The halogen (F, Cl, Br, I) and H₂O systematics of Samoan lavas: Assimilated-seawater, EM2 and high-³He/⁴He components

The Samoan mantle plume samples two or more mantle components including an extreme EM2 composition with ⁸⁷Sr/⁸⁶Sr > 0.720 and a primitive component with high ³He/⁴He. The high ⁸⁷Sr/⁸⁶Sr melts have a unique potential to constrain the composition of the EM2 mantle end-member that is commonly attributed to subduction recycling. However, a previous study of H₂O, CO₂, S, F and Cl in Samoan glasses was hampered by the presence of unresolved assimilated sea water. The current study builds on the earlier work by extending the volatile database to include the trace halogens Br and I, and reporting new volatile data for additional glasses with ⁸⁷Sr/⁸⁶Sr up to 0.7125 and ³He/⁴He up to 15 Ra (Ra is the atmospheric ³He/⁴He ratio of 1.39×10^-6).The selected glasses with MgO of 4.0 to 6.5 wt.% have CO₂ concentrations of 4 to 200 ppm that reflect degassing of CO₂ on the seafloor. The glasses contain 0.7-1.9 wt.% H₂O, 880-1870 ppm F, 490-1790 ppm Cl, 1.9-7.2 ppm Br, 19-130 ppb I and 0.6-1.7 wt.% K. Correlations between the concentrations of these elements suggest the melts retain H₂O concentrations close to pre-degassing values and demonstrate the melts have been variably affected by assimilation of seawater-derived brines. The brines are indicated to have had salinities of 55±15 wt.% salt, F/Cl ratios close to seawater (e.g. <0.0001), Br/Cl ratios ~40% higher than seawater, and I/Cl ratios ten times the seawater value. It is calculated the melts assimilated brine fractions of 0 to 0.45 wt.%, which contributed up to ~30% of the total H₂O and up to ~70% of the total Cl in the melts.After accounting for the effects of brine assimilation, the Samoan melts are suggested to have a fairly constant magmatic Cl/K of 0.05±0.2, which is lower than the median MORB value. Assimilation-corrected H₂O/Ce values are not correlated with ⁸⁷Sr/⁸⁶Sr, but the sample with the highest ⁸⁷Sr/⁸⁶Sr of 0.7125 has an assimilation-corrected H₂O/Ce of 83±3, that is significantly lower than typical MORB values of 150-300. These data confirm that the Samoan EM2 source is depleted in H₂O, and perhaps Cl, relative to lithophile elements of similar compatibility.The glasses deemed free of seawater components, including the least enriched sample with ³He/⁴He of 15 Ra and the most enriched sample with ⁸⁷Sr/⁸⁶Sr of 0.7125, have indistinguishable Br/Cl and I/Cl that are within the narrow range determined for EM1 and EM2 glasses from Pitcairn and Society seamounts and mid-ocean ridge basalt glasses (e.g. Br/Cl=0.0028±0.0006; I/Cl=0.00006±0.00003; 2σ). The uniformity of mantle Br/Cl and I/Cl could be explained if halogens are not significantly subducted into the mantle. However, an alternative possibility is that the subduction zone 'filter' controls the abundance ratios of halogens recycled into the mantle, and recycled halogens with associated H₂O, have been circulated throughout the mantle.