Vol 27, No 4 (2019)

The paper describes a methodology of applying X-ray computed tomography (CT) in studying textural–morphological characteristics of sulfide-bearing ultramafic rocks from the Yoko-Dovyren layered massif in the northern Baikal area, Buryatia, Russia. The dunites are used to illustrate the applicability of a reliable technique for distinguishing between grains of sulfides and spinel. The technique enables obtaining statistical characteristics of the 3D distribution and size of the mineral phases. The method of 3D reconstructions is demonstrated to be applicable at very low concentrations of sulfides: no less than 0.1–0.2 vol %. Differences between 3D models are determined for sulfide segregations of different size, in some instances with features of their orientation suggesting the direction of percolation and accumulation of the sulfide liquids. These data are consistent with the morphology of the largest sulfide segregations, whose concave parts adjoin the surface of the cumulus olivine and simultaneously grow into grains of the poikilitic plagioclase. Detailed information of these features is useful to identify fingerprints of infiltration and concentration of protosulfide liquids in highly crystallized cumulate systems.

A numerical thermodynamic model is proposed for one of the most important geological fluid system, ternary H₂O–CO₂–CaCl₂ system, at P-T conditions of the middle and lower crust and crust–mantle boundary. The model is based on the previously proposed equation for concentration dependence of the excess Gibbs free energy and on the first obtained P-T dependencies of the coefficients of the equation of state (EOS) expressed via molar volumes of the components. The EOS allows to predict the properties of the fluid participating in the majority of deep petrogenetic processes: its phase state (homogeneous or multi-phase), densities of fluid phases, concentrations of components in the co-existing phases, and the chemical activities of the components. The model precisely reproduces all available experimental data on the phase state of the ternary H₂O–CO₂–CaCl₂ fluid system in the ranges of temperatures 773.15–1073.15 K and pressures 0.1–0.9 GPa and also allows the correct application of the EOS beyond the experimentally studied range of temperatures and pressures up to P = 2 GPa and T = 1673.15 K. The possibility of the correct extrapolation of our EOS is ensured by using the parametrization of P-T dependencies via the molar volume of water. The latter remains in the experimental domain of values or falls slightly beyond its boundaries, when increasing temperatures and pressures.

This paper investigates the Jigede gabbros from the Shalazhashan tectonic belt, Alxa Block. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating reveals that they were emplaced in the Middle Permian (ca. 262 Ma). All the gabbro samples collected from the intrusion exhibit low contents of TiO₂ (0.24–0.37 wt %) and \({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{{\text{3}}}^{{\text{T}}}\) (4.87–5.41 wt %), but high levels of MgO (11.15–12.15 wt %), CaO (11.45–12.75 wt %), Al₂O₃ (14.18–17.08 wt %) and high Mg# (0.81–0.83). Relative to HREEs and LILEs, the gabbros are enriched in LREEs and depleted in Th, Nb, Ce, Zr, Hf, and Ti, with positive Eu, Sr and U anomalies. In contrast to MORB, the Jigede intrusion has higher initial ⁸⁷Sr/⁸⁶Sr values of 0.7046 to 0.7054 and lower ε_Nd(T) of + 1.8 to + 4.8. Elemental and isotopic data suggest that the intrusion was likely to have been produced by partial melting of a shallow lithospheric mantle source modified by slab-derived fluids, with subsequent cumulation of plagioclase. Thus, the Jigede intrusion was emplaced in a back-arc setting, in response to the Paleo-Asian oceanic subduction. In addition, the final closure of the Paleo-Asian Ocean did not occur prior to the Middle Permian.