A plethora of magmatic processing occurs in magma reservoirs, where melts are stored prior to eruption. Magma reservoirs are complex, open systems, and often multiple reservoirs are partially inter-connected from source to surface, giving rise to the term 'volcanic plumbing system'. Parental melts feeding these reservoirs can have diverse and distinct geochemical and petrological characteristics, and be variably evolved or enriched. These melts can also bring with them a crystal cargo that may remain in equilibrium in the magma reservoir, but may also be modified by reaction, resorption, crystallisation and diffusion. Melts and crystals can be transported between reservoirs, from the upper mantle and through the crust, leading to melt mixing, reactions and volatile exsolution. Basaltic volcanic systems are fed by primitive melts, and due to the rapid ascent of melts and short magma storage times, these volcanoes provide the best means of unravelling the mantle and crustal contribution to geochemical heterogeneity observed in erupted samples.

Despite the potential chemical complexity of a magma reservoir, evidence for magma processing and reaction can be preserved in melt inclusion suites and the compositional structure of their host crystals. Magmatic processes during storage and transport at two basaltic volcanoes are investigated using two carefully selected eruptions: the 1669 eruption at Mt. Etna, and the 2007 Father's Day eruption at Kīlauea. A suite of diverse geochemical, petrological and petrographical observations, made at a range of length-scales, are combined and interpreted in tandem with geophysical monitoring data. The conclusions of these studies shed light on the architecture of each volcano's plumbing systems and basaltic plumbing systems in general.

This thesis is divided into two parts. The first study unravels the crustal and mantle processes controlling melt geochemical heterogeneity at Mt. Etna, Sicily, during the 1669 eruption, the largest eruption in historical times. The 1669 melt inclusion suite arises from the mixing of two basaltic melts with similar major element compositions but very different trace and volatile element compositions. The melt geochemistry suggests that at least one end-member melt has been heavily influenced by assimilation of carbonate in the crust. The elevation in alkalis, caused by carbonate assimilation, enhances carbon and sulfur solubility in one end member. The melt inclusion suite indicates that mixing of these melts occurred in the shallow crust shortly before eruption and this mixing may be the cause of the enhanced $CO2$ fluxes prior to eruptions at Mt. Etna.

The second study is split into two parts. Each uses the eruptive products of the Father's Day eruption at Kīlauea and aims to unravel the connectivity of the plumbing system between the summit and East Rift Zone, with a focus on timescales of storage and transport. The first part investigates the melt geochemistry in terms of heterogeneity and volatile composition, and the second investigates the crystal cargo in terms of features of the macro-scale crystal cargo distribution and the micro-scale geochemical zoning of individual crystals. The integration of observations and models from these two studies constrains the pressure, temperature and composition of source magma feeding the Father's Day eruption. The eruption is investigated in the context of the "magma surge'' event that preceded the intrusion, as well as within the context of the longer-term trends in Kīlauea geochemistry at the summit and East Rift Zone. Melt inclusion and matrix glass volatile systematics provide insights into the degassing path of the magma and the duration of magma transport to the surface is constrained by diffusion modelling. Estimated timescales for ascent by diffusion modelling of macrocryst major element composition, melt inclusion water content and the melt Fe$\mathrm{<mrow\; data-mjx-texclass="ORD">3+</mrow>}$/Fe ratio are in agreement with timescales observed from the geophysical data of $<$8 hours from reservoir depth to eruption.

Both studies emphasise how petrological observations can supplement geophysical monitoring datasets collected at the surface to aid our interpretation of volcanic behaviour and eruption forecasting.