Aleksandr S. Stepanov

The key role of mica during igneous concentration of tantalum

Abstract Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta.

Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds

The transition zone of the Earth’s mantle (the depth interval between two major seismic discontinuities at 410 km and 660 km) is critical to understanding our planet’s evolution. Some diamonds are thought to have originated in the transition zone and the inclusions found in them are the only samples of material directly extracted from this depth range. By comparing natural majorite garnet inclusions in diamonds with the compositions of experimentally crystallized majorite garnets, we determine two major compositional trends, the pure metabasitic (or eclogitic) trend and the combined metaperidotitic and metapyroxenitic trend, that are strongly correlated with their preferred substitution mechanisms during majorite formation. Based on these trends, we demonstrate that the majority of the reported majorite inclusions in natural diamonds formed neither in a pure metabasite nor in a metaperidotite lithology, but in fact crystallized from a wide range of compositions intermediate between conventional basaltic and peridotitic, referred to here as metapyroxenitic. Given the dominance of metapyroxenite-type majorite diamond inclusions and their inferred syngenetic origin, we argue that a significant fraction of metapyroxenite rock is present within Earth’s transition zone and is important in the diamond-forming process. This is in agreement with recent self-consistent seismological and/or mineral physics studies that support models of a lithologically heterogeneous transition zone. From trace element and carbon isotope features, we infer a crustal origin for these rocks.

Contrasting P-T paths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust

Abstract The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models.

Discussion: “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust” by Wang et al. 2016 (Contributions to Mineralogy and Petrology) 171:62

Wang et al. (Contrib Mineral Petrol 171:62, 2016a) present data on composition of xenolith from Southern Tibet and conclude that ulrapotassic melts from the region formed by melting mantle, and complex interaction with a crustal component. In this discussion we demonstrate that numerous observations presented by Wang et al. (2016a) can be explained by partial melting of crust followed by interaction between that melt and the mantle. We show that this model can explain the variability of magmas in such suits without evoking occurrence of coincidental, unrelated events. Moreover we demonstrate that our model of a crustal origin of the proto-shoshonite melts is now supported by independent lines of evidence such as geochemistry of restites after high- and ultrahigh- pressure melting and melt inclusion studies.

Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition: COMMENT

On the basis of geochemical and mineralogical features, Ballouard et al. (2016) argue that magmatic fractionation alone cannot explain the formation of leucogranites with low Nb/Ta ratios. Instead, they propose that low ratios are better explained by post-magmatic interaction with F-bearing fluids. Although fluids may certainly play an important role in the formation of rare metal leucogranites, the model proposed by Ballouard et al. lacks key details. We contend that the principal features they attribute to magmatic-hydrothermal processes are better explained by the magmatic fractionation model.

Brown diamonds from an eclogite xenolith from Udachnaya kimberlite, Yakutia, Russia.

We have performed petrographic and spectroscopic studies of brown diamonds from an eclogite xenolith from the Udachnaya pipe (Yakutia, Russia). Brown diamonds are randomly intermixed with colorless ones in the rock and often located at the grain boundaries of clinopyroxene and garnet. Brown diamonds can be characterized by a set of defects (H4, N2D and a line at 490.7 nm) which are absent in colorless diamonds. This set of defects is typical for plastically deformed diamonds and indicates that diamonds were likely annealed for a relatively short period after deformation had occurred. Excitation of brown colored zones with a 632.8 nm He-Ne laser produced the typical diamond band plus two additional bands at 1730 cm(-1) and 3350 cm(-1). These spectral features are not genuine Raman bands, and can be attributed to photoluminescence at ∼710 nm (1.75 eV) and ∼802 nm (1.54 eV). No Raman peak corresponding to graphite was observed in regions of brown coloration. Comparison with previous reports of brown diamonds from eclogites showed our eclogitic sample to have a typical structure without signs of apparent deformation. Two mechanisms with regard to diamond deformation are proposed: deformation of eclogite by external forces followed by subsequent recrystallization of silicates or, alternatively, deformation by local stress arising due to decompression and expansion of silicates during ascent of the xenolith to surface conditions.

