Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 19 (2009): Q02001, doi:10.1029/2008GC002236.Ocean intraplate volcanoes (OIVs) are formed in a sequence of stages, from large to small, that involve a systematic progression in mantle melting in terms of volumes and melt fractions with concomitant distinct mantle source signatures. The Hawaiian volcanoes are the best-known example of this type of evolution, even though they are extraordinarily large. We explore the Pb-Sr-Nd-Hf isotopic evolution of much smaller OIVs in the Fieberling-Guadalupe Seamount Trail (FGST) and small, near-ridge generated seamounts in the same region. In particular, we investigate whether we can extend the Hawaiian models to Jasper Seamount in the FGST, which displays three distinct volcanic stages. Each stage has characteristic variations in Pb-Sr-Nd-Hf isotopic composition and trace element enrichment that are remarkably similar to the systematics observed in Hawaii: (1) The most voluminous, basal “shield building” stage, the Flank Transitional Series (FTS), displays slightly isotopically enriched compositions compared to the common component C and the least enriched trace elements (143Nd/144Nd: 0.512866–0.512909, 206Pb/204Pb: 18.904–19.054; La/Sm: 3.71–4.82). (2) The younger and substantially less voluminous Flank Alkalic Series (FAS) is comparatively depleted in Sr, Nd, and Hf isotope compositions plotting on the side of C, near the least extreme values for the Austral Islands and St. Helena. Trace elements are highly enriched (143Nd/144Nd: 0.512912–0.512948, 206Pb/204Pb: 19.959–20.185; La/Sm: 9.24). (3) The Summit Alkalic Series (SAS) displays the most depleted Sr, Nd, and Hf isotope ratios and is very close in isotopic composition to the nearby near-ridge seamounts but with highly enriched trace elements (143Nd/144Nd: 0.512999–0.513050, 206Pb/204Pb: 19.080–19.237; La/Sm: 5.73–8.61). These data fit well with proposed multicomponent melting models for Hawaii, where source lithology controls melt productivity. We examine the effect of melting a source with dry peridotite, wet peridotite, and pyroxenite, calculating melt productivity functions with depth to evaluate the effect of potential temperature and lithospheric thickness. This type of melting model appears to explain the isotopic variation in a range of small to large OIVs, in particular for OIVs occurring far from the complicating effects of plate boundaries and continental crust, constraining their geodynamic origin.JBT acknowledges financial support from the French Institut National des Sciences de l’Univers. The isotope work at SDSU was made possible by NSF and Keck grants to BBH

Cartographie prédictive du potentiel d'exhalation du radon-222 à la surface des sols: Exemple d'application dans le Massif Armoricain

International audienceRadon-222 is a radioactive natural gas produced by the decay of radium-226, itself produced by the decay of uranium-238 naturally present in rocks and soil. It can accumulate in buildings, and inhalation of this gas and its decay products is a potential human health risk. Effective risk management needs to determine in advance the areas in which the density of buildings with high radon levels is likely to be highest. Research programs over the past several years have successfully developed a methodology for predicting and mapping the radon exhalation potential at the soil surface. This approach, based on quantification of the radon flux at the surface, starts from a precise characterization of the main local geological and pedological variables that influence the radon source and its transport to the soil/atmosphere interface. The methodology crosses the cartographic analysis into a Geographic Information System (GIS) and a simplified model for vertical transport of radon by diffusion through pores in the soil. This code, called TRACHGEO, calculates the radon flux density as a function of the chemical and physical properties of the rock and the subjacent soil. In this paper, we discuss the results from the application of this approach to an area (3,000 km2) located in Brittany (Western France). We discuss the validity of our forecasts and the use of such predictive maps as guides for radon risk management in existing or future buildings

Prediction of areas presenting a high radon exhalation potential: A new methodology based on the properties of geological formations and soils

