Konstantin D. Litasov

The deep water cycle and flood basalt volcanism

Experimental data combined with numerical simulations suggest that fast-subducting oceanic slabs are cold enough to carry a significant amount of H2O into the deep mantle in antigorite; with increasing depth, this mineral undergoes solid–solid transitions to phase A and then to phase E and/or wadsleyite. Ice VII and clathrate hydrates can also be stable under pressure and temperature (PT) conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Cold slabs are expected to stagnate in the mantle transition zone. With time, they are heated to the temperature of the ambient transition zone and release excess water as a H2O-bearing fluid. This may cause voluminous melting of overlying upper mantle rocks. If such a process operates in nature, magmas geochemically similar to island-arc basalts are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the southeastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with the fast (cold) subduction of the Mongolia–Okhotsk oceanic slab, and originated by the dehydration of the stagnant slab in the transition zone. We show that the influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually diminished with increasing distance from the subduction zone. Thus, the unique size and volume of the Siberian flood basalt province could have originated due to long-term underflow beneath Siberia with or without the existence of a lower mantle plume.

The system K2CO3-MgCO3 at 6 GPa and 900–1450 °C

Abstract Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1450 °C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 °C. We propose that K2Mg(CO3)2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K2Mg(CO3)2+MgCO3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900-1000 °C. Thus, the K2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K2Mg(CO3)2 compound was studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K2Mg(CO3)2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)2 exhibited a trigonal R3̅m structure previously established at ambient conditions.

Melting and subsolidus phase relations in the system Na2CO3-MgCO3±H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle

Abstract Phase relations in the Na2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900-1400 °C temperatures. Sub-solidus assemblages are represented by Na2CO3+Na2Mg(CO3)2 and Na2Mg(CO3)2+MgCO3, with the transition boundary near 50 mol% MgCO3 in the system. The Na2CO3-Na2Mg(CO3)2 eutectic is established at 1200 °C and 29 mol% MgCO3. Melting of Na2CO3 occurs between 1350 and 1400 °C. We propose that Na2Mg(CO3)2 disappears between 1200 and 1250 °C via congruent melting. Magnesite remains as a liquidus phase above 1300 °C. Measurable amounts of Mg in Na2CO3 suggest an existence of MgCO3 solid-solutions in Na2CO3 at given experimental conditions. The maximum MgCO3solubility in Na-carbonate of about 9 mol% was established at 1100 and 1200 °C. The Na2CO3 and Na2Mg(CO3)2 compounds have been studied using in situ X‑ray coupled with a DIA-type multi-anvil apparatus. The studies showed that eitelite is a stable polymorph of Na2Mg(CO3)2 at least up to 6.6 GPa and 1000 °C. In contrast, natrite, γ-Na2CO3, is not stable at high pressure and is replaced by β-Na2CO3. The latter was found to be stable at pressures up to 11.7 GPa at 27 °C and up to 15.2 GPa at 1200 °C and temperatures at least up to 800 °C at 2.5 GPa and up to 1000 °C at 6.4 GPa. The X‑ray and Raman study of recovered samples showed that, under ambient conditions, β-Na2CO3 transforms back to γ-Na2CO3. Eitelite [Na2Mg(CO3)2] would be an important mineral controlling insipient melting in subducting slab and upwelling mantle. At 6 GPa, melting of the Na2Mg(CO3)2+MgCO3 assemblage can be initiated, either by heating to 1300 °C under “dry” conditions or at 900-1100 °C under hydrous conditions. Thus, the Na2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the Na- and Mg-rich carbonatite melts similar to those found as inclusions in olivines from kimberlites and the deepest known mantle rock samples-sheared peridotite xenoliths (190-230 km depth).

Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds

The transition zone of the Earth’s mantle (the depth interval between two major seismic discontinuities at 410 km and 660 km) is critical to understanding our planet’s evolution. Some diamonds are thought to have originated in the transition zone and the inclusions found in them are the only samples of material directly extracted from this depth range. By comparing natural majorite garnet inclusions in diamonds with the compositions of experimentally crystallized majorite garnets, we determine two major compositional trends, the pure metabasitic (or eclogitic) trend and the combined metaperidotitic and metapyroxenitic trend, that are strongly correlated with their preferred substitution mechanisms during majorite formation. Based on these trends, we demonstrate that the majority of the reported majorite inclusions in natural diamonds formed neither in a pure metabasite nor in a metaperidotite lithology, but in fact crystallized from a wide range of compositions intermediate between conventional basaltic and peridotitic, referred to here as metapyroxenitic. Given the dominance of metapyroxenite-type majorite diamond inclusions and their inferred syngenetic origin, we argue that a significant fraction of metapyroxenite rock is present within Earth’s transition zone and is important in the diamond-forming process. This is in agreement with recent self-consistent seismological and/or mineral physics studies that support models of a lithologically heterogeneous transition zone. From trace element and carbon isotope features, we infer a crustal origin for these rocks.

