Experimental partitioning of Rb, Cs, Sr, and Ba between alkali feldspar and peraluminous melt Available to Purchase

Hydrous partial melting experiments performed between 650 and 750 °C at 200 MPa (H₂O) on synthetic metapelite compositions (quartz + albite + muscovite + biotite mineral mixtures) doped with Ba, Sr, Rb, and Cs yielded alkali feldspar crystals with a wide range of compositions in equilibrium at their rims with peraluminous melt. Measured partition coefficients for normally trace lithophile elements between feldspar and melt [D(M)^Fsp/gl,M = Ba, Sr, Rb, Cs] do not depend on either temperature or bulk composition of melt for the compositions studied. Values of D(Sr)^Fsp/gl are between 10 and 14 and appear to be independent of the albite and orthoclase contents of the feldspar crystals. In contrast, values of D(Ba)^Fsp/gl and D(Rb)^Fsp/gl are strongly dependent on the orthoclase content of feldspar, relationships that can be expressed by the following linear equations: D(Ba)^Fsp/gl= 0.07 + 0.25(orthoclase) and D(Rb)^Fsp/gl = 0.03 + 0.0 1(orthoclase), where orthoclase is in mole percent. These equations reproduce the range of previously reported values for D(Ba) and D(Rb) determined on natural and synthetic samples. A single partition coefficient for Cs was also determined at D(CS)^Fsp/gl = 0.13.

These data can be used in conjunction with recently published partition coefficients for muscovite, biotite, and plagioclase feldspars (Blundy and Wood 1991; Icenhower and London 1995) to model quantitatively the trace element signatures of peraluminous magmas during anatexis and crystallization. Depending on bulk composition and pressuretemperature trajectories during episodes of partial melting, Ba and Sr (and to a lesser extent, Rb) are retained at the source while Cs and Li are enriched in the first melts. These results explain why peraluminous granitic magmas with S-type (sedimentary) sources carry a distinctive enrichment in Li and Cs.