Zhu and Ogasawara (2002) interpret the textures and mineralogy of dolomitic marbles from the Kokchetav massif in terms of pressure-induced dissociation of dolomite (dol) to aragonite (ara) + magnesite (mag). On the basis of this assumption they postulate a minimum metamorphic pressure of 7.85 GPa, corresponding to subduction of at least 250 km depth.
I maintain that dolomite remained stable throughout the entire subduction-exhumation cycle in the dolomitic marble and that peak pressures reached at most 6 GPa. Dolomite is the main matrix mineral as well as the main mineral in inclusions in both garnet and zircon from the dolomitic marbles. Although magnesite is rarely found as inclusion (Shatsky et al., 1995), the assemblage ara + mag has never been observed so far. Zhu and Ogasawara (2002) do not report any magnesite. They suggest that magnesite survived as a metastable phase to conditions of about 2 GPa, 650 °C, where it partly reacted to MgO + C + O2. The product MgO reacted then with majoritic garnet to form retrograde clinochlore. Apart from the resulting clinochlore, there is no other evidence supporting such an unnecessary complex process. Clinochlore is a very common mineral found as a result of retrogression of garnet and in this case can be attributed to the reaction: pyrope + 2 dolomite + 4 H2O = clinochlore + 2 calcite + 2 CO2. In fact, this reaction is in better agreement with the observed corrosion of garnet and dolomite and the equilibrium of newly formed calcite and clinochlore. The reported aragonite (Fig. 1E of Zhu and Osagawara, 2002) is questionable. Calcite and aragonite have the same average atomic number (Z) and hence their backscatter emission should be similar, noting that garnet with a higher Z has a significantly lower backscatter emission than the supposed aragonite. Nevertheless, the reported carbonates with very low Mg and Fe contents probably originate from aragonite. The peak metamorphic assemblage is therefore garnet + clinopyroxene + dolomite ± aragonite ± diamond as previously suggested by Ogasawara et al. (2000). Peak metamorphic temperatures of ~950 °C in these dolomitic marbles are well constrained by garnet-clinopyroxene thermometry and are independently confirmed by the nitrogen aggregation state of diamond (eg., Hermann et al., 2001, and references therein). Figure 1 compiles relevant experimental phase relations. At 950 °C the assemblage dol + ara has a stability field confined to pressures below 5.5–6 GPa. Dol + ara react to Mg-calcite at 950–1000 °C, further confirming the peak temperature and minimum pressure of 4.5 GPa at 900 °C, within the stability field of diamond. The resulting field of peak metamorphism (A′) is in strong contrast to the peak metamorphism (A) suggested by Zhu and Ogasawara (2002).
Zhu and Ogasawara (2002) use a complex reaction to explain the observed difference between peak and retrograde garnet compositions. This is again unnecessary because the retrograde garnet is buffered by coexisting calcite, dolomite, and chlorite according to the equilibria CaMg−1garnet = CaMg−1carbonates, and FeMg−1garnet =FeMg−1chlorite. The supposed majoritic character of garnet is not convincing because a Si content of 3.02 is within uncertainty of the analyses and the normalization process is not distinguishable from a non-majoritic garnet. The difference in Si contents between peak and retrograde garnet might be real and is easily explained by the retrograde equilibrium MgSiAl−2garnet =MgSiAl−2chlorite. Hence, also on the basis of garnet composition, there is no need to have free MgO and consequently to propose metastable magnesite. The observed retrograde graphite can be explained by retrogression of diamond or by reduction of CO2 from the metamorphic fluid. The retrogression likely occurred at pressure <1.0 GPa and temperature ~600 °C (B′), in agreement with observed fluid influx in the country rocks of the dolomitic marble. The proposed field of retrogression (B) given by Zhu and Ogasawara (2002) is situated in the aragonite stability field and hence is inconsistent with their observation of stable calcite.
I conclude that the textures and mineral compositions presented by Zhu and Ogasawara (2002) do not support a minimum pressure of 7.85 GPa but a maximum pressure of 6 GPa. The described retrograde assemblage with calcite and clinochlore originates from decomposition of garnet and dolomite in the presence of fluids and does not require magnesite and MgO as precursors. Hence there is no evidence for dolomite dissociation in the Kokchetav dolomitic marbles.