https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 1 Formation and origin of Fe-Si oxyhydroxide deposits at the ultra-slow spreading 2 Southwest Indian Ridge 3 4 Authors: 5 Kaiwen Ta1, 2, Zijun Wu1*, Xiaotong Peng2 and Zhaofu Luan1 6 7 1School of Ocean and Earth Science and State Key Laboratory of Marine Geology, 8 Tongji University, Shanghai, China, 9 2Deep Sea Science Division, Institute of Deep Sea Science and Engineering, Chinese 10 Academy of Sciences, Sanya, China. 11 12 Corresponding author: 13 Zijun Wu, ([email protected]) 14 15 Abstract 16 Low-temperature hydrothermal system is dominated by Fe-Si oxyhydroxide 17 deposits. However, the formation process and mechanism on modern hydrothermal 18 Fe-Si oxyhydroxides at ultra-slow spreading centers remain poorly understood. The 19 investigation presented in this paper focuses on six Fe-Si deposits collected from 20 different sites at the Southwest Indian Ridge (SWIR). The mineralogical and 21 geochemical evidence showed significant characteristics of a low-temperature 22 hydrothermal origin. The Mössbauer spectra and iron speciation data further provided 1 https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 23 an insight into iron-bearing phases in all deposits. Two different types of 24 biomineralized forms were discovered in these deposits by Scanning Electron 25 Microscopy analysis. Energy-dispersive X-ray spectrometry and nano secondary ion 26 mass spectrometry revealed that distinct biogenic structures were mainly composed of 27 Fe, Si, and O, together with some trace elements. The Sr and Nd isotope compositions 28 of Fe-Si deposits at the SWIR were closely related to interaction between 29 hydrothermal fluids and seawater. The remarkably homogeneous Pb isotope 30 compositions can be attributed to hydrothermal circulation. Based on these findings, 31 we suggest that microbial activity plays a significant role in the formation of Fe-Si 32 oxyhydroxides at the at ultra-slow spreading SWIR. Biogenic Fe-Si oxyhydroxides 33 potentially provide insights into the origin and evolution of life in the geologic record. 34 35 36 1 Introduction 37 Hydrothermal Fe-Si-oxyhydroxide deposits are widespread in many geological 38 settings, such as mid-ocean ridges (Alt, 1988; Benjamin et al., 2006; Dekov et al., 39 2010; Peng et al., 2015), back-arc spreading centers (Iizasa et al,. 1998; Hein et al., 40 2008; Sun et al., 2012), seamounts (Karl et al., 1989; Boyd and Scott, 2001; Emerson 41 and Moyer, 2002; Singer et al., 2011), and intra-plate submarine volcanoes (Edwards 42 et al., 2011; Fleming et al., 2013). Fe-Si oxyhydroxides in the form of yellowish to 43 brown chimneys, mounds and flat-lying deposits have often been observed in 44 low-temperature hydrothermal fields (Emerson and Moyer, 2002; Peng et al., 2015; 45 Johannessen et al., 2016; Ta et al., 2017). In general, modern low-temperature 2 https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 46 hydrothermal systems are the product of diffuse hydrothermal fluids and/or the 47 conductive cooling of high-temperature hydrothermal fluids mixed with seawater 48 (German et al., 1990). A pronounced excess of ferrous iron and dissolved silica are 49 typical characteristics of hydrothermal vent fluids (Tivey, 2007). Low-temperature 50 hydrothermal Fe-Si oxyhydroxides are considered as important hydrothermal products 51 which reflect the diffusion and evolution of hydrothermal fluids. The mineralogical 52 and geochemical compositions of such deposits have provided insight into their 53 formation mechanisms (Dekov et al., 2010; Sun et al., 2015; Ta et al., 2017). Recently, 54 studies focused on the origin of Fe-Si-oxyhydroxides have been increasing, however 55 little is currently known about the links between these deposits and microbial activity 56 at the ultra-slow spreading Southwest Indian Ridge (SWIR). 57 The ultra-slow spreading SWIR represents the longest segment of the world’s 58 slowest-spreading ridge (German et al., 2010; Husson et al., 2015). The SWIR is 59 characterized by a spreading rate of approximately 12–15 mm/yr, a lack of transform 60 faults, and extensive exposures of mantle peridotites (Dick et al., 2003; Niu et al., 61 2015). Compared to fast-spreading ridge systems, hydrothermal activity along the 62 ultra-slow spreading SWIR may have greater chemical and thermal fluxes (German et 63 al., 2010). However, the composition and depth of oceanic crust at the SWIR seems to 64 be different in many respects from average oceanic crust, and the data suggest the 65 presence of thickened crust or a large thermal anomaly in the region (Sauter et al., 66 2009; Zhang et al., 2013; Niu et al. 2015). The diversity in styles of hydrothermal 67 vents at the SWIR has been of particular interest, particularly the activity of 3 https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 68 high-temperature and low-temperature vents (German et al., 1998; Tao et al., 2012). 