O. G. Safonov

Crystal-melt equilibria involving potassium-bearing clinopyroxene as indicator of mantle-derived ultrahigh-potassic liquids: an analytical review

Abstract Crystal–liquid equilibria, including phase relationships of minerals with silicate and/or carbonate melts, are reviewed in order to understand the occurrence of clinopyroxene with up to 2 wt.% K 2 O ( KCpx ). This mineral occurs as inclusions in diamond from kimberlite pipes and in garnet from garnet–clinopyroxene potassium-poor silicate rocks intercalated with diamondiferous silicate–carbonate rocks of the Kokchetav Complex, northern Kazakhstan. The analysis of the available experimental data allowed estimation of the effect of P , T and compositional parameters on the equilibrium of KAlSi 2 O 6 (in Cpx )=1/4K 4 Si 2 O 6 +3/4Al 4/3 Si 2 O 6 (in melt ) in various silicate systems. A strong dependence of the K 2 O partition coefficient both on pressure and SiO 2 and Al 2 O 3 contents in the melt was identified and thermodynamically described. The resulting thermodynamic equation allows the calculation of pressure of the KCpx formation within interval 50–100 kbar for known melt compositions. A model for the formation of KCpx in deep-mantle potassium-rich carbonate–silicate magmas was derived using the available experimental and petrologic data. The formation of KCpx from any potassium-poor (K 2 O K p Cpx / L =K 2 O in Cpx /K 2 O of melt is much smaller then 1. The presence of Cpx with 1 wt.% of K 2 O as inclusions in Grt from potassium-poor rocks could be explained only by the crystallization of KCpx from potassium-rich silicate or silicate–carbonate magma in the presence of KCl brine at P >70 kbar. This chemical zoning of potassium-bearing clinopyroxene suggests its crystallization together with feldspar from silicate melt during rapid ascent of melt toward the Earths surface from depth about 200 km. Since Tschermak-type substitution has not been observed in either pyroxene, we suggested that feldspar resulted from the peritectic reaction KAlSi 2 O 6 +[SiO 2 ] L / fl =KAlSi 3 O 8 at relatively shallow mantle levels. A hypothetical phase diagram for the system Ca(Mg,Fe)Si 2 O 6 –KAlSi 2 O 6 is presented.

Response to comments by Nicoli et al. on paper by Belyanin et al. (2012)

The basis of the Nicoli’s et al. (this issue) comment is a critique of the temperature of *1,100 C estimated by Belyanin et al. (2012) from rare Al–Mg-rich mineral assemblages in sample DR19 from the Southern Marginal Zone (SMZ) Limpopo Complex (LC). However, these authors used this critique to promote the argument that UHT conditions are impossible (their terminology) in the SMZ and that Tmax could not have been much higher than 850 C. In the reply, we address major issues highlighted by Nicoli’s et al. (this issue), namely that of biotite stability and dehydration melting of metapelite, and provide new P–T data based on combination of PERPLE_X pseudosection modeling and TWQ conventional thermobarometry for sample DR19. We provide unequivocal evidence that TMax in the SMZ must have been at least 900 C in accordance with the requirement for UHT conditions (e.g., Harley 2008). In fact, higher temperatures could easily have been attained (e.g., Tsunogae et al. 2004). Discussion of the Nicoli’s et al. comments

Ultrapotassic clinopyroxene from the Kumdy-Kol microdiamond mine, Kokchetav Complex, Kazakhstan: Occurrence, composition and crystal-chemical characterization

Abstract We report data on the composition and crystal structure of the most K-rich (3.61 wt% K2O) natural clinopyroxene yet discovered. The studied crystal was found as a tiny inclusion in garnet from a garnet-clinopyroxene rock of the Kumdy-Kol microdiamond mine, Kokchetav complex, Northern Kazakhstan. Microprobe analysis yields the formula (Ca0.61Fe0.13Mg0.04Mn0.01K0.17Na0.05) (Al0.61Mg0.39)(Si1.61Al0.39)O6.00. Lattice parameters are: a = 9.773(1), b = 8.926(1), c = 5.269(1) Å, β = 105.75(1)°. The structure was refined up to Rall = 2.42% using 982 independent reflections. Substitution of K for Ca causes significant modification of the average structure. No evidence for an additional M2’ position was found. Crystal-chemical characteristics are compared with published data on both natural and synthetic K-bearing clinopyroxenes.

