ALBERT

Description: The phase relations in the K 2 CO 3 –FeCO 3 system were studied in multianvil experiments using graphite capsules at 6 GPa and 900–1400°C. Subsolidus assemblages comprise the stability fields of K 2 CO 3 + K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (K 2 CO 3 ) = 50 mol%. The K 2 CO 3 –K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 –FeCO 3 eutectics are established at 1100°C and 65 mol% and at ~1150°C and 46 mol% K 2 CO 3 , respectively. Siderite is a subliquidus phase at 1400°C at X (K 2 CO 3 ) 〈 24 mol%. Similar phase relations were established in the K 2 CO 3 –MgCO 3 system, which has two eutectics at 1200°C and 74 mol% and at 1250°C and 48 mol% K 2 CO 3 , respectively. The natural siderite used in the present study contained 6 mol% MnCO 3 and 7 mol% MgCO 3 . Although the obtained Fe-bearing carbonate phases exhibit uniform Mn/(Fe + Mn + Mg) ratio, magnesium tends to redistribute into the solid phases K 2 Fe(CO 3 ) 2 or siderite. At 1200°C and X (K 2 CO 3 ) = 50 mol%, the K 2 Fe 0.88 Mn 0.06 Mg 0.06 (CO 3 ) 2 melt coexists with the K 2 Fe 0.78 Mn 0.06 Mg 0.16 (CO 3 ) 2 compound. Assuming continuous solid solution between K 2 Fe(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 , the K 2 Fe(CO 3 ) 2 end-member should melt congruently slightly below 1200°C, which is about 50° lower than the melting point of K 2 Mg(CO 3 ) 2 . The siderite–magnesite system was studied at 6 GPa and 900–1700°C. Complete solid solution is recorded between Fe 0.94 Mn 0.06 CO 3 siderite and magnesite. At X (MgCO 3 ) = 7 mol% and 1600°C, the (Fe 0.90 Mn 0.06 Mg 0.04 )CO 3 partial melt coexists with (Fe 0.86 Mn 0.06 Mg 0.08 )CO 3 siderite, whereas at X (MgCO 3 ) = 26 and 35 mol%, the (Fe 0.71 Mn 0.06 Mg 0.23 )CO 3 partial melt coexists with (Fe 0.51 Mn 0.06 Mg 0.43 )CO 3 siderite. Based on these data, Fe 0.94 Mn 0.06 CO 3 siderite should melt slightly below 1600°C, i.e . 300° lower than magnesite. Development of bubbles in the quenched melt at X (MgCO 3 ) = 7 mol% and 1700°C suggests incongruent melting of siderite according to the reaction: siderite = liquid + CO 2 fluid.

Description: The new Na 4 Ca(CO 3 ) 3 , Na 2 Ca 3 (CO 3 ) 4 and Na 2 Ca 4 (CO 3 ) 5 compounds were synthesized in the system Na 2 CO 3 –CaCO 3 in multianvil experiments at 6 GPa and characterized by Raman spectroscopy. In addition, the Na 2 Ca 3 (CO 3 ) 4 compound was studied using in situ energy dispersive and single-crystal X-ray diffraction. Single bands in the CO 3 2– symmetric stretching region ( v 1 ) and out-of-plane bending region ( v 2 ) in the Na 4 Ca(CO 3 ) 3 Raman spectrum suggest a single crystallographically distinct carbonate group in the structure. In contrast, the spectra of Na 2 Ca 3 (CO 3 ) 4 and Na 2 Ca 4 (CO 3 ) 5 show two and three bands, respectively, in both the symmetric stretching region ( v 1 ) and out-of-plane bending region ( v 2 ), suggesting more than one crystallographically distinct carbonate group in the unit cell. Raman activity in the forbidden v 2 mode and multiple bands are observed in the in-plane bending region ( v 4 ) for the three compounds, proving the reduction of site symmetry of the CO 3 2– ions with the loss of the threefold rotation axis ( D 3h -〉 D 2h or C s ). Such a decrease in symmetry suggests distortion of the group itself, but may be attained by rearrangements of the coordinated metal cations as in the aragonite-group carbonates. At 6.5 GPa and 1000°C, the structure of Na 2 Ca 3 (CO 3 ) 4 was found to be orthorhombic or monoclinic with a β angle close to 90° and the lattice parameters: a = 7.3357(6) Å, b = 8.0377(9) Å, and c = 31.5322 (32) Å, with V = 929.59(14) Å 3 . No structural changes were observed during pressure decrease down to 1 GPa, while a discontinuous increase in unit-cell parameters and volume was observed upon decompression from 1 GPa at room temperature. This indicates a pressure-induced phase transition to a structurally related ambient-pressure phase. The abnormally long c -parameter and proximity of the β-angle to 90° of Na 2 Ca 3 (CO 3 ) 4 at ambient conditions suggest that, in the monoclinic system, the metric symmetry is higher than the Laue symmetry, which is a common sign for merohedral twinning.

