19 research outputs found
Recommended from our members
An investigation of a polymetamorphic terrain using <sup>40</sup>Ar-<sup>39</sup>Ar geochronology
This research is an exploration both of the Precambrian metamorphic geology of south-western Montana and the Wlaser ablation microprobe 40At-39At geochronological technique. Using the high spatial resolution of the UV laser microprobe it has been possible to produce systematic 40At-39Ar ages within individual mica grains. Further, the UV laser has been used to drill into biotite inclusions within garnet to look at the shielding effect of the garnet lattice, and to drill depth profiles into garnet to measure helium diffusion in laboratory experiments.
The south-western Montana region of the Precambrian Wyoming Province was believed to have undergone at least three separate periods of metamorphism: M1- granulite facies in the Archaean, M2 - amphibolite facies in the Early Proterozoic, and M3 - greenschist to epidote-amphibolite facies in the Mid-Proterozoic. Because of the relatively low blocking temperature of the 40Ar- 39Ar geochronological system in mica (c.300-350 °C), it is readily reset by regional metamorphic events and previous K-Ar and 4OAr_38 Atgeochronology had found that the Early Proterozoic event dominated the ages obtained.
UV laser 4OAt-39At dating of the matrix mica constrained the timing of cooling from the Early Proterozoic metamorphic event to between 1780 to 1740 Ma with a cooling rate between 1 and 8 °C/m.y. 40Ar-39Ar analyses of individual biotite inclusions in garnet also produced similar ages. However, Pb-Pb step leach dating of a small subset of garnet yielded ages between 1808 and 1765 Ma, demonstrating that the garnet did not grow during an Archaean event but, rather, during the Early Proterozoic metamorphic event. Thus, the shielding properties of garnet on biotite inclusions could not be easily assessed in these samples. However, where matrix biotite had been partially reset by the Cretaceous plutons, there was some evidence to suggest that a minority of the biotite inclusions in the same sample had been shielded from resetting. The influence of fractures, defects and other fast diffusion paths is believed to have prevented most of the inclusions from being shielded. No evidence was found to show that the rocks in south-western Montana were metamorphosed during the Archaean and it seems likely that the M1 and M2 events were not greatly separated in time and were both Early ProterozoiC in age.
The high spatial resolution of the UV laser microprobe was used in order to date highly altered biotite within rocks that had undergone later (M3) greenschist metamorphism. Biotite was interlayered with chlorite, clinozoisite and prehnite but using the UV laser it was possible to separately analyse areas of unaltered biotite and areas of alteration within a single mineral and produce ages from both. It was thus possible to measure two ages from one sample: an unaltered age consistent with the timing of the Early Proterozoic metamorphism, and a younger age that could be linked to the c.1500-1360 Ma timing of Belt basin formation to the north and west of the region. This technique therefore was able to overcome the difficulties associated with producing meaningful ages from altered samples whilst constraining the timing of the M3 metamorphic event
The role of dynamical polarization of the ligand to metal charge transfer excitations in {\em ab initio} determination of effective exchange parameters
The role of the bridging ligand on the effective Heisenberg coupling
parameters is analyzed in detail. This analysis strongly suggests that the
ligand-to-metal charge transfer excitations are responsible for a large part of
the final value of the magnetic coupling constant. This permits to suggest a
new variant of the Difference Dedicated Configuration Interaction (DDCI)
method, presently one of the most accurate and reliable for the evaluation of
magnetic effective interactions. This new method treats the bridging ligand
orbitals mediating the interaction at the same level than the magnetic orbitals
and preserves the high quality of the DDCI results while being much less
computationally demanding. The numerical accuracy of the new approach is
illustrated on various systems with one or two magnetic electrons per magnetic
center. The fact that accurate results can be obtained using a rather reduced
configuration interaction space opens the possibility to study more complex
systems with many magnetic centers and/or many electrons per center.Comment: 7 pages, 4 figure
Derivation of spin Hamiltonians from the exact Hamiltonian: Application to systems with two unpaired electrons per magnetic site
The foundations and limits of S=1/2 and S=1 spin Hamiltonians for systems with two unpaired electrons in two well-defined orbitals per site are discussed by merging accurate ab initio calculations in binuclear systems with the effective Hamiltonian theory. It is shown that, beyond the usual JijSi.Sj terms, the effective spin Hamiltonian necessarily introduces four-body spin operators in the S=1/2 case and biquadratic terms in the S=1 formalism. The order of magnitude of these additional terms can be rationalized from a quasidegenerate perturbation theory expansion starting from a Hubbard-type Hamiltonian. This permits to discuss the physical mechanisms governing the reduction from the all electron Hamiltonian to the spin-only Hamiltonians and the conditions under which a further reduction from a spin Hamiltonian to the simplest Heisenberg-Dirac-Van Vleck form is possible. The overall discussion is illustrated by numerical calculations of the magnetic coupling between two Ni2+ cations in the K2NiF4 perovskite and between triply bonded carbon atoms in poly-ynes
Adsorption and inhibition effect of 2,4-diamino-6-hydroxypyrimidine for mild steel corrosion in HCl medium: experimental and theoretical investigation, Ionics
2,4-Diamino-6-hydroxypyrimidine (2D6H) was examined as corrosion inhibitor of mild steel (MS) in 0.1 M HCl using potentiodynamic
measurements, linear polarization resistance (LPR), scanning electron microscopy, electrochemical experiments, and
quantum chemical calculations. All measurements show that the corrosion inhibition effectiveness is forthright compared to the
concentration of 2D6H ranging from 0.5 to 10.0 mM. Adsorption of 2D6H on the MS surface in the presence of HCl is
determined to obey Langmuir adsorption isotherm. The electronic features elucidated by quantum chemical calculations were
associated with the experimental inhibition productivities. The mechanism of inhibition was revealed by Epzc measurements