Mantle–slab interaction and redox mechanism of diamond formation

Abstract

Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle-slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle-slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.carbonate-iron interaction | high-pressure experiment | mantle mineralogy | deep carbon cycle S ubduction of crustal material plays an important role in the global carbon cycle (1-6). Depending on oxygen fugacity and pressure-temperature (P-T) conditions, carbon exists in the Earth's interior in the form of carbides, diamond, graphite, hydrocarbons, carbonates, and CO 2 (7-11). In the upper mantle, the oxygen fugacity (fO 2 ) varies from one to five log units below the fayalitemagnetite-quartz (FMQ) buffer, with a trend of a decrease with depth (6,(12)(13)(14)(15). At a depth of ∼250 km, mantle is reported to become metal saturated (16, 17), which holds true for all mantle regions below, including the transition zone and lower mantle. The subduction of the oxidized crustal material occurs to depths greater than 600 km (4-6). The main carbon-bearing minerals of the subducted materials are carbonates, which are thermodynamically stable up to P-T conditions of the lower mantle (10,11,18). As evidenced by the compositions of inclusions in diamond, which vary from strongly reduced, e.g., metallic iron and carbides (19-23), to oxidized, e.g., carbonates and CO 2 (6,20,(24)(25)(26)(27)(28), carbonates may be involved in the reactions with reduced deep-seated rocks, including Fe 0 -bearing species (29-31). A scale of these reactions is determined mainly by the capacity of subducted carbonate-bearing domains. An important consequence of such an interaction is that it can produce diamond. However, studies on diamond synthesis via the reactions between oxidized and reduced phases are limited (32-35).To understand the mechanisms of the interaction of carbonbearing oxidized-and reduced-mineral assemblages, we performed high-pressure experiments with an iron-carbonate system; an approach was used that enabled the creation of an oxygen fugacity gradient in the capsules (Materials and Methods and SI Materials and Methods). Results and DiscussionThe experimental results and the phase compositions are given in Table 1 and Table S1, respectively. At temperatures of 1,000 and 1,100°C, the iron-car...

Experimental and Theoretical Evidence for Surface-Induced Carbon and Nitrogen Fractionation during Diamond Crystallization at High Temperatures and High Pressures

“…As can be seen from experiments, metal-carbon systems are most conducive for a stable growth of cube faces and surfaces other than octahedra. A third such environment would be melts from subducted carbonates containing minor quantities of silicates and relatively high amount of biogenic carbon, such as those generating carbonaceous diapirs [35] or during interaction of carbonates with reduced mantle material [36]. Such setting would have the potential to provide conditions that would form diamond crystals with the surface-induced compositional inhomogeneity, such as we observed in our experiments including the carbonate system.…”

High-Pressure and High-Temperature Carbon Isotope Fractionation in the Fe-C System: Implications for Carbon Dynamics in the Deep Earth

“…2), show the presence of distinct fractionation between graphite/diamond and iron carbide melt, where lighter carbon isotope fractionate to the iron carbide relative to graphite/diamond. A quantitative estimation of the carbon isotope fractionation between iron carbide and graphite/diamond (gr/dia) was made for the first time in that study which was supported by several later studies at varying temperature and pressure conditions [22][23][24][25].…”

The Many Facets of Diamond Crystals

“…Vadim Reutsky, Piotr Kowalski, Yuri Palyanov, EIMF, and Michael Wiedenbeck in their contribution provided clear evidence of carbon and nitrogen fractionation related to the growing surfaces of a diamond [14]. This work is a vivid example of the importance of the experimental modelling contribution to solving the problems of natural diamond genesis [15][16][17][18]. By studying diamonds produced in high pressure high temperature experiments with various growth systems, the authors demonstrate that regardless of the bulk composition of the system, there exists a measurable fractionation of carbon isotopes and nitrogen impurity on the surface of diamond itself.…”