Melting of kimberlite of the Udachnaya-East pipe: Experimental study at 3–6.5 GPa and 900–1500°C

200 Kimberlites are the products of crystallization of the deepest magmas generated in the Earth’s mantle (below 150 km) and are the main sources of natural diamonds. In spite of significant progress in under� standing kimberlite petrogenesis, many problems, such as reconstruction of the compositions of primary kimberlite melts and their evolution during intrusion, are still debatable. The main problem is that the com� position of kimberlites does not correspond to the composition of parental melts. First, kimberlites are contaminated by a significant portion of xenogenic material represented by xenoliths and their fragments (xenocrysts). Second, most kimberlites worldwide underwent postmagmatic alterations to various

Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900–1700 °C and its relation to the system CaCO3-FeCO3-MgCO3

Abstract The subsolidus and melting phase relations in the CaCO3-siderite system have been studied in multianvil 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 R3c, while CaCO3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO3 from aragonite, Pmcn, to calcite, R3c, structure, i.e., the presence of FeCO3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO3-CaCO3 join extends from 1580 °C (FeCO3) to 1670 °C (CaCO3) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO3. 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 CO2 in (Fe,Ca)CO3 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 CaCO3- MgCO3-FeCO3 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)CO3 phase relations.

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.

Optical properties of siderite (FeCO3) across the spin transition: Crossover to iron-rich carbonates in the lower mantle

Abstract Upper mantle carbonates are thought to be iron poor and magnesium rich. However, at lower mantle conditions spin-pairing transitions in iron-bearing phases may trigger iron redistribution between the minerals. Here, using visible and near infrared absorption measurements, we examine the siderite crystal field up to 65 GPa. Optical spectrum of siderite at 1 bar has an absorption band at 10 325 cm-1 corresponding to the crystal field splitting energy (10Dq) of ferrous iron in an octahedral field. This band intensifies and blue-shifts (86 cm-1/GPa) with pressure, but disappears abruptly at 44 GPa signaling the spin transition. Simultaneously, a new absorption band centered at 15 629 cm-1 (88 cm-1/GPa) appears in the spectrum. Tanabe-Sugano diagram analysis allowed assigning the observed absorption bands to 5T2g → 5Eg and 1A1g → 1T1g electronic transitions in high- and low-spin siderite, respectively. Similarly, we evaluate the crystal field splitting energy of low-spin siderite 10Dq = 17 600 cm-1 (45 GPa), as well as the Racah parameters B = 747 cm-1 and C = 3080 cm-1. We find that the crystal field stabilization energy (CFSE) of ferrous iron in low-spin siderite (45 700 cm-1 at 45 GPa) is an order of magnitude higher than that in the high-spin phase (4130 cm-1 at 1 bar). From the derived CFSE values we estimate the iron-partitioning coefficient for the carbonate-perovskite system and show that lowspin carbonates are iron rich and magnesium poor. We also show that the color of siderite is governed by the 1Ag → 1T1g absorption band and the Fe-O charge transfer.

Microsoft excel spreadsheets for calculation of P-V-T relations and thermodynamic properties from equations of state of MgO, diamond and nine metals as pressure markers in high-pressure and high-temperature experiments

We present Microsoft Excel spreadsheets for calculation of thermodynamic functions and P-V-T properties of MgO, diamond and 9 metals, Al, Cu, Ag, Au, Pt, Nb, Ta, Mo, and W, depending on temperature and volume or temperature and pressure. The spreadsheets include the most common pressure markers used in in situ experiments with diamond anvil cell and multianvil techniques. The calculations are based on the equation of state formalism via the Helmholtz free energy. The program was developed using Visual Basic for Applications in Microsoft Excel and is a time-efficient tool to evaluate volume, pressure and other thermodynamic functions using T-P and T-V data only as input parameters. This application is aimed to solve practical issues of high pressure experiments in geosciences and mineral physics. The program to calculate P-V-T properties of pressure markers is presented.The program was developed using VBA module in MS Excel.The calculation scheme is based on the formalism of equations of state.Thermodynamic and P-V-T properties of MgO, diamond and 9 metals is calculated.

Revised calibration of the Sm:SrB4O7 pressure sensor using the Sm-doped yttrium-aluminum garnet primary pressure scale

The pressure-induced shift of Sm:SrB4O7 fluorescence was calibrated in a quasi-hydrostatic helium medium up to 60 GPa using the recent Sm-doped yttrium-aluminum garnet primary pressure scale as a reference. The resulting calibration can be written as P = −2836/14.3 [(1 + Δλ/685.51)−14.3 − 1]. Previous calibrations based on the internally inconsistent primary scales are revised, and, after appropriate correction, found to agree with the proposed one. The calibration extended to 120 GPa was also performed using corrected previous data and can be written as P = 4.20 Δλ (1 + 0.020 Δλ)/(1 + 0.036 Δλ).