Partitioning of trace elements between plagioclase, clinopyroxene and melt

Abstract

This study focusses on the partitioning of trace elements between plagioclase, clinopyroxene and equilibrium melt. Such mineral/melt partition coefficients are widely used to model petrogenetic processes in igneous systems. However, theoretical considerations lead us to expect that the values of partition coefficients will change with many variables, including both mineral and melt compositions, as well as temperature and pressure. Plagioclase and clinopyroxene are two of the most common minerals in the Earth’s crust and if we can understand what controls the partitioning of trace elements into common rock forming phases, we can more accurately model these processes. To examine the major controls of partitioning in plagioclase and clinopyroxene, these two phases were grown experimentally at controlled pressure, temperature and oxygen fugacity. These minerals were grown from a wide range of simple, synthetic systems, mostly focussing on variations in CaO-MgO-Al2O3-SiO2-Na2O±Fe2O3 (CMASN±F) compositional space. 102 successful experiments are included in this thesis, 57 of these contain clinopyroxene and melt, 76 contain plagioclase and melt and 34 of these experiments contain both plagioclase and clinopyroxene and melt. This allows for the partitioning of each phase with their equilibrium melt to be well constrained before comparing the partitioning between the solid phases. Melt composition is shown to play a significant role in the partitioning of trace elements in both phases, especially when the substituting trace element has a different charge to the element it replaces. Even-though melt composition plays a key role in the partitioning of trace elements in both phases, if the substitution and charge balancing mechanisms are the same in both minerals, the effect of melt composition will be cancelled out. Such is the case for the partitioning of the rare earth elements (REEs) between plagioclase and clinopyroxene. This is advantageous as in natural samples, the equilibrium melt is rarely preserved, so partitioning between the solid phases is much easier to measure than the mineral/melt partition coefficients. The partitioning of the REEs between plagioclase and clinopyroxene has been used to calibrate a geothermometer. The geothermometer has been applied to a selection of natural coexisting plagioclase and clinopyroxene pairs, with ambiguous results.