The Earth's mantle plays a key role in the long-term carbon cycle, serving as a reservoir for carbon storage and transport between the planetary interiors and the surface. Iron sulfides e.g., pyrite(FeS2), pyrrhotite(Fe1-xS), and troilite(FeS), interact with carbon under the high-pressure and high-temperature conditions. The redox state, governed by iron sulfides, determines the form of carbon storage by controlling carbon phase transitions. In a reducing environment, FeS phases can catalyze the conversion of CO2 to CH4, playing a significant role in the formation and dynamics of deep mantle carbon reservoirs.

To investigate sulfides' role in carbon storage, thermodynamic models of Fe-S-C containing system were developed[1,2]. Carbon solubility in Fe-Ni-S melts affects diamond crystallization, and at greater depths, these melts act as carbon concentrators[3]. Sulfide inclusions in diamonds from the Yubileinaya pipe further support their role in the carbon cycle[4].

To explore the role of sulfides in carbon storage, we examined reactivity between FeS2 or FeS and CO2 under elevated conditions in laser-heated diamond anvil cells. The products were analyzed using in situ multi-grain single-crystal XRD at pressures up to 130 GPa and ~2000 K. Pyrite remained stable after heating in CO2 up to the highest studied pressure, while heating of FeS resulted in Fe-bearing sulfide phases (FeS polymorphs and FeS2). At 35–45 GPa, FeS-III and FeS-VI appeared together, due to temperature gradients during the laser heating, while at 60–90 GPa and 1500–2000 K, only FeS-VI was synthesized. The formation conditions for FeS polymorphs align well with previously reported phase diagram[5]. These findings provide insights into sulfur behavior in Fe-S-C systems, advancing our understanding of deep iron-sulfur-carbon cycle.

1.Tsuno & Dasgupta., EPSL, 412,132-142(2015).

2.Zhang et al., GCA, 225,66–79(2018).

3.Zhang et al., EPSL, 520,164–174(2019).

4.Klein-Bendavid et al., IKC Ext. Abstr., 8(2003).

5.Edmund et al. EPSL, 646,118959(2024).