Boron isotope composition of secondary smectite in suevites at the Ries crater, Germany: Boron fractionation in weathering and hydrothermal processes

The 24-km diameter Ries crater, in Germany, is one of the best-preserved terrestrial complex impact structure and clay minerals in the groundmass of the Ries suevites have been recognized for several decades. It is generally accepted that these clays were formed by post-impact aqueous alteration of impact-generated glasses and/or finely comminuted crystalline basement material at temperatures above ambient conditions. The Ries impact structure provides a good opportunity to study the evolution of post-impact development of impact craters. Here we present a study of boron isotopic compositions (delta B-11) and B concentrations of the secondary smectite clay fraction in fall-out and crater suevites from the Ries crater (Germany). These data were used to study the boron fractionation in clay mineral precipitation processes and to model, using B-isotope fractionation, the characteristics (pH, temperature, and salinity) of fluids during fluid-rock interaction at different positions within the crater. The results of this study show that the boron isotopic composition of smectite in fall-out suevites sampled from 4 different locations (average -24.5 +/- 1.8 parts per thousand) was significantly different from that of the smectite in crater suevites sampled from the Nordlingen 1973 drill core at different depth intervals, from 370 m to 525 m (average -4.07 parts per thousand). Similarly, the delta B-11 composition of the fluids responsible for the alteration calculated from smectite isotopic composition in the fall-out suevites (+7.5 +/- 1.6 parts per thousand), differed from the alteration fluid composition in crater suevites ( +17.6 +/- 10.8 parts per thousand); indicating a different origin of the fluids responsible for the alteration of the fall-out and crater suevites in the Ries crater. Our results suggest that the alteration in fall-out suevites occurred at lower temperatures than the crater suevites and at a lower pH, which is consistent with the smectite precipitation in equilibrium with meteoritic fluids. The boron isotopic composition of smectite in the crater suevites, suggests secondary clay formation at higher temperatures and/or at elevated pH (>8-9). Low delta B-11 values of fixed boron in secondary smectite in fall-out suevites suggests that B was incorporated into the mineral structure during the precipitation of authigenic smectite; whereas the smectite precipitated in equilibrium with the boron and hydrogen isotopic composition in the meteoric water that percolated through the system. Our study provides new constraints on the origin and evolution of the (geothermal) fluids that were involved in the formation of altered minerals at the Ries crater and suggests that boron isotope composition studies of the secondary clay phases associated with post-impact cooling may have important implications for the changes in fluid flow and changes in geochemical signatures of the fluids, and provide new constraints on fluid/rock interaction and mass transport during the cooling of the impact crater. (C) 2011 Elsevier B.V. All rights reserved.