Hydrothermal modification of the Sikhote-Alin iron meteorite under low pH geothermal environments. A plausibly prebiotic route to activated phosphorus on the early Earth

The Sikhote-Alin (SA) meteorite is an example of a type IIAB octahedrite iron meteorite with ca. 0.5wt% phosphorus (P) content principally in the form of the siderophilic mineral schreibersite (Fe,Ni)₃P. Meteoritic in-fall to the early Earth would have added significantly to the inventory of such siderophilic P. Subsequent anaerobic corrosion in the presence of a suitable electrolyte would produce P in a form different to that normally found within endogenous geochemistry which could then be released into the environment. One environment of specific interest includes the low pH conditions found in fumaroles or volcanically heated geothermal waters in which anodic oxidation of Fe metal to ferrous (Fe²⁺) and ferric (Fe³⁺) would be coupled with cathodic reduction of a suitable electron acceptor. In the absence of aerobic dioxygen (Eᵒ=+1.229V), the proton would provide an effective final electron acceptor, being converted to dihydrogen gas (Eᵒ=0V). Here we explore the hydrothermal modification of sectioned samples of the Sikhote-Alin meteorite in which siderophilic P-phases are exposed. We report on both, (i) simulated volcanic conditions using low pH distilled water and (ii) geothermally heated sub-glacial fluids from the northern Kverkfjöll volcanic region of the Icelandic Vatnajoküll glacier. A combination of X-ray photoelectron spectroscopy (XPS) and electrochemical measurements using the scanning Kelvin probe (SKP) method reveals that schreibersite inclusions are significantly less susceptible to anodic oxidation than their surrounding Fe–Ni matrix, being some 550mV nobler than matrix material. This results in preferential corrosion of the matrix at the matrix-inclusion boundary as confirmed using topological mapping via infinite focus microscopy and chemical mapping through Raman spectroscopy. The significance of these observations from a chemical perspective is that electrochemically noble inclusions such as schreibersite are likely to have been released into the geological environment through an undermining corrosion of the surrounding matrix, thus affording localised sources of available water-soluble, chemically reactive P in the form of H-phosphite [H₂PO₃ ⁻, Pi(III) as determined by ³¹P NMR spectroscopy]. This compound has been shown to have considerable prebiotic chemical potential as a source of condensed P-oxyacids. Here we demonstrate that Pi(III) resulting from the hydrothermal modification of Sikhote-Alin by sub-glacial geothermal fluids can be readily dehydrated into the condensed P-oxyacid pyrophosphite [H₂P₂O₅ ²⁻, PPi(III)] by dry-heating under mild (85°C) conditions. The potential significance of this latter condensed P-compound for prebiotic chemistry is discussed in the light of its modified chemical properties compared to pyrophosphate [H₂P₂O₇ ²⁻, PPi(V)].