The Ordovician chondrite from Brunflo, central Sweden III. Geochemistry ofterrestrial alteration

The fossil H chondrite Brunflo, found in a slab of Ordovician limestone fro m central Sweden, is pervasively altered to an assemblage dominated by calc ite and barite. The meteorite is surrounded by a 15-20 cm wide zone of ligh ter colors than the unaffected limestone due to dissolution of hematite. He re we present detailed geochemical analyses of two meteorite samples, 14 li mestone samples at distances from 0 to 29 cm along two profiles from the me teorite, and a reference sample of Brunflo limestone. Element concentration s in Brunflo and surrounding bleached limestone have been strongly disturbe d during two stages of alteration (early oxygenated and deep burial). In th e meteorite, the Ni/Co ratio has changed from an initial value of 20 to 0.8 and redox sensitive elements like V, As, Mo, Re and U are strongly enriche d. The sulfur isotope composition of barite from Brunflo (delta(34)S = +35 parts per thousand) indicates initial loss of meteoritic sulfide, followed by later accumulation of sea water sulfate as barite. During deep burial un der more reducing conditions, reduction processes supported by an externall y derived reductant possibly derived from alum shale underlying the limesto ne, were largely responsible for the observed redox phenomena. In spite of massive redistribution of many elements, concentrations of Pt, Ir and Au re main at chondritic levels. The geochemistry and mineralogy of alteration de termined for Brunflo are similar to those in "reduction spots" in red beds, where accumulation of a similar suite of elements (except Mo, Re) occurred as a result of isolated reduction activity. (C) 2000 Elsevier Science B.V. All rights reserved.