Andrey V. Korsakov

On the occurrence and boron isotopic composition of tourmaline in (ultra)high-pressure metamorphic rocks

Abstract: The extensive P–T stability and the high chemical variability of tourmaline (Tur) together with its common occurrence in metasediments proves its high potential for petrological and (isotope) geochemical studies on fluid–rock interaction in subduction- and collision-related rocks. This paper reviews the occurrence, major element chemistry and boron isotopic composition of Tur in high- and ultrahigh-pressure metamorphic (UHPM) rocks. In addition, it presents a new discovery of coesite-bearing Tur (schorl) from the Erzgebirge (Germany), as well as Tur (dravite) related to the retrograde history of coesite- and diamond-bearing rocks from the Erzgebirge and the Kokchetav Massif (Kazakhstan). The scarce data on worldwide occurrences of (U)HPM Tur reveal a high occupation of the crystallographic X-site (dominated by Na) and the possible presence of excess B, with little further distinctiveness in its major element composition when compared with Tur from medium-grade rocks. High K2O contents in Tur are probably not related to UHP growth or equilibration. The B isotopic composition of (U)HPM Tur ranges in δ11B from −16 to +1‰, with many samples in or below the range of continental crust. In contrast, Tur formed during retrograde fluid influx typically shows high δ11B values (up to +28‰), suggesting heavy-B fluids infiltrating the exhuming (U)HPM units. Coesite inclusions in Tur, characterized by Raman spectroscopy, are regarded as the best indicator for its UHP stability. Supplementary material: Analytical methods, tourmaline compositions and boron isotope values are available at http://www.geolsoc.org.uk/SUP18354.

Raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh-pressure metamorphic rocks from diamonds in sample preparation materials

Abstract Natural diamond from three ultrahigh-pressure metamorphic (UHPM) terranes (Erzgebirge Massif, Germany; Kokchetav Massif, Northern Kazakhstan; Rhodope Metamorphic Province, Greece) and synthetic diamond from cutting and polishing materials (paste, spray, saw blade) were studied by means of optical microscopy and Raman microspectroscopy, to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond. Several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones [i.e., diamond as residual particles (contaminants) from the cutting and polishing material] such as the diamond size, the presence of inclusions, coatings, or coexistent phases and two diamond Raman band parameters, i.e., the Raman shift and the full-width at half maximum height (FWHM).

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

Laser Raman microspectroscopy was applied to quartz inclusions in coesite- and diamond-grade metapelites from the Kokchetav ultrahigh-pressure metamorphic (UHPM) complex, Northern Kazakhstan, and diamond-grade eclogite xenoliths from the Mir kimberlite pipe, Yakutiya, Russia to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnets have a higher frequency shift than those in the matrix. Residual pressures retained by quartz inclusions depend on the metamorphic history of the garnet host. The Raman frequency shift of quartz inclusions in garnet from coesite-grade and diamond-grade metamorphic rocks shows no systematic change with increasing peak metamorphic pressures. The highest shifts of the main Raman bands of quartz were documented for monocrystalline quartz inclusions in garnets from a diamond-grade eclogite xenolith. Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1–0.6 GPa for quartz inclusions from coesite-grade metapelites from Kokchetav, 0.1–0.3 GPa for quartz inclusions from diamond-grade metapelites from Kokchetav, and 1.0–1.2 GPa for quartz inclusions from the diamond-grade eclogite xenoliths from the Mir kimberlite pipe. Normal stresses and internal (residual) pressures of quartz inclusions in garnet were numerically simulated with a 3-shell elastic model. Estimated values of residual pressures are inconsistent with the residual pressures estimated from the frequency shifts. Residual pressure slightly depends on P–T conditions at peak metamorphic stage. Laser Raman microspectroscopic analysis of quartz is a potentially powerful method for recovering an ultrahigh pressure metamorphic event. Monocrystalline quartz inclusions yielding a residual pressure greater than 2.5 GPa might indicate the presence of a former coesite.

Aragonite-calcite-dolomite relationships in UHPM polycrystalline carbonate inclusions from the Kokchetav Massif, northern Kazakhstan

The presence of aragonite in polycrystalline carbonate inclusions in garnet in diamond-grade metamorphic rocks from the Kokchetav Massif, N. Kazakhstan was identified for the first time by means of Raman analyses and mapping, cathodoluminescence images and optical and scanning electron microscopy. Aragonite appears within the inclusions as dirty, chaotically oriented materials surrounded by a clean monocrystalline calcite shell; the grain boundary between the host-garnet and the aragonite-bearing inclusions is often characterized by a wavy or amoeboid shape; no cracks occur around the aragonite-bearing inclusions; no significant shift in the main aragonite Raman band was measured. These observations indicate that residual pressure within the inclusion is minor. These features are inconsistent with an origin of aragonite at peak metamorphic conditions (6 GPa) by decomposition of dolomite.

