I'm studying a rhyolitic tuff that displays zirconium enrichment upsection, and am uncertain why. Lithologically the upper tuff consists of bedded block and ash flow deposits and fall out, while the lower tuff is a massive pumice rich tuff. Does differences reflect zoning in the tuff/magma chamber? Or is it more plausible that the two tuffs were derived under different processes? All other elemental concentrations are similar and overlap, which suggests they were derived from a similar or same souce. Any guidance is much appreciated.
Similar and overlapping elemental concentrations point to a closed system, so that Zr differences are more likely to reflect zoning in the magma chamber rather than different origin of Lower and Upper Tuff.
If the Lower Tuff represents the content of the shallower part of the magma chamber (ejected first), enrichment in high density and very high melting point Zircon in the Upper Tuff (ejected later in time, from the deeper part of the chamber) could be due to partial gravitational segregation of Zircon crystals in the chamber.
See also this abstract http://geology.gsapubs.org/content/35/1/73.abstract for ideas and comparison.
You may also want to consider contamination of the tuff during eruption. The major element concentration is a little different too. Is there any physical evidence in the rocks themselves that suggest wall rock incorporation in the erupted components. The enrichment is not huge in terms of ppm, and a wall rock component with not that much additional zircon could account for it. Finding an additional zircon component in imaged/dated heavy mineral separates may identify this if the field evidence isnt there.
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