Triple-halogen (Cl-Br-I) fluid inclusion LA-ICP-MS microanalysis to unravel iodine behavior and sources during marine fluid infiltration into the basement in unconformity settings

Abstract

Halogen signatures of hydrothermal fluids are a powerful tool for deciphering fluid provenance and reconstruction of key ore-forming mechanisms such as fluid mixing processes. Analytical protocols for LA-ICP-MS fluid inclusion analysis of Cl and Br concentrations have existed for over 10 years, but Br/Cl ratios alone do not provide unambiguous evidence of fluid provenance. However, recent methodological advances now allow also for quantification of I by LA-ICP-MS analysis in single fluid inclusions, significantly expanding the applicability of halogens for unambiguous fluid provenance analysis. However, owing to the novelty of the fluid inclusion triple-halogen LA-ICP-MS method, and due to the notorious difficulties caused by the generally low I concentrations in hydrothermal fluids, reference Cl-Br-I data of many crustal fluid types remains scarce, even when including data from bulk analytical techniques. Single fluid inclusion halogene analytics as are used here are vital due to the lack of sampling bias inherent to bulk analytical techniques that narrow the analyzed compositional range.The evaluation of Cl, Br and I systematics of fluid inclusions from unconformity-related hydrothermal veins in the Schwarzwald, Germany, provides new insight into the behavior of I and the evolution and source of the fluids involved in formation of unconformity-type vein deposits in central Europe during the Triassic to Cretaceous. While the variation in Br/Cl (molar ratio: 0.1 × 10−3–8.3 × 10−3) is consistent with previously established models of mixing processes between two fluids with a salinity source in seawater evaporation (modified bittern brine) and halite dissolution (modified halite dissolution brine), respectively, the large scattering of I/Cl (molar ratio: 1.4 × 10−6–330 × 10−6) cannot be attributed to the salinity source alone. Instead, I must have been derived from interaction with organic matter, which appears to have been more pronounced within the modified bittern brine fluid endmember. During fluid infiltration into the basement and associated fluid-rock interaction, I/Cl and Br/Cl acted conservatively on analytically detectable scales and large inhomogeneities in source I signatures remained intact. This finding also presents direct evidence of local fluid composition inhomogeneity and absence of internal mixing of basement reservoirs for long time periods (more than 100 Ma).