ALBERT

Description: Sulfur, along with hydrogen, oxygen, and carbon, is one of the most common volatiles in magmatic mantle processes. As a redox-sensitive element, sulfur can have a direct influence on the redox evolution of mantle rocks, melts, and fluids, and participate in processes of mantle metasomatism. Modern concepts suggest that subduction processes play a key role in the global sulfur cycle. We report the results of the first high-pressure–high-temperature experiments in olivine-sulfur and olivine-pyrite systems aimed at modeling sulfidation processes in a silicate mantle with involvement of S-bearing fluids or melts and determining a potential mechanism of sulfide formation under deep subduction conditions. It was found that at the lower temperature stage of sulfidation, the partial recrystallization of olivine was accompanied by extraction of Fe and Ni into an S-bearing fluid and, finally, an olivine, orthopyroxene, and pyrite assemblage was formed; i.e., sulfide mineralization of an ultramafic rock occurred. At higher temperatures, the complete sulfidation and recrystallization of olivine resulted in the formation of forsterite and enstatite, containing inclusions of Ni-rich sulfide melt. Strong enrichment of S-bearing fluids in Fe and Ni led to a further sulfide melt generation. It is thus experimentally demonstrated that the influence of ephemeral S-bearing fluids on ultramafic mantle rocks results in an extraction of base metals from the solid-phase silicates, modifying their mineral and chemical compositions, and providing conditions for mobilization of an ore material in the form of sulfides at pressure-temperature conditions of the lithospheric mantle.

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: 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: Multiple saturation experiments have been performed in a multicomponent system at 6.3 to 7.5 GPa and 1400–1670 °C using a split-sphere multi-anvil apparatus to constrain the conditions of kimberlite magma generation. The starting bulk compositions of samples corresponded to the average group II kimberlite (orangeite), with water contents varying from 5 to 9 wt% H 2 O and the CO 2 /(CO 2 +H 2 O) molar ratio from 0.37 to 0.24. The charges were placed inside graphite liners sealed in Pt capsules to avoid Fe loss. Oxygen fugacity ( f O 2 ) during the experiment was buffered by the equilibrium between graphite and a hydrous carbonate-silicate melt about EMOG/D. As water in the starting kimberlite increased from 5 to 9 wt%, the temperature of its complete melting became ~100 °C lower (relative to 1670 °C), both in the 6.3 and 7.5 GPa runs. Orthopyroxene was stable just below the liquidus at all pressures and H 2 O concentrations applied in the experiments. An olivine + garnet + orthopyroxene assemblage was present at ≤100 °C below the liquidus when H 2 O was 5 wt%. At 7 and 9 wt% H 2 O, the same assemblage appeared at 100–150 and 〉200 °C below the liquidus, respectively. In no experiment was clinopyroxene observed as a run product. Olivine, garnet, and orthopyroxene stable in the multiply saturated melt were compositionally similar to mantle peridotite minerals found as xenoliths in kimberlites worldwide. Thus we infer that generation of group II kimberlite magma may occur by partial melting of carbonated (metasomatized) garnet harzburgite at pressures from 6.3 to 7.5 GPa, temperatures about 1500–1600 °C, and no more than 5 wt% H 2 O in the melt. Water, in the amounts required to produce this magma, may come from interaction of K-Ca-rich carbonatite melt, infiltrating from a deeper mantle source, with a peridotite protolith containing H 2 O in nominally anhydrous minerals and, possibly, also in phlogopite.

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.

Description: Phase relations in the system K 2 CO 3 -CaCO 3 have been studied in the compositional range, X (K 2 CO 3 ), from 100 to 10 mol%, at 6.0 GPa and 900–1450 °C. At 900–950 °C, the system has three intermediate compounds: K 6 Ca 2 (CO 3 ) 5 , K 2 Ca(CO 3 ) 2 , and K 2 Ca 3 (CO 3 ) 4 . The K 2 Ca(CO 3 ) 2 compound decomposes to the K 6 Ca 2 (CO 3 ) 5 + K 2 Ca 3 (CO 3 ) 4 assembly above 950 °C. The K 6 Ca 2 (CO 3 ) 5 and K 2 Ca 3 (CO 3 ) 4 compounds melt congruently slightly above 1200 and 1300 °C, respectively. The eutectics were established at 64 and 44 mol% near 1200 °C and at 23 mol% near 1300 °C. K 2 CO 3 remains as a liquidus phase at 1300 °C and 75 mol% and melts at 1425 ± 20 °C. Aragonite remains as a liquidus phase at 1300 °C and 20 mol% and at 1400 °C and 10 mol%. CaCO 3 solubility in K 2 CO 3 and K 2 CO 3 solubility in aragonite are below the detection limit (〈0.5 mol%). Infiltration of subduction-derived K-rich Ca-Mg-Fe-carbonatite into the Fe 0 -saturated mantle causes the extraction of (Mg,Fe)CO 3 components from the melt, which shifts its composition toward K-Ca-carbonatite. According to our data this melt can be stable at the P-T conditions of subcratonic lithosphere with geothermal gradient of 40 mW/m 2 corresponding to temperature of 1200 °C at 6 GPa.