Experimental Modeling of the Origin of Native Metals (Fe) in the Earth’s Crust under Reducing Conditions

Abstract

The origin of native Fe in the Earth’s crust was experimentally modeled by reproducing interaction between basalt melts and fluid (H₂ and/or H₂ + CH₄) at a temperature of 1100–1250°C, fluid pressure of 1–100 MPa, and strongly reducing conditions with fO₂ = 10⁻¹² to 10⁻¹⁴ bar. The experiments were carried out in an IHPV equipped with a unit of original design for conducting long lasting experiments under a high pressure of reduced fluid. The experiments were done using samples of natural magmatic rock: magnesian basalt from Tolbachik volcano, Kamchatka, and this magnesian basalt enriched in Ni and Co oxides. The experimental results highlight the following features of interaction between reducing fluid and basalt melt: (1) In spite of the high reducing potential of the system of H₂ or (H₂ + CH₄) fluid with magmatic melt, hydrogen oxidation and reduction reactions of metals of variable valence are not completed in the system. Redox reactions in the basalt melt are terminated because H₂O forms in the system and buffers the redox potential of H₂ of the (H₂ + CH₄) mixture. (2) The initially homogeneous magmatic melt becomes heterogeneous: the newly formed H₂O dissolves in the melt and partially in the fluid phase, and this results in more silicic melt and small metal segregations morphologically resembling liquid immiscibility textures. (3) The onset of metal–silicate liquid immiscibility in magmatic melts interacting with reducing fluid can occur at geologically realistic temperatures (≤1250°C), which are much lower than the melting temperatures of iron and its alloys with nickel and cobalt. (4) Carbon, which is formed in the experiments by methane pyrolysis, is dissolved in the metal phase. Our experimental results illustrate a mechanism explaining the occurrence of carbon in natural native iron. (5) The texture and size of the experimentally reproduced metal segregations are consistent with data on naturally occurring native metals, primarily, iron and its alloys with nickel and cobalt, in magmatic rocks of various composition and genesis.