Origin of Cl-bearing silica-rich melt inclusions in diamonds: Experimental evidence for an eclogite connection

Melting phase relations of a model chloride- and carbonate-bearing eclogite have been studied at 7.0–10.5 GPa and 1200–1675 °C. The mineral assemblage coexisting with partial melts is garnet, omphacite, kyanite, and coesite or stishovite. At temperatures of 1200–1400 °C, the partial melt has an SiO 2 -poor, Cl-bearing carbonatite composition. With increasing temperature, it becomes progressively SiO 2 rich, and at temperatures of 1500–1700 °C contains as much as 53 wt% SiO 2 . The compositions of the melts are comparable with those of silicic end members, of inclusions in fibrous and/or cloudy diamonds worldwide, implying that they may be produced via chemical reactions of alkalic chloride-carbonate liquids with mantle eclogites. Our experiments reproduced the trends of the compositional variations of the fluid or melt inclusions within eclogitic diamonds, and thus suggest a reliable model for their origin.

Ultrahigh potassium content in the clinopyroxene structure: an X-ray single-crystal study

Thecrystalstructuresandchemicalcompositionsoftwocrystalsofclinopyroxene(labelledK1andK2)synthesizedinthe model system CaMgSi2O6 - KAlSi2O6 at 7 GPa, have been investigated. Quantitative analysis leads to the following formulae: (Ca0.75K0.23Mg0.02)(Mg0.73Al0.27)(Si1.97Al0.03)O6.00and (Ca0.90K0.07Mg0.03)(Mg0.86Al0.14)(Si1.94Al0.06)O6.00, for K1 and K2 respectively. Latticeparametersare:a =9.803(2),b=8.985(2),c =5.263(1)A, =105.69(1)° forK1anda =9.744(1),b=8.904(2),c =5.273(1) A, b = 106.14(1)° for K2. The structures were refined to Rall = 1.16 and 2.02 %, respectively, using 982 independent reflections. Substitution of K for Ca causes strong modifications to the average structure, mainly a lengthening of the M2-O3C1 bond distance and a shortening of the T-O3A1 bond distance. The crystal K1 shows the largest volume for a M2 polyhedron ever reported for the clinopyroxene structure withdivalentcations inthissite. No evidence was found fora smallcation such as Mg coexistingwithCa in the M2 site that might act as a structural stabilizer for the accommodation of K in the clinopyroxene structure.

Synthetic hypersilicic Cl-bearing mica in the phlogopite-celadonite join : A multimethodical characterization of the missing link between di-and tri-octahedral micas at high pressures

Abstract A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 °C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + VIAl + VI □ = IVAl + VIMg) and a chemical formula: K1.01(Mg2.45Al0.19□ 0.35)Σ=3(Si3.52Al0.48)Σ=4O10[(OH,O)1.66Cl0.34)]Σ=2. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm-1 in the Raman spectra. Singlecrystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) Å, β =100.06(4)°, V = 489.1(4) Å3. The structure refinement shows the presence of vacancies on the octahedral sites (15% for M1 and 6.5% for M2). Chlorine occupies a position about 0.5 Å from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle α is 0°. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the α angle and expands its stability field at high pressure. High aK₂O conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Alceladonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

Abstract The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin diamond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) Å3, K0= 129(1) GPa, K’ = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compressibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show significant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to fit into the pyroxene structure.

Potassium-bearing clinopyroxene: a review of experimental, crystal chemical and thermodynamic data with petrological applications

Abstract Available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic end member with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside-K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation (ΔfH0 = -2932.7 kJ/mol), the standard volume (V0 = 6.479 J mol1 bar1) and the isothermal bulk modulus (K0 = 145 GPa) of K-jadeite were calculated from first principles, and the standard entropy (S0 = 141.24 J mol-1 K-1) and thermal-expansion coefficient (α = 3.3 × 10-5 K-1) of the K-jadeite end member were estimated using quasiharmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wide P-T interval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.

Indicator reactions of K and Na activities in the upper mantle: Natural mineral assemblages, experimental data, and thermodynamic modeling

The paper presents a review of data on mineral assemblages and reactions that are potential indicators of K and Na activities in upper mantle fluids and melts modifying upper mantle rocks in the course of mantle metasomatism. Results of experimental modeling of these reactions are discussed. These data are utilized to calculate phase reactions in