I. D. Ryabchikov

Physicochemical conditions of magma formation at the base of the Siberian plume: Insight from the investigation of melt inclusions in the meymechites and alkali picrites of the Maimecha-Kotui province

Based on the compositions of melt inclusions and coexisting minerals from meymechites and alkali picrites, the temperatures and pressures of the ascending material of the Siberian plume were estimated at the level of the lithosphere-asthenosphere boundary. The melts trapped in olivine show high contents of titanium and other incompatible elements. The rocks crystallized under high oxygen fugacity conditions. The calculated compositions of primary magmas are similar to the compositions of near-solidus melts derived from a dry fertile lherzolite at 7 GPa. The estimated potential temperature is close to 1650°C, which is much higher than the potential temperature of plumes that generate the primary basaltic magmas of mid-ocean ridges. The obtained data show that, during the activity of the giant magma-generating system of the Siberian trap province, hot peridotite masses ascended probably from the core-mantle boundary up to the base of the continental lithosphere. Our results are at odds with the suggestion that the basalt flows of the Siberian and other large igneous provinces are not related to mantle plumes.

Mineral Microinclusions in Cubic Diamonds from the Yakutian Deposits Based on Analytical Electron Microscopy Data

A great number of publications are devoted to inclusions in natural diamonds, because these data allow us to reconstruct the crystallization conditions of diamonds at a great depth and to obtain unique information on deep geologic processes as a whole. The main attention of researchers has been focused on perfect octahedral diamond crystals, whereas inclusions of diamond crystals of cubic habit (hereafter, cubic diamonds) remained poorly studied for many years. This was caused by a low transparency of most cubic crystals, their micrometer size, and the presence of heterogeneous inclusions. These attributes substantially complicate the traditional study with an electron microprobe [1]. Recent investigations of cubic diamond crystals from various deposits all over the world have shown that they contain many unusual melt or fluid microinclusions of complex carbonatite composition [1‐4]. Melt sulfide microinclusions were revealed in cubic diamond crystals from the Yakutian deposits [3]. However, the published information on the composition of specific mineral phases contained in cubic diamond crystals as inclusions remains scanty. Analytic electron microscopy is a very efficient method of studying microinclusions incorporated into translucent crystals. This method is a combination of transmission and scanning electron microscopy with energy-dispersive analysis. We used this method to study mineral inclusions in imperfect translucent diamond crystals of various types [5 and references therein]. In particular, the inclusions in a few cubic crystals from deposits in northern China were investigated with this method [6, 7]. The aim of this study was to use analytical electron microscopy for the examination the mineral microinclusions in cubic diamond crystals from deposits in the Siberian Craton. In the course of this study, we examined whitish gray crystals from the Yubileinaya kimberlite pipe (samples 1 and 2), gray and greenish yellow crystals from the Aikhal pipe (samples 3 and 4), black crystals

Composition of hydrous melts in equilibrium with quartz eclogites

SummaryCompositions of the hydrous melts in equilibrium with garnet, omphacitic clinopyroxene and quartz have been investigated experimentally at 28.5 and 35 kbar. They are represented by silica-rich liquids (> 70% SiO2) with low MgO, FeO and CaO contents. The removal of ca 10–15% of the magma of this composition may be sufficient to convert quartz eclogite formed after subduction of altered MORB into a quartz-free bimineralic eclogite assemblage, which is a common type of xenoliths in kimberlites.At 28.5 kbar the solidus temperature is between 700 and 750° C in the system quartz eclogite—water, and the high pressure amphibole-out boundary lies at ca 25 kbar in accord with the previous studies.ZusammenfassungUm Prozesse zu simulieren, die bei der Subduktion von Ozeanbodenbasalten durch partielle Anatexis im Stabilitätsfeld von Eklogiten ablaufen, wurde die Zusammensetzung wasserhältiger Schmelzen in Gleichgewicht mit Granat, Omphacit und Quarz bei 28.5 und 35 Kbar experimentell untersucht. Diese Schmelzen sind reich an SiO2 (> 70 Gew%) und arm an Mg0, Fe0 and CaO. Die Extraktion von ca. 10–15% derartiger Schmelzen würde genügen, um quarzführende Eklogite, die durch die Subduktion von alteriertem MORB Material entstanden sind, in quarzfreie bimineralische Eklogite umzuwandeln wie sie häufig als Xenolithe in Kimberliten beobachtet werden.Im System Quarz-Eklogit-Wasser liegt die Solidustemperatur bei 28.5 Kbar zwischen 700 und 750°C. Die obere Stabilitätsgrenze von Amphibol liegt in diesem Temperaturbereich bei ca. 25 Kbar.

Oxygen potential of diamond formation in the lower mantle

Thermodynamic calculations have shown that when a metallic phase arising due to ferroan ion disproportionation is contained in lower-mantle rocks, carbon occurs as iron carbide and the oxygen fugacity corresponds to the equilibrium of ferropericlase with Fe-Ni alloy. The typical values of oxygen fugacity in zones of diamond formation in the lower mantle lie between the iron-wüstite buffer and six logarithmic units above this level. The processes that proceed in the lower mantle give rise to variation of

Redox Equilibria in Alkaline Lavas from Trindade Island, Brazil

f_{O_2 }

Partitioning of Rare Earth Elements Between Phosphate-rich Carbonatite Melts and Mantle Peridotites

within several orders of magnitude above the elevated

Origin of carbonatite magma during the evolution of ultrapotassic basite magma

f_{O_2 }