Subgrain 40Ar/39Ar dating of museum-quality micas reveals intragrain heterogeneity

International audienceMuseum-grade mica megacrysts of the Phalaborwa phlogopite (ca. 2 Ga) and the Rubikon pegmatite (ca. 0.5 Ga) were screened for intra-grain compositional and chronological heterogeneities by electron probe microanalysis and 40Ar/39Ar dating, respectively. Both micas were known to have Rbsingle bondSr ages indistinguishable from Usingle bondPb ages. Even though step heating plateaus were obtained for nearly a hundred 100 μm scale subgrain chips, age variations of several percents were observed among individual chips. The age variations were unrelated to the position within the megacryst and to the position in the irradiation canister. Element mapping parallel and perpendicular to the (001) plane showed significant variations in concentrations of major elements Ti, Fe, Mn, K, Al. Cation transport, element redistribution and retrograde mineral formation that escaped visual detection require open-system, fluid-assisted chemical reactions. These recrystallization processes occurred during one or several post-magmatic hydrothermal event(s). The spatial distribution of sub-grain ages show that 40Ar/39Ar dating does not provide a simple, diffusion-controlled cooling age, but rather apparent ages controlled by mineral retrogression and recrystallization. The result was variable loss of Ar at the subgrain scale. These observations show that retrogression events cannot be detected by the presence or absence of plateau ages. Instead, recrystallization can be diagnosed (1) by a thorough petrological investigation based on microchemical maps, and (2) by Cl/K and Ca/K constraints from 40Ar/39Ar systematics. The quest for a natural mineral sufficiently homogeneous to act as a precise calibrator for the 40K half-life remains unsatisfactory

Petrochronology of the Terre Adélie Craton (East Antarctica) evidences a long-lasting Proterozoic (1.7–1.5 Ga) tectono-metamorphic evolution — Insights for the connections with the Gawler Craton and Laurentia

The Terre Adélie Craton displays superimposed strain fields related to the Neoarchean (2.6–2.4 Ga, M1) and Paleo-Mesoproterozoic (1.7–1.5 Ga, M2) metamorphic events. M1 is a regional granulite facies event, constrained by P-T modelling at ~0.8–1.0 GPa – 800–850 °C, followed by a decompressional retrogression in the upper amphibolite facies at ~0.6 GPa – 750 °C. M2 Stage 1 P-T peak is constrained at 0.6–0.7 GPa – 670–700 °C, followed by a steep P-T path down to 0.3 GPa – 550 °C. Retrogression after M2 PT peak occurred in a context of dextral shearing along the Mertz Shear Zone along with thrust motions within the eastern Terre Adélie Craton. In this paper, we present a series of 63 new 40Ar/39Ar ages of biotite and amphibole pairs in mafic rocks from a complete traverse of the Terre Adélie Craton. 40Ar/39Ar dating constrains M2 amphibolite facies metamorphism at a regional scale between 1700 and 1650 Ma, during stage 1 peak metamorphism. During retrogression, lower amphibolite facies recrystallization mainly occurred along vertical shear zones and mafic dykes between 1650 and 1600 Ma (Stage 2), followed by amphibolite to greenschist facies metamorphism until after 1500 Ma (Stage 3). At the scale of the Mawson continent, this event is related to the growth of an active margin above an oblique subduction zone. The supra-subduction model best explains opening of Dumont D'Urville and Hunter basins at 1.71 Ga followed by their rapid closure and metamorphism at 1.70 Ga. In this context, episodic shear zone reactivation and magmatic dyke emplacement led to a partial reequilibration of the 40Ar/39Ar system until <1500 Ma. This latter phase of mafic magmatism largely coincides with a hot spot event at the scale of the Gawler Craton and western Laurentia paleocontinent

Masjarakat Bodjonegoro

This article reports the findings of a policy survey designed to establish research priorities to inform future research strategy and advance nursing home practice. The survey was administered in 2 rounds during 2013, and involved a combination of open questions and ranking exercises to move toward consensus on the research priorities. A key finding was the prioritization of research to underpin the care of people with cognitive impairment/dementia and of the management of the behavioral and psychological symptoms of dementia within the nursing home. Other important areas were end-of-life care, nutrition, polypharmacy, and developing new approaches to putting evidence-based practices into routine practice in nursing homes. It explores possible innovative educational approaches, reasons why best practices are difficult to implement, and challenges faced in developing high-quality nursing home research

Nursing home research: the first international association of gerontology and geriatrics (IAGG) research conference

The International Association of Gerontology and Geriatrics held its first conference on nursing home research in St Louis, MO, in November 2013. This article provides a summary of the presentations.Yves Rolland, Barbara Resnick, Paul R.Katz, Milta O.Little, Joseph G.Ouslander, Alice Bonner … et al