The tales of disequilibrium and equilibrium crystallization of rare metal minerals: Data from new experiments

The metal tantalum (Ta) is becoming increasingly valued due to its use in modern technology such as mobile phones and tablets. The major application of this metal is in tantalum capacitors, which have unrivaled performance-for-size and high reliability. Ta is typically hosted in columbite-group minerals (CGMs), which are also known colloquially as “coltan” (columbite-tantalite) in Central Africa. Economic deposits of Ta are rare, and commercial production of the metal comes from a limited number of countries, hence leading to the classification of Ta as a “strategic resource” (Linnen et al. 2012). Significant production of Ta originates from war-torn regions of Central Africa, leading some countries—including the U.S.A.—to introduce legal requirements on tracing the origin of Ta-concentrates. These requirements have led to projects attempting to mineralogically and geochemically fingerprint CGMs from various deposits (Melcher et al. 2015). The common feature of CGMs is compositional zonation expressed as Ta/(Ta+Nb) and Mn/(Mn+Fe) ratios. The origin of this zonation in CGMs is enigmatic because it records intense fractionation of chemically similar elements on a very fine scale and is one of the key characteristics that can be used for identification of the petrogenetic sources of the minerals. Mechanisms proposed to explain this phenomenon require the involvement of melts and fluids of contrasting compositions, both internally and externally derived (e.g., Neiva et al. 2015). The CGMs are usually found in pegmatites: granitic rocks containing very large crystals (London 2008). While the origin of pegmatites has been debated over the years, the currently accepted theory states that pegmatites crystallize from super-cooled granitic melts (London 2008). Instead of compositional characteristics, such as high volatile contents, the theory emphasizes the role of the thermal history of the intrusions. Pegmatites can be formed from a melt of ordinary granitic compositions without anything more than moderate water content. This theory explains giant crystal size, graphic-intergrowth of K-feldspar and quartz and mineralogical zonation of associated, evolved-intrusions. However, the relationship between the crystallization of super-cooled melts and the textures of CGMs so far remains elusive. Van Lichtervelde et al. (2018, in the this issue) demonstrate that complex zonation of natural CGMs could be reproduced by experiments at supersaturated conditions. They found that within a single experiment, composition of CGMs crystals could vary widely, and to an amazing extent, they were able to reproduce the range of compositions observed in natural CGMs worldwide. The zonation of crystals is explained by super-saturation in the melt, coupled with slow lattice diffusion post-crystallization. Highly zoned crystals form in a closed system without evidence of liquation or fluid separation, thus suggesting that it could be an entirely magmatic phenomenon. While some of the compositions used in the experiments contain fluxing elements (i.e., F and P) it seems that these components were not essential for the development of zonation. In parallel with the model of London (2008), the emphasis has shifted to thermal history, rather than compositional characteristics of the melts. Another intriguing finding of the study is the observation in experimental CGMs of ordering-disordering phenomena: occurrence of Ta and Nb in Fe and Mn sites and vice versa. These compositional features might prove instrumental in constraining the conditions of formation of pegmatite minerals. Equilibrium and disequilibrium could be closely related phenomena. Van Lichtervelde et al. (2018) found that while grains of CGMs could be intensively zoned, the compositional ranges are not random. Coexisting crystals of columbite-tantalite and tapiolite form tight clusters with end-member compositions which systematical shifts in experimental data and natural samples. This suggests that equilibrium was established between some zones of two minerals while other zones grew with metastable compositions. While it is clear that new data present a significant advancement in understanding the crystallization of pegmatitic systems in the context of CGM, many related questions require further research. What is the role of ordering-disordering phenomena in compositional zonation and stability of CGM? How often do CGMs reach saturation in granites beyond their occurrence in rare metal-enriched pegmatites? What is the significance of these minerals for crustal-scale Nb-Ta fractionation? Further studies are necessary and new experimental approaches and ideas may well pave the way for explaining well-known features of these important and enigmatic minerals.