Trace-element partitioning in pantellerites and trachytes

Abstract

In order to investigate the effect of increasing melt peralkalinity on partitioning, partition coefficients have been determined using neutron activation analyses of coexisting phenocrysts and glass of five samples from Pantelleria spanning the range trachyte to pantellerite. Alkali feldspar partition coefficients for Fe, Rb, Ba, Sr, and Eu vary with melt peralkalinity due to changes in melt polymerization and to the systematic increase in X or and decrease in X an of the feldspar. In going from trachyte to pantellerite, Fe partition coefficients increase from 0.04 to 0.10, presumably because Fe +3 increasingly substitutes in the feldspar tetrahedral site as melt activity of Al declines and Fe concentrations increase. Partition coefficients for trivalent light REEs decrease and the partitioning pattern becomes flatter, the most evolved samples having some of the lowest published values for feldspar. The hundredfold decline in Eu partition coefficients (2.5 to 0.024) and the decrease in the size of the positive partitioning anomaly are attributed to increasing Eu 3+/Eu 2+ in the melt as it becomes more peralkaline, as well as to concomitant decrease in the Ca content of feldspar. As a result, the behavior of Eu during fractional crystallization of peralkaline suites is fundamentally different from that in metaluminous suites; absolute abundances rise and the size of the negative Eu anomaly changes little with fractionation beyond pantelleritic trachyte. Barium and strontium do not decline to extremely low concentrations in pantellerites, as they do in highly evolved metaluminous rhyolites, because the feldspar partition coefficients drop precipitously with peralkalinity. Similarly, Rb contents do not get large because the partition coefficient increases slightly (0.11 to 0.25) with peralkalinity and is sufficient to make Rb only weakly incompatible in the feldspar-dominated assemblage. Partition coefficients calculated for Sr and Ba from natural phenocryst-matrix pairs can be gross over-estimates in these feldspar-rich felsic rocks if phenocrysts are zoned. This effect is severe for assemblages with bulk distribution coefficients ( D) greater than 4 even at very small crystal contents, and for 4 > D > 2 it is significant if crystal contents exceed about 25%. Some of the very high values and much of the variability reported in the literature for Ba and Sr feldspar partition coefficients may be a result of this effect. In contrast to feldspar, partition coefficients for apatite and mafic phases change little with peralkalinity. Partition coefficients for REEs and the compatible elements Sc, Co, and Cr are all less than half those in Fe-rich clinopyroxene and olivine of similar composition in high-silica rhyolite. Although absolute levels of partition coefficients are a function of melt polymerization, the shapes of the REE partitioning patterns appear to be related to the composition of the clinopyroxene. Scandium is most strongly concentrated in clinopyroxene (partition coefficient = 27–60), having larger values in more Fe-rich compositions. Apatite shows a wide range in composition within the suite, the concentrations of Si, Fe, REEs, and Y increasing and Mg, Ca, and P decreasing in more evolved samples. These trends, however, parallel those in coexisting glasses, so that partition coefficients are essentially constant. In peralkaline suites, Cs is the most incompatible of commonly analyzed elements, but its great mobility during alteration and weathering limits its use as a differentiation index. Next in order of incompatibility are Th, U, Hf, Zr, and Ta, which have small, but finite, values of D due to the partitioning of Th and U into apatite (~2), the accommodation of Zr and Hf by clinopyroxene (~0.4) and titanomagnetite (~0.2), and relatively high concentrations of Ta in cossyrite and Fe-Ti oxides. The next most incompatible are the lightest and the heaviest REEs, and, finally, the middle trivalent REEs and Rb. Plots of pairs of relatively incompatible elements form straight-line trends over large compositional ranges in peralkaline suites because the replacement of olivine + titanomagnetite by cossyrite, as the melt becomes more peralkaline, causes little change in D; the REE patterns of titanomagnetite and cossyrite are both approximately flat at values less than 0.1, and the relatively high titanomagnetite partition coefficients for Ta (~2), Zr, and Hf, when combined with the very low partition coefficients for these elements for olivine, approximate the moderate partition coefficients for cossyrite.