Oxygen thermobarometry of orogenic lherzolite massifs

Alan B. Woodland*, Jacques Kornprobst, Bernard J. Wood

*Corresponding author for this work

    Research output: Contribution to journalArticle

    78 Citations (Scopus)

    Abstract

    The oxidation state has been determined for spinel peridotites from 13 orogenic lherzolite massifs including Beni Bousera, Ronda, and 11 smaller massifs in the French Pyrenees. The oxygen fugacity (fo2) was calculated for 67 samples from microprobe analyses using a set of secondary spinel standards to correct the ferric iron content in the spinels. The utility of this method is confirmed by the good agreement between the calculated values and those determined by Mossbauer spectroscopy on 28 samples.The spinel peridotites of Ronda and Beni Bousera are relatively reduced, averaging -11 and -1.5 log units relative to fayalite-magnetite-quartz (FMQ) respectively, which is in agreement with values from abyssal peridotites and mid-ocean ridge basalt (MORB) glasses. The Pyrenean massifs are relatively oxidized [ΔlogfO2(FMQ)=-0.4] and are intermediate between the abyssal peridotites and continental xenolith suites. No systematic gradients are observable. Instead, variations of up to 2 log units in fo2 occur at a localized scale. This type of variation is also observed for trace elements and radiogenic isotopes. The harzburgites at Beni Bousera record the most reduced conditions. Local oxidation coincides with the appearance of amphibolc, indicating that metasomatizing fluids or melts are generally oxidized compared with the host peridotites. Partial re-equilibration in the plagioclase peridotite fades has occurred at Ronda, causing the spinels to become Cr rich. Re-equilibration is extremely heterogeneous. Mild oxidation appears to attend the crystallization of fine-grained plagioclase.The similarity in fo2 values at Beni Bousera and Ronda indicates a fairly uniform oxidation state at a scale of ̃ 200 km. This scale of homogeneity is also observed in the Pyrenees, where no significant variation in fo2 is apparent along 200 km of strike in the Northern Pyrenean Zone.

    Original languageEnglish
    Pages (from-to)203-230
    Number of pages28
    JournalJournal of Petrology
    Volume33
    Issue number1
    DOIs
    Publication statusPublished - Feb 1992

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