247 research outputs found
Early formation and long-term stability of continents resulting from decompression melting in a convecting mantle
The origin of stable old continental cratonic roots is still debated. We present numerical modelling results which
show rapid initial formation during the Archaean of continental roots of ca. 200 km thick. These results have been
obtained from an upper mantle thermal convection model including differentiation by pressure release partial melting
of mantle peridotite. The upper mantle model includes time-dependent radiogenic heat production and thermal
coupling with a heat reservoir representing the Earths lower mantle and core. This allows for model experiments
including secular cooling on a time-scale comparable to the age of the Earth. The model results show an initial phase
of rapid continental root growth of ca. 0.1 billion year, followed by a more gradual increase of continental volume
by addition of depleted material produced through hot diapiric, convective upwellings which penetrate the continental
root from below. Within ca. 0.6 Ga after the start of the experiment, secular cooling of the mantle brings the average
geotherm below the peridotite solidus thereby switching off further continental growth. At this time the thickness of
the continental root has grown to ca. 200 km. After 1 Ga of secular cooling small scale thermal instabilities develop
at the bottom of the continental root causing continental delamination without breaking up the large scale layering.
This delaminated material remixes with the deeper layers. Two more periods, each with a duration of ca. 0.5 Ga and
separated by quiescent periods were observed when melting and continental growth was reactivated. Melting ends at
3 Ga. Thereafter secular cooling proceeds and the compositionally buoyant continental root is stabilized further
through the increase in mechanical strength induced by the increase of the temperature dependent mantle viscosity.
Fluctuating convective velocity amplitudes decrease to below 10 mma Ă 1 and the volume average temperature of the
sub-continental convecting mantle has decreased ca. 340 K after 4 Ga. Surface heatflow values decrease from 120 to
40 mW m Ă 2 during the 4 Ga model evolution. The surface heatflow contribution from an almost constant secular
cooling rate was estimated to be 6 mW m Ă 2, in line with recent observational evidence. The modelling results show
that the combined effects of compositional buoyancy and strong temperature dependent rheology result in continents
which overall remain stable for a duration longer than the age of the Earth. Tracer particles have been used for
studying the patterns of mantle differentiation in greater detail. The observed ( p, T, F, t)-paths are consistent with
proposed stratification and thermo-mechanical history of the depleted continental root, which have been inferred
from mantle xenoliths and other upper mantle samples. In addition, the particle tracers have been used to derive the
thermal age of the modelled continental root, defined by a hypothetical closing temperature. © 2000 Elsevier Science
B.V. All rights reserved
Stability and growth of continental shields in mantle convection models including recurrent melt production
The long-term growth and stability of compositionally layered continental upper mantle has been investigated by
numerical modelling. We present the first numerical model of a convecting mantle including differentiation through
partial melting resulting in a stable compositionally layered continental upper mantle structure. This structure includes
a continental root extending to a depth of about 200 km. The model covers the upper mantle including the crust and
incorporates physical features important for the study of the continental upper mantle during secular cooling of the
Earth since the Archaean. Among these features are: a partial melt generation mechanism allowing consistent recurrent
melting, time-dependent non-uniform radiogenic heat production, and a temperature- and pressure-dependent rheology. The
numerical results reveal a long-term growth mechanism of the continental compositional root. This mechanism operates
through episodical injection of small diapiric upwellings from the deep layer of undepleted mantle into the continental root
which consists of compositionally distinct depleted mantle material. Our modelling results show the layered continental
structure to remain stable during at least 1.5 Ga. After this period mantle differentiation through partial melting ceases due
to the prolonged secular cooling and small-scale instabilities set in through continental delamination. This stable period of
1.5 Ga is related to a number of limitations in our model. By improving on these limitations in the future this stable period
will be extended to more realistic values. Ă 1998 Elsevier Science B.V. All rights reserved
Can Lower Mantle Slab-like Seismic Anomalies be Explained by Thermal Coupling Between the Upper and Lower Mantles?
Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalies are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However, thermal coupling between two ow systems separated by an impermeable interface might provide an al ternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmet ric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermeable interface, our results demonstrate the dominant role of mechanical coupling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect of mechanical coupling can be significantly weakened if a narrow low viscosity zone exists beneath the 660-km discontinuity. In such a case, both thermally induced `slabs' in the lower mantle and thermally activated plumes that rise from the upper/lower mantle boundary are observed even though mass transfer between the shells does not exist
Improved modelling of lifestyle changes in Integrated Assessment Models : Cross-disciplinary insights from methodologies and theories
Recent studies show that lifestyle changes can provide an essential contribution to achieving the Paris climate targets. While some efforts have been made to incorporate lifestyle changes into model-based scenarios, the attempts are currently very stylised and included exogenously. This paper discusses current efforts to represent lifestyle change in models, and analyses potential insights from relevant scientific disciplines to improve the representation of lifestyle changes in models â including modelling specific behaviour changes, identifying cross-cutting lifestyle solutions, representing the intentions behind the changes and quantifying their impacts. As such, this research attempts to bridge the gap between qualitative and quantitative theories and methodologies. Based on the results of this literature analysis, we recommend defining lifestyle changes more harmoniously, exploring an expanded range of approaches, domains and transformative solutions, adopting a whole-systems approach, and addressing the trade-offs between the use of exogenous inputs and endogenous modelling. © 2019 The AuthorsPeer reviewe
Flooding and Phytophthora cinnamomi : effects on photosynthesis and chlorophyll fluorescence in shoots of non-grafted Persea americana (Mill.) rootstocks differing in tolerance to Phytophthora root rot
Please read abstract in the article.The Hans-Merensky Foundation and the National Research Fund (NRF) through the THRIP programme (Department of Science and Technology, South Africa).http://www.elsevier.com/locate/sajbhj201
Generalized Bose-Einstein Condensation
Generalized Bose-Einstein condensation (GBEC) involves condensates appearing
simultaneously in multiple states. We review examples of the three types in an
ideal Bose gas with different geometries. In Type I there is a discrete number
of quantum states each having macroscopic occupation; Type II has condensation
into a continuous band of states, with each state having macroscopic
occupation; in Type III each state is microscopically occupied while the entire
condensate band is macroscopically occupied. We begin by discussing Type I or
"normal" BEC into a single state for an isotropic harmonic oscillator
potential. Other geometries and external potentials are then considered: the
{}"channel" potential (harmonic in one dimension and hard-wall in the other),
which displays Type II, the {}"cigar trap" (anisotropic harmonic potential),
and the "Casimir prism" (an elongated box), the latter two having Type III
condensations. General box geometries are considered in an appendix. We
particularly focus on the cigar trap, which Van Druten and Ketterle first
showed had a two-step condensation: a GBEC into a band of states at a
temperature and another "one-dimensional" transition at a lower
temperature into the ground state. In a thermodynamic limit in which
the ratio of the dimensions of the anisotropic harmonic trap is kept fixed,
merges with the upper transition, which then becomes a normal BEC.
However, in the thermodynamic limit of Beau and Zagrebnov, in which the ratio
of the boundary lengths increases exponentially, becomes fixed at the
temperature of a true Type I phase transition. The effects of interactions on
GBEC are discussed and we show that there is evidence that Type III
condensation may have been observed in the cigar trap.Comment: 17 pages; 6 figures. Intended for American Journal of Physic
T-Cell activation: a queuing theory analysis at low agonist density
We analyze a simple linear triggering model of the T-cell receptor (TCR) within the framework of queuing theory, in which TCRs enter the queue upon full activation and exit by downregulation. We fit our model to four experimentally characterized threshold activation criteria and analyze their specificity and sensitivity: the initial calcium spike, cytotoxicity, immunological synapse formation, and cytokine secretion. Specificity characteristics improve as the time window for detection increases, saturating for time periods on the timescale of downregulation; thus, the calcium spike (30 s) has low specificity but a sensitivity to single-peptide MHC ligands, while the cytokine threshold (1 h) can distinguish ligands with a 30% variation in the complex lifetime. However, a robustness analysis shows that these properties are degraded when the queue parameters are subject to variationâfor example, under stochasticity in the ligand number in the cell-cell interface and population variation in the cellular threshold. A time integration of the queue over a period of hours is shown to be able to control parameter noise efficiently for realistic parameter values when integrated over sufficiently long time periods (hours), the discrimination characteristics being determined by the TCR signal cascade kinetics (a kinetic proofreading scheme). Therefore, through a combination of thresholds and signal integration, a T cell can be responsive to low ligand density and specific to agonist quality. We suggest that multiple threshold mechanisms are employed to establish the conditions for efficient signal integration, i.e., coordinate the formation of a stable contact interface
Search for the standard model Higgs boson in tau final states
We present a search for the standard model Higgs boson using hadronically
decaying tau leptons, in 1 inverse femtobarn of data collected with the D0
detector at the Fermilab Tevatron ppbar collider. We select two final states:
tau plus missing transverse energy and b jets, and tau+ tau- plus jets. These
final states are sensitive to a combination of associated W/Z boson plus Higgs
boson, vector boson fusion and gluon-gluon fusion production processes. The
observed ratio of the combined limit on the Higgs production cross section at
the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of
115 GeV.Comment: publication versio
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt{s} = 1.96 TeV using Kinematic Characteristics of Lepton + Jets Events
We present a measurement of the top quark pair ttbar production cross section
in ppbar collisions at a center-of-mass energy of 1.96 TeV using 230 pb**{-1}
of data collected by the DO detector at the Fermilab Tevatron Collider. We
select events with one charged lepton (electron or muon), large missing
transverse energy, and at least four jets, and extract the ttbar content of the
sample based on the kinematic characteristics of the events. For a top quark
mass of 175 GeV, we measure sigma(ttbar) = 6.7 {+1.4-1.3} (stat) {+1.6- 1.1}
(syst) +/-0.4 (lumi) pb, in good agreement with the standard model prediction.Comment: submitted to Phys.Rev.Let
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