ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract 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 δ 18O 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 (δ 18O=+5.5 to 7.0‰) and mantle megacrysts (δ 18O=+ 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00371848
