Halogen systematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study

The abundance of halogens (Cl, Br and I) in the Earth's mantle has the potential to provide information about mantle metasomatism and volatile recycling in subduction zones. Basalt melts sample different parts of the Earth's mantle, but few data are available for Br or I in basalt melts, and the relative partitioning behaviour of these elements has not been investigated rigorously. To address these issues, we determined the abundances of Cl, Br and I in enriched Mid-Ocean Ridge Basalt (E-MORB) glasses from Macquarie Island in the southwest Pacific. The Macquarie Island glasses are fairly typical of Pacific MORB with MgO of ∼5.5. to 9wt%, ⁸⁷Sr/ ⁸⁶Sr of 0.70257-0.70276, ¹⁴³Nd/ ¹⁴⁴Nd of 0.51300-0.51306 and ³He/ ⁴He of ∼8Ra. These glasses provide a unique opportunity to investigate halogen partitioning behaviour, because their variable MgO contents and trace element signatures (e.g. La/Sm ∼1.4-7.9) result from different degrees of partial melting and fractional crystallisation. The combined measurement of Br/Cl, I/Cl and K/Cl, together with correlations between Cl and other trace elements, demonstrate that the halogens in the Macquarie Island glasses had a mantle source and were not influenced by seawater contamination. Log-log correlation diagrams indicate that Cl, Br and K were not statistically fractionated during partial melting, crystallisation or degassing of CO ₂ from these melts. The behaviour of I is less well constrained and minor fractionation of I/Cl cannot be precluded during multi-stage melting and enrichment processes. The data indicate the mantle source of the Macquarie Island glasses was characterised by K/Cl of 13±4, Br/Cl of (3.7±0.5)×10 ^-3 and I/Cl of (130±100)×10 ^-6 (2σ uncertainties; weight ratios). The K/Cl ratio of the Macquarie Island glasses is equivalent to the median of all published MORB data. This suggests the Br/Cl and I/Cl values may also be representative of average MORB mantle values.