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Can experimental oxygen fugacity be controlled?

More precise data is required to determine whether a novel experimental technique designed to control experimental fO2 is effective under H2O-undersaturated conditions.

Piston cylinder experiments typically employ noble metals as sample containers due to their low reactivity and high melting temperature. Many years of experimental research has demonstrated that choice of capsule metal often involves a payoff between melting temperature and the ability to control important compositional parameters (e.g., loss of Fe and H2O).

Oxygen fugacity (ƒO2) is intrinsically linked to these variables and is a key property of an experiment because it controls the valence of multivalent elements. In turn, this alters phase relations and mineral compositions, and affects speciation of other volatiles elements such as sulphur.

Earlier work by Jakobsson (2012) presented a novel experimental technique for controlling ƒO2 by physically separating a redox buffer from an Au-Pd alloy inner capsule with a hydrogen-permeable barrier. This technique relies on hydrogen fugacity being equal in both in the outer and inner capsule as fixed by the solid buffer; however, the original study failed to take into account the H2O-undersaturated nature of the experimental melts, which actually act to reduce the ƒO2 imposed on the inner capsule.

Jakobsson 12 capsule setup

Diagram of the Jakobsson (2012) capsule setup

This amendment acknowledges this oversight, and corrects the measured ƒO2 in the original study for H2O-undersaturation. The authors conclude that whilst this sample assembly is capable of controlling fO2 in H2O-saturated runs, more precise analysis of other parameters (such as the activity of Fe and H2O in the melt) are needed to assess whether the same holds true for H2O-undersaturated variants.

Jakobsson, S., Blundy, J., & Moore, G. (2014). Oxygen fugacity control in piston-cylinder experiments: a re-evaluation. Contributions to Mineralogy and Petrology, 167(6), 1-4. http://dx.doi.org/10.1007/s00410-014-1007-5

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Cumulate xenoliths betray small scale changes in melt composition and magma storage conditions

A texturally diverse suite of cumulates beneath Grenada, Lesser Antilles, are produced at shallow depths and show marked differences from comparable rocks in the same volcanic arc

Primitive melts produced beneath island arc volcanoes are rarely erupted at the surface in their original form, instead charting a huge variety of evolved compositions and testifying to the influence of intracrustal processing during magmatic ascent. The study of cumulates (coarse-grained igneous rocks) that sample directly from magma storage regions offers a chance to glimpse a ‘snapshot’ of this magmatic evolution.

A new CRITMAG-funded study by Stamper and co-workers combines major element analysis of mineral compositions in plutonic xenoliths and volcanic rocks with data from previous experimental studies. The data is used to explore the differentiation of mantle-derived magmas beneath volcanic island of Grenada, Lesser Antilles.

Photomicrograph (PPL) of poikilitic hornblende gabbro. Hornblende oikocrysts containing inclusions of clinopyroxene, spinel and iddingsitised olivine, with interstitial plagioclase.

Photomicrograph (PPL) of poikilitic hornblende gabbro. Hornblende oikocrysts containing inclusions of clinopyroxene, spinel and iddingsitised olivine, with interstitial plagioclase.

They find that observed diversity in cumulate assemblage and texture is caused by variability in parental melt composition and post-cumulus interaction with hydrous evolved melts. The whole plutonic suite is produced in a narrow pressure window (P = 0.2 – 0.5 GPa) at ∼ 850 – 1050◦C, tracing a shallow (depth ≤15km) section of a vertically extensive volcanic system. Major element barometers and experimental phase relations indicate that the source magma underwent equilibration with a garnet lherzolite source at depth of ≥55 km.

Grenada cumulates are notably different from those found on the neighbouring island of St Vincent, which lies only 120 km to the north. At Grenada, lower magmatic H2O contents are manifest are in plagioclase-rich cumulates and aluminous spinels. The contrast in assemblages and mineral chemistry of cumulate xenoliths from the two islands demonstrate the effect of small scale changes in melt composition and magma storage conditions.

Stamper CC, Blundy JD, Arculus RJ, & Melekhova E. (2014) ‘Petrology of Plutonic Xenoliths and Volcanic Rocks from Grenada, Lesser Antilles’. Journal of Petrology, 55(7), 1353-1387. http://dx.doi.org/10.1093/petrology/egu027

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