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Theses

Caractérisation des conditions redox du manteau terrestre dans différents contextes géodynamiques

Abstract : Many studies have demonstrated that arc basalts are more oxidized (~ ∆QFM+1.5 ± 1) than mid-ocean ridge basalts (MORB; ∆QFM-1.2 à ~ ∆QFM). However, the oxidation state of the sub-arc mantle is still debated. The use of various proxies on the same set of samples aims to determine the oxidation state of magmas, the effects of secondary processes on their oxidation state and to better constrain the fO2 of primary magmas from different geodynamic settings and that of their mantle source. For that purpose, Fe3+/ΣFe ratios, V partition coefficients between silicate melts and olivines (DvOl/Melt) as well as V/Sc and V/Yb ratios were measured in melt inclusions from high-Mg olivines (Fo > 80 %) from subduction zones, mid-oceanic ridges and hot spots. The analysis of Fe3+/ΣFe ratios by XANES spectroscopy using analytical conditions that prevent beam damage during analysis of hydrous glasses and by Mössbauer spectroscopy confirmed that arc magmas are more oxidized than those from hot spots and mid-ocean ridges. A positive correlation between Fe3+/ΣFe ratios and water contents in melt inclusions suggests that the oxidation state recorded in primitive arc magmas is likely to be due to changes in the oxidation state of the mantle beneath arcs because of subduction processes. The calculation of the fO2 from Fe3+/ΣFe ratios also indicates that the range of oxidation state recorded in arc melt inclusions varies from one subduction zone to another. An evaluation of the effects of volatile degassing and differentiation processes on the magmatic fO2 was carried out on each melt inclusion population. fO2 values were then estimated for reconstructed primary melt compositions using the different redox proxies. Results obtained from Fe3+/ΣFe ratios, DvOl/Melt partition coefficients and V/Yb ratios suggest that arc primary magmas are more oxidized than those from hot spots and mid-ocean ridges. However, fO2 values differ from one proxy to another. Furthermore, the positive correlation between the Fe3+/ΣFe ratios, the fO2 calculated from these ratios and the water contents of primary magmas suggests that the difference in the oxidation state of magmas is not due to secondary processes. However, the comparison between the Fe3+/ΣFe ratios, the fO2 values calculated from these ratios and Th/La and Ba/La ratios of primary magmas does not confirm that the observed changes in redox state can be simply linked to the influence of silicate melt or aqueous fluids coming from the slab to the mantle wedge. On the opposite, fO2 values calculated from V/Sc ratios suggest that primary arc magmas are comparable to those from mid-ocean ridges and hot spots.
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Submitted on : Friday, June 25, 2021 - 11:07:46 AM
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2019CLFAC084_GABORIEAU.pdf
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  • HAL Id : tel-03270888, version 1

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Marion Gaborieau. Caractérisation des conditions redox du manteau terrestre dans différents contextes géodynamiques. Sciences de la Terre. Université Clermont Auvergne, 2019. Français. ⟨NNT : 2019CLFAC084⟩. ⟨tel-03270888⟩

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