Computer Science
Scientific paper
Sep 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..75...37w&link_type=abstract
Earth and Planetary Science Letters, Volume 75, Issue 1, p. 37-49.
Computer Science
14
Scientific paper
The Laacher See phonolite tephra sequence (11,000 years B.P.) of the Quaternary East Eifel volcanic field (West Germany) represents an inverted, chemically zoned magma column. Mafic and differentiated phonolites, respectively, represent the lowermost and uppermost erupted portion of the Laacher See magma chamber. Sr and Nd isotopic compositions of whole rocks, matrices and phenocrysts have been analyzed in order to provide constraints for open versus closed system evolution of the Laacher See magma chamber. 87Sr/86Sr isotope ratios of mafic phonolites and their phenocrysts are slightly more radiogenic than parental East Eifel basanite magmas. Bulk rock samples show a drastic increase in 87Sr/86Sr from mafic towards the most differentiated compositions that were erupted from the top of the magma chamber. Glass matrix separates show a parallel, but less pronounced, increase in 87Sr/86Sr. Phenocrysts, in contrast, show a narrow range in 87Sr/86Sr with a slight, but significant, increase towards the top of the magma chamber. Phenocrysts from the uppermost portion of the magma column were not in isotopic (or chemical) equilibrium with their host matrices.
143Nd/144Nd isotope ratios for whole rocks, matrices, and phenocrysts fall within a restricted range similar to that of East Eifel mafic magmas. A representative suite of crustal rocks (lower crustal granulites, quartzo-feldspathic gneisses, mica schists, Devonian slates and graywacke) was also analyzed in order to permit an evaluation of possible assimilation models.
Our results are consistent with chemical evolution of the zoned Laacher See magma chamber mainly through crystal fractionation accompanied by minor amounts of assimilation. Slight contamination of the magma system may have involved (a) the assimilation of gneisses (?) and mica schists during the initial stage of magma chamber evolution (basanite-mafic phonolite), (b) combined assimilation-fractional crystallization (AFC) concurrent with the second differentiation stage (mafic phonolite-zoned phonolite), and (c) post-crystallization assimilation of the most fractionated volatile-rich melts of the top magma layers during a late, liquid-state (?) differentiation stage. The latter possibly involved fluid transport and/or exchange with the surrounding (partially molten) country rocks. Open system evolution of the Laacher See magma chamber is further indicated by magma mixing, as confirmed by our isotope data, leakage of volatiles from the cupola and metasomatism of wall rocks.
Staudigel Hubert
Wörner Gerhard
Zindler Alan
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