AbstractsEarth & Environmental Science

The Earth’s mantle cannot be viewed directly by geoscientists. To understand the chemistry and physical properties of the mantle we can use data from pieces of the mantle brought to the surface by volcanic eruptions, obducted mantle segments, experimentally reproducing the conditions of the deep earth, computational models and by seismology. It is widely accepted that the majority of the upper region of the mantle consists of olivine, orthopyroxene, clinopyroxene and an Al phase (plagioclase, spinel or garnet). Any loss or gain of chemical components mediated by a fluid, melt or diffusion is referred to as metasomatism. Investigating metasomatic processes that operate in the mantle is vital to understand the compositional and mineralogical variability of the mantle and the origins of mantle derived magmas. Much work has been conducted on this topic over the last decades but there are still many unanswered questions. This thesis takes a detailed look at how extremely alkali enriched silicate melts react with mafic upper mantle minerals during metasomatism. Such extreme melt compositions are typically formed at much shallower pressures in the Earth’s crust, however there is a large body of evidence to suggest that in some cases evolved alkali rich melts can be present at much higher pressures in the mantle. These melts may then metasomatise the mantle producing secondary mineral assemblages dominated by amphibole, phlogopite and diopside. Natural samples from the Heldburg Phonolite, Central Germany, where fragments (xenoliths) from the upper mantle that have been incorporated into an alkali rich evolved phonolite melt, are studied to observe how phonolite melts may react with mantle rocks. The mafic mantle minerals of olivine, orthopyroxene and spinel are consumed by the melt to form secondary polymineralic reaction rims. In addition to this, experiments were conducted at upper mantle pressures and temperatures to reconstruct the textures, compositions and mineralogy of the naturally occurring reaction rims. Pressure, temperature, time and H2O content of the melt were varied to provide constrains on the conditions of formation of the reaction rims observed in the Heldburg Phonolite. A range of analytical techniques and calculations were used to gain insights into how chemical components are exchanged between the mafic mantle phases within xenoliths and the host phonolite melt. Rates of reaction rim growth calculated from the experiments are then used to give quantitative estimates of the residence times of xenolithic material in the Heldburg Phonolite. To understand the processes by which minerals react with melts, the experimental samples were viewed at the nano-scale using transmission electron microscopy. This technique was used to view the micro-structures the reaction rims and the role of intergranular fluid during mineral replacement reactions. These findings have implications beyond the specific system studied and provide a potential mechanism of…