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

Control and monitoring of oxygen fugacity in piston cylinder experiments

“…Polished cross-sections of post-run experimental assemblies revealed clouds of graphite in the Pyrex adjacent to furnace damage, consistent with interpretations from Brooker et al (1998). Even when experiments are pressurised carefully and furnaces and spacer materials appear to remain intact, Stamper et al (2014) demonstrated that infiltration of carbon occurs through both walls of a double capsule surrounded by crushable alumina. Although their experiments were not specifically designed to test for carbon infiltration, the results were clearly not systematic, showing no correlation with run duration, P or T. This is also consistent with some sort of mechanical degradation, rather than simple diffusional control.…”

“…Grey area denotes the 'no hydrogen loss field' as discussed in Supplementary Material. a Despite obvious fH 2 -equilibrium between all sensors in Pyrex assemblies, slow hydrogen diffusion takes place and causes oxidation of about 0.25 log fO 2 units within ~12 h. In contrast, results for MgO in b showed clear evidence of faster hydrogen loss, in comparison with Pyrex, reaching equilibrium only after ~30 h and a resulting oxidation of ~0.5 log fO 2 units carbon (Brooker et al 1998(Brooker et al , 1999Stamper et al 2014). Brooker et al (1998) examined the phenomenon of sample 'blackening' in high-temperature experiments (>1,400 °C) and concluded that elemental carbon infiltrates from the graphite furnace, noting in particular that accidental direct contact between the metal capsule and graphite can greatly accelerate carbon infiltration.…”

“…The majority of the reference glasses were anhydrous, 1 atm charges (MORB glasses of Cottrell et al 2009), but the calibration also included Smithsonian standards VG-2 and A99 (natural basaltic glasses) and one hydrous, highpressure piston cylinder experiment (1 GPa, 1,350 °C, NNO). For further details of the µXANES analytical methods, including data processing, see Stamper et al (2014) and Supplementary Material therein.…”

“…Commonly expressed as oxygen fugacity (fO 2 ), it controls the valence of multivalent elements, which in turn influences their chemical and physical behaviour in magmas (Lee et al 2005;Mallmann and O'Neill 2009;Jenner et al 2010). Prevailing fO 2 also exerts a strong influence over phase relations and mineral compositions (Feig et al 2010;Stamper et al 2014). In the deep mantle, partial melting may be initiated by redox reactions (Frost and McCammon 2008), and thus, fO 2 can also affect rheological properties of the dynamic Earth (Taylor and Green 1988;Kress 1997;Rohrbach et al 2007;Foley 2011;Yaxley et al 2012).…”

Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths

“…7). This is reminiscent of the M-and C-series lavas of Grenada whose CaO versus MgO whole rock contents show two separate liquid lines of descent (Stamper et al 2014a), attributed to variations in the differentiation depth of a common picritic parent (Stamper et al 2014b). A geochemical boundary between St. Vincent and the Grenadines has been suggested by on the basis of the restriction of M-and C-series rocks to the southern part of this arc segment.…”