Siberian flood basalt magmatism and Mongolia-Okhotsk slab dehydration

Experimental data combined with numerical calculations suggest that fast subducting slabs are cold enough to carry into the deep mantle a significant portion of the water in antigorite, which transforms with increasing depth to phase A and then to phase E and/or wadsleyite by solid-solid phase transition. Clathrate hydrates and ice VII are also stable at PT conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Some cold slabs are expected to deflect while crossing the 410 km and stagnate in transition zone being unable to penetrate through 660 km discontinuity. In this way slabs can move a long way beneath continents after long-lived subduction. With time, the stagnant slabs are heated to the temperature of the ambient transition zone and release free H~2~O-bearing fluid. Combining with transition zone water filter model this may cause voluminous melting of overlying upper mantle rocks. If such process operates in nature, magmas geochemically similar to island-arc magmas are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the south-eastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with fast subduction of the Mongolia-Okhotsk slab and originated by dehydration of the stagnant slab in the transition zone. We show that influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually reduced with increasing distance from the subduction zone

Thermal equation of state and thermodynamic properties of iron carbide Fe 3 C to 31 GPa and 1473 K

Resent experimental and theoretical studies suggested preferential stability of Fe 3 C over Fe 7 C 3 at the condition of the Earth's inner core. Previous studies showed that Fe 3 C remains in an orthorhombic structure with the space group Pnma to 250 GPa, but it undergoes ferromagnetic (FM) to paramagnetic (PM) and PM to nonmagnetic (NM) phase transitions at 6–8 and 55–60 GPa, respectively. These transitions cause uncertainties in the calculation of the thermoelastic and thermodynamic parameters of Fe 3 C at core conditions. In this work we determined P‐V‐T equation of state of Fe 3 C using the multianvil technique and synchrotron radiation at pressures up to 31 GPa and temperatures up to 1473 K. A fit of our P‐V‐T data to a Mie‐Gruneisen‐Debye equation of state produce the following thermoelastic parameters for the PM‐phase of Fe 3 C: V 0  = 154.6 (1) Å 3 , K T 0 = 192 (3) GPa, K T ′ = 4.5 (1), γ 0 = 2.09 (4), θ 0  = 490 (120) К, and q  = −0.1 (3). Optimization of the P‐V‐T data for the PM phase along with existing reference data for thermal expansion and heat capacity using a Kunc‐Einstein equation of state yielded the following parameters: V 0  = 2.327 cm 3 /mol (154.56 Å 3 ), K T 0  = 190.8 GPa, K T ′ = 4.68, Θ E10  = 305 K (which corresponds to θ 0  = 407 K), γ 0  = 2.10, e 0  = 9.2 × 10 −5 K −1 , m  = 4.3, and g  = 0.66 with fixed parameters m E 1  = 3 n  = 12, γ ∞  = 0, β  = 0.3, and a 0  = 0. This formulation allows for calculations of any thermodynamic functions of Fe 3 C versus T and V or versus T and P . Assuming carbon as the sole light element in the inner core, extrapolation of our equation of state of the NM phase of Fe 3 C suggests that 3.3 ± 0.9 wt % С at 5000 К and 2.3 ± 0.8 wt % С at 7000 К matches the density at the inner core boundary. Key Points We present a P‐V‐T EOS for PM‐Fe 3 C with support from thermodynamic analyses We discuss uncertainties in magnetic transitions We applied EOS data for modeling carbon content in the corePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101805/1/jgrb50396.pd

