Experimental modeling of the transformation of H2O-CO2-CH4 inclusions during isobaric cooling and isothermal compression

The transformations (density, composition, or shape alterations) of fluid inclusions under isobaric cooling and isothermal compression have been experimentally modeled. The H₂O-CO₂-CH₄ inclusions used in the model experiments have been synthesized in quartz at 1 or 3 kbar and 700°C. The parameters of the model experiments were as follows: for isobaric cooling, 700–400°C at 1 or 3 kbar; for isothermal compression, 1–3 and 3–5 kbar at 400 and 700°C, respectively. The shape of the inclusion walls changed as a result of the experiments because of the dissolution and reprecipitation of the host phase. The intensity of these changes was directly affected by the pressure and temperature differences and the amount of the aqueous phase in the inclusions. Fluid densities increased in many inclusions in the run products: the increase ranged from insignificant (0.06 g/cm³) to fairly noticeable (0.15 g/cm³). The composition of the fluid inclusions also changed. The results of the experiments, in particular of those carried out with a labeled fluid, show that the penetration of a denser external fluid into inclusions with a lower fluid density is the basic reason for the increase in the density and change in the composition of fluid inclusions. In several experiments, the methane concentration changed with an attendant change in the melting temperature of carbon dioxide but without noticeable changes in the carbon dioxide homogenization temperature. Original Russian Text ? O.V. Vasyukova, V.I. Fonarev, 2006, published in Geokhimiya, 2006, No. 12, pp. 1266–1278.     

Experimental study of Fe-Mg distribution between biotite and orthopyroxene at P=490 MPa

Partitioning of Mg and Fe between coexisting biotite and orthopyroxene has been experimentally determined at temperatures 700, 750 and 800° C and 490 MPa total pressure in the system KAlO₂-MgO-FeO-SiO₂-H₂O. Oxygen fugacity was controlled by the QFM buffer. Starting materials were synthetic minerals of differing Fe/(Fe+Mg) values. Run products were analyzed for partitioning of components by a microprobe. Orthopyroxene was established to be notably inhomogeneous, whereas biotite was essentially homogeneous. To establish equilibrium relations, statistical treatment of the results of each experiment in addition to the whole complex of experimental data was applied. The regression equations for isotherms of the Fe-Mg partitioning between the minerals studied have been obtained. As a result, the equation for a two-dimensional regression may be written as: $$\begin{gathered} Y = (A + A_1 t + A_2 t^2 )(X - X^4 ) + (B + B_1 t + B_1 t^2 )(X^2 - X^4 ) + \hfill \\ (C + C_1 t + C_1 t^2 )(X^3 - X^4 ) + X^4 {\text{ where }}Y = X_{{\text{Opx}}}^{{\text{Fe}}} ;{\text{ X}} = {\text{X}}_{{\text{Bi}}}^{{\text{Fe}}} ; \hfill \\ t = 1000/T,K, \hfill \\ \begin{array}{*{20}c} {A = {\text{ }}4.59398,} & {A_1 = - {\text{ }}8.29838,} & {A_2 = {\text{ }}4.97316,} \\ {B = - 11.13731,} & {B_1 = {\text{ }}28.19304,} & {B_2 = - 20.98240,} \\ {A = {\text{ }}8.25072,} & {C_1 = - 20.80485,} & {C_2 = {\text{ }}15.35967} \\ \end{array} \hfill \\ {\text{ }}\sigma = 0.0143{\text{ }} \hfill \\ \end{gathered}$$ . This equation enables extrapolation of partitioning isotherms over a wide range of temperatures.     

Multistage Metamorphic Evolution of the Trivandrum Granulite Block, Southern India

The Trivandrum Granulite Block (TGB), southern India records evidence for three distinct stages of evolution (M1–;M3) during the Pan-African high grade metamorphism, with possible temperature gradient from north to south of the terrain as detected from mineral phase equilibria thermobarometry in three classic localities, namely Nuliyam, Kunnanpara and Nellikkala. The charnockites, both incipient and massive, were formed during the first stage (M1) at temperatures higher than their host rocks, and at appreciably lower pressures. Charnockite formation was dominantly controlled by an increase in partial pressure of CO₂, along structural locales during subisothermal decompression, although an increase of potash activity could have also been an important factor in this process. The charnockites at Nellikkala in the northern margin of TGB were formed under appreciably more H₂O-rich conditions (X_H2O = 0.53±0.03) than those at Nuliyam (X_H2O = 0.25±0.02) in the southern margin. It is inferred that during the period between the metamorphic stages M1 and M2, the terrain experienced subisobaric cooling. Comparison of results from thermobarometry with data on absolute age determinations from geochronology of the metamorphic rocks in TGB allows the interpretation that the M1 metamorphic event took place during 540–;600 Ma, M2 at about 530 Ma and M3 in the interval of 440–;470 Ma. Mineralogic and thermobarometric evidence for earlier high-grade metamorphic processes, if any, have been erased from these rocks. The processes of charnockite formation and post-peak retrograde metamorphism in the TGB took place under high geothermal gradients (40–;150°/km). This probably testifies to the existence of a local heat source, either magmas at depth or mantle (plume) beneath the region. The general metamorphic cycle in the TGB is estimated to be ca. 100–;160 Ma, which is much shorter in time span than that in the other regions of southern Peninsular India such as the Karnataka Craton and the Eastern Ghats Mobile Belt. During this period, the terrain experienced rapid exhumation of approximately 6–;7 cm/year, with the total amplitude of vertical movements estimated to be about 16–;17 km.