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Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles

DOI: 10.1007/s00410-013-0954-6 DOI Help

Authors: Charlotte Stamper (University of Bristol) , Elena Melekhova (University of Bristol) , J. D. Blundy (University of Bristol) , R. J. Arculus (Australian National University) , M. C. S. Humphreys (Durham University) , Richard Brooker (University of Bristol)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Contributions To Mineralogy And Petrology , VOL 167 (1)

State: Published (Approved)
Published: January 2014
Diamond Proposal Number(s): 8008 , 8726

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. Fe 3+ /?Fe of high-pressure experimental glasses as measured by ?XANES 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. ?XANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of K D Fe?Mg ol?melt when all Fe is assumed to be in the Fe2+ state (K Fe T D ). 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.

Journal Keywords: Oxidised Phase Relations Of A Primitive Basalt From Grenada; Lesser Antilles

Subject Areas: Environment

Instruments: I18-Microfocus Spectroscopy

Added On: 22/01/2014 11:27

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