An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. Chromian spinels

 In order to develop a model for simulating naturally occurring chromian spinel compositions, we have processed published experimental data on chromian spinel-melt equilibrium. Out of 259 co-existing spinel-melt experiments reported in the literature, we have selected 118 compositions on the basis of run time, melt composition and experimental technique. These data cover a range of temperatures 1150–1500° C, oxygen fugacities of −13<log f _O2< −0.7, and bulk compositions ranging from basalt and norite, to komatiite. Six major spinel components with Cr³⁺, Al³⁺, Ti⁴⁺, Mg²⁺, Fe³⁺ and Fe²⁺-bearing end-members were considered for the purpose of describing chromite saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure (0.101 MPa). The empirically calibrated mineral-melt expression based on multiple linear regressions is: K ^Sp _i =A/T(K)+B log f _O2+C ln (Fe³⁺/Fe²⁺)_L+D ln R _L +E, where K ^Sp _i is an equilibrium constant and R _L is a melt structure-chemical parameter (MSCP). Twenty-eight forms of equilibrium constants were considered, including single distribution coefficients, exchange equilibrium constants, formation constants for AB₂O₄ components, as well as simple “spinel cation ratios”. For each form of the equilibrium constants, a set of 16 combinations of the MSCPs have been investigated. The MSCP is present in the form of composite ratios [e.g., Si/O, NBO/T,(Al+Si)/Si, or (Na+K)/Al] or as simple cation ratios (e.g., Mg/Fe²⁺). For the calculation of Fe³⁺ and Fe²⁺ species in silicate melts, we used existing equations, whereas the Fe³⁺/Fe²⁺ ratio of spinels was calculated from the spinel stoichiometry. The regression parameters that best repoduce the experimental data were for the following constants: (Fe³⁺/Fe²⁺) _Sp , (Mg/Fe²⁺) _Sp /(Mg/Fe²⁺) _L , (Cr/Al) _Sp / (Cr/Al) _L , K _FeCr2O4, and Ti _Sp /Ti _L . These expressions have been combined into a single program called SPINMELT, which calculates chromite crystallization temperature and composition at a given f _O2 with an average accuracy of ∼10° C and 1–2 mol%. An example of the use of SPINMELT is presented for a magma parental to the Bushveld Complex. Received: 30 May 1995/Accepted: 1 November 1995     

Spectroscopic constraints on growth of Siberian mixed-habit diamonds

Notable within-crystal variability of mineralogical and geochemical properties of single natural diamonds are commonly attributed to changing chemistry of parental fluids, sources of carbon and redox conditions of diamond precipitation. A distinct type of compositional heterogeneity (mixed-habit structure) is well-known to occur in diamonds as well as in many other minerals due to purely “structural” reasons that are unequal crystal chemistry of crystallographically different faces and selective absorption and fractionation of impurities between adjacent growth pyramids. Based on the combined cathodoluminescence, Fourier-transformed infrared spectroscopy and photoluminescence spectroscopy, study of nine diamond crystals with different growth histories and external morphology, but all showing mixed-habit patterns at different growth stages, we show that mixed-diamonds may grow in closed system conditions or with a slowly decreasing growth rate from a media with a much lower impurity content than previously thought. Intracrystal nitrogen distribution seems to be a function of growth rate even in the cases of unusual impurity partitioning between growth sectors. Generally poor with IR-active hydrogen at moderate nitrogen aggregation parameters, studied diamonds likely resemble the low hydrogen content from the growth medium that, for cubic diamonds, was typically suggested hydrogen-rich and a crucial factor for growth of cubic and mixed-habit diamonds. We also show that mixed-habit diamond growth may occur not only in peridotitic suite but also in an extended field of geochemical affinities from high-Ni to low-Ni or maybe even Ni-free environments, such as pyroxenitic or eclogitic.     

Proterozoic mafic magmatism in Siberian craton: An overview and implications for paleocontinental reconstruction

We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton, including recently published U–Pb and ⁴⁰Ar–³⁹Ar dates. These new precise ages suggest that at least some of the previously published K–Ar ages of Siberian mafic bodies should be ignored. The time–space geochronological chart, or the ‘barcode’ of mafic magmatic events shows significant differences between northern and southern Siberia. Both are characterized by ∼1900–1700 Ma magmatic events, but then there was an almost 1 Ga mafic magmatic ‘pause’ in south Siberia until ∼800 Ma. Meanwhile there are indications of multiple mafic magmatic events in North Siberia (Anabar shield and Olenek uplift) between ∼1600 and 1000 Ma. A series of magmatic events probably related to the breakup of Rodinia occurred in southern Siberia after ∼800 Ma. So far, there are no indications of late Neoproterozoic mafic magmatism in North Siberia. Ca. 1000–950 Ma mafic sills were reported from Meso- to Neo-Proterozoic sedimentary successions in the Sette-Daban area on the east side of the Siberian craton, but their tectonic setting is debated. Recent Ar–Ar dates of ∼1750 Ma for NW-trending dykes in the Aldan and Anabar shields, together with similar-age NNE-trending Baikal uplift dykes in south-eastern Siberia suggest the existence of a giant radial dyke swarm possibly related to a mantle plume centred in the Vilyui River area.     

Woolly rhino discovery in the lower Kolyma River

A nearly complete frozen mummy of a woolly rhinoceros (Coelodonta antiquitatis Blum., 1799) was discovered in a gold mine on the lower reaches of the Kolyma River, north–eastern Siberia. This is the first find of the whole body of woolly rhino in permafrost. A large part of the mummified body was preserved, including the left part of the body, covered by skin, including skin of the head and ear, fore and hind legs. The skull with 2 horns and the lower jaw were also preserved. Most of the internal organs were lost, except the intestines, stomach, and their contents. A rib fragment from this individual was dated by AMS-radiocarbon method to 39,140 ± 390 years BP (OxA-18755). Spore and pollen analyses of the stomach contents indicate that grasses and sagebrushes formed the main part of the diet of C. antiquitatis in this region of Arctic Siberia.     

Velocities of Present-Day Horizontal Movements in the Central Sector of the Greater Caucasus according to GPS Observations and Their Relation to Tectonics and the Deep Structure of the Earth’s Crust

The results of the first GPS measurements along the geophysical profile that intersects all major geological structures of the Osetiya region of the Greater Caucasus are presented. The results of the measurements are interpreted in comparison with those of neotectonic studies and data on the deep structure.