Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path

Abstract

Experimental simulation of rhodochrosite-involving decarbonation reactions resulting in the formation of spessartine and CO2-fluid was performed in a wide range of pressures (P) and temperatures (T) corresponding to a hot subduction P-T path. Experiments were carried out using a multi-anvil high-pressure apparatus of a “split-sphere” type (BARS) in an MnCO3–SiO2–Al2O3 system (3.0–7.5 GPa, 850–1250 °C and 40–100 h.) with a specially designed high-pressure hematite buffered cell. It was experimentally demonstrated that decarbonation in the MnCO3–SiO2–Al2O3 system occurred at 870 ± 20 °C (3.0 GPa), 1070 ± 20 °C (6.3 GPa), and 1170 ± 20 °C (7.5 GPa). Main Raman spectroscopic modes of the synthesized spessartine were 349–350 (R), 552(υ2), and 906–907 (υ1) cm−1. As evidenced by mass spectrometry (IRMS) analysis, the fluid composition corresponded to pure CO2. It has been experimentally shown that rhodochrosite consumption to form spessartine + CO2 can occur at conditions close to those of a hot subduction P-T path but are 300–350 °C lower than pyrope + CO2 formation parameters at constant pressures. We suppose that the presence of rhodocrosite in the subducting slab, even as solid solution with Mg,Ca-carbonates, would result in a decrease of the decarbonation temperatures. Rhodochrosite decarbonation is an important reaction to explain the relationship between Mn-rich garnets and diamonds with subduction/crustal isotopic signature.

Experimental Modeling of Decarbonation Reactions, Resulting in the Formation of CO2 Fluid and Garnets of Model Carbonated Eclogites under Lithospheric Mantle P,T-Parameters

“…It has been experimentally established that at temperatures below the onset of decarbonation reactions, coesite and corundum interaction results in the formation of kyanite (over the entire pressure range of 3.0-7.5 GPa and temperatures of 1050-1450 °C). A similar process occurred in all our earlier experimental studies on the modeling of decarbonation reactions with the formation of CO2 fluid in association with pyrope, pyrope-almandine, pyrope-grossular, pyrope-almandine-grossular, and spessartine [65][66][67], as well as in the carbonate-oxide-sulfide system (MgCO3-SiO2-Al2O3-FeS system, 6.3 GPa, 1250-1450 °C) [94]. Synthesized kyanite contains FeO impurities (0.4-2.6 wt.%), which indicate exchange reactions of kyanite with magnesiosiderite-the only iron-bearing phase in the reaction volume of the capsules.…”

“…In previous experimental research devoted to the reconstruction of decarbonation reactions resulting in the garnet + CO 2 formation, carbonate + SiO 2 + Al 2 O 3 or carbonate + kyanite were used as starting materials. As evident from our previous experiments [65][66][67] and from pioneer research [42], the SiO 2 + Al 2 O 3 assemblage is not stable under high pressures and high temperatures. SiO 2 reacts with Al 2 O 3 to form kyanite.…”

“…Taking into account our previous experience of the studies on carbonate-oxide systems under high pressures and temperatures [65][66][67][85][86][87], platinum was chosen as the capsule material. The capsule volumes were selected according to the sizes of the high-pressure cell and to ensure the possibility of a number of modern analytical studies for each sample.…”

“…The first experimental modeling of decarbonation reactions associated with the formation of pyrope and CO2-fluid, and simulating the interaction in eclogite rocks-CO2 systems under conditions of the upper mantle, was performed in [42] (Figure 1b, reaction (11) in Supplementary Materials). Later, we carried out systematic experimental studies to determine the position of decarbonation lines in the P,T field, and established the stability regions of the CO2-fluid in association with pyrope, pyrope-almandine, pyrope-grossular, pyrope-almandine-grossular, and spessartine (Figures 1b and 2, reactions ( 12)- (16) in Supplementary Materials) [64][65][66][67]. These experimental works were continued in studies [68,69].…”

Shifted excitation Raman difference spectroscopy for soil component identification and soil carbonate determination in the presence of strong fluorescence interference

“…Two weaker Raman signals are located at 553 and 353 cm À1 and correspond to the O-Si-O symmetric bending mode and SiO 4 rotation, respectively. [39][40][41] Almandine as another garnet with different composition has its Si-O symmetric stretching vibration located at 915 cm À1 . Furthermore, weaker bands at 863 and 1036 cm À1 due to O-Si-O asymmetric stretching, at 554 cm À1 attributed to O-Si-O symmetric bending vibrations and at 501 cm À1 due to O-Si-O asymmetric bending vibrations can be identified.…”