francois nauret - Academia.edu (original) (raw)

Papers by francois nauret

EGS - AGU - EUG Joint Assembly, Apr 1, 2003

EGU General Assembly Conference Abstracts, Apr 1, 2016

Journal of Volcanology and Geothermal Research, May 1, 2022

HAL (Le Centre pour la Communication Scientifique Directe), 2010

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Copyright

Geochemistry Geophysics Geosystems, Sep 1, 2022

Tephra layers preserved in marine sediments are strong tools to study the frequency, magnitude an... more Tephra layers preserved in marine sediments are strong tools to study the frequency, magnitude and source of past major explosive eruptions. Thirty‐seven volcanoes from the Ecuadorian and Colombian arc, in the northern Andes, experienced at least one eruption during the Holocene. The volcanic hazard is therefore particularly high for the populated areas of the Andes and in particular cases for the coastal region, and it is crucial to document such events to improve hazard assessment. The age and distribution of deposits from major Holocene eruptions have been studied in the Cordillera, but no descriptions of distal fallouts have been published. In this study, we focused on 28 Holocene tephra layers recorded in marine sediment cores collected along the northern Ecuador—Southern Colombia margin. New lithological, geochemical and isotope data together with 14C datings on foraminifers allow us to determine the age and volcanic source of marine tephra, and to propose a first land‐sea correlation of distal tephra fallouts. We show that at least seven explosive eruptions from Guagua Pichincha, Atacazo‐Ninahuilca, Cotopaxi, and Cerro Machín volcanoes left tephra deposits recorded in marine cores over 250 km away from their source. Volume estimates of emitted tephra range between 1.3 and 6.0 km3 for the tenth century Guagua Pichincha, ∼5 ka Atacazo‐Ninahuilca, ∼6.7 and ∼7.9 ka Cotopaxi events, suggesting that they were eruptions of Volcanic Explosivity Index of 5. The distribution of these deposits also brings new constraints for a better evaluation of the volcanic hazard in Ecuador.

Journal of South American Earth Sciences, Aug 1, 2020

In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust ... more In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust 30 Ma ago led to the production of increasingly differentiated magmas. Better constrain on the interactions between ascending magmas and the thick continental crust are needed to understand the contamination processes and their implications on the geochemical diversity of arc magmas. Lascar volcano figures among the most active Quaternary volcano of the CVZ and yet geochemical studies on its volcanic products remain scarce. We lead major-, trace elements and Pb-Sr isotopes measurements on 10 Lascar samples representative of the different petrologic types (andesitic to dacitic) in order to better characterize the Pb-Sr isotopic signature and diversity of Lascar magmas yet defined based on only 8 samples.

Geochemistry Geophysics Geosystems, Jun 1, 2010

Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been much less studied than those from ... more Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been much less studied than those from the Indian, Atlantic, and Pacific due to the difficulty of access related to ice cover. In 2001 and 2004 the Arctic ridges (Gakkel Ridge and Lena Trough) were intensively sampled. In this study we present the first helium, neon, and argon concentrations and isotopic ratios in a suite of samples from the ultraslow spreading Lena Trough (∼0.75 cm/yr effective full rate). Central Lena Trough (CLT) lavas display 4 He/ 3 He between 89,710 and 97,530 (R/Ra between 7.4 and 8.1), similar to the mean MORB ratio of 90,000 ± 10,000 (R/Ra = 8 ± 1). In a three neon isotope diagram, the samples fall on the MORB line, without showing any excess of nucleogenic 21 Ne. The 40 Ar/ 36 Ar ratios vary from 349 to 6964. CLT samples have a typical MORB He and Ne isotopic composition. Rare gases do not indicate any mantle heterogeneities or contribution of subcontinental lithospheric mantle, although this has been suggested previously on the basis of the Sr-Nd and Pb isotopic systems. Based on noble gas systematics, a DUPAL-like anomaly is not observed in the Arctic Ocean. We propose two possible models which reconcile the rare gases with these previous studies. The first is that the Lena Trough mantle has a marble cake structure with small-scale heterogeneities (<1 km), allowing rapid diffusion and homogenization of rare gases compared to elements such as Sr, Nd, and Pb. The second model proposes that the recycled component identified by other isotopic systems was fully degassed at a recent date. It would therefore have a negligible mass budget of rare gases compared to other isotopic systems. This would suggest that the mantle enrichment beneath Lena Trough was generated by rift-forming processes and not by recycling.

