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Contributions to Mineralogy and Petrology

, Volume 149, Issue 5, pp 600–612 | Cite as

Melt inclusions in scoria and associated mantle xenoliths of Puy Beaunit Volcano, Chaîne des Puys, Massif Central, France

  • Séverine Jannot
  • Pierre Schiano
  • Pierre Boivin
Original Paper

Abstract

In order to characterize the composition of the parental melts of intracontinental alkali-basalts, we have undertaken a study of melt and fluid inclusions in olivine crystals in basaltic scoria and associated upper mantle nodules from Puy Beaunit, a volcano from the Chaîne des Puys volcanic province of the French Massif Central (West-European Rift system). Certain melt inclusions were experimentally homogenised by heating-stage experiments and analysed to obtain major- and trace-element compositions. In basaltic scoria, olivine-hosted melt inclusions occur as primary isolated inclusions formed during growth of the host phase. Some melt inclusions contain both glass and daughter minerals that formed during closed-system crystallisation of the inclusion and consist mainly of clinopyroxene, plagioclase and rhönite crystals. Experimentally rehomogenised and naturally quenched, glassy inclusions have alkali-basalt compositions (with SiO2 content as low as 42 wt%, MgO>6 wt%, Na2O+K2O>5 wt%, Cl~1,000–3,000 ppm and S~400–2,000 ppm), which are consistent with those expected for the parental magmas of the Chaîne des Puys magmatic suites. Their trace-element signature is characterized by high concentration(s) of LILE and high LREE/HREE ratios, implying an enriched source likely to have incorporated small amounts of recycled sediments. In olivine porphyroclasts of the spinel peridotite nodules, silicate melt inclusions are secondary in nature and form trails along fracture planes. They are generally associated with secondary CO2 fluid inclusions containing coexisting vapour and liquid phases in the same trail. This observation and the existence of multiphase inclusions consisting of silicate glass and CO2-rich fluid suggest the former existence of a CO2-rich silicate melt phase. Unheated glass inclusions have silicic major-element compositions, with normative nepheline and olivine components, ~58 wt% SiO2, ~9 wt% total alkali oxides, <3 wt% FeO and MgO. They also have high chlorine levels (>3,000 ppm) but their sulphur concentrations are low (<200 ppm). Comparison with experimental isobaric trends for peridotite indicates that they represent high-pressure (~1.0 GPa) trapped aliquots of near-solidus partial melts of spinel peridotite. Following this hypothesis, their silica-rich compositions would reflect the effect of alkali oxides on the silica activity coefficient of the melt during the melting process. Indeed, the silica activity coefficient decreases with addition of alkalis around 1.0 GPa. For mantle melts coexisting with an olivine-orthopyroxene-bearing mineral assemblage buffering SiO2 activity, this decrease is therefore compensated by an increase in the SiO2 content of the melt. Because of their high viscosity and the low permeability of their matrix, these near-solidus peridotite melts show limited ability to segregate and migrate, which can explain the absence of a chemical relationship between the olivine-hosted melt inclusions in the nodules and in basaltic scoria.

Keywords

Olivine Fluid Inclusion Nepheline Mantle Xenolith Alkali Basalt 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

We thank Didier Laporte for supplying the nodule samples, and Jean-Luc Devidal and Michelle Veschambre for technical help. We also acknowledge F. Faure and R. Clocchiatti for helpful discussion. The LA-ICPMS analyses were carried out with the assistance of Gilles Chazot whose help is gratefully acknowledged. Journal reviews by P.J. Wallace and R.L. Nielsen are appreciated. Financial support was provided by the European Community’s Human Potential Programme under contract HPNR-CT-2002-0211 (Euromelt). S. Jannot is also grateful for the Conseil Régional d’Auvergne for funding her Ph.D. thesis.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Séverine Jannot
    • 1
  • Pierre Schiano
    • 1
  • Pierre Boivin
    • 1
  1. 1.Laboratoire Magmas et VolcansOPGC-Université Blaise Pascal-CNRSClermont-FerrandFrance

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