Rare earth elements in olivine:determination, occurrence and behaviour

Abstract

Olivine is a widely occurring mineral that offers great potential for improving our understanding of geological processes. Olivine is present across a range of geologically significant settings, occurring in stony-iron meteorites, as inclusions in diamonds and as a dominant phase in upper mantle peridotite. The utility of the trace elements in olivine has been established through their application as geothermobarometers and as a tool for discrimination of mantle lithologies. The study of the rare earth elements (REE) in olivine has lagged behind that of the trace elements due to their ultra-low abundances. However, the value of REE studies has been proven through their application in minerals such as garnet, upon which the field of mantle metasomatism, for example, has rapidly expanded, as well as Ca perovskite, which has added much to our understanding of the differentiation in the early earth. With the precedent set and rapidly advancing analytical capabilities, the REE in olivine are in a prime position for geochemical exploration.

This thesis presents a novel analytical technique for determination of the REE in olivine using laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-QICP-MS) and demonstrates the application of this technique for a global suite of olivine. Building upon this research, this thesis brings new insights into olivine-melt and olivine-mineral REE partitioning behaviour. Key findings from this aspect of the thesis are that: 1) the olivine Mg number (Mg#) exerts a strong control over REE partitioning via its influence over lattice elasticity; and 2) that the LREE in natural olivine may experience preferential diffusion of the light REE (LREE) from neighbouring phases or grain boundary fluids/melts. This work thus provides a framework within which the REE in olivine can be interpreted. With this framework established the final aspect of this thesis uses the study of the REE in olivine to assess the metasomatic model for Archaean peridotite. Mass balance modelling of the REE in olivine, orthopyroxene and garnet with a range of reasonable metasomatic agents has shown that the conventional metasomatic model cannot replicate all three phases. In doing so this work highlights the importance of analysing all the major phases in such studies. Furthermore this thesis has revealed olivine to be an important host of the REE in Archaean peridotite. This paves the way for future studies to exploit the REE contents of olivine and bolster geochemical studies of the subcontinental lithospheric mantle (SCLM).