Fe and S-isotope compositions of hydrothermal deposits from Kings Triple Junction, Lau Basin, southwest Pacific Ocean

Abstract

We analyzed iron and sulfur isotopic compositions of polymetallic sulfides from an extinct hydrothermal mound near the Kings Triple Junction of the northern Lau basin belonging to a back-arc basin for understanding the origin and mineral zoning processes in this study. Sulfides from the hydrothermal mound�s pedestal slab consisting of chalcopyrite and pyrite show enrichment of heavy S-isotope (?34SV-CDT = +8.3 to +9.6�). Sphalerite-dominated chimney on the mound�s slope also has variable ?34S values between +5.5 and + 8.3� from central orifice to outer wall. These isotopic ratios indicate the substantial contribution of heavy isotope rich sulfate-S in seawater circulating within the mound. Low ?33S values in these sulfides also indicate that the sulfate reduction is of thermal origin, with minimal or no role for microbes. Circulating seawater induced changes in fluid composition and post-depositional zone refinement of certain minerals can explain the variation in ?34S in different layers within the chimney and pedestal slab. The Fe-isotopic compositions (?56FeIRMM-014) show a range from ?1.1 to ?0.3� in the pedestal slab while they vary between ?1.7 and ? 0.6� at different parts of the peripheral chimney section. We explain such changes are mostly due to redox processes and mineral zoning development during these sulfide deposit formation. The tree-like FeMn oxide growth structures in the vicinity of the hydrothermal mound having a thin birnessite top layer and amorphous iron oxide bottom part show distinct ?56Fe values of ?3.0 and ? 0.8�, respectively.