TRACE-ELEMENT PARTITIONING BETWEEN AMPHIBOLE, PHLOGOPITE, AND BASANITE MELT

We have measured amphibole-melt and phlogopite-melt partition coeffici ents (D) for 22 trace elements in experimentally crystallized natural basanites with the ion microprobe, The synthesized phases display an e xceptional degree of homogeneity for both major and trace elements, as demonstrated by the ratio of the standard deviation to the mean count ing statistics uncertainty of the measurements, In pargasitic hornblen de, actinides are highly incompatible (D = 0.001), LILE and HFSE are m ildly incompatible(D = 0.04-0.2 and 0.1-0.2, respectively); and REE pa rtition coefficients vary from 0.05 to 0.6, with a maximum near Ho. Ex cept for the LILE(D = 0.1-3.7), phlogopite partition coefficients are generally lower, especially the REE (D approximate to 0.01). The parti tioning results are consistent with a model in which the variation in partition coefficient with ionic radius results from the crystal latti ce strain induced by the size misfit of the substituting trace element , This result predicts a decrease in Young's Modulus (E) with increasi ng size of the cation sites in the crystal lattice, and E derived for the largest site in both amphibole and phlogopite agree well with expe rimentally determined bulk mineral values, The ability to model partit ioning with an elastic strain model provides an important link between trace element partitioning and the macroscopic properties of minerals , Relative to an anhydrous peridotite, partial melting of an amphibole or phlogopite bearing peridotite will result in no Th-U fractionation , slight LILE depletions, and, aside from Ti, no significant HFSE depl etions. Thus, barring the addition of any slab components besides H2O, partial melting of hydrated peridotite is not a plausible explanation for any of the geochemical features commonly associated with subducti on zone magmas.