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| Titel |
Oxidation state of the lithospheric mantle beneath the Massif Central,France |
| VerfasserIn |
L. Uenver-Thiele, A. B. Woodland, H. Downes, R. Altherr  |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062673
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| Zusammenfassung |
| The Tertiary and Quaternary volcanism of the French Massif Central sampled the underlying
subcontinental lithospheric mantle (SCLM) in the form of xenoliths over a wide geographic
area of ~20.000km2. Such an extensive distribution of xenoliths provides an unique
opportunity to investigate regional variations in mantle structure and composition. On the
basis of textural and geochemical differences, Lenoir et al. (2000) and later Downes et al.
(2003) identified two distinct domains in the SCLM lying north and south of latitude 45Ë 30’
N, respectively. The northern domain is relatively refractory, but has experienced pervasive
enrichment of LREE. The southern domain is generally more fertile, exhibiting depletion in
LREE. A metasomatic overprint has developed to variable extents in many xenolith
suites.
The different histories of these two juxtaposed blocks of SCLM should also be
reflected in their oxidation state, with local variations also to be expected due to
metasomatic interactions. For example, if carbonate-melt metasomatism played a role in
the LREE enrichment of the northern domain (Lenoir et al. 2000; Downes et al.
2003), then such mantle should be relatively oxidised. Since surprisingly little redox
data are currently available, we are undertaking a study to determine the oxidation
state of the SCLM beneath the Massif Central over the largest geographical area
possible.
All xenoliths investigated are spinel peridotites, mostly with protogranular textures
(although some samples are porphyroclastic or equigranular). Most samples are nominally
anhydrous although minor amphibole is present in some xenolith suites. Major element
compositions of the individual minerals were determined by microprobe. Two-pyroxene
temperatures (BKN) range from 750Ë to ~1200Ë C.
Ferric iron contents of spinel were determined by Mössbauer spectroscopy and gave a
range of Fe3+/ Fetot from 0.191 to 0.418, with a conservative uncertainty of ±0.02. These
data were used to calculate oxygen fugacity (fO2) of the peridotites using the Nell-Wood
calibration for the equilibrium between olivine, orthopyroxene and spinel (Wood et al. 1990)
and are referenced to the fayalite-magnetite-quartz (FMQ) redox buffer. Preliminary results
yield Δlog(fO2) values between FMQ-0.17 and FMQ+1.65 log units. In this fO2 range
propagated uncertainties are on the order of 0.1 log units. Although there is some overlap,
localities from the northern block tend to record higher values (>FMW+0.9). In the
south, fO2values from a number of localities cluster around FMQ+0.3 to FMQ+0.6,
with higher values associated with the occurrence of amphibole in the xenoliths.
The higher values (>FMQ+1) testify to localised metasomatic interaction in the
SCLM, possibly related to melt migration during earlier phases of magmatic activity
in the region. This hypothesis is also consistent with lower fO2 values observed
at one locality (Fraisse) on the northern block that is significantly older and thus
sampled the mantle prior to the subsequent metasomatic activity in this block of
SCLM.
Downes H., Reichow M.K., Mason P.R.D., Beard A.D., Thirlwall M.F. (2003) Mantle
domains in the lithosphere beneath the French Massif Central: trace element and isotopic
evidence from mantle clinopyroxenes. Chem. Geol., 200, 71-87.
Lenoir, X., Garrido, C.J., Bodinier, J.-L., Dautria, J.-M. (2000) Contrasting lithospheric
mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite
xenoliths. Earth Planet. Sci. Lett. 181, 359–375.
Wood B.J., Bryndzia L.T., Johnson K.E. (1990) Mantle oxidation state and its relation to
tectonic environment. Science, 248, 337-345. |
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