Contributions to Mineralogy and Petrology

, Volume 95, Issue 3, pp 343–349 | Cite as

Stable isotope relations in an open magma system, Laacher See, Eifel (FRG)

  • G. Wörner
  • R. S. Harmon
  • J. Hoefs


18O/16O and D/H ratios have been measured for matrix glasses and phenocrysts from the zoned phonolitic Laacher See tephra sequence (11000 y.b.p., East Eifel volcanic field, FRG) to study open-system behaviour of the associated magma system. Mineral and glass δ18O values appear to be largely undisturbed by low-temperature, secondary alteration, record isotopic equilibrium and confirm previous conclusions, based on radiogenic isotope evidence, of early, small-scale crustal assimilation during differentiation of parental magmas in a crustal magma chamber. One sanidine-glass pair possibly documents the late stage influx of meteoric fluids into the topmost magma layer prior to eruption. A sealing carapace of chilled magma, which itself was strongly contaminated, prevented large-scale fluid exchange up to the point prior to eruption when this carapace was fractured and meteoric water gained access to parts of the magma system. D/H measurements of various glass types (glass inclusions, dense and pumiceous glass) and amphiboles gave conflicting results suggesting a combination of degassing, volatile exchange with country rocks and hydration. Stable isotope ratios for primitive parental magmas (δ18O=+5.5 to 7.0‰) and mantle megacrysts (δ18O=+ 5.5 to +6.0‰, δD=−21 to −38‰, for amphiboles and phlogopite, resp.) suggest a rather variable fluid composition for the sub-Eifel mantle.


Tephra Meteoric Water Parental Magma Magma System Volcanic Field 
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Copyright information

© Springer-Verlag 1987

Authors and Affiliations

  • G. Wörner
    • 1
  • R. S. Harmon
    • 2
    • 3
  • J. Hoefs
    • 3
  1. 1.Institut f. MineralogieRuhr UniversitätBochumFederal Republic of Germany
  2. 2.Department of Geological SciencesSouthern Methodist UniversityDallasUSA
  3. 3.Geochemisches InstitutUniversität GöttingenGöttingenFederal Republic of Germany

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