Abstract
The oxidation states of magmas provide valuable information about the release and speciation of volatile elements during volcanic eruptions, metallogenesis, source rock compositions, open system magmatic processes, tectonic settings and potentially titanium (Ti) activity in chemical systems used for Ti-dependent geothermometers and geobarometers. In this paper we explore the use of Mn in apatite as an oxybarometer in intermediate and silicic igneous rocks. Increased Mn concentrations in apatite in granitic rocks from the zoned Criffell granitic pluton (southern Scotland) correlate with decreasing Fe2O3 (Fe3+) and Mn in the whole-rock and likely reflect increased Mn2+/Mn3+ and greater compatibility of Mn2+ relative to Mn3+ in apatite under reduced conditions. Fe3+/Fe2+ ratios in biotites have previously been used to calculate oxygen fugacities (fO2) in the outer zone granodiorites and inner zone granites where redox conditions have been shown to change from close to the magnetite-hematite buffer to close to the nickel-nickel oxide buffer respectively (Stephens et al., 1985). This trend is apparent in apatite Mn concentrations from a range of intermediate to silicic volcanic rocks that exhibit varying redox states and are shown to vary linearly and negatively with log fO2, such that logfO2=-0.0022(±0.0003)Mn(ppm)-9.75(±0.46)Variations in the Mn concentration of apatites appear to be largely independent of differences in the Mn concentration of the melt. Apatite Mn concentrations may therefore provide an independent oxybarometer that is amenable to experimental calibration, with major relevance to studies on detrital mineral suites, particularly those containing a record of early Earth redox conditions, and on the climatic impact of historic volcanic eruptions.
Highlights
43 Redox conditions play a pivotal role in determining the metallogenic properties of plutonic bodies (Ishihara, 1977, 1981; Belvin, 2004) and they control the molecular speciation of volcanic gases containing sulphur, carbon and hydrogen exsolved on eruption (e.g. Scaillet et al, 1998)
For example Ce anomalies in zircon have recently been experimentally calibrated at varying fO2 conditions by Trail et al, (2011) in order to estimate the oxidation state of Hadean magmas. 90 In this paper, we have evaluated the redox conditions in granitic rocks from the post91 Caledonian Criffell pluton in southern Scotland using Ce anomalies in zircon and Fe3+/Fe2+ 92 ratios in biotite (Stephens et al, 1985)
We investigate the use of Mn concentrations in 94 coexisting apatites to record changing redox conditions in this pluton
Summary
43 Redox conditions play a pivotal role in determining the metallogenic properties of plutonic bodies (Ishihara, 1977, 1981; Belvin, 2004) and they control the molecular speciation of volcanic gases containing sulphur, carbon and hydrogen exsolved on eruption (e.g. Scaillet et al, 1998). It is widely accepted that calc-alkaline magmas, found mainly around active subduction zones, are more oxidised than their tholeiitic counterparts (Gill, 1981; Arculus, 2003; Carmichael, 1991; Kelly and Cottrell, 2009; Lee et al, 2010) This is reflected in Fe depletion during differentiation of calc-alkaline magmas that results from the stabilisation of Fe3+-bearing oxide minerals under more oxidising conditions (Gill, 1981). 90 In this paper, we have evaluated the redox conditions in granitic rocks from the post Caledonian Criffell pluton in southern Scotland using Ce anomalies in zircon and Fe3+/Fe2+ ratios in biotite (Stephens et al, 1985) The former method is shown to result in large uncertainties in calculated fO2 values. Reliable estimates of the redox conditions of historic volcanic eruptions may enable better estimates of their climatic impact
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