“Xeno-pumice” describes a pumice-like material, high in silica content and vesiculation, found as a xenolith in a more mafic rock. A xeno-pumice is an indicator of magma-crust interaction; however, the origin, nature, and processes behind this xenolith are still debated. Xeno-pumice has been described in a few places worldwide, including the Canary Islands in Spain, Indonesia, Iceland, the USA, Chile and Mexico. This thesis, for the first time, presents and analyzes the mineralogy, textural features, whole-rock geochemistry (major and trace element), and oxygen isotopes of xeno-pumice samples found in Harrat Rahat, Saudi Arabia. Harrat Rahat is a volcanic field whose last eruption was in 1256A.D. and reached the outskirts of Madinah, one of the main cities in Saudi Arabia. Harrat Rahat is characterized by a wide range of volcanic products: from basalts to trachyte. Previous studies suggested that this chemical variation has its source in the mantle and minor crustal contamination; however, the xeno-pumice samples found indicate magma-crust interaction. Thus, in this thesis, the crust-melt interaction hypothesis is addressed as a process that could modify the composition of the melt and thus the resulting volcanic products and eruptive style of the volcanic field. Indeed, the chemistry and oxygen isotope values of the studied volcanic rocks show a variation in composition, which is suggested to be the consequence of crust-melt interaction. The petrology, chemistry and oxygen isotope values suggest that the melt interacted with either the metamorphosed plutonic portion of the upper Arabian crust or with (meta-) sediments below Harrat Rahat. Finally, this thesis proved that magma-crust interaction occurred at Harrat Rahat, which has important implications for interpreting eruption mechanisms and mantle sources.

Knowledge of the provenance of crustal volatiles and the processes by which they are released is extremely important for the dynamics of magmatic systems. Presented here are the results of multiple investigations, which aim to understand magmatic volatile contamination from contrasting but complementary perspectives. The main methodologies used include He and C isotope values and CO2/3He ratios of volcanic gases and fluids; simulation of magma-carbonate interaction using high-pressure high-temperature experimental petrology; X-ray microtomography of vesiculated xenoliths and computer modeling. Findings show that the contribution from upper crustal volatiles can be substantial, and is dependant on the upper crustal lithology on which a volcano lies, as well as the composition of the magma supplied. Carbonate dissolution in particular is strongly controlled by the viscosity of the host magma. The details of the breakdown of vesiculated xenoliths is complex but has wide reaching implications, ranging from the dissemination of crustally derived materials through a magma body to highlighting that crustal volatiles are largely unaccounted for in both individual volcano and global volatile budgets. In synthesizing the conclusions from each of the individual perspectives presented, I propose the contribution of volatiles from crustal sources to play a significant role in many geological systems. This volatile component should be taken into consideration in future research efforts.