Cosmogenic 3He exposure ages and geochemistry of basalts from Ascension Island,Atlantic Ocean

The chronology and origin of volcanism of Ascension Island, South Atlantic Ocean, is poorly resolved. Here we use in situ produced cosmogenic ³He in olivine and pyroxene phenocrysts from well-preserved lava flows to date the main sub-aerial basalt volcanism on the island. Etching olivine separates in HF/HNO₃ appears to remove a significant proportion of the implanted radiogenic ⁴He contribution. Average exposure ages of each flow corrected for radiogenic He range from 328 ka to 186 ka and are used to refine the chronology and stratigraphy of the island. Magmatic ³He/⁴He ratios derived from in vacuo crushing are in the range of 6.3–7.3 R_A. This range is lower than the neighbouring Mid-Atlantic Ridge segment (6–8°S) but slightly higher than measured in regional ocean islands of St. Helena, Tristan da Cunha and Gough. Combining these data with new trace element data and published radiogenic isotope ratios it appears that the Ascension Island magmatism is a mix of HIMU mantle material, typified by basalts from St. Helena, and depleted MORB-source mantle.