20,799 research outputs found
Granitic rocks and metasediments in Archean crust, Rainy Lake area, Ontario: ND isotope evidence for mantle-like SM/ND sources
Granitoids, felsic volcanic rocks and clastic metasediments are typical rocks in Archean granite-greenstone belts that could have formed from preexisting continentasl crust. The petrogenesis of such rocks is assessed to determine the relative roles of new crust formation or old crust formation or old crust recycling in the formation of granite-greenstone belts
The Kolar Schist Belt: A possible Archaean suture zone
The Kolar Schist Belt represents a N-S trending discontinuity in the structures, lithologies, and emplacement and metamorphic ages of late Archean gneisses. The suggestion of a much older basement on the west side of the belt is not seen on the east. Within the schist belt amphibolites from each side have distinctly different chemical characteristics, suggesting different sources at similar mantle depths. These amphibolites were probably not part of a single volcanic sequence, but may have formed about the same time in two completely different settings. Could the amphibolites with depleted light REE patterns represent Archean ocean floor volcanics which are derived from a mantle source with a long term depletion of the light REE? Why are the amphibolites giving an age which may be older than the exposed gneisses immediately on either side of the belt? These results suggest that it is necessary to seriously consider whether the Kolar Schist Belt may be a suture between two late Archean continental terranes
Malmquist Bias and the Distance to the Virgo Cluster
This paper investigates the impact of Malmquist bias on the distance to the
Virgo cluster determined by the H_0 Key Project using M100, and consequently on
the derived value of H_0. Malmquist bias is a volume-induced statistical effect
which causes the most probable distance to be different from the raw distance
measured. Consideration of the bias in the distance to the Virgo cluster raises
this distance and lowers the calculated value of H_0. Monte Carlo simulations
of the cluster have been run for several possible distributions of spirals
within the cluster and of clusters in the local universe. Simulations
consistent with known information regarding the cluster and the errors of
measurement result in a bias of about 6.5%-8.5%. This corresponds to an
unbiased distance of 17.2-17.4 Mpc and a value of H_0 in the range 80-82
km/s/Mpc.
The problem of determining the bias to Virgo illustrates several key points
regarding Malmquist bias. Essentially all conventional astronomical distance
measurements are subject to this bias. In addition, the bias accumulates when
an attempt is made to construct "distance ladders" from measurements which are
individually biased. As will be shown in the case of Virgo, the magnitude and
direction of the bias are sensitive to the spatial distribution of the parent
poputation from which the observed object is drawn - a distribution which is
often poorly known. This leads to uncertainty in the magnitude of the bias, and
adds to the importance of minimizing the number of steps in "distance ladders".Comment: 19 pages, 3 figures, Latex, To appear in Ap
The hydrology of prehistoric farming systems in a central Arizona ecotone
The prehistoric land use and water management in the semi-arid Southwest was examined. Remote sensing data, geology, hydrology and biology are discussed along with an evaluation of remote sensing contributions, recommendations for applications, and proposed future remote sensing studies
Unidirectional and diffractionless surface plasmon-polaritons on three-dimensional nonreciprocal plasmonic platforms
Light-matter interactions in conventional nanophotonic structures typically
lack directionality. Furthermore, surface waves supported by conventional
material substrates do not usually have a preferential direction of
propagation, and their wavefront tends to spread as it propagates along the
surface, unless the surface or the excitation are properly engineered and
structured. In this article, we theoretically demonstrate the possibility of
realizing \emph{unidirectional and diffractionless surface-plasmon-polariton
modes} on a nonreciprocal platform, namely, a gyrotropic magnetized plasma.
Based on a rigorous Green function approach, we provide a comprehensive and
systematic analysis of all the available physical mechanisms that may bestow
the system with directionality, both in the sense of one-way excitation of
surface waves, and in the sense of directive diffractionless propagation along
the surface. The considered mechanisms include (i) the effect of strong and
weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like
topology of the modal dispersion surfaces, and (iii) the source polarization
state, with the associated possibility of chiral surface-wave excitation
governed by angular-momentum matching. We find that three-dimensional
gyrotropic plasmonic platforms support a previously-unnoticed wave-propagation
regime that exhibit several of these physical mechanisms simultaneously,
allowing us to theoretically demonstrate, for the first time, unidirectional
surface-plasmon-polariton modes that propagate as a single ultra-narrow
diffractionless beam. We also assess the impact of dissipation and nonlocal
effects. Our theoretical findings may enable a new generation of plasmonic
structures and devices with highly directional response
Asymptotic behavior of the least common multiple of consecutive arithmetic progression terms
Let and be two integers with , and let and be
integers with and . In this paper, we prove that , where is a constant depending on and .Comment: 8 pages. To appear in Archiv der Mathemati
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