Stable isotope composition of impact glasses from the Nordlinger Ries impact crater, Germany


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Stable isotope composition of impact glasses from the Nordlinger Ries impact crater, Germany
Geochimica et Cosmochimica Acta
Vennemann T.W., Morlok A., Von Engelhardt W., Kyser K.
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The Ries impact, which today is represented by a 24-km-diameter complex
crater, occurred at 15 Ma. It is estimated that a projectile of about 1
km diameter formed a transient crater of 6 to 7 km radius and 2.8 km
depth. Impact melt glasses investigated are found within the suevite,
the breccia that forms the uppermost layers of the ejecta blanket around
the crater. The glasses are considered to represent quenched melts
produced as a result of the impact. The oxygen isotope compositions of
several glasses sampled from widely spaced localities are very
homogeneous with delta O-18 values in the range of 6.7 to 7.4 parts per
thousand and delta O-17 of 3.3 to 3.7 parts per thousand. With
increasing devitrification and alteration the delta O-18 values increase
up to 15.8 parts per thousand, delta O-17 up to 8.1 parts per thousand.
Water content also increases with devitrification from fresh glasses
with about 1.3 wt.% to 3.8 wt.% in devitrified glass. deltaD values
decrease with increasing water content from about -87 to -127 parts per
thousand. Whole-rock oxygen isotope compositions of the sedimentary
cover sequence range from 17 to 27 parts per thousand, and crystalline
basement rocks range from 8.8 to 13.5 parts per thousand for granites
and gneisses, whereas amphibolites have delta O-18 values of 5.2 and 6.1
parts per thousand. Models suggesting that thr: glass in the suevite
represents a mixed melt derived from all the rocks present in the
suevite in proportion of their occurrence (Engelhardt, 1997) do not
agree with the oxygen isotope composition of the glass. The simplest
explanation for the homogeneous chemical and oxygen isotope composition
is that the glass represents quenched melts from a few closely spaced
lithologies only. High Fe and Ni contents in the glass, Cr/Ni and Cr/Co
ratios, and relatively low delta O-18 values indicate involvement of
amphibolites during melting. The balance of the melt was made up of
spatially associated granites or gneisses as indicated by the major
element and rare-earth element geochemistry. The best agreement between
major element and oxygen isotope composition of the glass and model
melts is obtained by mixtures of amphibolite and granite in proportions
similar to their average occurrence in the fallout suevite. Copyright
(C) 2001 Elsevier Science Ltd.
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