Anton V. Arefiev

Liquid immiscibility plays an important role in the formation of carbonatites and associated alkaline Si-undersaturated magmas. Experiments in the sodium carbonate-aluminosilicate systems suggest that the carbonate-silicate miscibility gap is limited by crustal and shallow mantle pressures (up to 2.5 GPa). Unlike in the potassium-rich carbonate-aluminosilicate systems, the carbonate-silicate miscibility gap was established at pressures of 3.5–6 GPa. It is therefore interesting to elucidate the immiscibility range under intermediate pressures, corresponding to 100–200 km depths. Here we conducted experiments over 3–6 GPa and 1050–1500 °C in the systems corresponding to immiscible melts obtained by partial melting of carbonated pelite (DG2) at 6 GPa and 1200 °C. We found that partial melting begins with the alkali-rich carbonatite melt, while immiscible phonolite melt appears over 1050–1200 °C at 3 GPa, 1200 °C at 4.5 GPa, and 1200–1500 °C at 6 GPa. As pressure decreases from 6 to 3 GPa, Na becomes less compatible, and the concentration of the jadeite component in clinopyroxene decreases by a factor of 1.5–6. As a result, the compositions of the immiscible phonolite and carbonatite melts evolve from ultrapotassic (K₂O/Na₂O weight ratio = 10–14) resembling silicic and carbonatitic micro-inclusions in diamonds from kimberlites and placers worldwide to moderately potassic (K₂O/Na₂O = 1–2), which may correspond to phonolitic and associated carbonatitic melts of the spinel facies of the shallow mantle.

Inclusions in mantle minerals and xenoliths from kimberlites worldwide derived from depths exceeding 100 km vary in composition from alkali-rich saline to carbonatitic. Despite the wide distribution of these melts and their geochemical importance as metasomatic agents that altered the mineralogy and geochemistry of mantle rocks, the P-T range of stability of these melts remains largely undefined. Here we report new experimental data on phase relations in the system KCl–CaCO₃–MgCO₃ at 3 GPa obtained using a multianvil press. We found that the KCl–CaCO₃ and KCl–MgCO₃ binaries have the eutectic type of T-X diagrams. The KCl-calcite eutectic is situated at K2# 56 and 1000 °C, while the KCl-magnesite eutectic is located at K2# 79 and 1100 °C, where K2# = 2KCl/(2KCl + CaCO₃ + MgCO₃) × 100 mol%. Just below solidus, the KCl–CaCO₃–MgCO₃ system is divided into two partial ternaries: KCl + magnesite + dolomite and KCl + calcite–dolomite solid solutions. Both ternaries start to melt near 1000 °C. The minimum on the liquidus/solidus surface corresponds to the KCl + Ca_0.73Mg_0.27CO₃ dolomite eutectic situated at K2#/Ca# 39/73, where Ca# = 100∙Ca/(Ca + Mg) × 100 mol%. At bulk Ca# ≤ 68, the melting is controlled by a ternary peritectic: KCl + dolomite = magnesite + liquid with K2#/Ca# 40/68. Based on our present and previous data, the KCl + dolomite melting reaction, expected to control solidus of KCl-bearing carbonated eclogite, passes through 1000 °C at 3 GPa and 1200 °C at 6 GPa and crossovers a 43-mW/m² geotherm at a depth of 120 km and 37-mW/m² geotherm at a depth of 190 km.

Melting phase relations in the eclogite-carbonate system were studied at 6 GPa and 900–1500 °C. Starting mixtures were prepared by blending natural bimineral eclogite group A (Ecl) with eutectic Na-Ca-Mg-Fe (N2) and K-Ca-Mg-Fe (K4) carbonate mixtures (systems Ecl-N2 and Ecl-K4). In the Ecl-N2 system, the subsolidus assemblage is represented by garnet, omphacite, eitelite, and a minor amount of Na₂Ca₄(CO₃)₅. In the Ecl-K4 system, the subsolidus assemblage includes garnet, clinopyroxene, K₂Mg(CO₃)₂, and magnesite. The solidus of both systems is located at 950 °C and is controlled by the following melting reaction: Ca₃Al₂Si₃O₁₂ (Grt) + 2(Na or K)₂Mg(CO₃)₂ (Eit) = Ca₂MgSi₃O₁₂ (Grt) + [2(Na or K)₂CO₃∙CaCO₃∙MgCO₃] (L). The silica content (in wt%) in the melt increases with temperature from < 1 at 950 °C to 3–7 at 1300 °C, and 7–12 at 1500 °C. Thus, no gradual transition from carbonate to kimberlite-like (20–32 wt% SiO₂) carbonate-silicate melt occurs even as temperature increases to mantle adiabat. This supports the hypothesis that the high silica content of kimberlite is the result of decarbonation at low pressure. As temperature increases from 950 to 1500 °C, the melt Ca# ranges from 58–60 to 42–46. The infiltration of such a melt into the peridotite mantle should lower its Ca# and causes refertilization from harzburgite to lherzolite and wehrlitization.