ПОЧТИ АБСОЛЮТНЫЕ УРАВНЕНИЯ СОСТОЯНИЯ АЛМАЗА, Ag, Al, Au, Cu, Mo, Nb, Pt, Ta, W ДЛЯ КВАЗИГИДРОСТАТИЧЕСКИХ УСЛОВИЙ

Using the modified formalism of [Dorogokupets, Oganov, 2005, 2007], equations of state are developed for diamond, Ag, Al, Au, Cu, Mo, Nb, Pt, Ta, and W by simultaneous optimization of shock-wave data, ultrasonic, X-ray, dilatometric and thermochemical measurements in the temperature range from ~100 K to the melting temperature and pressures up to several Mbar, depending on the substance. The room-temperature isotherm is given in two forms: (1) the equation from [Holzapfel, 2001, 2010] which is the interpolation between the low pressure (x≥1) and the pressure at infinite compression (x=0); it corresponds to the Thomas-Fermi model, and (2) the equation from [Vinet et al., 1987]. The volume dependence of the Grüneisen parameter is calculated according to equations from [Zharkov, Kalinin, 1971; Burakovsky, Preston, 2004] with adjustable parameters, t and δ. The room-temperature isotherm and the pressure on the Hugoniot adiabat are determined by three parameters, K', t and δ, and K0 is calculated from ultrasonic measurements. In our study, reasonably accurate descriptions of all of the basic thermodynamic functions of metals are derived from a simple equation of state with a minimal set of adjustable parameters.The pressure calculated from room-temperature isotherms can be correlated with a shift of the ruby R1 line. Simultaneous measurements of the shift and unit cell parameters of metals are conducted in mediums containing helium [Dewaele et al., 2004b; 2008; Takemura, Dewaele, 2008; Takemura, Singh, 2006], hydrogen [Chijioke et al., 2005] and argon [Tang et al., 2010]. According to [Takemura, 2001], the helium medium in diamond anvil cells provides for quasi-hydrostatic conditions; therefore, the ruby pressure scale, that is calibrated for the ten substances, can be considered close to equilibrium or almost absolute. The ruby pressure scale is given as P(GPa)=1870⋅Δλ/λ0⋅(1+6⋅Δλ/λ0). The room-temperature isotherms corrected with regard to the ruby scale can also be considered close to equilibrium or almost absolute. Therefore, the equations of state of the nine metals and diamond, which are developed in our study, can be viewed as almost absolute equations of state for the quasi-hydrostatic conditions. In other words, these equations agree with each other, with the ruby pressure scale, and they are close to equilibrium in terms of thermodynamics. The PVT relations derived from these equations can be used as mutually agreed pressure scales for diamond anvil cells in studies of PVT properties of minerals in a wide range of temperatures and pressures. The error of the recommended equations of the state of substances and the ruby pressure scale is about 2 or 3 per cent. Calculated PVT relations and thermodynamics data are available at http://labpet.crust.irk.ru.По единой схеме с использованием модифицированного формализма из [Dorogokupets, Oganov, 2005, 2007] построены уравнения состояния алмаза, Ag, Al, Au, Cu, Mo, Nb, Pt, Ta, W путем одновременной оптимизации ударных данных, ультразвуковых, рентгеновских, дилатометрических и термохимических измерений в диапазоне температур от ~100 К до температуры плавления и до давлений несколько Mbar в зависимости от вещества. Комнатная изотерма была задана двумя формами: уравнением В. Хольцапфеля [Holzapfel, 2001, 2010], которое является интерполяционным между низкими давлениями (x≥1) и давлением при бесконечном сжатии (x=0), соответствующим модели Томаса-Ферми, и уравнением П. Вине [Vinet et al., 1987]. Объемная зависимость параметра Грюнейзена рассчитана по соотношениям из [Zharkov, Kalinin, 1971; Burakovsky, Preston, 2004], в которых параметры t и δ являются подгоночными. Комнатная изотерма и давление на ударной адиабате определяются тремя параметрами: K', t и δ, а параметр K0 рассчитывается из ультразвуковых измерений. В результате нам удалось с разумной точностью описать все основные термодинамические функции металлов в рамках простого уравнения состояния с минимальным набором подгоночных параметров.Рассчитанное по комнатным изотермам давление можно сопоставить со сдвигом линии R1 люминесценции рубина, одновременные измерения которого и параметров ячейки металлов проведены в гелиевой [Dewaele et al., 2004b, 2008; Takemura, Dewaele, 2008; Takemura, Singh, 2006], водородной [Chijioke et al., 2005] и аргоновой средах [Tang et al., 2010]. Показано [Takemura, 2001], что гелиевая среда в алмазных наковальнях обеспечивает квазигидростатические условия, поэтому рубиновую шкалу, откалиброванную по десяти веществам, можно считать близкой к равновесной или почти абсолютной. Она имеет вид P(GPa)=1870⋅Δλ/λ0⋅(1+6⋅Δλ/λ0). Откорректированные по полученной рубиновой шкале комнатные изотермы других веществ также можно считать близкими к равновесным или почти абсолютным, поэтому построенные нами уравнения состояния девяти металлов и алмаза можно отнести к почти абсолютным уравнениям состояния для квазигидростатических условий. Другими словами, они являются взаимосогласованными между собой, с рубиновой шкалой давлений и близки к равновесным в термодинамическом смысле. Рассчитанные по ним P–V–T соотношения могут быть использованы в качестве взаимосогласованных шкал давления в алмазных наковальнях при изучении P–V–T свойств минералов в широкой области температур и давлений. Погрешность рекомендуемых уравнений состояния веществ и рубиновой шкалы составляет порядка 2–3 %. Расчет P–V–T соотношений и термодинамики доступен по адресу http://labpet.crust.irk.ru

