Geochemical characteristics of volcanic rocks from ascension island, South Atlantic Ocean

Abstract

The volcanic rocks of Ascension Island are a transitional to mildly alkaline basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite suite. Although the overall major element variations in the suite are consistent with derivation of the more evolved compositions by crystal fractionation from parental basalt magma, trace element variations among basalt and hawaiite compositions define four distinct magma types. Three of these types are discriminated by variations in Zr/Nb. A number of hawaiite scoria cones and associated flows restricted to the southwestern part of the island have low Zr/Nb (4.1), whereas basalt scoria and flows distributed over the southeastern part of the island have high Zr/Nb (≈6.0), and basalt and hawaiite scoria cones and associated flows widely distributed over the remainder of the island have intermediate Zr/Nb (≈ 5.0). The fourth magma type is a subset of the intermediate Zr/Nb group, but has high Ni and Sr relative to Zr compared to the rest of the group; the flows defining this magma type are related to a single vent, Dark Slope Crater, in the southwestern part of the island. The mugearite and benmoreite flows and scoria are exclusively derived by crystal fractionation of intermediate Zr/Nb group parent basalt magma. Field relationships suggest non-overlapping phases of eruption of the different mafic magma types. The oldest exposed mafic lava flows are of high Zr/Nb basalt; limited KAr age data suggest that this magma type may have erupted between ca 0.66 and 0.35 Ma. Subsequently, there was localised eruption of the Dark Slope Crater magma type, followed by equally localised eruption of the low Zr/Nb magma type. The most recent eruptions (which have continued to possibly within the last few hundred years) have been much more widespread and of the intermediate Zr/Nb magma type.