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

Abstract

1The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, 2 hosts a number of hydrothermal sites in both back-arc and island-arc settings. High 3 temperature (> 348 °C) 'black smoker' vents have been sampled at three locations along 4 segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as 'white smoker' (< 5 212 °C) and diffuse (< 28 °C) hydrothermal fluids from within the caldera of the Kemp 6 submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is 7 markedly different, with pH ranging from <1 to 3.4, [Cl -] from ~90 to 536 mM, [H 2 S] from 8 6.7 to ~200 mM and [F -] from 35 to ~1000 μM. All of the vent sites are basalt-to basaltic 9 andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on 10 rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have 11 total REE concentrations ranging from 7. 2). 16We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc 17 spreading ridge is variably influenced by ion exchange with host minerals, phase separation, 18 competitive complexation with ligands, and anhydrite deposition, whereas fluids from the 19 Kemp submarine volcano are also affected by the injection of magmatic volatiles which 20 enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite 21 deposits from Kemp differ from those of the present-day fluids, potentially providing critical 22 information about the nature of hydrothermal activity in the past, where access to 23 hydrothermal fluids is precluded. 24 25 3 INTRODUCTION 26The chemical properties of the rare earth elements (REEs) are fundamentally similar, 27 and differences in their behaviour in natural materials and fluids can usually be attributed to 28 atomic radii controls on their speciation and mobility (Elderfield et al., 1988). In 29 hydrothermal environments, the distribution of the REEs provides important information 30 about fluid evolution during subsurface circulation (Elderfield et al., 1988;Haas et al., 1995), 31 sources of fluid constituents and the extent of seawater mixing (Van Dover, 2000; Embley et 32 al., 2007;Craddock et al., 2010), conditions of mineral deposition and venting history 33 (Craddock and Bach, 2010) and the transport and fate of plume particulate material (German 34 et al., 1990;Bau and Dulski, 1999;Sherrell et al., 1999). 35Most hydrothermal fluids have remarkably uniform chondrite-normalised REE 36 (REE CN ) distribution patterns, with enrichment in the light-REEs relative to the heavy-REEs 37 and a positive europium anomaly (Michard and Albarède, 1986). This pattern has been 38 observed in fluids from basalt-hosted hydrothermal systems on the Mid-Atlantic Ridge 39 (MAR) (Mitra et al., 1994;James et al., 1995) and the East Pacific Rise (EPR) (Michard et 40 al., 1983;Michard and Albarède, 1986), but also in fluids venting through basaltic andesite 41 and andesite substrates in the Lau Basin , heavily-sedimented...

Sulfate minerals control dissolved rare earth element flux and Nd isotope signature of buoyant hydrothermal plume (EMSO-Azores, 37°N Mid-Atlantic Ridge)

“…As Eu 2+ and Sr 2+ have similar ionic radii (Shannon, 1976), Eu can be incorporated into the same secondary minerals. The concave downward-shaped DREE patterns of the Capelinhos fluid are likely related to anhydrite precipitation in agreement with previous anhydrite studies that show preferential uptake of LREE (Mills and Elderfield, 1995;Schmidt et al, 2010;Craddock et al, 2010;Cole et al, 2014).…”

“…They show light REEs (LREEs) enrichment over heavy REEs (HREEs), together with positive europium anomalies, and concentration enrichment factors of 10 to 10 3 compared to seawater concentrations (Michard and Albarède, 1986;Michard, 1989;Klinkhammer et al, 1983Klinkhammer et al, , 1994Mitra et al, 1994;Douville et al, 1999;Bau and Dulski, 1999;Schmidt et al, 2010). In comparison, REE patterns of black smoker fluids collected at back-arc basins and subduction zones exhibit either LREEs or MREEs enriched compared to HREEs (Craddock et al, 2010;Cole et al, 2014). The behavior of hydrothermal REEs once expelled into deep seawater has been inferred solely from the REE signatures of suspended particulate matter (SPM) and/or sediment cores in the vicinity of hydrothermal vents (Trocine and Trefry, 1988;German et al, 1990German et al, , 1991German et al, , 2002Rudnicki and Elderfield, 1993;Sherrell et al, 1999;Chavagnac et al, 2005Chavagnac et al, , 2006Chavagnac et al, , 2008German and Seyfried, 2014).…”

Environmental Change in the Deep Ocean

“…Vents show a great deal of heterogeneity in terms of their physical manifestations, temperatures, and chemistry that reflect underlying geology. For example, vents associated with island back arcs in subduction zones can be subject to high levels of volcanic disturbance, and vent fluids tend to be rich in volatiles [e.g., H 2 S, SO 2 , H 2 O, CO 2 , hydrogen chloride (HCl), and hydrogen fluoride (HF)] (56). Along mid-ocean ridges, the longevity and distribution of individual hydrothermal vent fields and their chemistry depend on their underlying geology (57).…”

Factors affecting the rare earth element compositions in massive sulfides from deep‐sea hydrothermal systems

“…Seafloor hydrothermal fluids have remarkably uniform REE distribution patterns, exhibiting greater enrichment in light REEs (LREEs) than heavy REEs (HREEs), and positive Eu anomalies [e.g., Michard and Albarède , ; Michard , ; Klinkhammer et al ., ; Mitra et al ., ; Douville et al ., ]. However, in some hydrothermal systems (e.g., the Comfortless Cove vent field, Mid‐Atlantic Ridge (MAR) near 5°S and the East Scotia subduction zone located in the Atlantic sector of the Southern Ocean), the hydrothermal fluids have extremely high REE concentrations (up to 123 nmol/kg), are enriched with mid‐REEs (MREEs), and have negative Eu anomalies due to the accumulation of particulate anhydrite, which is MREE enriched and sourced from black and white chimneys [ Schmidt et al ., ; Cole et al ., ]. In the Manus Basin, the REE patterns in different vent fluids range from LREE enriched, to MREE and HREE enriched, to flat, and show positive Eu anomalies due to the differences in the degassing of magmatic volatiles (i.e., HF and SO 2 ) and the precipitation of anhydrite in submarine hydrothermal systems [ Craddock et al ., ].…”

“…In addition, REE data may be used to evaluate the physicochemical conditions of hydrothermal fluids [Michard et al, 1983;Michard and Albarède, 1986;Michard, 1989;Klinkhammer et al, 1994;James et al, 1995;Douville et al, 1999;Bach et al, 2003;Mills and Elderfield, 1995]. As such, REE data provide important information about (1) the geochemical nature of ancient hydrothermal activity [Rimskaya-Korsakova and Dubinin, 2003;Schmidt et al, 2010;Cole et al, 2014]; (2) the impact of hydrothermal activity on the chemical mass balance of elements between sulfides and seawater [German et al, 1990;Mitra et al, 1994;Bau and Dulski, 1999;Sherrell et al, 1999]; (3) magmatic degassing in seafloor hydrothermal systems [Craddock et al, 2010]; and (4) the formation conditions and sources of seafloor sulfides, sulfates, and native sulfur balls and chimneys [e.g., Graf, 1977;Alt, 1988;Barrett et al, 1990;Gillis et al, 1990;Mills and Elderfield, 1995;Bach et al, 2003;Rimskaya-Korsakova and Dubinin, 2003;Zeng et al, 2007Zeng et al, , 2009Zeng et al, , 2010Zeng et al, , 2011.…”