oxygen, sulfur,and carbon isotopic variation relative to the mantle; and intermineralradiogenic isotopic equilibration. Several of these eclogitescould be derived from ancient, recycled, oceanic crust, butmany others exhibit no evidence for an oceanic crustal protolith.The apparent lack of stable-isotope variation in the Udachnayaeclogites could be due to the antiquity of the samples and consequentlack of deep oceanic and biogenically diverse environments atthat time. Those eclogites that are interpreted to be non-recycledhave compositions characteristic of Group A eclogites from otherlocalities that also have been interpreted as being directlyfrom the mantle. At least two separate and diverse isotopicreservoirs are suggested by Nd isotopic whole-rock reconstructions.Most samples were derived from typical depleted mantle. However,two groups of three samples each indicate both enriched mantleand possible ultra-depleted mantle present beneath Yakutia duringthe late Archean and early Proterozoic. The vast majority ofeclogites studied from the Obnazhennaya pipe also exhibit characteristicsof Group A eclogites and are probably derived directly fromthe mantle. However, the eclogites from the Mir kimberlite aremore typical of other eclogites world-wide and show convincingevidence of a recycled, oceanic crustal affinity. We concurwith the late Ted Ringwood that eclogites can be formed in avariety of ways, both within the mantle and from oceanic crustalresidues. KEY WORDS: diamonds; eclogite xenoliths; isotopic composition; REE; Yakutia     

Polygenetic tonalite-trondhjemite-granodiorite (TTG) magmatism in the Smartville Complex, Northern California with a note on LILE depletion in plagiogranites

Summary Tonalite, trondhjemite, and granodiorite (TTG) occur in dikes, plugs and tabular to equant plutons within the intrusive core of the Smartville Complex, a late Jurassic rifted arc. Two groups of TTG are recognized. A high-K group consisting of calc-alkaline tonalite to granodiorite is enriched in LILE and Th and depleted in Na, Y and HREE with respect to a more tholeiitic and trondhjemitic low-K group. Within the high-K group, Th, LIL, La, and La / Lu show a regional southward increase from biotite tonalite plutons in the north to granodiorite intrusions in the south. These regional chemical variations parallel regional chemical variations in older metavolcanic rocks and massive metadiabase that form the bulk of the basement into which the Smartville TTG were intruded. The geochemical and geological characteristics of most high-K group rocks are consistent with an origin by low-pressure (< 5 kb) partial melting of arc basement rocks. Some high-K group rocks, however, are strongly depleted in Y and HREE, suggestive of melting in the garnet stability field at P > 10 kb. Thus, the basement probed by the high-K group may be vertically, as well as laterally, extensive. A low-K group of largely tholeiitic tonalite to trondhjemite intrusions has lower LIL, Th, and La/Lu and higher Na, Y and HREE than the high-K group. Within this group, Y, Ga, and Na all increase to the west towards the Smartville sheeted dike complex. The westernmost intrusives in the low-K group have the chemical characteristics (e.g. high Y, Y/Nb and (Y+Nb)/Rb) of ocean ridge granites. The low-K group is best modeled by crystal fractionation of coeval, basaltic and andesitic magmas, although crustal assimilation also appears to be important in one of the intrusions.Like most oceanic tonalites (e.g. plagiogranites), the low-K group rocks are overdepleted in LIL elements. The over-depletion appears to be an intrinsic property of the low-K intrusives, unrelated to post-magmatic hydrothermal effects. It is proposed that LIL elements are lost from low-K rocks because they evolve a vapor phase prior to the fixing of LIL elements by crystallization of a phase such as biotite. The relative order of LIL over-depletion (Rb > K > Ba) is consistent with this interpretation. Polygenetischer Tonalit-Tronhjemit-Granodiorit (TTG)-Magmatism.us im Smartville Komplex, Nord-Kalifornien, mit einer Notiz Über LILE Verarmung

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Polgenetischer Tonalit-Tronjemit_Granodiorit (TTG)-Magmatismus im Smartville Komplex, Nord-kalifornien, mit einer Notiz über LILE Verarmung

