Mössbauer and Infrared Study of Clinopyroxenes Real Structure from Lamproites

Anton Antonov (avlan@aa2074.spb.edu), Nikita Ovchinnikov (nikita@ad.iggp.ras.spb.ru), Larisa Nikitina (nikita@ad.iggp.ras.spb.ru) & Miriam Babushkina (msb@mb2171.spb.edu)

Institute of Precambrian Geology and Geochronology, Russian Academy of Science, Makarova emb. 2, St-Petersburg, 199034, Russia

Clinopyroxenes (Cpx) diopside-augite (Ca(Mg,Fe)Si₂O₆ - (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆) series from mantle rocks - a transitional type lamproites and olivine lamproites of Baltic shield (Nikitina at al, 1998) were studied due to Mössbauer, infrared spectroscopy together with electron microprobe. X-ray diffraction analyses show no sign of impurities or inhomogeneity in Cpx. The degree of iron oxidation state in lamproite Cpx is much greater than in Cpx from metamorphic and effusive rocks (Nikitina et al, 1978). The amount of Fe³⁺ in lamproites Cpx are 20 - 68% from total iron and Fe³⁺ occupies M1 octahedral positions and replace silicon in tetrahedral (T) positions, as well. The substitution of Si by Fe³⁺ indicates in Cpx structure, together with aegirine component, the presence of ferri-Tschermak`s component - Ca Fe³⁺ Fe³⁺SiO₆, the existence of which was supposed by Morimoto (Morimoto, 1988). In Cpx structure the large cations Ca and Na occupy M2 position and the amount of vacancies (1-Ca-Na), suitable for Fe²⁺ and Mg occupation, is small. For Cpx lamproites we establish the preferable occupation of this vacancies by Fe²⁺. The same M2 occupation vacancies character is observed in effusive rocks Cpx (Nikitina et al., 1978), which show much greater ferrous iron disorder degree in lamproites and effusive clinopyroxenes, then in metamorphic ones. The occupation of M2 position by Ca, Na, Fe and Mg leads to the differences in cation environment composition for iron in M1 octahedron, which presents the additional doublets in Mössbauer spectra. In infrared spectra of Cpx the absorption bands of OH (3700 - 3530 cm^-1) and molecular water (3440 cm^-1) stretching vibrations are presented. The differences in position and number of absorption bands are connected with character of substitutions in crystallographic positions M1, M2 and T in structure of Cpx. Thus, in lamproitic clinopyroxene structures the large degree of iron oxidation and disordering of Fe³⁺ between octahedron and tetrahedron and Fe²⁺ between M1 and M2 positions are established and, also, the presence of OH-group and molecular water are detected.

Morimoto N, Amer. Miner., 73, 535-550, (1988).

Nikitina LP, Ekimov SP, Maslenikov AV, Osherovich EZ, Babushkina MS, Jemay AX, Cation distribution and thermodynamic of iron-magnesium silicate solid solutions, L: Nauka, 239, (1978).

Nikitina LP, Levskiy LK, Beliatsky BV, Juravlev VA, Lepekhina EN, Antonov AV, Abs. of Inter. Conf. Genesis problems of magmatic and metamorphic rocks, S. -Petersburg, 116-117, (1998).

Thermodynamic Mixing Properties of Ternary NaCl-H₂O-CO₂ Fluid Constrained from Phase Equilibrium Experiments

Leonid Y. Aranovich (aleonid@online.ru)¹, Taras V. Gerya (taras@iem.ac.ru)¹ & Robert C. Newton (rcnewton@midway.uchicago.edu)²

¹ Institute for Experimental Mineralogy, Chernogolovka, Moscow distr.142432, Russia

² Dept. of Earth and Space Sciences, UCLA, Los Angeles CA 90095, USA

H₂O activities in concentrated NaCl solutions were measured in the ranges 600-900^oC and 2-15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite - periclase dehydration equilibrium. Isobaric halite saturation points, which constrain the NaCl activity in the brine, were also determined at each experimental pressure. H₂O and CO₂ activities in the binary water-carbon dioxide fluid were constrained by experimentally bracketing equilibrium CO₂ mole fraction, X(CO₂), for simple dehydration and decarbonation reactions:

Talc = 3 Enstatite(En) + Quartz (Qz)+ H₂O;

Calcite + Qz = Wollastonite + CO₂;

En + Magnesite = Forsterite + CO₂,

in a broad range of pressure from 6-14 kbar, temperature from 600-1000^oC, and composition from X(CO₂)=.05-1.0. It was found that H₂O activity in the water-salt system decreases sharply with pressure at P above 4 kbar. The mixing effect in the H₂O-CO₂ system is positive up to the pressure of 14 kbar even at the highest experimental temperature of 1000^oC. A simple thermodynamic model of the concentrated H₂O-NaCl fluid has been developed, which treats the "configurational" part of the Gibbs free energy of mixing (G^mix) through the "effective degree of ionization", and also includes a non-configurational regular solution-type term. Five model parameters are calibrated based on the experimental data. Component activities in the H₂O-CO₂ binary are described by a simple van-Laar type equation with three empirical parameters calibrated to fit the experimental data. The resulting expression extrapolates well to agree with independent experimental activity-composition constraints at relatively low P-T (500^oC/0.5 kbar). The pressure derivatives of the G^mix agree very well with independent measurements of the volume of mixing in the binary in a wide P-T-X(CO₂) range. A simple additive model for the ternary system has been developed, with the NaCl-CO₂ binary represented by a sub-regular formalism with P-T dependent parameters. The 4 NaCl-CO₂ binary parameters are quantified to fit the experimental miscibility gap in the ternary system at 800^oC and 9 kbar (Shmulovich and Graham, 1999) along with a (very constraining) experimental observation that the NaCl melting temperature in pure CO₂ at 12 kbar is, within the ±5^oC uncertainty, the same as under dry conditions. Calculations with the model reproduce well experimental ternary phase diagram at 700^oC, 1 and 2 kbar. Adding strong electrolyte to the H₂O-CO₂ has a paramount effect on the isobaric T-X(CO₂) position of simple metamorphic reactions.

Shmulovich KI & Graham CM, Contrib. Mineral. Petrol., 136, 247-257