Halogens in Volcanic Systems

The halogens (Cl, Br, I) are moderately volatile elements that exhibit incompatible behaviour during melting, and are hydrophyllic - in addition, iodine is strongly fractionated by biological processes. Although the halogens share similar geochemical properties to the noble gases, the heavy halogens in particular have been underutilized as tracers, because of the analytical difficulties related to determining their low abundances in geological materials. This research presents the first known data on HIMU OIBs (Azores, Canary Islands) and extends the known range for EM1 (Tristan da Cunha) and EM2 (Tristan da Cunha and São Miguel) type basalts. Samples were selected from the Tristan da Cunha group (EM1/EM2), the Azores archipelago (HIMU and EM2), and the Canary Islands (HIMU). The samples were selected on the basis of mantle reservoir and 3He/4He (low, MORB-like (Mid-Ocean Ridge Basalt), moderately high RA), to enable a diverse suite of OIB samples to be analysed. Olivine and pyroxene mineral separates from basalts have been analysed from each of the ocean island groups. The halogens are assumed to be mainly sited in melt (Tristan, Azores, Canaries) and fluid (Canaries) inclusions observed within the mineral phases; noble gases were liberated by a combination of crushing and stepped heating. The Tristan da Cunha basalts show Br/Cl and I/Cl molar ratios that extend from the range previously determined for MORB samples up to the high values characteristic of marine pore fluids, with maximum values: Br/Cl = 3.56x10-3 and I/Cl = 3530x10-6. This range is interpreted to represent a mixing trend between a MORB-like and a subducted marine pore fluid or I-rich sediment signature, in the source of the Tristan da Cunha basalts. The Azores basalts yield I/Cl (0.54-1480)x10-6, with the Br/Cl ratios showing (0.39-3.37)x10-3. There appears to be some variation between islands, observed in the I/Cl values – with the samples from Graciosa showing the greatest range in I/Cl and Br/Cl ratios, and samples from the western islands (Corvo and Flores) showing the most I-rich ratios, the islands on the Terceira Rift showing the greatest range in halogen ratios, with the islands to the south of the Terceira Rift yielding MORB-like halogen ratios. The variation within the group is proposed to be controlled by the interacting nature of the plume and tectonic environment. Crushing analyses show that the Canaries basalts have a similar range in Br/Cl (0.32-2.00)x10-3 to MORB, but extend to much higher I/Cl values (154-18700)x10-6. The data overlap with data from marine sediments. Analyses indicate the presence of a fluid component, which is not seen in the other OIBs. Heating analyses show a more MORB-like component together with the presence of a marine pore fluid component, with Br/Cl (0.14-173)x10-3 and I/Cl values (12.7-4440)x10-6. The EM1 and EM2 basalts show evidence for recycling of metasomatised SCLM - with a marine pore fluid signature – mixing with a MORB-like component, whereas the HIMU basalts exhibit a three component system with a MORB, OIB, AOC, and marine pore fluid signature