Experimental Modeling of Decarbonation Reactions, Resulting in the Formation of CO[sub.2] Fluid and Garnets of Model Carbonated Eclogites under Lithospheric Mantle P,T-Parameters

• Published in: Minerals (Basel) 2023-06, Vol.13 (7)
• Main Authors: Bataleva, Yuliya V, Novoselov, Ivan D, Kruk, Aleksei N, Furman, Olga V, Palyanov, Yuri N
• Format: Article
• Language: English
• Subjects: Carbonates; Earth; Mantle
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min13070859

First experimental modeling of decarbonation reactions resulting in the formation of CO[sub.2]-fluid and Mg, Fe, Ca, and Mn garnets, with composition corresponding to the garnets of carbonated eclogites of types I and II (ECI and ECII), was carried out at a wide range of lithospheric mantle pressures and temperatures. Experimental studies were performed on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS), in (Mg, Fe, Ca, Mn)CO[sub.3]-Al[sub.2]O[sub.3]-SiO[sub.2] systems (with compositional variations according to those in ECI and ECII), in the pressure interval of 3.0–7.5 GPa and temperatures of 1050–1450 °C (t = 10–60 h). A specially designed high-pressure cell with a hematite buffering container—preventing the diffusion of hydrogen into the platinum capsule—was used, in order to control the fluid composition. Using the mass spectrometry method, it was proven that in all experiments, the fluid composition was pure CO[sub.2]. The resulting ECI garnet compositions were Prp[sub.48]Alm[sub.35]Grs[sub.15]Sps[sub.02]–Prp[sub.44]Alm[sub.40]Grs[sub.14]Sps[sub.02], and compositions of the ECII garnet were Prp[sub.57]Alm[sub.34]Grs[sub.08]Sps[sub.01]–Prp[sub.68]Alm[sub.23]Grs[sub.08]Sps[sub.01]. We established that the composition of the synthesized garnets corresponds strongly to natural garnets of carbonated eclogites of types I and II, as well as to garnets from xenoliths of diamondiferous eclogites from the Robert Victor kimberlite pipe; according to the Raman characteristics, the best match was found with garnets from inclusions in diamonds of eclogitic paragenesis. In this study, we demonstrated that the lower temperature boundary of the stability of natural garnets from carbonated eclogites in the presence of a CO[sub.2] fluid is 1000 (±20) °C at depths of ~90 km, 1150–1250 (±20) °C at 190 km, and 1400 (±20) °C at depths of about 225 km. The results make a significant contribution to the reconstruction of the fluid regime and processes of CO[sub.2]/carbonate-related mantle metasomatism in the lithospheric mantle.