Research

The minor element composition of olivine crystals are commonly used to evaluate the lithological properties of the underlying mantle. However, few studies consider the influence of crustal processes on the composition of magmatic olivines.

To address the influence of crustal processes, we developed a simple numerical model of magma recharge and diffusive re-equilibration. Results indicate that multiple episodes of magma recharge and mixing can lead to the formation of high-Ni and low-Ca olivine crystals, without the need to invoke a lithologically heterogeneous source.

Gleeson, M., and Gibson, S.A., (2019). Crustal controls on apparent mantle pyroxenite signals in ocean-island basalts. Geology, 47(4), pp.321-324, 10.1130/G45759.1

Variations in the Fe-isotope ratio of oceanic basalts have been hypothesised to results from variations in the melt fraction and/or oxygen fugacity of the mantle, the presence of garnet in the mantle residue, and the lithological properties of the mantle source.

Basalts from the plume-influenced Galapagos Spreading Centre display a large range in Fe-isotope ratios, comparable to that observed in a global compilation of MORB and OIBs. We use 2-component mantle melting models, coupled to a Markov Chain Monte Carlo algorithm, to show that the presence of a lithologically distinct component in the Galapagos mantle plume provides the best explanation to the Fe-isotope and trace element composition of the Galapagos Spreading Centre basalts. As such, we suggest that a recycled crustal component is present in the Galapagos plume.

Gleeson, M., Gibson, S.A. and Williams, H.M., (2020). Novel insights from Fe-isotopes into the lithological heterogeneity of Ocean Island Basalts and plume-influenced MORBs. Earth and Planetary Science Letters, 535, p. 1-13, 10.1016/j.epsl.2020.116114

The Galapagos mantle plume is high heterogeneous. However, only limited volatile data exists for several regions of the archipelago. We analysed melt inclusions and nominally anhydrous minerals from scoria and xenolith samples collected on Isla Floreana in the southern Galapagos, and compared our results to published data from Isla Fernandina in the western Galapagos.

Results indicate that there is little difference in the water content of magmas prior to eruption between the two locations, despite their highly contrasting trace element and isotopic compositions. Nevertheless, the nominally anhydrous mineral data, especially from the xenoliths, indicates that reactive porous flow leads to the generation of volatile-rich magmas in crystal-rich magma storage regions beneath Isla Floreana.

Gleeson, M., Gibson, S.A., Stock, M.J., and EIMF (in prep). New constraints on the volatile content of the Galápagos mantle plume from melt-inclusions and Nominally Anhydrous Minerals. In prep for submission to GCA.

The characteristics of magma storage regions beneath the most active volcanic systems in the world (such as Kilauea) are relatively well constrained. Far fewer constraints, however, exist for less frequently active volcanic systems that are supplied by a lower flux of magma from the underlying mantle. To address this, we carried out a detailed study of Isla Floreana in the southern Galapagos, which is supplied by a very low flux of magma from the underlying mantle.

After removing crystals that have been influenced by reactive porous flow processes in highly crystalline magma storage regions, we use mineral-melt barometers to show that magma beneath Isla Floreana is primarily stored below the Moho, in the upper mantle. Comparison of our results to a global compilation of barometric estimates from ocean island volcanoes worldwide indicates that the flux of magma from the underlying mantle likely represents a first-order control on the depth of magma storage.

Gleeson, M., Gibson, S.A., and Stock, M.J., (in review). Upper mantle mush zones beneath low melt flux ocean island volcanoes: insights from Isla Floreana, Galápagos. J.Pet. (preprint available on Earth ArXiv https://doi.org/10.31223/osf.io/4w8vr)

Fieldwork to the Galapagos Archipelago in 2017 was used to collect data and samples from the 2015 eruption of Volcan Wolf. Results indicate that the 2015 eruption was one of the largest eruptions in the Galapagos over the last 100 years.

