Discriminating hydrothermal and terrigenous clays in the Okinawa Trough, East China Sea: Evidences from mineralogy and geochemistry

Highlights

• We present mineral and chemical compositions in clays from the East China Sea.

• The terrigenous and hydrothermal clays have distinguishable compositions.

• Clay chemistry can indicate the evolution of hydrothermal fluids.

• Geologic setting determines the hydrothermal fluid–sediment reaction.

Abstract

The Okinawa Trough (OT) in the East Asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic–hydrothermal activities. In this study, the clays collected during IODP Expedition 331 to the middle OT (Iheya North Knoll) were analyzed for mineralogical and geochemical compositions. By comparing with the clays from the East China Sea shelf and surrounding rivers, we examine different clay origins. The hydrothermal field in the mid-OT is dominated by Mg-rich chlorite, while the recharge zone has clay mineral assemblages similar to the shelf and rivers, showing high content of illite, subordinate chlorite and kaolinite and scarce smectite. Compared to the terrigenous clays, the hydrothermal clays in the OT have high concentrations of Mg, Mn and Zr but low Fe, Na, K, Ca, Ba, Sr, P, Sc and Ti, while the hydrothermal clays in the mid-ocean ridge are relatively enriched in Fe and V and depleted in Al, Mg, Zr, Sc and Ti. Different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays, diagnostic of variable clay origins. We infer that the OT hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids. The remarkably different compositions of hydrothermal clays between the sediment-rich back arc basin like OT and the sediment-starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid–rock/sediment reactions under various geologic settings.

Introduction

Clay minerals in terrestrial and marine environments are widely used in the studies of paleoclimatic and paleoenvironmental reconstructions, because they have a direct bearing on climatic and environmental changes, weathering process and sediment provenance. In the marginal sea environment, the clay size particles predominantly come from the river input and eolian dust. Clay minerals including illite, chlorite, kaolinite and smectite and other rock-forming minerals such as quartz and plagioclase are thus the primary components of terrigenous clays in marginal seas and global ocean (Windom, 1976, Chamley, 1989). Almost all detrital clays are sourced from weathering crust by physical disintegration and chemical decomposition at low temperature. The detrital clay is relatively stable in terrestrial environments but may become metastable or unstable in marginal seas and oceans (Galán and Ferrell, 2013). Authigenic clays and alteration of detrital clays occur often in shallow sea and submarine hydrothermal areas where bio-agents and hydrothermal fluid play important roles in clay formation and alteration (Severmann et al., 2004, Dias and Barriga, 2006, Lackschewitz et al., 2006, Glenn and Filippelli, 2007, Snyder et al., 2007, Foustoukos et al., 2008, Cuadros et al., 2011, Manuella et al., 2012, Hazen et al., 2013, Nguetnkam et al., 2014).

Submarine hydrothermal venting is a widespread phenomenon in global ocean, particularly in the mid-ocean ridges (MOR) and back arc basins. Active seafloor hydrothermal systems have extraordinarily high fluxes of energy and matter and thus, are important for chemical evolution of global ocean (Chamley, 1989, Severmann et al., 2004, Dias and Barriga, 2006, Lackschewitz et al., 2006, Miyoshi et al., 2013). Due to the chemical alteration of detrital clays and the formation of clays in these extreme seafloor environments, the indication of clay minerals for paleoenvironmental and paleoclimatic changes may become insensitive and even produce misleading results.

The major sources of hydrothermal clays include submarine weathering of massive sulfides or metalliferous sediments (Haymon and Kastner, 1981, Hekinian et al., 1993), alteration products of oceanic crust (Humphris et al., 1980, Alt and Honnorez, 1984), and direct precipitation from low-temperature hydrothermal fluid (McMurtry et al., 1983). As tracers, mineralogy, chemistry and oxygen isotopic composition of hydrothermal clays have been used to investigate the formation mechanism of clays from various settings, and provide insights into the fluid chemistry and fluid–rock/sediment interaction (Marumo and Hattori, 1999). Mineral assemblages of clays in hydrothermal fields are overall different under various geologic settings, e.g., illite, paragonite, smectite, nontronite, chlorite, anhydrite, zeolite, epidote, titanite, quartz, feldspar, pyrite, hematite, magnetite, Mn-oxides and Fe-oxyhydroxides and metalliferous sulfides mostly occurring at MOR (Alt, 1995, Mills et al., 1996, Teagle et al., 1998, Lackschewitz et al., 2006, Wang et al., 2014), while mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, and montmorillonite are present in Jade hydrothermal area in the middle Okinawa Trough (OT), a back arc basin (Marumo and Hattori, 1999). In addition, previous studies also revealed the difference in elemental and isotopic compositions between terrigenous and hydrothermal clays in submarine environments (Elderfield et al., 1988, Mills and Elderfield, 1995, Sheppard and Gilg, 1996, Marumo and Hattori, 1999, Lackschewitz et al., 2006, Boström, 2009).

The Okinawa Trough is located in the southeast of the East China Sea (ECS) and Eurasian continental margin, and is regarded as an incipient intra-continental basin formed behind the Ryukyu arc–trench system (Fig. 1). The hydrothermal activities in the OT, especially in the middle part, have been widely documented (Ishibashi et al., 1988, Halbach et al., 1993, Marumo and Hattori, 1999, Glasby and Notsu, 2003, Hongo et al., 2007, Takai et al., 2011, Tsuji et al., 2012). Over the past decades, the provenances and environmental tracing application of clay minerals in East Asian rivers and marginal seas have been well investigated (Milliman et al., 1985, Yang et al., 2002, Dou et al., 2010a). The detrital clay sediments in the trough mainly derive from the continent via the inputs of the Changjiang (Yangtze River) and Huanghe (Yellow River) and eolian dust as well (Zhu et al., 1988, Dou et al., 2010a), while smectite may partly come from the submarine alteration of volcanic debris (Zhu et al., 1988). Nevertheless, the compositions of clay sediments in the continental margin should be further clarified in terms of their origins, sources and environmental implication. And particularly, the OT clays altered by the hydrothermal activities can provide valuable information about the evolution of hydrothermal activity in this back arc basin.

In 2010, one of the co-authors (S. Yang) participated in the Integrated Ocean Drilling Program (IODP) Expedition 331 to the mid-OT, and collected various hydrothermal samples from the Iheya North Knoll area (Fig. 1). In this study, we report mineralogical and elemental compositions of the clay sediments separated from two drilling sites of this expedition and from the ECS shelf and surrounding rivers. The paper aims to identify the clay origins in the mid-OT with special emphasis on the discrimination of terrigenous (detrital) clays sourced from the China continent or Taiwan Island and hydrothermal clays formed in the trough. The alteration of detrital clays subject to hydrothermal fluid activity will be further investigated. Moreover, the difference in chemical compositions of hydrothermal clays from various geologic settings including the mid-ocean ridge and back arc basin will be revealed in this study.