The transfer of crystals between magmas : a mechanistic and phenomenological study

Magma mixing and mingling processes are widely recognised in the textures and geochemistry of igneous rocks. A common feature of rocks from mixed magmas is the presence of magmatic xenocrysts, crystals that have been mechanically transferred from a magma of a different composition. This thesis investigates both the physical mechanisms of crystal transfer, and the physio-chemical response of crystals to the change in temperature and melt composition it experiences. Analysis of xenocrysts in lavas from Unzen, Japan, demonstrates how an understanding of these processes can inform interpretations from textural and geochemical analyses. Experimental and numerical studies investigate the fundamental process of low Reynolds number gravitational settling of spheres through fluid interfaces as an analogue for crystal transfer. Experiments reveal the dominant control on the volume of fluid entrained during the transfer is shown to be the viscosity ratio, with significant entrainment occurring when the viscosity of the upper fluid is comparable or greater than that of the lower. However, boundary integral method simulations suggest the density contrasts between crystals and melt are too small for gravitational settling to account for crystal transfer between magmas. Plagioclase and quartz xenocrysts in a mafic enclave erupted at 4-5 ka from Unzen are analysed to determine the effect of transfer on their textures and chemistry. Plagioclase xenocrysts contain resorbed interiors, surrounded by Fe-rich rims of clear growth, whilst quartz are rounded, heavily embayed and surrounded by coronas of hornblende, glass and vesicles. 50% of plagioclase phenocrysts in the host magma have signatures of previous resorption events, suggesting repeated episodes of injection and crystal recycling in the magma storage system prior to eruption. Erupted lavas from 1792 and 1991-1995 suggest such processes have been occurring for the last 300 years, but this study suggests they have possibly been ongoing for at least 5000 years.