Volcanic glass shards and banded pumice from the KBS tuff of northern Kenya exhibit marked variations in magnetic susceptibility and color (colorless to dark brown). The darker glass shards exhibit superparamagnetism, which we now know to be carried by a population of tiny microcrystals of Fe-rich cubic oxide, ∼20 to ∼100 A in size, thought to be magnetite. A theory for their origin is one of nucleation and growth (precipitation) in quenched homogeneous glass (colorless) at temperatures of ∼700–1000 °C during and immediately subsequent to eruption. Results from high-temperature heating experiments on KBS shards support this idea. The precipitate appears in the KBS shards as a consequence of their cooling history and is the origin of their coloring; the origin cannot lie in negligible compositional differences between the colorless and darkest shards. On the other hand, banded pumice from the KBS tuff has both dark and colorless glasses of differing compositions. The pumice appears banded because precipitation occurred preferentially in the Fe-rich glass. Although magma mixing may have played a role in the eruption of these materials, on the basis of our survey of distal eruptive products, it would appear that the volumetric amount of the mafic end member (dark pumice) was insubstantial.

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