Progressive carbonation and Ca-metasomatism of serpentinized ultramafic rocks: insights from natural occurrences and hydrothermal experiments

Contributions to Mineralogy and Petrology (2023) 178:38 https://doi.org/10.1007/s00410-023-02013-z ORIGINAL PAPER Progressive carbonation and Ca‑metasomatism of serpentinized ultramafic rocks: insights from natural occurrences and hydrothermal experiments Nomuulin Amarbayar1,2 · Otgonbayar Dandar1,2 · Jiajie Wang1 · Atsushi Okamoto1 · Masaoki Uno1 Undarmaa Batsaikhan2,3 · Hideko Takayanagi4 · Yasufumi Iryu4 · Noriyoshi Tsuchiya1 · Received: 6 July 2022 / Accepted: 13 April 2023 / Published online: 16 June 2023 © The Author(s) 2023 Abstract Hydration, carbonation, and related metasomatism of mantle peridotite play a significant role in the global geochemical cycle. In this study, we combined an analysis of carbonated serpentinite with hydrothermal experiments on carbonation and Ca-metasomatism for samples from the Manlay ophiolite, southern Mongolia to investigate that carbonation mechanism of the serpentinite body after serpentinization. Samples show that the serpentinite was either transected by calcite and dolomite veins or was completely replaced by carbonates (calcite with minor dolomite) and quartz, in which the original mesh texture of serpentinite was preserved. Carbonation occurred after low-temperature serpentinization (lizardite/chrysotile), suggesting that carbonation occurred at temperatures lower than 300 ˚C. Calcite in the serpentinite showed δ13 CVPDB values ranging from -8.83 to -5.11 ‰ and δ18 OVSMOW from + 20.1 to + 24.4 ‰, suggesting that CO2 in the fluids could be derived from the degradation of organic material or methanotrophic processes rather than the origin of seafloor limestone. Three batch-type experiments, i.e., single step experiments (1) Olivine + NaHCO3,aq + CaCl2,aq and (2) Chrysotile + NaHCO3,aq + wollastonite (Ca source), and two steps experiment (3) Olivine carbonation and Ca-metasomatism, were conducted at 275 °C and 5.7 MPa to constrain the mechanism of calcite replacement of serpentinite. We found that calcite precipitated from the solution directly in the first two experiments, but replacement of serpentinite by calcite was not observed. In contrast, the third experiment caused the initial carbonation to form magnesite and then changed to calcite by later alteration. The natural occurrences and experiments revealed the possibility that the carbonation of olivine followed by Ca-rich fluid infiltration produced calcite in the carbonated serpentinite. Such Ca-metasomatism of Mg carbonates could easily occur in the ultramafic bodies and significantly affect the global carbon cycle. Keywords Replacement · Manlay ophiolite · Carbonation · Ca-metasomatism · Carbon storage Introduction Communicated by Timm John. * Noriyoshi Tsuchiya 1 Graduate School of Environmental Studies, Tohoku University, Aoba 6‑6‑20, Aramaki Aoba‑ku, Sendai 980‑8579, Japan 2 Geoscience Center, School of Geology and Mining, Mongolian University of Science and Technology, Ulaanbaatar 14191, Mongolia 3 School of Geology and Mining, Mongolian University of Science and Technology, Ulaanbaatar 14191, Mongolia 4 Graduate School of Science, Tohoku University, Aobayama, Sendai 980‑8578, Japan Carbonation is a crucial alteration process that converts silicate and hydroxide minerals into carbonate minerals such as calcite, aragonite, dolomite, magnesite, and siderite, with implications from the global carbon cycle, formation of ore deposits, and CO2 sequestration (Phillips and Evans 2004; Kelemen and Matter 2008; Bjerga et al. 2015; Boskabadi et al. 2020; Oyanagi et al. 2021). The presence of silicate minerals, such as olivine and pyroxene, in mantle peridotite and basalt make them highly susceptible to carbonation reactions (Kelemen et al. 2011; Boskabadi et al. 2020) such as olivine reacting with a CO2-rich fluid to form magnesite (Eq. 1). In the MgO–SiO2–H2O–CO2 system, typical 13 Vol.:(0123456789) 38 Page 2 of 22 carbonation reactions after olivine is written as follows (e.g., Grozeva et al. 2017); Contributions to Mineralogy and Petrology (2023) 178:38 ophiolite in Mongolia. We present a unique example of pervasive serpentinite carbonation, where the serpentinite was Mg2 SiO4 (Olivine) + 2CO2 = 2MgCO3 (Magnesite) + SiO2 , (Quartz) The magnesite + quartz assemblage (known as listvenite) is commonly found in carbonated ultramafic rocks from numerous ophiolite complexes (e.g., Ulrich et al. 2014; Falk and Kelemen 2015; Menzel et al. 2018, 2022; Boskabadi et al. 2020). A similar reaction pathway is preserved in the carbonation of serpentinized ultramafic rock and serpentinite (Eq. 2; Kelemen and Hirth 2012; Menzel et al. 2022) as follows: replaced by calcite + quartz assemblage, but it preserves mesh textures of the original serpentinite. In this case, Mg in the serpentinites should be removed from the body. In order to understand the mechanism of the replacement of serpentinite by Ca-carbonate, we conducted three hydrothermal experiments (single step: Olivine + NaHCO3,aq + CaCl2,aq and Chrysotile + NaHCO3,aq + wollastonite, and two steps: Olivine carbonation and Ca-metasomatism). Combining nat- Mg3 Si2 O5 (OH)4 (Serpentine) + 3CO2 = 3MgCO3 (Magnesite) + 2SiO2 (Quartz) + 2H2 O. The fluid composition corresponding to carbonation reactions (Eq. 1 and Eq. 2) can be predicted by thermodynamic models (Klein and Garrido 2011). The formation of magnesite (Eqs. 1 and 2) can easily occur by reacting Mg2+ ion liberated from the Mg-bearing minerals in ultramafic rocks with external C O2 fluids. Cacarbonate (calcite) and Ca-Mg carbonate (dolomite) are also often observed in association with serpentinites. In these cases, as Ca is not always abundant in the serpentinites, Ca should be transported from the external sites as well as CO2 to form Ca-carbonates. Therefore, calcite and dolomite commonly occur in fractures and vugs within the ultramafic rocks (e.g. Falk et al. 2016; Hinsken et al. 2017; Lafay et al. 2017; Boskabadi et al. 2020; Okamoto et al. 2021). For example, the fully carbonated ultramafic body of the Oman ophiolite shows the cross-cutting relationship between carbonate minerals, with coarse dolomite and calcite veines cutting fine-grained magnesite-quartz listvenite (e.g., Falk and Kelemen 2015). In the Wadi Fins of the Oman ophiolite, the polygonal vein networks composed of serpentine + calcite were developed in the serpentinized peridotite body overlain by oceanic limestone (de Obeso and Kelemen 2018). Calcite, magnesite, and dolomite demonstrate the varying stability fields at elevated temperatures and pressures (e.g., Kerrick and Connelly 2001). Therefore, when they enter the Earth’s interior, the type of carbonate minerals (Ca-carbonates vs. Mg carbonates) formed into the ultramafic body during CO2 uptake near surface conditions could provide significant influences on the global carbon cycle (Galvez and Pubellier 2019). However, due to the complex vein structures composed of various carbonates in the carbonated serpentinites, the mechanism o (...truncated)