The halogens (F, Cl, Br and I) are a unique group of moderately volatile lithophile elements that are depleted on the Earth and behave as incompatible elements in the Earth’s mantle. The heavy halogens (Cl, Br and I) are strongly concentrated in Earth’s surface reservoirs and have high solubilities in aqueous fluids meaning that their geochemical cycles are more closely coupled to that of H2O than most other fluid mobile elements. This thesis takes advantage of recent analytical developments that enable the precise measurement of F, Cl and H2O by secondary ion mass spectrometry (SHRIMP), and the analysis of irradiation-produced noble gas isotopes to measure Cl, Br and I in mg-sized samples. The aims are to investigate halogens in both mantle melts and mantle rocks, thereby improving understanding of how halogens behave during subduction, mantle metasomatism and melting. In addition, an important secondary aim was to evaluate spot versus bulk analytical approaches for characterising mantle xenoliths. The thesis reports three major results chapters: 1) a suite of subduction-related glasses from the Mariana forearc, arc and backarc basin were investigated to test if the sources of halogens from within the slab change across the arc system, and if surface-derived halogens are significantly recycled back into the deeper mantle. 2) a suite of peridotite xenoliths were selected from Cenozoic volcanoes in three locations in Eastern Australia spanning roughly 2300 km: Atherton in NE Queensland, Toowoomba in SE Queensland, and western Victoria; were investigated to evaluate continent-scale mantle metasomatism possibly linked to subduction during Paleo-Mesozoic times. 3) a suite of peridotite xenoliths from the Tariat region of Mongolia were investigated to characterise the dominant host phases of halogens in a MORB-like reservoir and to evaluate the effects of melt-extraction.Nineteen submarine volcanic glasses from the Southeast Mariana Forearc Rift, and the Mariana arc and backarc basin showed halogen concentrations of 90-311 μg/g F, 71-1554 μg/g Cl, 131-5061 ng/g Br and 4-65 ng/g I. The glasses have Cl/Nb of 26-1417, Br/Nb of 0.07-3.69 and I/Nb of 0.001-0.069 that are up to 100x greater than MORB values and correlated with Ba/Nb. These data indicate slab-derived halogens account for 81-100% of the total Cl, Br and I in the arc and forearc glasses and 32-88% in backarc glasses. The glasses all have similar Br/Cl weight ratios of 0.0016-0.0037 l and I/Cl of (8.3-127) ×10−6. This range overlaps altered ocean crust ± lithospheric serpentinites but is much lower than forearc serpentinites, suggesting dehydration of the altered ocean crust and lithosphere as the dominant source of halogens and related slab-fluids. These results do not favour subduction of hydrated forearc mantle or the involvement of melange diapirs as important sources of subduction-related volcanism. The Br/Cl and I/Cl of the Marianas glasses are similar to those measured in subduction-related lavas from the SW Pacific, and MORB, which suggest halogens have similar origins in most subduction zones and a subducted origin in the MORB mantle.Peridotite xenoliths from Eastern Australia (n=12) and Tariat (n=6) were investigated by a combination of SHRIMP mineral spot analyses and bulk methods. Lherzolites from Eastern Australia and Tariat have measured bulk concentrations of 2.6-30 μg/g F that agree with estimates based on modal mineralogy and mineral spot analyses and overlap expected mantle values. This indicates that mantle F is predominantly hosted by clinopyroxene>orthopyroxene>olivine≥spinel. In contrast, measured Cl concentrations are much higher than estimates based on mineral spot analyses and modal mineralogy. The high measured values are partly ascribed to the presence of secondary alteration. However, the spot analyses of mantle minerals give estimated Cl concentration of 0.6-0.9 μg/g for Tariat and 0.5-1.9 μg/g for Australian lherzolites that are at the lower end of the expected mantle range. The lower-than-expected mineral Cl concentrations are interpreted to indicate that a significant portion of mantle Cl might be hosted by accessory minerals and/or in non-stochiometric sites such as fluid/melt inclusions or along grain boundaries as well as in the dominant mantle minerals of clinopyroxene, orthopyroxene, olivine and spinel.Fluorine and Cl have contrasting behaviours during melt depletion and metasomatism. The Tariat xenoliths show a broad decrease in F with increasing MgO that is a result of melt-depletion. Fluorine is correlated with 87Sr/86Sr and La/SmCPX showing progressive enrichment from metasomatism in Australian samples. In contrast with F, Cl is extremely depleted in most samples and does not vary systematically as a function of either MgO or 87Sr/86Sr and La/SmCPX. Nonetheless, Cl is enriched in amphibole-bearing lherzolites from Australia and phlogopite-bearing lherzolites from Tariat indicating a Cl-rich metasomatizing agent in both locations. The behaviour of Cl is therefore different to F during metasomatism as F will become pervasively enriched in the nominally anhydrous minerals of cryptically metasomatized mantle, while Cl will be dominantly restricted to hydrous phases in modally metasomatized mantle.