Implications for behavior of volatile elements during impacts—Zinc and copper systematics in sediments from the Ries impact structure and central European tektites

摘要

Moldavites are tektites genetically related to the Ries impact structure, located in Central Europe, but the source materials and the processes related to the chemical fractionation of moldavites are not fully constrained. To further understand moldavite genesis, the Cu and Zn abundances and isotope compositions were measured in a suite of tektites from four different substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia) and chemically diverse sediments from the surroundings of the Ries impact structure. Moldavites are slightly depleted in Zn (~10–20%) and distinctly depleted in Cu (>90%) relative to supposed sedimentary precursors. Moreover, the moldavites show a wide range in δZn values between 1.7 and 3.7‰ (relative to JMC 3-0749 Lyon) and δCu values between 1.6 and 12.5‰ (relative to NIST SRM 976) and are thus enriched in heavy isotopes relative to their possible parent sedimentary sources (δZn = −0.07 to +0.64‰; δCu = −0.4 to +0.7‰). In particular, the Cheb Basin moldavites show some of the highest δCu values (up to 12.5‰) ever observed in natural samples. The relative magnitude of isotope fractionation for Cu and Zn seen here is opposite to oxygen-poor environments such as the Moon where Zn is significantly more isotopically fractionated than Cu. One possibility is that monovalent Cu diffuses faster than divalent Zn in the reduced melt and diffusion will not affect the extent of Zn isotope fractionation. These observations imply that the capability of forming a redox environment may aid in volatilizing some elements, accompanied by isotope fractionation, during the impact process. The greater extent of elemental depletion, coupled with isotope fractionation of more refractory Cu relative to Zn, may also hinge on the presence of carbonyl species of transition metals and electromagnetic charge, which could exist in the impact-induced high-velocity jet of vapor and melts.