69 Isotope analyses of mid-ocean ridge basalts have revealed a clear distinction between 70 the Southwest Indian ridge and the Pacific and Atlantic ridge compositions (Hamelin 71 and Allègre, 1985; Vlastélic et al., 1999). Previous studies have confirmed that 72 plume-ridge interactions have produced geochemical and geophysical anomalies 73 along the Indomed and Gallieni fracture zones beneath the SWIR (Breton et al., 2013; 74 Yang et al., 2017). Recently, increasing attention has been paid to the petrology, 75 element geochemistry, microbial communities and biogeography of the SWIR (Li et 76 al., 2015; Chen et al., 2016; Ji et al., 2017; Zhou et al., 2018; Zhang et al., 2018). 77 Although microbial activity was revealed to play an important role in the formation of 78 Fe and Si minerals in low-temperature hydrothermal fields (Dekov et al., 2010), there 79 are a few studies that show the presence of biomineralized structures encrusted by 80 Fe-Si oxyhydroxides in the SWIR hydrothermal systems, indicating the oxyhydroxide 81 deposits may also be of biogenic origin (Peng et al., 2011; Sun et al., 2015). 82 Here, we report the geochemical and geomicrobiological characterization of 83 Fe-Si deposits from the ultra-slow spreading SWIR. Scanning electron microscope 84 (SEM), X-ray diffraction (XRD), inductively coupled plasma-mass spectrometry 85 (ICP-MS), nano secondary ion mass spectrometry (nanoSIMS), Pb-Sr-Nd-O isotopic 86 analysis, Mössbauer Spectroscopy, and sequential iron mineral extraction experiments 87 were used to investigate: (1) the geochemical and morphological characteristics of 88 biogenic Fe-Si oxyhydroxides, (2) the role of microbial activity in the formation of 4 https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 89 the Fe-Si oxyhydroxides, (3) the implications of the Sr-Nd-Pb isotopic content of the 90 low-temperature Fe-Si deposits at the ultra-slow spreading SWIR. 91 2 Geological Setting 92 This study is focused on the ultra-slow spreading Southwest Indian ridge 93 segments 27 and 28, as defined by Cannat et al. (1999), incorporating the Indomed 94 and Gallieni Fracture Zones (46.0 °E to 52.0 °E), where the central shallow section of 95 the SWIR has an overall 15° obliquity. Geophysical and geochemical data have 96 shown that this area is a V-shaped shallow domain associated with thickened crust and 97 robust magmatism (Lin and Zhang., 2006). Tectonic and volcanic processes result in 98 the growth of the oceanic crust at the SWIR (Niu et al. 2015; Li et al., 2015). Previous 99 geological surveys have indicated that the robust melt supply may be associated with 100 the Marion and Crozet hotspots (Georgen et al., 2001; Sauter et al., 2009). A study by 101 Yu et al. (2018) showed that segments 27 and 28 of the ridge had contrasting tectonic 102 and magmatic processes. An inactive hydrothermal field was discovered near the 103 center of segment 27 (Zhao et al., 2013). The axial depth of this segment has become 104 shallower and experienced a dramatic increase in magma supply since 8–11 Ma 105 (Sauter et al., 2009). However, the Longqi hydrothermal field of segment 28, at 106 49.6 °E, has been confirmed to remain active, at a water depth of around 2750 m (Tao 107 et al., 2012). 108 3 Materials and Methods 109 3.1 Sample Collection and Sample Descriptions 5 https://doi.org/10.5194/bg-2019-315 Preprint. Discussion started: 4 September 2019 c Author(s) 2019. CC BY 4.0 License. 110 Fragile and porous hydrothermal deposits (samples 20V-T8, 21V-T1, 21V-T7 111 and 33II-T2) were collected by a TV grabber during a cruise of the R/V DaYang 112 YiHao conducted by the China Ocean Mineral Resource R&D Association (COMRA) 113 at the SWIR, from 2008 to 2015. The yellowish 21V-T1 and brown 21V-T7 samples 114 were located ~20 km north of the SWIR, and the brown 20V-T8 sample was located 115 ~8 km to the north. The purple-red 34II-T22 sample was collected from the axis of the 116 SWIR. Sample DIV95 was recovered from the Longqi field by the Human Occupied 117 Vehicle (HOV) ‘Jiaolong’ diving cruise in 2015. This sample was very friable with a 118 layered structure. The layers were nearly parallel and displayed an obvious color 119 change (Fig. 2a).