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Experimental tracking of primitive magmas beneath Grenada, Lesser Antilles

High pressure experiments on a high-Mg basalt indicate parental magmas beneath Grenada are oxidised, and resolve the origin of two distinct lavas series

Experimental petrologists at the University of Bristol conducted experiments on lavas from Grenada using a range of experimental apparata to simulate to pressures and temperatures found beneath the island arc volcano. The redox conditions of the experimental runs were measured using the Diamond Light Source synchrotron, UK, and spanned a wide range of oxygen fugacities.  Synthetic replicas of natural rocks produced at moderately oxidising conditions were found to be comparable to the most primitive lavas erupted on Grenada.

Stamper and co-workers were able to use the composition of olivine crystals produced in experiments to calibrate a novel oxybarometer, which uses the partitioning of Fe and Mg between liquid and crystals to measure the oxygen fugacity of an olivine-bearing basalt.

Piston cylinder experiment from Stamper et al. 2014

A synthetic replica of a Grenadan magma produced during a high pressure experiment, as seen through a scanning electron microscope (gl: glass, ol: olivine, qu: quench, spl: splinel)

Experiments from this study also resolve the origin of the geochemically and petrographically distinct M- and C-series lavas, the latter type being unique to Grenada. At high pressures, experimental liquids are able to track the geochemical evolution of the highly magnesian M-series. In contrast, at lower pressures, clinopyroxene saturation is displaced to lower temperatures, relative to olivine, and so residual melts generated at these conditions become enriched in calcium, replicating the characteristic feature of the C-series.

Stamper CC, Melekhova E, Blundy JD, Arculus, RJ, Humphreys, MCS & Brooker, RA (2014) ‘Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles’, Contibutions to Mineralogy and Petrology, 167:954. http://dx.doi.org/10.1007/s00410-013-0954-6

SCIENTIFIC ABSTRACT

A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO2) from Grenada, Lesser Antilles, at anhydrous, H2O-undersaturated and H2O-saturated conditions in the pressure range 1 atm to 1.7 GPa. Fe3+/ΣFe of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO2) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO − 2.5 to + 3.8 show that increasing fO2 mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H2O, respectively. H2O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of KDFe−Mgol−melt when all Fe is assumed to be in the Fe2+ state ( KFeTD ). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO2 to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.

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