Abstract
Experimental modeling of ankerite–pyrite interaction was carried out on a multi-anvil high-pressure apparatus of a “split sphere” type (6.3 GPa, 1050–1550 °C, 20–60 h). At T ≤ 1250 °C, the formation of pyrrhotite, dolomite, magnesite, and metastable graphite was established. At higher temperatures, the generation of two immiscible melts (carbonate and sulfide ones), as well as graphite crystallization and diamond growth on seeds, occurred. It was established that the decrease in iron concentration in ankerite occurs by extraction of iron by sulfide and leads to the formation of pyrrhotite or sulfide melt, with corresponding ankerite breakdown into dolomite and magnesite. Further redox interaction of Ca,Mg,Fe carbonates with pyrrhotite (or between carbonate and sulfide melts) results in the carbonate reduction to C0 and metastable graphite formation (±diamond growth on seeds). It was established that the ankerite–pyrite interaction, which can occur in a downgoing slab, involves ankerite sulfidation that triggers further graphite-forming redox reactions and can be one of the scenarios of the elemental carbon formation under subduction settings.
Highlights
The existing ideas about the polygenic origin of diamond [1,2,3] imply various processes, mechanisms, and driving forces of diamond crystallization in nature, including redox reactions, changes in P–T conditions, the evolution of melt or fluid composition, etc. [1,2,3,4,5,6,7,8]
Redox reactions leading to the oxidation of hydrocarbons or the reduction of carbonates or CO2 to elemental carbon are considered as driving forces of diamond formation in these models [3,9,10]
An analysis of the experimental results has shown that even at relatively low temperatures (1050–1250 ◦ C), at which pyrite is stable in the solid-phase state, ankerite in the presence of pyrite is unstable and enters into reactions
Summary
The existing ideas about the polygenic origin of diamond [1,2,3] imply various processes, mechanisms, and driving forces of diamond crystallization in nature, including redox reactions, changes in P–T conditions, the evolution of melt or fluid composition, etc. [1,2,3,4,5,6,7,8]. It is known that some carbonates are thermodynamically stable up to P–T parameters of the lower mantle [28], and under subduction conditions, up to 80% of carbonates do not undergo decarbonation and partial(Figure melting. Determined parameters of melting and decomposition of Mg,Ca,Fe carbonates and iron sulfides: 1—siderite + aragonite = ankerite [31]; Figure 1. Our recent experimental studies in multicomponent Fe,Ni-olivineankerite-sulfur and Fe,Ni-olivine-ankerite-pyrite systems [43], aimed at modeling the reactions of silicates and carbonates sulfidation, showed the possibility of crystallization of elemental carbon (graphite) in these processes. Due to the complexity of the studied systems, a detailed reconstruction of the formation processes of elemental carbon phases during sulfidation of FeO-bearing carbonates was not possible In this regard, in this study, it seems relevant to investigate the processes of formation of graphite (±diamond), coupled with sulfidation of carbonate, in a relatively simple ankerite–pyrite system at P–T parameters of the lithospheric mantle
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