The Ordovician chondrite from Brunflo, central Sweden, II. Secondary minerals

Abstract

Brunflo is an altered H4-H5 stony meteorite that fell in calcareous mud on the continental shelf of the Iapetus Ocean about 460–470 Ma B.P. Except for relicts of chromite all the original minerals are replaced by calcite, barite, an illite-type mineral and some minor to rare phases: carbonate-fluorapatite, quartz, titanium dioxide, chalcopyrite, three different members of the cobaltite-gersdorffite series (including a phase with a composition close to CoNi(AsS) 2), rammelsbergite, safflorite, nickeline, maucherite, orcelite, native nickel, “anomalous bornite”, galena, pyrite, sphalerite and two Cu-phases (Cu 1.66S and Cu 4FeS 4). The secondary mineralogy varies rapidly at a mm-scale, indicating non-equilibrium conditions during the alteration. By the time the mud was cemented to limestone most of the original minerals might have been transformed to secondary phases. During subsequent stages of diagenesis the temperature remained within the 0–50°C range for millions of years, probably explaining the formation of the phases of unusual compositions. A temperature rise to 200–300°C chiefly due to regional heating during the Caledonian orogeny in Silurian-Devonian time might have modified the secondary mineralogy.