Petrological analysis of the erupted products indicates that magma was predominantly stored at >7 km depth prior to eruption. This indicates that magma storage beneath Volcan Wolf is not truly 'transcrustal', although a shalow sill at ~1 km depth was identified by InSAR analysis of volcanic inflation in the 20 years prior to eruption.

Stock, M.J., Bagnardi, M., Neave, D.A., Maclennan, J., Bernard, B., Buisman, I., Gleeson, M., and Geist, D., (2018). Integrated petrological and geophysical constraints on magma system architecture in the western Galápagos Archipelago: insights from Wolf volcano. Geochemistry, Geophysics, Geosystems, 19(12), pp.4722-4743, 10.1029/2018GC007936

Bernard, B., Stock, M.J., Coppola, D., Hidalgo, S., Bagnardi, M., Gibson, S., Hernandez, S., Ramón, P. and Gleeson, M., (2019). Chronology and phenomenology of the 1982 and 2015 Wolf volcano eruptions, Galápagos Archipelago. Journal of Volcanology and Geothermal Research, 374, pp.26-38, 10.1016/j.jvolgeores.2019.02.013

Many basaltic volcanoes worldwide erupt a very narrow range of magma compositions over hundreds of thousands of years. Termed 'Monotonous Basaltic Volcanoes', these systems were previously believed to be underlain by large magmatic mush systems that hold the composition of ascending magmas constant over a very narrow compositional range. However, a recent petrological investigation of lavas from Volcan Wolf challenges this view.

Plagioclase and clinopyroxene crystals display compositions that are more evolved than the calculated mineral compositions in equilibrium with the erupted lavas. Along with the presence of quartz in the Wolf tephra, this indicates that andesitic - rhyolitic magmas are present in the sub-volcanic system of Volcan Wolf. Additionally, this raises the possibility that, given a change in the flux of magma from the mantle, a transition to explosive, rhyolitic magmatism might occur in the future.

Stock, M.J., Geist, D., Neave, D.A., Gleeson, M., Bernard, B., Howard, K., Buisman, I. and Maclennan, J., (in press). The hidden evolved melts beneath monotonous basaltic shield volcanoes in the Galápagos Archipelago. Nature Communications.

The conditions of magma storage beneath volcanic centres along the MER have, until recently, remained poorly known. Using Rhyolite-MELTS, we performed a series of fractional crystallisation models at different conditions and used a simple statistical routine to determine the fit between the model predictions and the composition of erupted lavas at Aluto volcano, Ethiopia. In doing so, we were able to show that magma is primarily stored in the upper crust (~5 km), coinciding with prior geophysical estimates.

Our work also demonstrated that the major element composition of the evolved peralkaline rhyolites along the MER can be generated by fractional crystallisation alone, without the need for assimilation of continental crust.

Gleeson, M., Stock, M.J., Pyle, D.M., Mather, T.A., Hutchison, W., Yirgu, G. and Wade, J., (2017). Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models. Journal of Volcanology and Geothermal Research, 337, pp.44-61, 10.1016/j.jvolgeores.2017.02.026

The contribution of crustal assimilation to the generation of highly evolved peralkaline rhyolites was evaluated using new analyses of trace elements, radiogenic isotope ratios, and oxygen isotopes in basalts to pantellerites from a range of volcanic systems along the MER. Results indicate that the peralkaline rhyolite magmas are generated by protracted fractional crystallisation. Additionally, we demonstrate the the degree of fractionation is controlled by the flux of magma into the system.

Hutchison, W., Mather, T.A., Pyle, D.M., Boyce, A.J., Gleeson, M., Yirgu, G., Blundy, J.D., Ferguson, D.J., Vye-Brown, C., Millar, I.L. and Sims, K.W., (2018). The evolution of magma during continental rifting: New constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes. Earth and Planetary Science Letters, 489, pp.203-218, 10.1016/j.epsl.2018.02.027