HAL (Le Centre pour la Communication Scientifique Directe), Sep 28, 2022

HAL (Le Centre pour la Communication Scientifique Directe), Sep 24, 2019

International audienceEcuadorian magmas result from processing through a thick crust a heterogene... more International audienceEcuadorian magmas result from processing through a thick crust a heterogeneous range of primitive melts. This results in a complex geochemical signal at the surface, where source and processes footprints are intricate. We propose here to compare two approaches at different scales. The first one aims to understand the general trends in Ecuadorian magmatism looking at the geochemistry of whole-rocks at the scale of the arc to decipher continental crust and slab component imprints. The second approach is based on the idea that whole-rock compositions represent rough averages of all processes taking place from source to eruption, but that those processes could be sorted out studying different minerals present in the rocks. For the first approach, 71 samples were analysed for major and trace elements as well as Sr, Nd and Pb isotopes to complete the existing geochemical dataset. This study confirms the already identified across-arc geochemical trends described in the literature that reflect the decrease in mantle melting and the slab dehydration away from the trench. Along-arc geochemical trends were identified for the first time, especially marked in the front arc which encompasses 99% and 71% of the total variation in 206Pb/204Pb and 87Sr/86Sr ratios of Quaternary Ecuadorian volcanics. These trends suggest (1) more extensive crustal contamination of magma in the southern part (up to 14%); and (2) a changing nature of metasomatism in the subarc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in magmas erupted in the southern part of the arc. The second approach focuses on one volcanic edifice, the Guagua Pichincha. 42 minerals (amphibole, plagioclase, pyroxene) were picked from two dacite samples and analysed for major and trace elements, and Pb isotopes. Early crystallized, high-Al amphiboles with Al2O3>=9.8 wt.% and Eu/Eu*>0.7 have the lowest and most heterogeneous 206Pb/204Pb (18.816-18.999), whereas plagioclases have the highest and most homogeneous 206Pb/204Pb (19.003-19.023). Low-Al amphiboles and pyroxenes display intermediate compositions and variability (18.934-19.007). The large isotopic disequilibrium between mineral phases and whole-rocks show that a rock sample is a complex mix of minerals and melts recording different parts of the magmatic system. Trace element and isotopic data show that primitive melts feeding the young Guagua Pichincha are heterogeneous. These melts seem to be contaminated by the Western Cordillera basement comprised by accreted ocean terrains on their way to surface. Minerals are precious insights of magmas' history as mass balance calculation reveals that in the case of these 2 dacites, the whole-rock Pb isotope composition is completely buffered by the matrix composition. Combining these two scales allows us to better understand the scale of mantle heterogeneity under the Ecuadorian arc and the significance of whole-rock compositions in arc magmas

Instituto Geológico, Minero y Metalúrgico – INGEMMET, 2016

Journal of Volcanology and Geothermal Research, Jun 1, 2023

HAL (Le Centre pour la Communication Scientifique Directe), Oct 11, 2022

Journal of Volcanology and Geothermal Research, 2018

Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in c... more Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in convergent arc settings generate voluminous and explosive eruptions that can strongly affect human activity and have significant environmental impacts. It is therefore crucial to understand how these magmas are generated in order to anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites) are derived from the mantle wedge and they are progressively modified by physical and chemical processes operating between the melting zone and the surface to produce silica-rich magmas. In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters remain constant. Petrology and majortrace element compositions of these lavas have already been examinated, and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope compositions range from 18.889 to 19.154 for 206 Pb/ 204 Pb, from 15.658 to 15.696 for 207 Pb/ 204 Pb, and from 38.752 to 38.918 for 208 Pb/ 204 Pb. Dacites display a remarkably homogeneous Pb isotopic composition, with higher 206 Pb/ 204 Pb values for a given 207-208 Pb/ 204 Pb compared to andesites. Andesites show notable 207 Pb/ 206 Pb variations for a given SiO 2 content, whereas dacites have lower and homogenous 207 Pb/ 206 Pb values. Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plotting at the high SiO 2 and 87 Sr/ 86 Sr and low 207 Pb/ 206 Pb fields. Based on our new dataset, we show that at least 3 different components are required to explain the Tungurahua compositional and isotope variation: one corresponds to the mantle, the second has a deep origin (slab component or lower crust), and

Earth and Planetary Science Letters

HAL (Le Centre pour la Communication Scientifique Directe), Oct 3, 2022

Journal of Volcanology and Geothermal Research

Repositorio Institucional INGEMMET, 2012

HAL (Le Centre pour la Communication Scientifique Directe), Dec 1, 2020