A research program carried out since 1997 produced a methodology for predicting areas with a strong potential for radon exhalation at the soil surface. This methodology is based on a quantification of the Rn exhalation rate, from a precise characterization of the main local geological and pedological parameters that control the radon source and its transport to the soil/atmosphere interface. It combines a cross mapping analysis of parameters used in a Geographic Information System with a model of the vertical transport of Rn by diffusion trough the soil. This code (TRACHGEO) calculates the radon flux density at the surface as a function of the properties of the rock and the soil. This approach is validated in 4 typical areas with different geological contexts, starting from in situ measurements of radon fluxes and of radon concentrations in dwellings. A lithogeochemical classification of the geological formations as a function of their U contents and their confrontation to Rn level measurements demonstrate the primordial influence of the U content of the basement on Rn exhalation. This study leads to an initial map of the exhalation potential by assigning a potential class to each lithogeochemistry. Nevertheless, in situ radon measurements reveal a high spatial variability on uraniferous lithologies. Tests made by the TRACHGEO tool show the need to take account of spatial heterogeneity of soils (in addition of geochemistry) to improve the mapping resolution. The TRACHGEO forecasts explain the variability of the Rn exhalation on a larger scale

Mapping of the geogenic radon potential in France to improve radon risk management: Methodology and first application to region Bourgogne

cited By 68In order to improve regulatory tools for radon risk management in France, a harmonised methodology to derive a single map of the geogenic radon potential has been developed. This approach consists of determining the capacity of the geological units to produce radon and to facilitate its transfer to the atmosphere, based on the interpretation of existing geological data. This approach is firstly based on a classification of the geological units according to their uranium (U) content, to create a radon source potential map. This initial map is then improved by taking into account the main additional parameters, such as fault lines, which control the preferential pathways of radon through the ground and which can increase the radon levels in soils. The implementation of this methodology to the whole French territory is currently in progress. We present here the results obtained in one region (Bourgogne, Massif Central) which displays significant variations of the geogenic radon potential. The map obtained leads to a more precise zoning than the scale of the existing map of radon priority areas currently based solely on administrative boundaries. © 2010 Elsevier Ltd

Géochimie des basaltes de l'île de Ua Huka (archipel des Marquises) : variation du taux de fusion partielle et hétérogénéité de la source mantellique

L'île de Ua Huka (archipel des Marquises) comporte un volcan-bouclier principal, édifié entre 3,2 et 2,4 Ma, puis immédiatement affecté par un double effondrement caldeirique. Après un arrêt de l'activité volcanique pendant 0,9 Ma, deux petits volcans se sont mis en place au sud-ouest de l'île entre 1,5 et 0,75 Ma. Les différents types de basaltes constituant ces édifices (tholéiites à olivine, basaltes alcalins et basanites) résulteraient de la fusion partielle d'une source chimiquement hétérogène, dont le taux aurait diminué au cours du temps. La variation temporelle de la composition de la source mantellique est tout à fait originale par rapport à l'évolution généralement observée aux Marquises. Elle implique en effet une contribution de plus en plus importante d'un composant de type manteau appauvri (DMM), probablement lithosphérique, dans la genèse des termes basanitiques les plus tardifs. (Résumé d'auteur

Estimation and mapping of uranium content of geological units in France

International audienceIn France, natural radiation accounts for most of the population exposure to ionizing radiation. The Institute for Radiological Protection and Nuclear Safety (IRSN) carries out studies to evaluate the variability of natural radioactivity over the French territory. In this framework, the present study consisted in the evaluation of uranium concentrations in bedrocks. The objective was to provide estimate of uranium content of each geological unit defined in the geological map of France (11,000,000). The methodology was based on the interpretation of existing geochemical data (results of whole rock sample analysis) and the knowledge of petrology and lithology of the geological units, which allowed obtaining a first estimate of the uranium content of rocks. Then, this first estimate was improved thanks to some additional information. For example, some particular or regional sedimentary rocks which could present uranium contents higher than those generally observed for these lithologies, were identified. Moreover, databases on mining provided information on the location of uranium and coal/lignite mines and thus indicated the location of particular uranium-rich rocks. The geological units, defined from their boundaries extracted from the geological map of France (11,000,000), were finally classified into 5 categories based on their mean uranium content. The map obtained provided useful data for establishing the geogenic radon map of France, but also for mapping countrywide exposure to terrestrial radiation and for the evaluation of background levels of natural radioactivity used for impact assessment of anthropogenic activities. © 2016 Elsevier Lt

Response to comment on “Indoor terrestrial gamma dose rate mapping in France: A case study using two different geostatistical models” by Warnery et al.