First finding of burkeite in melt inclusions in olivine from sheared lherzolite xenoliths.

For the first time burkeite was observed as a daughter phase in the melt inclusions in olivine by Raman spectroscopy. The olivine comes from sheared lherzolite xenoliths from the Udachnaya-East kimberlite pipe (Yakutia, Russia). This anhydrous sulfate-carbonate mineral (Na(6)(CO(3))(SO(4))(2)) is generally considered to be a characteristic mineral in saline soils or in continental lacustrine evaporite deposits. Recently, however, this mineral was identified in hydrothermal fluids. Our observations indicate that burkeite can also be formed from a mantle-derived melt.

Age and thermal history of the Maksyutov metamorphic complex: 40Ar/39Ar evidence

The 40Ar/39Ar method with stepwise heating was used to date phengite and glaucophane in the contact zone of garnet glaucophanite an omphacite-garnet rock (eclogite) from the lower unit of the Maksyutov metamorphic complex. The correlation of the measured age and the sizes of the phengite flakes indicates that the behavior of radiogenic Ar in them was controlled by the mechanisms of volumetric diffusion. Taking into account the fact that all of the rocks have the same thermal history, the dates most close to the age of metamorphism are those of the largest phengite flakes from garnet glaucophanite: 392 Ma. The age values obtained on phengite from an omphacite-garnet rock sampled at the maximum distance from the contact are equal to 378 Ma and correspond to the time when the rocks cooled to temperatures below 350°C. The results of numerical simulations indicate that the metamorphic age is no younger than 400 Ma, and the linear cooling rate can be estimated at 3.40 −0.75/+1.24°C/m.y. The maximum values of the phengite ages are consistent with the dates of glaucophane from three rock samples: 389–411 Ma.

Eitelite in sheared peridotite xenoliths from Udachnaya-East kimberlite pipe (Russia) – a new locality and host rock type

For the first time eitelite Na 2 Mg(CO 3 ) 2 was observed as a daughter phase in the melt inclusions in olivine from one of the deepest known mantle rocks sampled by kimberlite magma – sheared peridotite xenoliths (190 – 230 km), taken from the Devonian (~370 Ma) Udachnaya-East kimberlite pipe (Siberian craton, Russia). Eitelite was identified by confocal Raman spectroscopy and confirmed by energy-dispersive X-ray spectroscopy. Raman spectra of eitelite in the melt inclusions are characterized by a very strong band at 1105 cm −1 attributed to CO 3 2− symmetric stretching, and weaker bands at 207–208 and 260–263 cm −1 due to lattice vibration. Our findings of eitelite in the melt inclusions entrapped by olivine of mantle xenoliths indicate that this rare carbonate can crystallise from primitive mantle-derived alkaline carbonatite melt.

Raman-based geobarometry of ultrahigh-pressure metamorphic rocks: applications, problems, and perspectives

Raman-based geobarometry has recently become increasingly popular because it is an elegant way to obtain information on peak metamorphic conditions or the entire pressure-temperature-time (P-T-t) path of metamorphic rocks, especially those formed under ultrahigh-pressure (UHP) conditions. However, several problems need to be solved to get reliable estimates of metamorphic conditions. In this paper we present some examples of difficulties which can arise during the Raman spectroscopy study of solid inclusions from ultrahigh-pressure metamorphic rocks.

Origin of K-cymrite and kokchetavite in the polyphase mineral inclusions from Kokchetav UHP calc-silicate rocks: evidence from confocal Raman imaging

Confocal Raman Imaging revealed the coexistence of kokchetavite and K-cymrite (KAlSi 3 O 8 ·H 2 O) in association with muscovite/phengite, lollingite, calcite and α-cristobalite in polyphase mineral inclusions in clinopyroxene of calc-silicate ultrahigh-pressure metamorphic (UHPM) rocks from the Kokchetav Complex (northern Kazakhstan). Experiments have shown that K-cymrite is stable under high pressure conditions; however, this phase never had been reported from UHPM rocks before. The mode of occurrence of K-cymrite and kokchetavite in these rocks is strong evidence for kokchetavite formation through the dehydration of K-cymrite. The presence of fluid inclusions and K-cymrite in the polyphase mineral inclusions testify for water-saturated conditions in the rock-forming environment near peak metamorphic conditions.

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.