Origin of Differentiated Volcanic and Plutonic Rocks from Ascension Island, South Atlantic Ocean

• Published in: Journal of Petrology
• Main Authors: Kar, A., Weaver, B., Davidson, J., Colucci, M.
• Format: Text
• Language: English
• Published: Oxford University Press 1998
• Subjects: Articles; South Atlantic Ocean
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/39/5/1009; https://doi.org/10.1093/petroj/39.5.1009

The first phase of felsic magmatism on Ascension Island, in the form of trachyte and rhyolite domes, coulées, lava flows, and pyroclastic deposits, created the central and eastern parts of the island between about 1.0 and 0.56 my ago. The geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallization of high Zr/Nb mafic magmas as evidenced by identical 143Nd/144Nd and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitute one of the four distinct basalt and hawaiite suites identified from Ascension based on trace element characteristics. Syenite, monzonite, and granite xenoliths associated with the felsic magmatism are interpreted as cumulate rocks from, and intrusive equivalents of, fractionating felsic magmas. Many of the felsic rocks are characterized by high 87Sr/86Sr (>0.704) compared with mafic rocks (87Sr/86Sr <0.703), even when corrected for in situ decay of 87Rb since eruption. Such high 87Sr/86Sr coupled with high 143Nd/144Nd signatures do not correspond to known suboceanic mantle reservoirs and in the most part appear to reflect sub-solidus addition of a high 87Sr/86Sr component. This component is probably a seawater-derived fluid that might be added at the surface from wind-blown spray, or, more likely, at depth through hydrothermal circulation (fluids with high Sr contents have been recovered from fractures in a 3126-m-deep geothermal well). In either case, the extremely low Sr contents of the felsic rocks make them particularly susceptibleto Sr-isotope modification. Internal (mineral) isochrons for two granite xenoliths give ages of ∼0.9 and ∼1.2 Ma, with initial 87Sr/86Sr >0.705. Even though the high 87Sr/86Sr signature of most of the volcanic rocks is demonstrably introduced after solidification, the high initial 87Sr/86Sr values of the granite xenoliths suggest that hydrothermally altered pre-existing volcanic basement may have been melted or assimilated during differentiation of some of the felsic magmas.