Sulfide trace element signatures as a tracer for low- and high-temperature processes in serpentinites

Abstract

<p>Serpentinization of ultramafic rocks at mid oceanic spreading ridges is an important sink for reduced seawater sulfate. During the dehydration of serpentinites in subduction zones volatile phases like sulfur can be leached out or further transported into the mantle. For understanding the fate of sulfur in subduction zones, it is crucial to identify the signature of sulfides during the process of hydration. The reduction of seawater sulfate takes place under isotopic fractionation of sulfur. Generally, positive δ<sup>34</sup>S values are a result of high temperature thermochemical reduction of SO<sub>4</sub> to H<sub>2</sub>S while negative δ<sup>34</sup>S are a result of microbial sulfate reduction under low temperatures. High temperature fluids are typically derived from interaction with gabbroic intrusions before they alter the ultramafic host rocks. In contrast, low temperature fluids can be derived from relatively unmodified seawater. Therefore, these fluids imprint different metal concentrations during concurrent sulfide precipitation. Here we study ODP (Ocean Drilling Program) samples from the Iberian Margin and 15°20'N Fracture Zone along the Mid-Atlantic Ridge together with samples of ophiolitic serpentinites from the Northern Apennines. We present in situ δ<sup>34</sup>S and trace element signatures in sulfides to provide new insights into fluid sources and temperature conditions during water-peridotite interaction. High temperature fluids precipitate sulfides with stronger enrichments in metals like Cu, Co and Ni compared to sulfides formed from low temperature fluids. In respect to mafic dominated systems, sulfides from high temperature fluids of ultramafic systems show distinct enrichments in Ni. As a consequence, the combination of variations in δ<sup>34</sup>S and trace element signatures can be used to trace back the tectonic environment in which the sulfides formed and maybe used as a tracer for sulfide formation in subduction zones.</p>