Halogen and trace element analysis of carbonate-veins and Fe-oxyhydroxide by LA-ICPMS : Implications for seafloor alteration, Atlantis Bank, SW Indian Ridge

Halogens are critical elements for transporting metals in hydrothermal solution and tracing the sources of fluids and volatiles in subduction zones. This study tested the suitability of LA-ICPMS for simultaneous measurement of Cl, Br and I with selected trace elements in altered gabbros recovered from the Atlantis Bank core complex close to the SW Indian Ridge. The aim was to better understand the causes of I-enrichment in low temperature carbonate and Fe-oxyhydroxide alteration, which is intensely developed in core recovered from between ~100 and ~600 mbsf down Hole U1473A drilled during Expedition 360 of the International Ocean Discovery Program (IODP). The analysis of reference materials including scapolites, Durango apatite, USGS and NIST reference glasses and volcanic glasses demonstrate Cl, Br and I can be measured in most minerals, but accuracy is limited by matrix dependent laser backgrounds (false signals of Cl, Br and I generated during sample ablation). Silicate reference glasses define reproducible ‘laser backgrounds’ of ~320 μg/g Cl and ~1.7 μg/g Br that point to limits of quantification of ~200 μg/g Cl and ~1 μg/g Br in silicate glasses after correction for laser backgrounds. In contrast, laser backgrounds are much lower and possibly negligible for carbonate, suggesting limits of quantification as low as ~50 μg/g Cl and 0.1 μg/g for Br, dependent on instrument sensitivity. Laser backgrounds for I varied between minerals and analytical sessions, there was no laser background for I in apatite and under favourable conditions silicate glasses have laser backgrounds of <1 μg/g. In addition to the laser backgrounds, we report a correction for interference of doubly charged rare earth elements on Br measurement, which significantly improves data quality for some samples. The Atlantis Bank alteration minerals investigated include calcite veins and Fe-oxyhydroxide, which is intermixed with, and cut by veinlets of, calcite and smectite. The Fe-oxyhydroxide contains 100 s–1000s μg/g Cl, 10s μg/g Br and 5–30 μg/g I and it is strongly enriched in other trace elements including B and As. Most calcite veins contain <100–200 μg/g Cl, <0.4–0.9 μg/g Br and 5–26 μg/g I. Among the halogens, I is uniquely correlated with divalent cations in calcite including Mg and Sr, consistent with its incorporation into the carbonate lattice as iodate. In most cases smectite contains less I than coexisting calcite or Fe-oxyhydroxide, but three smectite grains in one sample are exceptionally enriched with 150–240 μg/g I. The in situ analyses support the attribution of exceptional iodine enrichment of Atlantis Bank lithologies to preferential adsorption of iodate over halides by Fe-oxyhydroxides and subsequent remobilisation with preferential fixing of iodate in carbonate-dominated alteration. The data demonstrate the utility of LA-ICPMS for in situ halogen studies. Iodine in carbonate veins can provide information about fluid oxidation state and fluid-rock reaction history. However, further work is required to test if Cl and Br in carbonate can provide information about fluid sources.