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| Titel |
Relationship between oxidation state and texture and the influence of metasomatism in the lithospheric mantle beneath the Massif Central, France |
| VerfasserIn |
Laura Uenver-Thiele, Alan Woodland, Hilary Downes, Rainer Altherr  |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082385
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| Zusammenfassung |
| Investigating regional variations in structure and composition of the subcontinental
lithospheric mantle (SCLM) is usually hampered by the lack of samples. However, the
numerous volcanic centers of the French Massif Central have brought samples of the
SCLM to the surface over a geographic area of ~20.000km2. Lenoir et al. (2000)
and Downes et al. (2003) identified textural and geochemical differences between
two domains lying north and south of 45Ë 30’ latitude, respectively. The northern
domain is relatively refractory, but has experienced pervasive enrichment of LREE.
The southern domain is generally more fertile, although the peridotites are LREE
depleted. Many xenolith suites have undergone variable degrees of metasomatism.
Variations in oxidation state might be expected from the differing histories of these two
domains.
We have undertaken an extensive study to determine the oxidation state of the SCLM
beneath the Massif Central over the largest geographical area possible, including 140
peridotite xenoliths from 45 localities. All xenoliths are spinel peridotites and vary in
composition from lherzolites to harzburgites. Using the nomenclature of Mercier and Nicolas
(1975) the xenoliths are mostly protogranular or protogranular-porphyroclastic, although
some are porphyroclastic or equigranular. Small amounts of amphibole or biotite occur in
some xenoliths, particularly in the southern domain, reflecting modal metasomatism.
These metasomatic phases are found predominantly in peridotites with protogranular
textures.
Oxidation state was determined using the equilibrium between the Fe-bearing
components of olivine and pyroxene and the magnetite component in spinel (i.e. Wood et al.
1990). Major element compositions of the individual minerals were determined by
microprobe. Ferric iron contents of spinel were determined by Mössbauer spectroscopy and
gave values of Fe3+/ Fetot from 0.159 to 0.459, with a conservative uncertainty of ±0.02.
Oxygen fugacity (fO2) of the peridotites was determined using the Nell-Wood calibration of
the above equilibrium (Wood et al. 1990) and are referenced to the fayalite-magnetite-quartz
(FMQ) redox buffer. Generally, Δlog(fO2) values lie between FMQ-0.47 and FMQ+1.66 log
units for the entire data set. In this fO2 range propagated uncertainties are ~ 0.1 log
units.
The texture - fO2 systematics differ between the northern and southern domains.
Protogranular lherzolites from the northern domain record fO2values >FMQ+1.25. In
contrast, the protogranular harzburgites exhibit values >FMQ+0.9 whereas the
protogranular-porphyroclastic or porphyroclastic harzburgites record lower values. The
texture - fO2 systematics of southern domain harzburgites are directly reversed. Southern
domain lherzolites record fO2 values ~FMQ+0.6 ±0.3. These systematic variations between
texture and oxidation state are particularly evident for a given locality. Metasomatic
interaction in the SCLM has produced notable changes in redox state at the regional as well
as local scale, generally through oxidation. However, the most oxidized samples are not
those containing amphibole, suggesting that cryptic metasomatism, (carbonate-melt
metasomatism?) occurred in both domains. At certain localities, both modal and cryptic
metasomatic events can be observed.
Downes H. et al. (2003) Chem. Geol., 200, 71-87.
Lenoir, X. et al. (2000) Earth Planet. Sci. Lett. 181, 359–375.
Wood B.J. et al. (1990) Science, 248, 337-345. |
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