Inclusions of former alkaline carbonate melts occur in igneous minerals, mantle xenoliths, and diamonds from kimberlites and lamproites. However, the identification of most daughter phases remains undetermined owing to the lack of their Raman spectra. Yet, the phase compositions of the alkali carbonate melt at different pressure are interesting to know because Na‐Ca, Na‐Mg, K‐Ca, and K‐Mg double carbonates have a number of pressure‐induced phase transitions. Here, we studied the quench products of K‐Ca, K‐Mg, and Na‐Ca carbonate melts obtained at 3 and 6 GPa. Scanning electron microscopy coupled with energy‐dispersive spectroscopy with silicon drift detector (EDS‐SDD) and Raman spectroscopy were employed for phase characterization. The following carbonates were detected among the quench products: γ‐Na₂CO₃, Na₄Ca(CO₃)₃ (Ia3d), Na₂Ca₃(CO₃)₃ (P1n1), K₂Ca(CO₃)₂ ( ) bütschliite, disordered K₂Ca₃(CO₃)₄ (Pnam), K₂Mg(CO₃)₂ ( ), aragonite, magnesite at 6 GPa; and CaCO₃‐bearing K₂CO₃, K₂Ca(CO₃)₂ ( ) bütschliite, K₂Ca₂(CO₃)₃ (R3), and ordered K₂Ca₃(CO₃)₄ (P2₁2₁2₁) at 3 GPa. The results indicate that the quench products of carbonate melt consist of carbonates thermodynamically stable at the pressure of quenching. Thus, the daughter phases in carbonatitic inclusions could be identified by obtained Raman spectra and reflect the pressure conditions of their entrapment.

To clarify the effect of water on carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments in the system KAlSi3O8-CaMgSi2O6-NaAlSi2O6-CaMg(CO3)2 under nominally dry and hydrous conditions by adding 1.5 wt% H2O at a pressure of 6 GPa and temperatures of 1000 to 1500 °C. Both systems start to melt at 1050–1100 °C. Under anhydrous condition the melting occurs via the following reaction: 6KAlSi3O8 (K-feldspar) + 6CaMg(CO3)2 (dolomite) = 2(Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11SiO2 (coesite) + 3 K2(Ca1-n, Mgn)2(CO3)3 (carbonatitic melt) + 3CO2 (fluid and/or liquid), whereñ 0.3–0.4. The carbonatitic melt has the following composition 38(K0.92Na0.08)2CO3⋅62Ca0.62Mg0.38CO3. A second immiscible silicic melt containing (in wt%, volatile free) SiO2 = 68.8, Al2O3 = 12.6, CaO = 3.7, MgO = 2.4, Na2O = 1.1, and K2O = 11.3 appears at 1250 °C. Both melts remain stable up to 1500 °C and coexist with the clinopyroxene ± garnet ± coesite residue. In the presence of water stored away in phengite, the melting begins with silicic melt, which contains (in wt%, volatile free) SiO2 = 61.4, Al2O3 = 15.3, CaO = 4.8, MgO = 3.0, Na2O = 2.2, and K2O = 13.3, and coexists with phengite, dolomite, clinopyroxene, and coesite. The phengite + dolomite assemblage remains to 1100 °C and disappears at 1200 °C producing two immiscible melts carbonatitic with approximate composition, 19(K0.89Na0.11)2CO3⋅81Ca0.57Mg0.43CO3, and silicic containing (in wt%, volatile free) SiO2 = 63.3, Al2O3 = 15.6, CaO = 4.5, MgO = 3.0, Na2O = 2.0, K2O = 11.6. The present results imply that partial melting of continental material subducted to a depth of 200 km can yield simultaneous formation of two immiscible melts, K-dolomitic and K-aluminosilicate. Under dry conditions, carbonatitic melt appears earlier (at a lower temperature). Given the low density and high mobility of this melt, it must rapidly percolate upward, leaving a refractory eclogite-like residue and leaving no chance for the formation of a second aluminosilicate melt. However, under hydrous conditions silicate melt appears earlier than carbonatitic melt, leaving a phengite- and dolomite-bearing residue, which finally yields the formation of two immiscible silicic and carbonatitic melts. The compositions of these melts fall in the compositional range of carbonatitic and silicic high-density fluids (HDFs) in diamonds worldwide. Thus, we suggest that the presence of water is a necessary requirement for the formation of immiscible HDFs inclusions in diamonds, and this suggestion is strongly supported by natural data from HDFs.