Spatial and temporal variations in mantle redox state beneath the Massif Central, France
Zusammenfassung der Projektergebnisse
The goals of this study were i) to determine the oxidation state of the lithospheric mantle beneath the Massif Central region, in central France, ii) to trace element compositions for selected samples to monitor metasomatic effects and iii) to combine the trace element results with the oxidation state data in order to better characterise how metasomatism (and partial melting) affects the lithospheric mantle. A suite of 122 mantle xenoliths from 45 volcanic centers distributed around the Massif Central were investigated in this study. Recorded ΔlogƒO2 values range between FMQ-0.47 and FMQ+1.66. Of the two mantle domains identified by Lenoir et al. (2000), the northern domain records generally higher ΔlogƒO2 values than southern domain samples. Harzburgites also tend be have higher ΔlogƒO2 values than lherzolites, reflecting their ease of being reset during metasomatic overprinting. Unlike previous studies (e.g. Woodland et al. 1992), the presence of amphibole does not correlate with the highest ∆logƒO2 values in our suite. Trace element concentrations in clinopyroxene were measured by LA-ICP-MS for a significant subset of samples. These samples can to be subdivided into four broad groups based upon their REE signatures. All samples have experienced various degrees of partial melting (1-15 %) followed by metasomatic overprinting. One group of samples from the northern domain (group D) apparently underwent partial melting within the stability field of garnet peridotite prior to being uplifted and re-equilibrated in the spinel peridotite facies. The others underwent partial melting in the presence of spinel. The other three groups (A, B, C) appear to represent different stages of chromatographic reactions with percolating melts/fluids that also illustrate variable intensities of metasomatic interactions. The oxidation state recorded in the xenolith suite also reflects this intensity or proximity to a conduit of migrating metasomatic melts; stronger trace element enrichment correlates with higher ∆logƒO2 values. However, a direct one to one correlation is complicated by the fact that the pre-metasomatic signature of samples were different, depending on degree of partial melting and whether melting occurred in the garnet or spinel facies. Metasomatic overprinting has led to the Massif Central lithospheric mantle having some of the highest oxidation states recorded in such a tectonic environment. In fact such high oxidation states are more reminiscent of the mantle wedge above subduction zones. Using this regional study as a basis it would be worthwhile investigating a large number of xenoliths from several chosen localities from the northern and southern domains, such as is currently available only for the Ray Pic locality. This would help constrain the extent of variations in metasomatic overprinting at a more localised level and allow more quantitiative modelling of this process. There would also be the potential of studying variations in oxidation state and metasomatism as a function of depth, permitting the assessment of whether the lithospheric mantle is vertically stratified.
Projektbezogene Publikationen (Auswahl)
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(2012) Oxidation state of the lithospheric mantle beneath the Massif Central, France. European Geosciences Union meeting, Vienna, Austria, April 2012, EGU2012-4763
Uenver-Thiele, L., Woodland, A.B., Downes, H., Altherr R.
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(2012) Oxidation state of the lithospheric mantle beneath the Massif Central, France. European Mineralogical Conference, Frankfurt, Germany, Sept. 2012 EMC2012-232
Uenver-Thiele, L., Woodland, A.B., Downes, H., Altherr R.
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(2013) Relationship between oxidation state and texture and the influence of metasomatism in the lithospheric mantle beneath the Massif Central, France; EGU General Assembly meeting, Vienna, Austria. 7-12 April, 2013 EGU2013-11398
Uenver-Thiele L, Woodland A, Downes H & Altherr R
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(2013) The relation between metasomatism and redox state of the upper mantle below the Massif Central, France; Mineralogical Magazine, 77(5) 2373
Uenver-Thiele L, Woodland A, Downes H & Altherr R
