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Geological Society, London, Special Publications; 1987; v. 30; p. 269-272;
DOI: 10.1144/GSL.SP.1987.030.01.12
© 1987 Geological Society of London

Magma and fluid evolution in the lavas and associated granite xenoliths of Ascension Island

Chris Harris*

Department of Earth Sciences, Parks Road, Oxford OX1 3PR, U.K.
Centre de Recherches Petrographiques et Geochimiques, BP20, 54501 Vandoeuvre-les-Nancy Cédex, France

Simon M.F. Sheppard

Centre de Recherches Petrographiques et Geochimiques, BP20, 54501 Vandoeuvres-les-Nancy Cédex, France

The mildly alkaline lavas of Ascension Island evolved in a H2O-undersaturated environment. H2O comprises less than 0.4 wt.% of glassy equivalents of the most evolved lavas (comendites) for which there is no evidence that it has been lost by degassing. In contrast, peralkaline granite xenoliths which are chemically similar to the evolved lavas contain silicate glass and fluid inclusions which demonstrate that the original granite body crystallized from a water-saturated magma. Microprobe analysis of the glass inclusions gives low totals suggesting that the H2O content was high (5–8 wt.%). Amphibole {delta}D values for the granite xenoliths are significantly different from the whole-rock {delta}D values of the comenditic obsidians while whole-rock {delta}18O values are similar. It is suggested that the high water content in the original granite magma is due to sea-water addition and that this explains these differences. Sea-water interaction with high-level magma chambers on Ascension may have been the cause of the extensive pyroclastic volcanism which occurred on the island.


* Department of Geochemistry, University of Cape Town, Rondebosch 7700, South Africa