Constraining temporal variations in metal and sulfur sources using high-resolution mineral-scale analysis of pyrite: evidence from the Brothers volcano, Kermadec arc, New Zealand

Variations in trace metal contents and sulfur isotope ratios (δ34S) within pyrite, at the scale of individual mineral grains, preserves a record of temporal fluctuations in the source of metals and sulfur as well as changes in the chemical composition and temperature of hydrothermal fluid during the evolution of the Brothers volcano, Kermadec arc, New Zealand. In this study, we analyzed pyrite from drill core recovered from two geochemically distinct hydrothermal systems at the Brothers volcano, the seawater-influenced NW Caldera (Site U1530) and magmatic-volatile-dominated Upper Cone (Site U1528) during the International Ocean Discovery Program’s Expedition 376. At the NW Caldera site, from 189 m below the seafloor, a seawater-derived hydrothermal fluid forming chlorite-rich alteration overprints early pyrophyllite + illite alteration. Within ~ 30 m of the seafloor at this same site, pyrite contains zones of high As content with a variable δ34S signature that ranges from -4.5 to 3.4‰ (n = 26). Values for δ34S > 0‰ record shallow mixing of seawater with upwelling hydrothermal fluids. In deeper parts of the system, but still within the chlorite-rich alteration zone, δ34S values > 0‰ are absent, indicating that relatively more sulfur is contributed from magmatic volatile degassing and SO2 disproportionation. In the pyrophyllite-rich alteration zone, pyrite contains Co-enriched cores that correspond to sharp changes in δ34S values from -5.3‰ to 4.6‰ (n = 68). Cobalt enrichment occurs in response to the mixing of seawater-derived hydrothermal fluid with Co-rich magmatic brines. At the Upper Cone site, a relatively constant supply of a low-salinity magmatic fluid results in pyrite grains that rarely exhibit any internal zonation in trace metal content. In pyrite where zonation does exist, a correlation between Cu and Sb and uniformly low δ34S values (< 0‰) indicates a link between metal enrichment, the pulsed degassing of magmatic volatiles, and SO2 disproportionation