Magnetic properties of K/T and E/O microspherules: origin by combustion?

Abstract

Magnetic spherules and spheroids are found in association with Ir anomalies at the Cretaceous/Tertiary (K/T) boundary and near to the Eocene/Oligocene (E/O) boundary. Detailed magnetic analyses have been performed on these particles, and on bulk K/T boundary clay from marine and continental sections. Coercivity spectra, and blocking and Curie temperatures of the spheroids and K/T boundary clays have been determined, and natural remanence and susceptibility measured for the bulk clay and associated strata. Marine K/T boundary clays are strongly magnetic, with remanent coercivities similar to those of the spheroids. In contrast, the continental clays are diamagnetic, except for the thin, overlying “impact layer”, which contains abundant paramagnetic jarosite, along with shocked quartz and feldspar. The morphologies, mineralogies, and magnetic properties of the K/T magnetic spheroids are closely analogous to magnetic fly ash spherules. This similarity may be related to the presence of carbonaceous “soot” particles in some marine K/T boundary clays, and the recent identification of plerospheres and cenospheres in continental boundary clay. In contrast, tektites and microtektites differ from the K/T magnetic spherules in a number of fundamental ways. The late Eocene particles are magnetically distinct from the K/T spheroids, both in their coercivity and thermal spectra. INAA results on Upper Cretaceous combustible shales indicate that the anomalous abundances of certain chalcophile and lithophile elements at the K/T boundary may be explained in terms of the natural combustion of bituminous sediments.