Inferences on the Influences of Age & Porosity on Oxidative Weathering of Massive Sulfides at the Endeavour Segment of Juan de Fuca Ridge

Abstract

Hydrothermal activity at mid-ocean ridge spreading centers occurs during the formation of new oceanic crust and is responsible for the accumulation of mineral deposits comprised mainly of inorganic metal sulfides that precipitate from mixtures of seawater and high-temperature, sulfide-rich, oxygen-poor vent fluid. These mineral aggregates are known as seafloor massive sulfide deposits and occupy unique biogeochemical niches that remain largely unexplored. Upon the cessation of hydrothermal activity, massive sulfide deposits undergo alteration via both biotically- and abiotically-mediated geochemical reactions. These processes are collectively described as oxidative weathering. While the observed textures of these deposits suggest significant variation in weathering rates, neither the causes of this variation nor the drivers that govern biogeochemical oxidation of massive sulfides are well-characterized. To begin to describe the mechanisms that dictate these processes, massive sulfide samples were collected from deposits along the Endeavour Segment of the Juan de Fuca Ridge. Coupled synchrotron-based X-ray Absorption Near Edge Spectroscopy (XANES) and X-Ray Fluorescence (XRF) microscopy were utilized to create comprehensive redox maps that allow for characterization of the localized redox environment and identification of weathering products. These techniques are a powerful and so far underutilized tool with which to examine the geochemical landscapes of seafloor massive sulfide deposits. Mineral identifications and spatial distributions were corroborated with optical microscopy and X-Ray Diffraction (XRD). The Juan de Fuca Ridge massive sulfide samples are composed of iron-sulfide phases, primarily pyrite (FeS₂), with minor amounts of other metal-bearing sulfides, such as sphalerite ((Zn,Fe)S₂) , wurtzite ((Zn,Fe)S₂), and cubanite (CuFe₂S₃). The samples contain rinds comprised of oxides and (primarily iron-bearing) clays that occur along massive sulfide exteriors and within pore channels. Greater amounts of secondary oxides and clays are observed concurrent with increased porosity and internal pore distribution and are inferred to be products of weathering. This study contributes to current understanding of the mineralogy and composition of seafloor massive sulfide deposits and provides new insight into relationships between age, porosity, and oxidative weathering.