Zusammenfassung Tonalit, Trondhjemit und Granodiorit (TTG) kommen als Gänge, Schlote und flächige Plutone im intrusiven Kern des Smartville Komplexes, einem spät-Jurassischen Riftbogen vor. Zwei Gruppen von TTG liegen vor: eine K-reiche Gruppe, die aus Kalk-alkalischen Tonaliten bis Granodioriten besteht, ist LILE und Th angereichert, jedoch an Na, Y und HREE, verglichen mit der mehr tholeiitischen und trondhjemitischen K-armen Gruppen, verarmt. Innerhalb der K-reichen Gruppe zeigen Th, LIL, La und La/Lu eine Zunahme von Biotit-Tonalit-Plutonen im Norden zu Granodiorit-Intrusionen im Süden. Diese regionalen chemischen Variationen in älteren, metavulkanischen Gesteinen sind mit jenen in massiven Metadiabasen parallel. Letztere bilden den Großteil des Basements, in welches die Smartville TTG intrudierten. Die geochemischen und geologischen Charakteristika der K-reichsten Gruppe sind in Übereinstimmung mit einem Ursprung durch teilweise Aufschmelzung unter niedrigem Druck (< 5 kb) der Basement Gesteine des Bogens. Einige K-reiche Gesteine sind jedoch stärker an Y und HREE angereichert, was auf Aufschmelzung in Stabilitätsfeld von Granat bei P > 10 kb hinweist. Das Basement, von dem Teile in die K-reiche Gruppe aufgenommen wurden, dürfte daher sowohl vertikal wie lateral ausgedehnt sein. Eine K-arme Gruppe von großteils tholeiitischen Tonalit bis Trondhjemit-Intrusionen hat niedrige LIL, Th und La/Lu und höhere Na, Y und HREE als die K-reiche Gruppe. Innerhalb dieser nehmen Y, Ga und Na nach Westen gegen den Smartville sheeted dike Komplex zu. Die westlichsten Intrusiva in der K-armen Gruppe haben chemische Charakteristika (z.B. hohes Y, Y/Nb und (Y + Nb)/Rb) von Graniten ozeanischer Rücken. Die K-arme Gruppe läßt sich am besten durch Kristallfraktionierung gleich alter, basaltischer und andesitischer Magmen modellieren, obwohl Assimilation von Krustenmaterial in einer der Intrusionen auch von Bedeutung zu sein scheint.Wie die meisten ozeanischen Tonalite (Plagiogranite) sind auch die Kali-armen Gesteine besonders an LIL-Elementen verarmt. Diese besondere Verarmung scheint eine charakteristische Eigenschaft von Kali-armen Intrusiven zu sein, die nicht in Beziehung zu postmagmatischen, hydrothermalen Erscheinungen steht. Wir nehmen an, daß LIL-Elemente von den Kali-armen Gesteinen in einer Dampfphase entfernt werden, bevor sie durch Kristallisation von Mineralen wie Biotit fixiert werden können. Die relative Ordnung der intensiven Abreicherung der LIL (Rb > K > Ba) stimmt mit dieser Interpretation überein.

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With 7 Figures     

Stable Fe isotope fractionations produced by aqueous Fe(II)-hematite surface interactions

Stable Fe isotope fractionations were investigated during exposure of hematite to aqueous Fe(II) under conditions of variable Fe(II)/hematite ratios, the presence/absence of dissolved Si, and neutral versus alkaline pH. When Fe(II) undergoes electron transfer to hematite, Fe(II) is initially oxidized to Fe(III), and structural Fe(III) on the hematite surface is reduced to Fe(II). During this redox reaction, the newly formed reactive Fe(III) layer becomes enriched in heavy Fe isotopes and light Fe isotopes partition into aqueous and sorbed Fe(II). Our results indicate that in most cases the reactive Fe(III) that undergoes isotopic exchange accounts for less than one octahedral layer on the hematite surface. With higher Fe(II)/hematite molar ratios, and the presence of dissolved Si at alkaline pH, stable Fe isotope fractionations move away from those expected for equilibrium between aqueous Fe(II) and hematite, towards those expected for aqueous Fe(II) and goethite. These results point to formation of new phases on the hematite surface as a result of distortion of Fe-O bonds and Si polymerization at high pH. Our findings demonstrate how stable Fe isotope fractionations can be used to investigate changes in surface Fe phases during exposure of Fe(III) oxides to aqueous Fe(II) under different environmental conditions. These results confirm the coupled electron and atom exchange mechanism proposed to explain Fe isotope fractionation during dissimilatory iron reduction (DIR). Although abiologic Fe(II)_aq - oxide interaction will produce low δ⁵⁶Fe values for Fe(II)_aq, similar to that produced by Fe(II) oxidation, only small quantities of low-δ⁵⁶Fe Fe(II)_aq are formed by these processes. In contrast, DIR, which continually exposes new surface Fe(III) atoms during reduction, as well as production of Fe(II), remains the most efficient mechanism for generating large quantities of low-δ⁵⁶Fe aqueous Fe(II) in many natural systems.     

The stoichiometric effects of ferric iron substitutions in serpentine from microprobe data

ABSTRACT

Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric ‘brucite’ may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96–98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.     

Practical flood frequency estimates

The best information on which to base estimates of future flood frequencies is records of past flood events. Where there is a substantial record at the location for which estimates are desired the estimation process is generally straighforward, although a variety of methods is used and there is major uncertainty in the estimates. In general, the frequency of future events is assumed to be indicated by the observed frequency of past events under constant controlling watershed conditions.Techniques are available for using information on historical (pre-record) flood data to improve the reliability of flood frequency estimates. There are methods for detecting and managing extremely unusual actual events (outliers) and for improving the reliability of short-record estimates based on long-record data at related locations. Regional correlation analysis is usable for establishing flood frequency estimates for locations where records are not available.Detailed hydrologic analysis, usually involving rainfall-runoff studies, is required for establishing flood frequency relationships for modified conditions of the watershed or, in many cases, for establishing flood frequency estimates for newly formed drainage systems such as in urban areas and airports.The principal use of flood frequency functions is to compare expected changes in flood damages (due to a contemplated action) with the economic and social costs or benefits of the contemplated action.