Geologic origin of magnetic volcanic glasses in the KBS tuff.

Volcanic glass shards and banded pumices from the KBS tuff of northern Kenya exhibit variation in magnetic susceptibility, and are colourless to dark brown. The darker glass shards exhibit superparamagnetism due to microcrystals of magnetite, approx 20-100 A in size. High T heating experiments on KBS shards support their origin as one of nucleation and growth (precipitation) in quenched homogeneous glass (colourless) at 700-1000oC during and immediately subsequent to eruption. Banded pumice, however, has both dark and colourless glasses of differing compositions. The pumice appears banded because precipitation occurred preferentially in the Fe-rich glass. The role played by magma mixing in the eruption of these materials is regarded as insignificant.-L.C.H.