A new study shows that substitution of aluminium for silicon in clinopyroxene tetrahedra increases during cooling. The resulting local charge balance favours REE incorporation into the crystal lattice.
A new study by Scarlato and co-workers shines the spotlight on clinopyroxenes in a trachybasaltic dyke in the Valle del Bove depression on Mt Etna, Italy. Previous studies by Mollo et al. were able to quantify how the the cooling rate of the crystals increased from 0.02ºC/min to 1.13ºC/min from the core to rim of the dyke. The fact that the solidification path in this igneous feature was already well-constrained provided a perfect platform for this study, which explores the effect of temperature variations on the partitioning of rare earth elements (REE) between clinopyroxene and melt.
The structure of the clinopyroxene mineral consists of interlocking silicon tetrahedra, and two sites where metal cations can be incorporated (called M1 and M2). The study found that a higher cooling rate favours the substitution of tetrahedral aluminium (AlIV) for silicon. AlIV acts as a local charge balance for REE ions; as a consequence, REE are more easily accommodated into the M2 site of the mineral lattice.
Scarlato et al.’s measurements from this natural laboratory match the theoretical predictions of REE behaviour made using the ‘lattice strain model’ of Blundy & Wood (1994). This demonstrates that the REE partitioning between melt and clinopyroxene in naturally cooled magmas is controlled by local charge balance and cation substitutions, rather than kinetic parameters or diffusion.
Scarlato P, Mollo S, Blundy JD, Iezzi G, & Tiepolo M (2014) ‘The role of natural solidification paths on REE partitioning between clinopyroxene and melt’ Bulletin of Volcanology, 76(3), 1-4. http://dx.doi.org/10.1007/s00445-014-0810-1