The generation of sodic granite magmas, western Palmer Land, Antarctic Peninsula

Subduction-related Mesozoic to Cainozoic granites s.l. in western Palmer Land, Antarctic Peninsula, have similar chemical compositions to Archean tonalite-trondhjemite-granodiorite (TTG) suites, Phanerozoic slab-melts (adakites), and to experimental partial melts of basaltic material in equilibrium with amphibole ± pyroxene ± garnet. They are predominantly sodic, metaluminous and most have Al₂O₃ > 15 wt% and Y < 18 ppm. All are light rare earth element (LREE)-enriched (2 < La/Yb_n <30) and most have small Eu anomalies. They have a wide range of initial ɛNd_(t) (−6.8 to +4.5) and ɛSr_(t) (+293.4 to −3.7), but most Pb isotope compositions deviate by < 0.3% from their mean. The Pb isotope data indicate a crustal component to all the granites, which Sr and Nd isotope variations suggest is pre-Triassic–Triassic. The ²⁰⁷Pb/²⁰⁴Pb_(t) range from 15.602 to 15.666 and appear to preclude a significant Proterozoic, or older, crustal component. The granites have chemical and isotopic compositions that suggest they are not partial melts of subducted oceanic lithosphere, as has been suggested for some Archean and Phanerozoic TTG magmas. We conclude that they were produced by mixing between basaltic-andesitic arc magmas, partial melts of juvenile basaltic lower crust and pre-Triassic crust. The low H(heavy)REE+Y content of some of the granites requires that garnet was a residual phase in the crust during partial melting, indicating a crustal thickness of >36 km. Between Triassic and Tertiary times the initial ɛNd_(t) of the magmatism increased and ɛSr_(t) decreased, suggesting that new continental crust was produced during this period. Underplating by mafic magma was an important crustal growth mechanism in the arc: the generation of abnormally thick crust, and its subse quent fusion, is considered to be a consequence of ca. ≥ 180 Ma of subduction and associated magmatism in the region. An implication of the model is that dense garnet-amphibolite and eclogite residues from partial melting of the lower crust will accumulate. In theory, the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle. Such a process would leave the rest of the crust enriched in large ion lithophile elements/LREE, but depleted in HREE+Y. Received: 2 October 1995 / Accepted: 5 January 1997     

Sampling‐surface orientation and clast macrofabric in periglacial colluvium

To isolate the influence of sampling‐surface orientation on the macrofabric of periglacial colluvial deposits, clast orientation measurements were obtained from seven paired horizontal and vertical exposures in turf‐banked solifluction lobes on Niwot Ridge, Colorado Front Range. Most samples form moderately strong, upslope‐plunging clusters aligned with the local slope orientation. Fabrics obtained from vertical faces were stronger than those from horizontal exposures in six of the pairs. Near‐horizontal sampling surfaces yield less biased results than vertical exposures, owing to operator perceptions, procedural difficulties, and the relative thinness of the layer affected by colluvial processes. Sampling procedures must be standardized before comparative studies of colluvium can yield reliable results. Copyright © 2001 John Wiley & Sons, Ltd.     

Silicon monoxide: A component of interstellar grains?

Gürtleret al. (1981) have argued that SiO cannot be the material responsible for the interstellar 9.7 m feature. We discuss their arguments and show them to be questionable. Experimental data on the thermal stability of SiO under reducing conditions typical of circumstellar regions could help decide whether SiO is an important constituent of interstellar grains.     

The temperature of interstellar oxide grains

Grain temperatures have been derived for small oxide grains in a typical interstellar radiation field. Detailed Mie calculations have been used to determineQ _abs from 1000 Å to 30 m. Longward of 30 m, there is no optical data available but the fact that such grains are likely highly defected implies that a ^–1 dependence in the far-infrared may be appropriate.     

Transient events in the COBE database simultaneous with gamma ray bursts

We are investigating the COBE DMR data at instances of known -ray bursts (GRBs) when any of the six DMR horn directions was coincident with the direction of a burst. The BATSE instrument on board GRO has detected 207 bursts during the eight-month period of overlap corresponding to the current release of COBE data. The odds of a GRB occurring within the DMR field of view are near one coincidence per year. Here we report on one such serendipitous observation in 1991, GRB 911226, for which a detailed analysis is currently in progress.     