Description: In situ X-ray diffraction study of the pyroxene to majorite transition in Na 2 MgSi 5 O 12 was carried out in Kawai-type high-pressure apparatus coupled with synchrotron radiation. The phase boundary between Na-pyroxene and Na-majorite was determined over the temperature interval of 1073–1973 K and was described by a linear equation P (GPa) = 12.39 + 0.0018 x T (K). The Clapeyron slope (d P /d T ) determined in this study is similar to the one predicted by computer simulations ( Vinograd et al. 2011 ) but smoother than the one obtained by quenched experiments ( Dymshits et al. 2010 ). The presence of sodium in the system lowers the pressure of pyroxene-to-majorite transformation. For the first time Na-majorite was characterized using Raman spectroscopy. Raman peaks of Na-majorite are broader than pyrope due to the substitution of Mg 2+ for Na + at the X site. Both Si-O symmetric stretching ( A 1 g - 1 ) and O-Si-O symmetric bending ( A 1 g - 2 ) modes of Na-majorite are significantly shifted to higher frequencies relative to corresponding bands of pyrope. In contrast the A 1 g -R (SiO 4 ) mode of Na-majorite (342 cm –1 ) displays a lower frequency than that of pyrope (365 cm –1 ). Our results enable further understanding of the mechanisms responsible for phase transformations in the Earth’s transition zone and lower mantle.

