Variations in REE and Sr isotope compositions of altered rocks from magmatic- and seawater-influenced hydrothermal systems at Brothers volcano, Kermadec Arc, New Zealand (IODP Expedition 376)

IODP Expedition 376 drilled five sites on Brothers volcano to enable characterization of the internal structure of the hydrothermal activity at this submarine arc volcano. We have conducted REE and Sr isotope analyses of core samples from two locations that host distinctly different hydrothermal systems.

Site U1527 is located on the top of the northwest caldera wall. It is characterized by high temperature (up to 320°C) venting of moderately acidic fluids, and Cu-Zn-Au-rich sulfide chimneys. Alteration of the dacitic volcanics has resulted in a secondary mineral assemblage primarily of chlorite + quartz (± zeolite in the upper part of the hole). Total REE concentrations are 72.9-129.0 ppm and the chondrite-normalized REE patterns exhibit LREE-enrichment and pronounced negative Eu anomalies. Sr isotope ratios vary between 0.704172 and 0.704577 indicative of interactions of the rock with a seawater-dominated fluid.

Site U1530 is also located on the northwest caldera wall on a narrow bench 400 m east of Site U1527. While the upper part of the hole is characterized by alteration reflecting reaction with seawater, the lower part exhibits an alteration assemblage characteristic of reaction of rocks with acid-sulfate fluids later overprinted by chlorite- and illite-bearing assemblages. These differences are reflected in the large range in total REE concentrations (29.3-209.3 ppm), with the lowest totals observed at depths greater than 160 mbsf. This mobility of the REEs is also reflected in the highly varying chondrite-normalized REE patterns that display both positive and negative Eu anomalies. Sr isotope ratios are also highly variable ranging from 0.705162 to 0.707183.

Site U1528 is located on the summit of a resurgent cone within the caldera. It is characterized by lower temperature (≤120°C) venting of very low pH (to 1.9), gassy fluids, with native sulfur chimneys and extensive Fe-oxyhydroxide crusts. The dacitic volcanics at this site are pervasively altered to an alteration mineral assemblage consisting of natroalunite + pyrophyllite + sulfur + rutile. Total REE concentrations are similar (73.7 to 147.2 ppm) to those at Site U1527 but the chondrite-normalized REE patterns exhibit considerable variation with both negative and no Eu anomalies. Sr isotope ratios vary between 0.704057 and 0.706258.