Earth's Mantle Melting in the Presence of C-O-H-Bearing Fluid

This chapter reviews recent experimental data on phase transformations and melting in peridotite and eclogite systems with a C–O–H fluid at pressures up to about 30 GPa with special attention to the effect of redox conditions. It outlines the fundamental differences for partial melting in systems with H2O, CO2 and reduced C–O–H fluid (CH4–H2O–H2). Melting in the H2O‐bearing systems is controlled by hydrogen solubility in nominally anhydrous silicates and occurs when silicates are supersaturated with H2O at definite P, T, X, and fO2. Melting in CO2‐bearing systems is determined by alkali carbonate stability and controlled mainly by Na2O, K2O, and H2O. The chapter also argues that subducted carbonates should play a major role in the “big mantle wedge” model for stagnant or deeply‐sinking slabs and proposes a new mechanism for generating slab‐derived carbonate bearing diapirs in the transition zone

Phase relations and melting of carbonated peridotite between 10 and 20 GPa: A proxy for alkali- and CO2-rich silicate melts in the deep mantle

We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite + 5.0 wt % CO2 and PERC: fertile peridotite + 2.5 wt % CO2) at 10-20 GPa and 1,500-2,100 °C and constrain isopleths of the CO2 contents in the silicate melts in the deep mantle. At 10-20 GPa, near-solidus (ACP: 1,400-1,630 °C) carbonatitic melts with 40 wt % CO2 gradually change to carbonated silicate melts with > 25 wt % SiO2 and 15 GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10-20 GPa. The liquidus is likely to be at ~ 2,050 °C or higher at pressures of the present study, which gives a melting interval of more than 670 °C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20 GPa at near mantle adiabat or even at plume temperature. These alkali- and CO2-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the zone of metal saturation, CO2 and H2O flux melting can take place and kimberlite parental magmas can be generated. These mechanisms might be important for mantle dynamics and are potentially effective metasomatic processes in the deep mantle. © 2014 Springer-Verlag Berlin Heidelberg.ISSN:0010-7999ISSN:1432-096

Quench Products of K-Ca-Mg Carbonate Melt at 3 and 6 GPa: Implications for Carbonatite Inclusions in Mantle Minerals

Alkali-rich carbonate melts are found as inclusions in magmatic minerals, mantle xenoliths, and diamonds from kimberlites and lamproites worldwide. However, the depth of their origin and bulk melt composition remains unclear. Here, we studied quench products of K-Ca-Mg carbonate melt at 3 and 6 GPa. The following carbonates were detected at 3 GPa: K2CO3, K2Ca(CO3)2 bütschliite (R3¯2/m), o-K2Ca3(CO3)4 (P212121), K2Ca2(CO3)3 (R3), K2Mg(CO3)2 (R3¯m), Mg-bearing calcite, dolomite, and magnesite. At 6 GPa, the variety of quench carbonate phases includes K2CO3, K2Ca(CO3)2 bütschliite (R3¯2/m), d-K2Ca3(CO3)4 (Pnam), K2Mg(CO3)2 (R3¯m), aragonite, Mg-bearing calcite, dolomite, and magnesite. The data obtained indicate that alkali-bearing carbonate melts quench to the alkaline earth and double carbonates that are thermodynamically stable at quenching pressure and can be used as markers reflecting the pressure of their entrapment. Further, in this study, we established the fields of melt compositions corresponding to the distinct quench assemblages of carbonate minerals, which can be used for the reconstruction of the composition of carbonatitic melts entrapped by mantle minerals

Melt Composition and Phase Equilibria in the Eclogite-Carbonate System at 6 GPa and 900–1500 °C

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 2) 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