Structure and chemistry in the hot molecular core G34.3+0.15

New multifrequency spatial and spectral studies of the hot molecular core associated with the ultracompact HII region G34.3+0.15 have demonstrated an extremely rich chemistry in this archetypal hot core and revealed differing spatial structure between certain species which may be a dynamical effect of chemical evolution. The structure of the hot core has been studied with the JCMT in the high excitation J=19-18 and J=18-17 lines of CH₃CN and with the Nobeyama Millimetre Array at 4 arc resolution in the J=6-5 transition. Comparison with a VLA NH₃(3,3) map shows a displacement between peak emission in the two chemical species which is consistent with chemical processing on a time scale comparable to the dynamical time scale of 10⁵ yrs.A 330-360 GHz spectral survey of the hot core with the JCMT has detected 358 spectral lines from at least 46 distinct chemical species, including many typical of shocked chemistry while other species indicate abundances that reflect the chemistry of a previous cold phase. The first unambiguous detection of ethanol in hot gas has been made. Observations of 14 rotational transitions of this molecule yield a temperature of 125 K and column density 2×10¹⁵ cm^–2. This large abundance cannot be made by purely gas-phase processes and it is concluded that ethanol must have formed by grain surface chemistry.     

Simulation of a Ground Water Recirculation Well with a Dual-Column Laboratory Setup

This paper describes a dual-column laboratory setup consisting of a glass column and a stainless-steel column filled with aquifer material. The setup was used to replicate a ground water recirculation well that serves as an in situ reactor and a combined injection/withdrawal well. The treatment solution consisted of a buffered titanium (III) citrate/vitamin B₁₂ mixture. The first column, representing the well, was made of glass, allowing for visual inspection of the mixing. The stainless-steel column was instrumented with redox (Eh) probes to monitor the changes in redox conditions. The redox measurement showed that, although the sand contained large quantities of iron oxides, the oxidation rate was relatively slow and the titanium solution would remain reduced for some time in the aquifer, continuing to react with the contaminants. This laboratory setup was used to optimize the reagent concentrations and rate of delivery for field implementation. It was found that 4 mM titanium citrate and 3 mg/L vitamin B₁₂ were sufficient to degrade 1,1,2,2-tetrachloroethane and carbon tetrachloride within one day, but not trichloroethylene, which required five days with 10 mM titanium citrate and 5 mg/L vitamin B₁₂.     

Theoretical regime equations for mobile gravel-bed rivers with stable banks

A system of rational regime equations is developed for gravel-bed rivers with stable banks using the optimality theory (OT). The optimality theory is based on the premise that equilibrium river geometry is characterised by an optimum configuration, defined here as maximum sediment-transport efficiency. Theoretical dimensionless equations are derived for width, depth, slope, width/depth ratio, and meandering–braiding transition. Independent dimensionless variables comprise discharge, sediment concentration, and relative bank strength, μ′, which is defined as the ratio of the critical shear stresses for the bank and bed sediments. Discharge exponents and general form of the equations agree well with previously developed empirical relations. Relative bank strength, μ′, is used to parameterise the influence of riparian vegetation on bank strength and is evaluated by calibrating against observed width/depth ratio. Once calibrated, the hydraulic geometry of natural gravel rivers is well described by the theoretical equations, including discrimination between meandering and braiding channels. The results provide strong support for the assumption that equilibrium or regime river behavior is equivalent to an optimal state and underline the importance of bank strength and sediment load as controls on hydraulic geometry.     

Rational regime model of alluvial channel morphology and response

A theoretical model is developed for predicting equilibrium alluvial channel form. The concept of greatest relative stability, achieved by maximizing resistance to ?ow in the ?uvial system, is presented as the basis for an optimization condition for alluvial systems. Discharge, sediment supply (quantity and calibre) and valley gradient are accepted as independent governing variates. The model is used to de?ne a dimensionless alluvial state space characterized by aspect ratio (W/d), relative roughness (D/d), and dimensionless shear stress (τ*) or, equivalently, channel slope (S). Each alluvial state exhibits unique values of Froude number and sediment concentration. The range of alluvial states for constant values of relative bank strength (parameterized by an apparent friction angle, ?′) forms a single plane in the state space (W/d, D/d, τ* or S). The scaling relations produced by the model are consistent with laboratory channels exhibiting a range of bank strengths, and with the behaviour of natural channels. Copyright © 2004 John Wiley & Sons, Ltd.     

Molecule formation and cloud collapse

Ultraviolet spectra ofo And, CrB, Ori A and Vir, which were obtained with the S59 spectrometer at a resolution of 1.7 Å in three 100 Å wide regions centered at 2110, 2454 and 2825 Å, have been studied for the presence or the absence of the effects due to their binary nature. As may have been anticipated from their orbital and other characteristics, no indication of strong binary interactions were seen in these observations. However, there are certain spectral peculiarities suggesting the possibility of modifications of spectral classifications for some of these objects. A rather unusual spectral behavior in Vir is also noted. In addition, based primarily on the reviewing of the available literature, we draw new attention to a remarkable property of the third component in Ori A.