A Quantitative Study of Magmatic Inclusions in the Plutonic Ejecta of Ascension Island

Plutonic blocks in the volcanic and pyroclastic rocks of AscensionIsland contain a variety of silicate melt and fluid inclusions,mainly in the intermediate (monzonite-syenite) and granite types.The silicate melt inclusions consist of trapped granitic meltwith a small contraction bubble. The fluid inclusions are: (i)highly saline (40 to > 67 wt. per cent NaCI) aqueous inclusions;(ii) low density CO₂ + H₂O inclusions, predominantly vapourand (iii) high density approximately pure H₂O inclusions. Mixedsilicate melt-saline aqueous fluid inclusions also occur showingthat the original magma became saturated with respect to salineaqueous fluid and a magmatic origin is thus indicated for thesaline fluid inclusions. Microprobe analysis of the silicatemelt inclusions suggests that the original magma may have containedgreater than 5 wt. per cent H₂O before saturation was reachedand homogenization temperatures show the trapping of the silicatemelt took place between 715-790?C. Separated amphiboles fromthe granites have D values which are significantly differentfrom the whole rock D values of the dry but otherwise chemicallysimilar obsidians. Sheppard & Harris (1985) interpret thisdifference as reflecting a sea water source for the water inthe gramtes. Qualitative and quantitative study of the fluidinclusions (extending the earlier work of Roedder & Coombs,1967) suggests that at least one episode of fluid immiscibility(into aqueous liquid+vapour, i.e. ‘boiling’) occurred.This resulted in a loss of H₂O via the vapour and increasedthe salinity of the remaining liquid. The initial fluids belongedto the system H₂O-CO₂-NaCl-KCl and the CO₂ was strongly partitionedinto the vapour phase during fluid unmixing, hence X_co2 decreasedas the fluid evolved. A suite of gabbroic blocks from Dark SlopeCrater contain pure CO fluid inclusions of just higher thancritical density. Assuming a trapping temperature of 1000–1200?Cthe pressure must have been > 18 kb–28 kb or 7-11 kmin depth, i.e. well below the base of the Ascension Cone. Thepure CO₂ inclusions in this suite of gabbros and the fluidsin the H₂O-CO₂NaCl-KCl system would seem to require entirelydifferent origins and it may be significant that these gabbrosshow a different isotopic Pb signature than do the volcanicsand other blocks.