An investigation of a polymetamorphic terrain using ⁴⁰Ar-³⁹Ar 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

Not just a number: examining coverage and content of antenatal care in low-income and middle-income countries.

Introduction: Antenatal care (ANC) provides a critical opportunity for women and babies to benefit from good-quality maternal care. Using 10 countries as an illustrative analysis, we described ANC coverage (number of visits and timing of first visit) and operationalised indicators for content of care as available in population surveys, and examined how these two approaches are related. Methods: We used the most recent Demographic and Health Survey to analyse ANC related to women's most recent live birth up to 3 years preceding the survey. Content of care was assessed using six components routinely measured across all countries, and a further one to eight additional country-specific components. We estimated the percentage of women in need of ANC, and using ANC, who received each component, the six routine components and all components. Results: In all 10 countries, the majority of women in need of ANC reported 1+ ANC visits and over two-fifths reported 4+ visits. Receipt of the six routine components varied widely; blood pressure measurement was the most commonly reported component, and urine test and information on complications the least. Among the subset of women starting ANC in the first trimester and receiving 4+ visits, the percentage receiving all six routinely measured ANC components was low, ranging from 10% (Jordan) to around 50% in Nigeria, Nepal, Colombia and Haiti. Conclusion: Our findings suggest that even among women with patterns of care that complied with global recommendations, the content of care was poor. Efficient and effective action to improve care quality relies on development of suitable content of care indicators

On the orthogonality of states with approximate wavefunctions

International audienc

Determination of the

The shielding anisotropy (SA) of the 13C-nucleus Δσ in COOH groups of some molecules could be deduced, in solution, from the measurements of the relaxation time T1 for different values of the magnetic field (contribution of the SA mechanism to the total relaxation rate) and from the determination of the reorientational correlation time of these species. Δσ was determined in function of the solvent for benzoic acid, in function of pH for glycine and for free and bound (to Mg2+) ethylene diamine tetraacetate (EDTA) ion. For benzoic acid dissolved in benzene the results were compared to those obtained in an aqueous solution and in the solid state, and in diamine tetraacetate (EDTA) ion. For benzoic acid dissolved in benzene the results were compared to those obtained in an aqueous solution and in the solid state, and in order to try to explain the observed variations, theoretical calculations of Δσ were performed. 17O NMR allowed us also to obtain the 17O quadrupolar coupling constant

On the suitability of strictly localized orbitals for hybrid QM/MM calculations

In the QM/MM method we have developed (LSCF/MM), the QM and the MM parts are held together by means of strictly localized bonding orbitals (SLBOs). Generally these SLBOs are derived from localized bond orbitals (LBOs) that undergo tails deletion, resulting in a nonpredictable change of their properties. An alternative set of SLBOs is provided by the extremely localized molecular orbitals (ELMOs) approach, where the orbitals are rigorously localized on some prefixed atoms without tails on the other atoms of the molecule. A comparative study of SLBOs arising from various localization schemes and ELMOs is presented to test the reliability and the transferability of these functions within the Local Self-Consistent Field (LSCF) framework. Two types of chemical bonds were considered: C - C and C - O single bonds. The localized functions are obtained on the ethane and the methanol molecules, and are tested on \u3b2-alanine and diethyl ether molecules. Moreover, the various protonation forms of \u3b2-alanine have been investigated to illustrate how well the polarity variation of the chemical bond can be handled throughout a chemical process. At last, rotation energy profiles around C - C and C - O bonds are reproduced for butane and fluoromethanol. Energetic, geometric, as well as electronic factors all indicate that ELMO functions are much more transferable from one molecule to another, leading to results closer to the usual SCF reference than any other calculations involving any other localized orbitals. When the shape of the orbital is the most important factor then ELMO functions will perform as well as any other localized orbital

Complexation of Ca2+ cation by the lateral chain of Paclitaxel (N-Benzoyl-ß-phenylisoserine): A theoretical study

International audienc

Frozen core orbitals as an alternative to specific frontier bond potential in hybrid quantum mechanics/molecular mechanics methods

The use of Frozen Core Orbitals (FCOs) at the frontier atom is proposed as an alternative to the specific frontier bond potential in the Local-SCF/MM method. The calculations illustrate that frontier bond length is correctly recovered without adding any specific force-field potential. It is shown that the same accuracy is reached for calculations using FCOs arising either from an atomic (ROHF) or from a molecular calculation using Extremely Localized Molecular Orbitals (ELMOs). In addition FCOs do not modify the description of the remaining of the QM subsystem from both an energetic and/or geometric point of view

Removing extra frontier parameters in QM/MM methods : a tentative with the Local Self-Consistent Field approach

We present a new quantum mechanics/mol. mechanics (QM/MM) scheme based on the Local-SCF method that avoids the extra-parametrization of the QM/MM frontier by taking into account the core electrons of the hybrid frontier atoms by means of self-consistent core orbitals (SCCO). This study follows and extends our previous investigation based on frozen core orbitals (FCO). Test calcns. on small org. compds. show that the most common atoms found in biochem. systems can be used as frontier atoms without adding any parameters, and that only relatively small geometric and energetic deviations compared to full-QM calcns. are generated