Rare earth elements as indicators of hydrothermal processes within the East Scotia subduction zone system

Abstract

The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, hosts a number of hydrothermal sites in both back-arc and island-arc settings. High temperature (>348°C) ‘black smoker’ vents have been sampled at three locations along segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as ‘white smoker’ (<212°C) and diffuse (<28°C) hydrothermal fluids from within the caldera of the Kemp submarine volcano. The composition of the endmember fluids (Mg=0mmol/kg) is markedly different, with pH ranging from <1 to 3.4, [Cl−] from ∼90 to 536mM, [H2S] from 6.7 to ∼200mM and [F−] from 35 to ∼1000μM. All of the vent sites are basalt- to basaltic andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have total REE concentrations ranging from 7.3 to 123nmol/kg, and chondrite-normalised distribution patterns are either light REE-enriched (LaCN/YbCN=12.8–30.0) with a positive europium anomaly (EuCN/Eu∗CN=3.45–59.5), or mid REE-enriched (LaCN/NdCN=0.61) with a negative Eu anomaly (EuCN/Eu∗CN=0.59). By contrast, fluids from the Kemp Caldera have almost flat REE patterns (LaCN/YbCN=2.1–2.2; EuCN/Eu∗CN=1.2–2.2).We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc spreading ridge is variably influenced by ion exchange with host minerals, phase separation, competitive complexation with ligands, and anhydrite deposition, whereas fluids from the Kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite deposits from Kemp differ from those of the present-day fluids, potentially providing critical information about the nature of hydrothermal activity in the past, where access to hydrothermal fluids is precluded.