Constraints on the behaviour and content of volatiles in Galápagos magmas from melt inclusions and nominally anhydrous minerals

Despite their relatively low concentration in most oceanic basalts, volatile species (e.g. H2O, CO2 and S) have a disproportionately large influence on a wide range of mantle and magmatic processes. However, constraining the concentration of H2O (and other volatiles) in basaltic magmas is not straightforward as submarine glass analyses are influenced by assimilation of hydrothermal brines, and the melt inclusion record is often reset by post-entrapment processes. Nevertheless, in this study we show that it is possible to reconstruct a detailed history of the volatile content of basaltic magmas through integration of multiple discreet volatile records and careful consideration of secondary processes. We present new analyses of volatiles in olivine-hosted melt inclusions, melt embayments and nominally anhydrous minerals (NAMS, clinopyroxene and orthopyroxene) found in basalts erupted on Floreana Island in the south-eastern Galápagos Archipelago. Our results indicate that the Floreana magmas, which are characterised by the most radiogenic Pb and Sr isotope signatures in the Galápagos Archipelago, contain H2O concentrations between 0.4 and 0.8 wt% (at a melt Mg# of 0.65, where Mg# = Mg/(Mg + Fe) molar). These are marginally greater than the H2O contents of magmas beneath Fernandina in the western Galápagos Archipelago (cf. 0.2–0.7 wt% H2O at Mg# = 0.65). While the volatile content of magmas from the western archipelago follow trends defined by concurrent mixing and crystallisation, NAMs from Floreana reveal the presence of rare, volatile-rich magmas (~2 wt% H2O) that form as a consequence of reactive porous flow in mush-dominated magmatic systems beneath the south-eastern Galápagos. Furthermore, the Floreana magmas have similar H2O/light Rare Earth Element ratios to basalts from the western Galápagos but contain F/Nd and Cl/K ratios that are ~2 – 3 times greater, indicating that the mantle source of the Floreana lavas might represent an important halogen reservoir in the Galápagos mantle plume.

Eclogite xenoliths document water cycling at the lithosphere-asthenosphere boundary

<p>The amount of water stored as OH-defects in nominally anhydrous minerals in the deep mantle is poorly constrained and its direct quantification can only be accessed by the analysis of mantle xenoliths. While the vast majority of xenoliths are peridotites and minor pyroxenites, some very rare xenoliths found in kimberlite pipes display an eclogitic mineral assemblage. We investigated three eclogite xenoliths from the 128 m.y. old Robert Victor kimberlite from South Africa that display an assemblage of garnet and omphacite with two samples showing additional kyanite, suggesting low-pressure gabbroic rock as protolith. Thermobarometry estimations based on Fe-Mg partitioning between garnet and pyroxene gives temperatures of 1100-1250 °C. When projected on the cratonic geotherm (Griffin & O’Reilly 2007) an equilibrium depth of 200-210 km is obtained, confirming that these rocks come from the lithosphere-asthenosphere boundary. Therefore these fragments might be key witnesses to understand the deep cycling of water in the mantle. <br><br>This study focuses on the H<sub>2</sub>O quantification in the three rock-forming minerals using Fourier transform infrared spectroscopy (FTIR). Omphacite contains 50-250 ppm H<sub>2</sub>O, kyanite contains 40-60 ppm H<sub>2</sub>O and garnet of only one eclogite contains 40 ppm H<sub>2</sub>O. Garnet and omphacite with the highest OH content are enriched in Ca.<br><br></p><p>The use of advanced mapping and profiling techniques enabled the investigation of the spatial repartition of the OH component in these minerals. High-resolution mapping (5.6 µm) of kyanite reveals diffusive gain of OH at the rim of the crystal that is interpreted as hydration during interaction with the kimberlitic melt. The OH plateau in the core of kyanite must therefore have been acquired previously, suggesting that this is residual OH that has been transported by subduction to the lithosphere-asthenosphere boundary by a once hydrated gabbroic protolith. Our results have implications for the retention of hydrogen over long timescale at the lithosphere-asthenosphere boundary and suggest that the deep cycling of water has been running since Archean times.</p><p> </p><p>Griffin, W. L., & O'Reilly, S. Y. (2007). Cratonic lithospheric mantle: is anything subducted?. <em>Episodes,</em> 30(1), 43-53.</p>