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.
Highlights
Manganese is a 3d transition metal that can have various valences (1, 2, 3, 4, 6, 7) and spin states; this element is one of the most common in the Earth’s crust and mantle (12th and 11th in terms of abundance) and in the bulk Earth (12th in terms of abundance [1,2,3,4])
Taking into account the previously developed approach and published results [23], the appearance of spessartine and CO2 fluid in the reaction volume was considered as the main criterion for the decarbonation reaction
Most existing thermal and thermodynamic models [48,49,50,51] predict that decarbonation involving Mg and Ca carbonates is rarely realized along most subduction P-T paths (Figure 5b)
Summary
Manganese is a 3d transition metal that can have various valences (1, 2, 3, 4, 6, 7) and spin states; this element is one of the most common in the Earth’s crust and mantle (12th and 11th in terms of abundance) and in the bulk Earth (12th in terms of abundance [1,2,3,4]). During subduction of the oceanic crust, Mn-rich oxides and rhodochrosite are transported to the mantle but interact with mantle rocks, leaving characteristic chemical “traces” in mantle rocks, most pronounced in garnet-bearing assemblages. These “traces” consist of a sharp increase in the concentrations of the spessartine component (as well as almandine) in garnets, the presence of carbon phases (diamond, graphite, C-O-H fluid), as well as the “subduction carbon isotopic signature” or “crustal oxygen isotopic signature” [13,14,15,16,17]. Similar “traces” are described in diamond-bearing rocks of ultra-high pressure (UHP) terranes [18,19,20]
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