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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.



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.