Halogens in Ocean Island Basalts

The halogen composition of thirteen basalts from ocean island settings, including nine samples from Tristan da Cunha, have been analysed using an extension of the Ar-Ar technique. The ocean island basalts (OIBs) have a subducted halogen signature, showing a strong overlap with I/Cl and Br/Cl in marine pore fluids. The OIBs show a mixing between mid-ocean ridge basalt (MORB) / bulk Earth (BE) and sediment, or marine pore fluid end-members. I/Cl values for the OIBs extend up to 4.1 x 10-3; Br/Cl values range up to 3.4 x 10-3. The highest values are observed in the Inaccessible Island samples, suggesting that the mantle source has evolved over time. Petrological analysis suggests a similar composition for the Tristan and Inaccessible Island basalts; however, Tristan eruptions appear to have been more effusive and less gaseous, due to the absence of vesicles. Cl/K values observed show a mixing between MORB and OIB. The low 3He/4He values suggest that U-rich sediment is being recycled into the mantle source. The 40Ar/36Ar values are air-like in composition, suggesting that dissolved air is being recycled within the sediment pore fluid. Estimates of halogen abundances in the Tristan mantle source are 90 ppm Cl, 0.46 ppm Br and 0.25 ppm I, giving Br/Cl 2.3 x 10-3 and I/Cl 776 x 10-5. A maximum of 400 ppm sediment in the Tristan mantle source is needed to obtain the halogen ratios observed