Partition coefficients for trace elements between melt and its crystallizing minerals can be used as additional source of information for changes of the status of the melt, such as changes in melt structure and/or changes in complexing behavior of trace elements. Melt structure control and volatile complexing have been proposed to be important processes in the evolution of strongly trace-element zoned high-silica rhyolite magmas („ 75 wt.% SiO2) as mainly preserved in ash-flow tuffs. Evolutionary models for zoned high-silica rhyolite magmas have been controversial due to difficulties in reconciling commonly observed strong trace-element gradients over small major-element variations.
Minerals of the 7 Ma Rattlesnake Tuff („ 280 km3 DRE) from eastern Oregon are well suited for partition coefficient investigations because important geologic prerequisites for imaging differentiation processes related to changes in the melt through natural partition coefficients are fulfilled:
i) existence of minerals from several coexisting high-silica rhyolites that make up a well characterized and strongly zoned rhyolite magma; ii) phyric rhyolite magmas are crystal poor (0.1-1.5 wt.%) and crystals are virtually unzoned suggesting minerals are equilibrium phases recording magma conditions after zonation was generated;
iii) compelling evidence for magma evolution models through some type of differentiation excluding mixing
or melting scenarios as being responsible for chemical
gradients within Rattlesnake Tuff high-silica rhyolites.
Partition coefficients (K's) were obtained from bulk analyses of alkali feldspar (fsp), Fe-rich clinopyroxene (cpx), titanomagnetite (tmt), and glass from pumices representing the progressively more evolved high-silica rhyolite groups. Systematic changes in partition coefficients with higher differentiation degree of the rhyolites are observed in Rb, Ba, Sr, Eu, and possibly LREE for fsp, Sc, Mn, Cr and possibly Zn and Ta for cpx, and Ta and Mn for tmt; all systematic variations principally correlate with crystal chemical changes based on nearly constant compound partition coefficients (e.g. (Eu/Ca)fsp/(Eu/Ca)glass). All other K's are either constant, vary unsystematically, or interpretation is limited by potential element contributions from accessory phase inclusions to bulk mineral analysis. From this follows that there is no evidence for changes in K's that are induced by changes in the melt therefore suggesting no volatile induced polymerization decrease roofward in the magma chamber towards higher differentiation degree of the rhyolite magma.
In summary, Rattlesnake Tuff partition coefficients argue against that observed trace element variations within
high-silica rhyolites are controlled by melt structure and/or trace-element volatile complexes leaving crystal-liquid partitioning as main control in the differentiation process. This is also supported by internally consistent crystal fractionation models assuming observed mineral partition data are similar to K's of minerals involved in fractionation process.