The lavas and pyroclastics of Ascension Island contain a suite of coarse grained igneous blocks which range in composition from olivine-gabbro to peralkaline-granite paralleling, but extending beyond the compositional range of the volcanics. The lavas range from alkali-basalt through hawaiite, trachybasalt, trachyandesite and trachyte to comendite. True basalt is relatively rare and there is a scarcity of analyses with 57 < Si02 < 63 wt %. No high pressure mineral assemblages and hence no possible mantle fragments have been found. Petrographic and isotopic data suggest that a suite of gabbros from Dark Slope Crater crystallised from a magma derived from a MORB-like source. The remaining blocks and all the lavas evolved from magmas derived from a less depleted source. The chemical variation seen in the lavas and blocks is best explained by crystal fractionation mechanisms in a relatively shallow magma chamber. The gabbroic blocks exhibit cumulus textures suggesting that they formed by accumulation of settling crystals. The intermediate to acid blocks compare much more closely in composition with the evolved lavas and are probably their slowly cooled equivalents. There is petrographic evidence that partial melting of intermediate coarse grained material gave rise to melts of granitic composition but these are not chemically equivalent to the acid lavas and blocks. A pegmatoid body crystallised in situ and closed system crystal fractionation alone resulted in a very similar sequence of mineral assemblages to the blocks and lavas and a peralkaline final liquid. High 87 S4/ 86 Sr ratios in the evolved lavas and blocks are attributable to contamination by a small quantity of highly radiogenic oceanic sediment. Comparison with other oceanic volcanoes suggests that these differentiation processes are much less important in determining the evolutionary path of the magma than its apparent starting composition.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580775 |
| Date | January 1982 |
| Creators | Harris, Christopher |
| Contributors | Atkins, F. B. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:35be8567-3787-48b9-a985-50979064d89e |
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