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Ocean Islands Basalts provide important windows into the compositional variations of the Earth's mantle, which in tum constrains models for mantle convection and evolution. The Canary Islands show contrasting styles of eruption and evolution of magmas in an ocean island setting. U-Th-Ra disequilibrium have been used to constrain rates and timescales of melt generation and differentiation beneath ocean islands, and to estimate the buoyancy flux, potential mantle temperature and the depth and degree of melting. The Canary Islands provide a rare opportunity to observe U-Th-Ra disequilibrium, because they are underlain by a region of low buoyancy flux, and were expected to show significant disequilibrium. Tenerife is underlain by numerous magma chambers, in which magmas have time to differentiate from basanites to phonolites, erupting to form large strato-volcano complexes. The fissure and small vent eruptions of unusually primitive basanites and alkali basalts from Lanzarote show little evidence of magma chambers, unless of substantial size and longevity at depth. The U-Th results indicate that lavas underwent rapid transport from the melt region. The historic and recent pre-historic eruptions (1824, 1730-36, Corona) from Lanzarote have some of the most primitive compositions found on oceanic islands with low SiO² contents (< 51 %), Mg numbers of 67-74 and high Cr and Ni contents. The rocks are restricted in Sr, Nd and Pb isotopes, being displaced from MORB towards the HIMU om field. The major and trace elements have been modelled by mixing a deep smaller degree (1 %) melt and a shallower larger degree (4%) melt. Negative K anomalies were observed in the small degree melts indicating that melt generation may have continued at a shallow level, perhaps to within the lithospheric mantle with melting in the presence of residual phlogopite. The Lanzarote source was modelled as a mixture of HIMU and EMIl asthenospheric mantle, with a small contribution from a shallow, lithospheric source. Thermal erosion of the lithospheric mantle is required for melting at depths (58 and 73 km) modelled from the major and trace elements. The Lanzarote lavas exhibit significant (²³⁰Th/²³⁸U) disequilibrium with ²³⁰Th excesses of 6 - 81 %. This was modelled by dynamic melting giving a calculated melt rate of 0.125 x 10^-3 kg.m^-1.yr^-1, a timescale of melt generation (matrix transfer time) of 270 ka for the 1 % melt and 1,100 ka for the 4 % melt. A consistent upwelling rate of I cm.yr^-1 and an assumption that the melting process has remained consistent over tens of km at depth. The Teide-Pico Viejo complex lavas have undergone fractionation and mixing to form compositions from basanite to phonolite. Crystallising phases differ in the Pico Viejo series, where amphibole is dominant in the more evolved lavas, and Pico Teide series, where olivine in the major control. The more evolved lavas require assimilation and fractional crystallisation to explain the range in ⁸⁷Sr/⁸⁶Sr. (²³⁰Th/²³⁸U) ranges from 1.004-1.39 and gives information regarding the timescales of differentiation within the magma chambers, not least because the youngest mafic rocks have the highest (²³⁰Th/²³⁸U) and the most evolved phonolites have the lowest. The timescale of differentiation from basanite to phonolite is of the order of 150,000 years, which links to the periodicity of the eruption cycles on the island. A Ra-Th 'pseudo' whole rock isochron gave an age of fractionation for the Montafia Blanca eruption of 2.3 ka ± 80, which is a maximum of 300 years prior to eruption, indicating that fractionation of plagioclase as a possible trigger of an eruption.

Subduction zones are places where one hydrated oceanic plate goes underneath another plate, and releases its fluids into the overlying mantle wedge. Slab-derived fluids play a key role in subduction zone processes. They serpentinize the cold forearc mantle at shallow depths; and deeper, they trigger hydrous mantle melting beneath the arc volcanoes and sometimes at backarc basin (BAB) spreading center. Examining the composition of arc and BAB magmas helps understanding genesis of subduction-related magmas, nature and composition of their mantle sources and slab-derived fluids. However, investigating such processes at shallow subduction zones is challenging, because the cold forearc mantle generally does not melt. Here, I investigate an unusual region in the southernmost Mariana convergent margin in the Western Pacific, near the Challenger Deep. The SE Mariana forearc stretched to accommodate opening of the southernmost Mariana Trough ~5Ma ago, opening the SE Mariana forearc rift (SEMFR) and causing seafloor spreading ~2.7-3.7Ma ago. The subducted slab beneath SEMFR deepens from <50km to ~100km, thus studying SEMFR lavas provides a unique opportunity to understand shallow subduction processes. By examining the major and trace element composition, the Pb-Nd-Sr isotopic ratios and the volatile contents (H2O, CO2, Cl, S, F) of SEMFR basalts, associated glassy rinds and olivine-hosted melt inclusions (Ol-MI) collected during three cruises (YK08-08, YK10-12, TN273), I show that: (i) SEMFR lavas were produced by adiabatic decompression melting of depleted asthenospheric BAB-like mantle at ~30±6.6 km depth and 1224±40oC; (ii) Ol-MI represent hydrous melts trapped by forearc mantle olivines. Xenocrysts were entrained with SEMFR basalts during ascent; (iii) SEMFR mantle flowed from the forearc towards the arc volcanoes and was metasomatized by shallow aqueous fluids; (iv) SEMFR shallow fluids are more aqueous than the fluids released beneath the Mariana arc and Mariana BAB; (v) the aqueous slab-derived fluids and the volatile fluxes are greatest at ~50-100km slab depth, suggesting that the minerals from the subducting plate mostly broke down beneath the arc to release their fluids. Such results provide new insights into shallow subduction processes, as previous studies showed that volatile fluxes and aqueous slab-derived fluids should increase toward the trench.

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