Limits to Magma Mixing Based on Chemistry and Mineralogy of Pumice Fragments Erupted from a Chemically Zoned Magma Body

The chemical variation among pumice fragments from the Pahute Mesa Member of the Thirsty Canyon Tuff (Black Mountain volcanic center, southwestern Nevada) is consistent with magma withdrawal from a chemically zoned magma body. The top of this magma body contained little chemical variation, the lowest concentration of light REEs, and the highest concentrations of $$SiO_{2}$$, heavy REEs, and Th. The pumice fragments derived from the top of the magma body contain nearly pure ferrohedenbergite and fayalite. The next discrete zone in the magma body contained lower $$SiO_{2}$$, heavy REEs, and Th concentrations, and very high concentrations of light REEs. Many of the pumice fragments derived from this layer contain large chemical variations among the olivines and clinopyroxenes. The lowest erupted layer contained relatively low concentrations of $$SiO_{2}$$, Th, and light and heavy REEs. Many pumice fragments that erupted from this layer also contain a large chemical variation among mafic phenocrysts, most of which are unzoned. Pumice fragments with polymodal disequilibrium phenocryst populations are a priori evidence of magma mixing. The magma mixing process is constrained by: the systematic vertical distribution of chemically distinct pumice fragments throughout the ash-flow sheet; the presence of disequilibrium phenocrysts within some pumice fragments in all but the lowermost part of the sheet; and the presence of compositionally uniform glass in most pumice fragments, including those with widely varying phenocryst compositions. Negligible mixing occurred at the top of the magma body; limited mixing occurred in the second and third layers. Because mixing did not destroy the original layering, the amount of guest magma must have been small. In order for unzoned disequilibrium phenocrysts to not become zoned, they must have been preserved in the magma body only a short time. And yet, in order to produce the homogeneous liquid that surrounds these phenocrysts, mechanical mixing must have been very efficient.