Native nickel in the TAG hydrothermal field sediments (Mid‐Atlantic Ridge, 26°N): Space trotter, guest from mantle, or a widespread mineral, connected with serpentinization?

Abstract

[1] Tiny metallic particles have been found in the sediments of the Trans-Atlantic Geotraverse (TAG) hydrothermal field, Mid-Atlantic Ridge (MAR). The metallic grains are almost pure Ni and have compositional and structural characteristics similar to those of other terrestrial metallic Ni 0 occurrences. Native nickel grains, documented here for the first time in oceanic sediments, appear in calcareous oozes intercalated with metalliferous layers, layers with bedrock rubble, and sporadically scattered cosmic microspherules. There are three possibilities for the metallic Ni 0 genesis: (1) cosmic, (2) lower mantle at the core-mantle boundary, or (3) upper mantle/lower crust. The flake-like appearance of the Ni 0 grains rules out cosmic ablation origin. No unequivocal evidence was found that the Ni 0 particles could have formed under high-pressure conditions similar to those at the core-mantle boundary. Ni 0 formation through seawater serpentinization of the upper mantle/lower crust is the most plausible mechanism. At the slow spreading MAR, seawater penetrates down through highly fractured crest zone and serpentinizes the olivine and pyroxene of the upper mantle/lower crust ultramafics/mafics. Serpentinization results in the generation of highly reduced volatile species: H 2 and hydrocarbons. In such an extremely reduced environment, Fe and other transition metals (Co, Ni, Cu, Zn) released from olivines and pyroxenes tend to appear as native metals if fS 2 approaches 0. The lightened serpentinized blocks uplift and expose the serpentinized ultramafics/mafics at the rift valley's walls. I speculate that tectonic movements and weathering could liberate the native metallic grains scattered in the serpentinized rocks, which may then have been dispersed in the proximate sediments by near-bottom currents.

Global rate and distribution of H2 gas produced by serpentinization within oceanic lithosphere

“…Of the various processes that can generate free hydrogen gas (H 2 ) in the lithosphere, field evidence suggests that one of the principal mechanisms is the hydration of ferrous iron minerals or “serpentinization” reactions [ Apps and van de Kamp , ]. The H 2 produced by serpentinization in turn figures prominently in theories regarding (1) the origin and early evolution of life on Earth [e.g., Canfield et al , ; Sleep and Bird , ], (2) the basal fuel‐source sustaining the subsurface biosphere [e.g., Charlou et al , ; Kelley et al , ; Menez et al , ], (3) the formation of abiogenic hydrocarbons [ McCollom and Seewald , ; Sherwood Lollar et al , ; Proskurowski et al , ] as well as (4) native metal alloys [ Dekov , ; McCollom and Bach , ], (5) the biogeochemical cycling of elements such as sulfur and carbon [ Alt et al , ], and (6) the potential use of H 2 as a substitute for fossil fuels [ Smith et al , ].…”

A New Occurrence of Terrestrial Native Iron in the Earth’s Surface: The Ilia Thermogenic Travertine Case, Northwestern Euboea, Greece

“…Examples of elements present in the native (zero-valence) state include, among others, Al [1,2], Si [3], Ni [4], Au [5,6], Co [7], Cu and Ti [8]. Native iron is unstable under the oxygen fugacity conditions prevailing in the Earth's crust and mantle, with a few exceptions [9].…”

Three‐dimensional seismic structure of a Mid‐Atlantic Ridge segment characterized by active detachment faulting (Trans‐Atlantic Geotraverse, 25°55′N‐26°20′N)

“…The ratio of Total Dissolved Manganese (TDM) to methane (CH 4 ) concentration in fluids sampled from the water column above the TAG segment indicate that present‐day hydrothermal fluids principally interact with basalt as opposed to peridotite [ Campbell et al , 1988; Charlou and Donval , 1993]. However, native Ni 0 particles recovered from sediments surrounding the TAG hydrothermal field have been interpreted as resulting from the tectonic disturbance of a serpentinized fault zone [ Dekov , 2006], and low boron levels in TAG fluids have been attributed to serpentinization in the downgoing convection limb [ Palmer , 1996]. In addition, hydrothermal discharge at TAG has been episodic, with numerous periods of sporadic venting from the various mounds located on the detachment footwall over the past ∼140,000 yrs [ Lalou et al , 1995, 1993], and we do not know how the fluid chemistry varied over this period.…”

“…Vent fluid composition at TAG is generally similar to that of fluids sampled at the fast spreading East Pacific Rise [ Campbell et al , 1988; Edmonds et al , 1996], and the low fluid pH [ Campbell et al , 1988] suggests little interaction with ultramafic rocks. However, the presence of native Ni particles in sediments nearby have been interpreted as being derived from tectonically uplifted serpentinized ultramafics/mafics [ Dekov , 2006], and low boron levels in TAG fluids have likewise been interpreted to result from boron removal during the serpentinization of ultra‐mafic rocks [ Palmer , 1996]. Sr and O isotopes in fluid samples indicate that flow pathways in this system are longer, and probably penetrate deeper, than in other hydrothermal systems hosted in fast spreading MORs [ Bach and Humphris , 1999].…”