Description: To constrain the ternary K 2 CO 3 -CaCO 3 -MgCO 3 T-X diagram at 6 GPa and to expand upon the known K-Mg, K-Ca, and Ca-Mg binary systems we have carried out multi-anvil experiments along the K 2 CO 3 -Ca 0.5 Mg 0.5 CO 3 join. The diagram has primary phase fields for K 2 CO 3 , K 2 Mg(CO 3 ) 2 , K 2 Ca 0.1–0.5 Mg 0.9–0.5 (CO 3 ) 2 , K 4 CaMg(CO 3 ) 4 , Ca-magnesite, and dolomite. The system has two liquidus minima near 1000 °C. At one minimum, a liquid with the composition of 36 K 2 CO 3 ·64(Ca 0.65 Mg 0.35 )CO 3 is in equilibrium with three phases: Ca-magnesite, K 2 Ca 0.1–0.5 Mg 0.9–0.5 (CO 3 ) 2 , and K 6 Ca 2 (CO 3 ) 5 . The other minimum, a liquid with the composition of 62 K 2 CO 3 ·38Ca 0.72 Mg 0.28 CO 3 is in equilibrium with K 2 CO 3 , K 4 CaMg(CO 3 ) 4 , and K 6 Ca 2 (CO 3 ) 5 . At 900 °C, the ternary diagram contains two- and three-phase regions with Ca-magnesite, aragonite, K 2 Ca 3 (CO 3 ) 4 , K 2 Ca(CO 3 ) 2 , K 6 Ca 2 (CO 3 ) 5 , K 2 CO 3 , K 2 Ca 0.1–0.5 Mg 0.9–0.5 (CO 3 ) 2 solid solution, K 2 Mg 0.9 Ca 0.1 (CO 3 ) 2 , and K 4 CaMg(CO 3 ) 4 . We also expect an existence of primary phase fields for K 6 Ca 2 (CO 3 ) 5 , K 2 Ca 3 (CO 3 ) 4 and aragonite. We suggest that extraction of K from silicate to carbonate components should decrease the minimum melting temperature of dry carbonated mantle rocks up to 1000 °C at 6 GPa and yield ultrapotassic Ca-rich dolomite melt containing more than 10 mol% K 2 CO 3 . As temperature increases above 1200 °C the melt evolves toward an alkali-poor, dolomitic liquid if the bulk molar CaO/MgO ratio 〉1, or toward K-Mg-rich carbonatite if bulk CaO/MgO 〈 1. The majority of compositions of carbonatite inclusions in diamonds from around the world fall within the magnesite primary field between the 1300 and 1400 °C isotherms. These melts could be formed by partial melting of magnesite-bearing peridotite or eclogite with bulk Ca/Mg 〈1 at temperatures ≤1400 °C. A few compositions revealed in the Ebelyakh and Udachnaya diamonds (Yakutia) fall within the dolomite primary field close to the 1200 °C isotherm. These melts could be formed by partial melting of dolomite-bearing rocks, such as carbonated pelite or eclogite with bulk Ca/Mg 〉1 at temperatures ≤1200 °C.

Description: The subsolidus and melting phase relations in the CaCO 3 -siderite system have been studied in multi-anvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900–1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R c , while CaCO 3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO 3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900–1000 and 1600 °C, respectively. An increase of FeCO 3 content above the solubility limit at T 〉 1000 °C, leads to composition-induced phase transition in CaCO 3 from aragonite, Pmcn , to calcite, R c , structure, i.e., the presence of FeCO 3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO 3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO 3 -CaCO 3 join extends from 1580 °C (FeCO 3 ) to 1670 °C (CaCO 3 ) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO 3 . At X (Ca) = 0–30 mol%, 6 GPa and 1500–1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X (Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO 2 in (Fe,Ca)CO 3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite ( Hammouda 2003 ; Yaxley and Brey 2004 ) and peridotite ( Dasgupta and Hirschmann 2007 ; Brey et al. 2008 ) systems are broadly consistent with the topology of the melting loop in the CaCO 3 -MgCO 3 -FeCO 3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO 〉 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO 〈 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO 3 phase relations.

Description: Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca 2.85 Mg 0.96 Fe 0.11 Si 2.04 O 8 ) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the São Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO 3 in the core and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO 3 and merwinite) and diamond at an early stage, and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

Description: The phase relations in the Na 2 CO 3 -(Fe 0.87 Mn 0.06 Mg 0.07 )CO 3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900–1400 °C. Subsolidus assemblages comprise the stability fields of Na 2 CO 3 + Na 2 Fe(CO 3 ) 2 and Na 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (Na 2 CO 3 ) = 50 mol%. Intermediate Na 2 Fe(CO 3 ) 2 compound has rhombohedral R eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) Å, V = 354.61(22). The Na 2 CO 3 -Na 2 Fe(CO 3 ) 2 eutectic is established at 1000 °C and 66 mol% Na 2 CO 3 . Na 2 Fe(CO 3 ) 2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na 2 CO 3 . Siderite remains a subliquidus phase at 1400 °C at X (Na 2 CO 3 ) ≤ 30 mol%. The ternary Na 2 CO 3 -FeCO 3 -MgCO 3 system can be built up from the corresponding binary systems: two systems with intermediate Na 2 (Mg,Fe)(CO 3 ) 2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO 3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO 3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.