Noble gas and major volatile systematics of the Manus back-arc basin and the Central American arc: Tracing source mixing, contamination and degassing

Magmatic volatiles are key components of volcanic systems. They can be utilised to investigate magma mixing and elemental cycling as well as contamination, degassing and eruption processes. This dissertation focuses on volatiles (noble gases and major volatiles) in two contrasting volcanic settings: the Manus back-arc basin and the Central American volcanic arc.; The Manus basin is a back-arc spreading environment where lavas are characterised by high 3He/4He ratios, indicative of a hotspot component contributing to its source. Submarine glasses erupted along the Manus spreading centre and the extensional transform zone have anomalously nucleogenic neon isotopes, which are decoupled from helium isotopes, challenging the classic “solar” hypothesis. This unusual observation is attributed to either a prior degassing event, resulting in preferential loss of neon followed by in-growth of nucleogenic neon, or a deep mantle source with unique characteristics inherited from Earth's accretion. The effects of diffusion, contamination and degassing were assessed using He-Ar-CO₂ relationships. The glass phase of the samples show high CO₂/3He ratios and low 4He/40Ar* ratios, suggesting modification of original source values by contamination or diffusional He loss. Although the vesicle phase of the samples show varying degrees of degassing, this can be corrected to estimate the CO₂/3He of the Manus plume (= 1.1 × 109 − slightly lower than average MORB values = 2 × 109).; Combined helium and carbon studies of geothermal fluids from the Central American arc were carried out in order to assess the relative contributions of carbon from various sources (subducting sediments, altered oceanic crust, the mantle, …etc.) and to quantify the absolute volcanic flux of carbon (as CO₂) from the arc. The findings of this study indicate that most of the CO₂ emitted from volcanoes in Central America (∼80%) is derived from a subducted marine carbonate or a limestone sediment source and that the volcanic CO₂ output along the are represents only 14–18% of the total CO₂ input at the trench. This implies that a significant amount of CO₂ may be either lost in the forearc region, or potentially recycled into the deep mantle (beyond the zone of magma generation).