VARIATIONS IN ALKALI FELDSPAR/MELT TRACE-ELEMENT PARTITIONING DURING FRACTIONATION OF PERALKALIC QUARTZ TRACHYTE-RHYOLITE SUITES

Most trace-element models of igneous petrogenesis assume that trace-element partition coefficients remain constant during crystal fractionation or partial melting. Recent work has demonstrated, however, that trace-element partitioning of many incompatible elements (e.g., Rb, Y, Zr, and Nb) can be modeled as a function of whole-rock chemical parameters, becoming less incompatible with increasing SiO2 and decreasing agpaitic index [mol (Na+K)/Al]. Eu, however, becomes more incompatible with increasing agpaitic index. These results strongly suggest that partition coefficients do not remain constant during fractionation processes.

Variations in alkali feldspar/melt trace-element partition coefficients as a function of increasing differentiation for three peralkalic quartz trachyte-rhyolite suites are investigated. These suites include samples of pantellerite domes from the Cinque Denti caldera, Pantelleria, Italy; the Star Mountain Formation, Trans-Pecos Texas; and samples of the Gomez Tuff from the Buckhorn caldera, Trans-Pecos Texas. D(Rb) declines from 0.30 to 0.21 and D(Eu) declines from 0.36 to 0.02 in the strongly peralkalic suite from Pantelleria (~64 to 70 wt% SiO2), while D(Y), D(Zr), and D(Nb) remain fairly constant (~0.01 to 0.03). In the less peralkalic Star Mountain suite (~63 to 72 wt% SiO2), D(Rb) increases from 0.33 to 0.40, D(Eu) declines from 2.05 to 1.24, and D(Y), D(Zr), and D(Nb) each increase from ~0.02 to 0.07. Over a much smaller compositional range (~71.5 to 73.5 wt% SiO2), the pantelleritic Gomez Tuff from the Buckhorn Caldera demonstrates an increase in D(Rb) from 0.37 to 0.41, and an increase in D(Eu) from 0.35 to 1.17. In this study, it is shown that although it may be acceptable to assume constant trace-element partition coefficients for some trace-element models, for many others such an assumption should not be made.