International audienceElements to answer the points raised in the Letter in question are proposed, notably about exposure metrics and their influence on the results and statistical power of epidemiological studies. © 201

Indoor terrestrial gamma dose rate mapping in France: a case study using two different geostatistical models

International audienceTerrestrial gamma dose rates show important spatial variations in France. Previous studies resulted in maps of arithmetic means of indoor terrestrial gamma dose rates by “departement” (French district). However, numerous areas could not be characterized due to the lack of data. The aim of our work was to obtain more precise estimates of the spatial variability of indoor terrestrial gamma dose rates in France by using a more recent and complete data base and geostatistics. The study was based on the exploitation of 97 595 measurements results distributed in 17 404 locations covering all of France. Measurements were done by the Institute for Radioprotection and Nuclear Safety (IRSN) using RPL (Radio Photo Luminescent) dosimeters, exposed during several months between years 2011 and 2012 in French dentist surgeries and veterinary clinics. The data used came from dosimeters which were not exposed to anthropic sources. After removing the cosmic rays contribution in order to study only the telluric gamma radiation, it was decided to work with the arithmetic means of the time-series measurements, weighted by the time-exposure of the dosimeters, for each location. The values varied between 13 and 349 nSv/h, with an arithmetic mean of 76 nSv/h. The observed statistical distribution of the gamma dose rates was skewed to the right. Firstly, ordinary kriging was performed in order to predict the gamma dose rate on cells of 1*1 km2, all over the domain. The second step of the study was to use an auxiliary variable in estimates. The IRSN achieved in 2010 a classification of the French geological formations, characterizing their uranium potential on the bases of geology and local measurement results of rocks uranium content. This information is georeferenced in a map at the scale 1:1 000 000. The geological uranium potential (GUP) was classified in 5 qualitative categories. As telluric gamma rays mostly come from the progenies of the 238Uranium series present in rocks, this information, which is exhaustive throughout France, could help in estimating the telluric gamma dose rates. Such an approach is possible using multivariate geostatistics and cokriging. Multi-collocated cokriging has been performed on 1*1 km2 cells over the domain. This model used gamma dose rate measurement results and GUP classes. Our results provide useful information on the variability of the natural terrestrial gamma radiation in France (‘natural background’) and exposure data for epidemiological studies and risk assessment from low dose chronic exposures

A statistical evaluation of the influence of housing characteristics and geogenic radon potential on indoor radon concentrations in France

International audienceRadon-222 is a radioactive natural gas produced by the decay of radium-226, known to be the main contributor to natural background radiation exposure. Effective risk management needs to determine the areas in which the density of buildings with high radon levels is likely to be highest. Predicting radon exposure from the location and characteristics of a dwelling could also contribute to epidemiological studies. Beginning in the nineteen-eighties, a national radon survey consisting in more than 10,000 measurements of indoor radon concentrations was conducted in French dwellings by the Institute for Radiological Protection and Nuclear Safety (IRSN). Housing characteristics, which may influence radon accumulation in dwellings, were also collected. More recently, the IRSN generated a French geogenic radon potential map based on the interpretation of geological features. The present study analyzed the two datasets to investigate the factors influencing indoor radon concentrations using statistical modeling and to determine the optimum use of the information on geogenic radon potential that showed the best statistical association with indoor radon concentration. The results showed that the variables associated with indoor radon concentrations were geogenic radon potential, building material, year of construction, foundation type, building type and floor level. The model, which included the surrounding geogenic radon potential (i.e. the average geogenic radon potential within a disc of radius 20km centered on the indoor radon measurement point) and variables describing house-specific factors and lifestyle explained about 20% of the overall variability of the logarithm of radon concentration. The surrounding geogenic radon potential was fairly closely associated with the local average indoor radon concentration. The prevalence of exposure to radon above specific thresholds and the average exposures to radon clearly increased with increasing classes of geogenic radon potential. Combining the two datasets enabled improved assessment of radon exposure in a given area in France. © 2013 Elsevier Ltd