Origin of differentiated volcanic and plutonic rocks from Ascension Island, South Atlantic Ocean

The first phase of felsic magmatism on Ascension Island, in the Son of trachyte and rhyolite domes, coulees, 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 Nd-143/Nd-144 and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitute one of the four distinct basalt and hawaiite suites identified from Ascension bared 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 Sr-87/Sr-86 (>0.704) compared with mafic rocks (Sr-87/Sr-86 <0.703), even when corrected for in situ decay of Rb-87 since eruption. Such hh Sr-87/Sr-86 coupled tenth high Nd-143/Nd-144 signatures do not correspond to known suboceanic mantle reservoirs and in the most part appear to reflect sub-solidus addition of a high Sr-87/Sr-86 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 care, the extremely low Sr contents of the felsic rocks make them particularly susceptible to Sr-isotope modification. Internal (mineral) isochrons for two granite xenoliths give ages of similar to 0.9 and similar to 1.2 Ma, with initial Sr-87/Sr-86 >0.705. Even though the high Sr-87/Sr-86 signature of mast of the volcanic rocks is demonstrably introduced after solidification, the high initial Sr-87/Sr-86 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.