Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach

摘要

Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220,1,200, and l,180 deg C), and at three different oxygen fugacities (fO_2 = IW, QFM, air) in order to assess the effect of fO_2 on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS. Most trace element DS increase slightly as temperature decreases, except for D_(Zr) D_(Fe), D_(Eu) and D_(Cr) that vary systematically with fO_2. Applying the Lattice Strain Model to our data suggests the presence of Fe~(2+)entirely in the octahedral site at highly to moderate reducing conditions, while Fe(3+) was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO_2and confirm the greater compatibility of Eu~(2+), which has an ionic radius similar to Sr, relative to Eu~(3+) in plagioclase and the higher Eu~(2+)/ Eu~(3+) under reducing conditions. For petrogenetic basaltic processes, a combined fraetionation of Eu~(2+)-Sr and Fe-Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks.