K-cymrite (KAlSi3O8 & centerdot;H2O), a high-pressure phase implicated in volatile and large-ion lithophile elements (LILE) transport during subduction, exhibits a complex structural evolution under high pressure. This study investigates the incommensurate modulated high-pressure modification of K-cymrite, revealing a phase transition from the hexagonal P6/mmm structure to a monoclinic (3+1)-dimensional structure with the superspace group C2/m(0 beta 0)s0 above 8.5 GPa. Single-crystal X-ray diffraction data, collected up to 20.2 GPa, demonstrate the emergence of satellite reflections at 7.3-8.5 GPa, indicative of an incommensurate modulation that resembles a lock-in mechanism with transition to commensurate 3b phase at similar to 15 GPa, before reverting to incommensurability above 16.2 GPa. The modulation is characterized by wave-like deformation of double tetrahedral layers and ditrigonal-like distortion of six-membered rings, driven by the ordering of guest H2O molecules interacting with interlayer K+ cations. These findings underscore the need to incorporate modulation effects into thermodynamic models of crust subduction and volatile cycling.

This study describes the Serkinskaya faunal complex of the Siberian lower Cambrian Kessyusa Group, which includes a fragment of gnathobase, several scalids of priapulid-like organisms, various morphotypes of spines, organic films with outgrowths, and other organic remains. Organic microfossils that we identified as gnathobase fragments (a jaw-like fragment and a robust spine with porous structure) were studied via Raman spectroscopy, and phase distribution maps were constructed to identify different kerogens. The jaw-like fragment has heterogeneity on the surface of the microfossil and can be interpreted as reflecting differential preservation of two different layers of the cuticle. The age of these remains is estimated to be Cambrian Fortunian to Stage 2 (Vendian-Cambrian, possibly Tommotian), which makes the identification of gnathobase fragments one of the oldest known occurrences of evidence for durophagy. These findings confirm the presence of filter-feeding, deposit-feeding (priapulids), vertical mixing of the sediment in the first centimeter, predation pressure (cuticle with outgrowths), and possible durophagy in the earliest Cambrian.

Nitrogen-bearing K-cymrite (K, NH4)AlSi3O8·(N2,NH3,H2O), a clathrate phase that is stable in subduction zones, may play a crucial role in transporting potassium, water, and nitrogen to the depths of the mantle. Synchrotron X-ray diffraction in a diamond anvil cell with a methanol-ethanol medium was used to study its compressibility up to 10 GPa. N-bearing K-cymrite retains its hexagonal P6/mmm symmetry without signs of phase transitions across the pressure range. The third-order Birch-Murnaghan equation of state yielded the following values: V0 = 191.38(3) Å3, K0 = 47.1(2) GPa, and K0’ = 6.40(7). Unlike K-cymrite KAlSi3O8·H2O, which develops incommensurate modulation above 7–8 GPa, no satellite reflections appear in the diffraction patterns of N-bearing K-cymrite, even at 16 GPa. This suggests that N2 stabilizes the double tetrahedral layers against wave-like bending.

Anhydrous Na2SO4, known by phase transitions on heating, has been reported mostly as a low temperature structural form, mineral thenardite in widely diversified geological environments. Recently, coexisting high temperature Na2SO4 polymorphs were found in volcanic sublimates. Here we present the finding of polymorph Na2SO4(III) in sulfate-rich melt inclusions from chloride enclaves of the Udachnaya-East kimberlite (Siberia). A natural analog of synthetic metastable Na2SO4(III) is documented for the first time in melt inclusions. Phase III was stabilized by the incorporation of Ca-ions, based on SEM-EDS. Raman imaging highlighted the regular distribution of two Na2SO4 polymorphs in melt inclusions. Na2SO4(III) was found in a Ca-rich solid solution, in contrast to Na2SO4(V), located in a Ca-depleted sulfate mixture.Identifications of polymorphs in melt inclusions is of special importance, as solidified former liquid inclusions provide the only means to directly establish the physio-chemical conditions of paleo fluid.

Graphite is a common mineral occurring in different metamorphic rocks, including high/ultrahigh-temperature and high/ultrahigh-pressure rocks. Raman spectroscopic studies combined with carbon isotope composition may highlight the carbon source (crustal or mantle) and PT conditions of graphite crystallization. In the present study, seven graphite-bearing samples of felsic granulite xenoliths from the "Eclogitovaya" pipe (the Eastern Pamir, Tajikistan) have been studied by Raman spectroscopy and analyzed its carbon isotope composition. Graphite crystals occur in the matrix as well as inclusions in rock-forming minerals (e.g., garnet and kyanite), indicating their simultaneous crystallization. The graphite formed as a result of the metamorphic transformation of carbonaceous material (delta 13 & Scy; ranges from -14.5 parts per thousand to -20.1 parts per thousand) originally present in the rocks during subduction. Structural analysis of intact graphite inclusions in garnet revealed rims of disordered graphite surrounding well-ordered graphite cores. Composite inclusions consisting of disordered graphite, well-ordered graphite, and CO2 were also observed. However, any evidence for precipitation of disordered graphite from fluid has not been found. It is likely that the origin of disordered graphite in the Eastern Pamir granulite xenoliths is attributed to the stress at the graphite-garnet interface during the rapid exhumation of the xenoliths.

Conformation and compaction of nucleic acids (NAs), particularly DNA, are vital for any organism but are poorly studied in vivo because of the absence of noninvasive techniques. In this study, we introduce label-free low-frequency (LF) Raman imaging for mapping NA conformation and compaction in eukaryotic cells. Specifically, we find that the LF Raman spectrum in the nucleus strongly differs from that in the cytoplasm: first, by the prominent π-band indicating the ordered arrangement of nucleotides and second, by the low intensity highlighting the compact DNA packing. Using low-frequency imaging, we also reveal the difference in NA packing in normal cells and cells with DNA damage. The proposed technique is expected to complement widely used high-frequency Raman imaging: while the latter tells us "what is there?", our approach answers the question "how is it organized?", which is of paramount importance for cell biology. Moreover, this approach can be extended to studying other complex biomolecular systems.

Findings of primary carbonate-bearing inclusions in garnet porphyroblasts from the calc-silicate rock of the Kokchetav massif indicate the presence of hydrous carbonatitic melt at the UHP peak metamorphism. However, two contrasting hypotheses exist for the formation of these inclusions: (1) Inclusions were trapped by garnet in the form of primary carbonatitic melt at peak metamorphism, and (2) inclusions were trapped as mineral aggregate during the prograde metamorphic stage and underwent melting close to the UHP peak metamorphism. A thorough Raman study revealed that UHP secondary carbonate-bearing inclusions, located in healed cracks, contain calcite, dolomite, quartz, phengite/muscovite, graphite, and CH4, whereas primary carbonate-bearing inclusions exhibit a distinct mineral association comprising phlogopite, calcite, dolomite, graphite/diamond, allanite, and chlorite. Most likely, primary inclusions initially were trapped as hydrous carbonatitic melt/fluid + mineral aggregate +/- silicate melt in the garnet/clinopyroxene. Melting of this mineral aggregate in primary inclusions or decompression at the UHP conditions (P > 3.5 GPa) resulted in brittle failure of the host mineral and formation of secondary inclusions chains. Both types of inclusions contain daughter minerals and step-daughter silicates formed by the reaction of the carbonatitic melt/fluid and host garnet. In contrast to the primary inclusions, the secondary inclusions almost do not contain accidentally trapped and retrograde minerals and, thus, are the most suitable for the UHP carbonatitic melt/fluid reconstruction.

Mantle xenoliths provide a direct window into the subcontinental lithospheric mantle (SCLM) beneath Archean cratons. Age constraints on the melting, metasomatism and deformation that have affected the SCLM are crucial for developing insight into continental dynamics and craton development. Obtaining constraints is nevertheless difficult. Garnet geochronology - and Lu-Hf geochronology in particular - has good prospects in this regard, because it uniquely allows dating of a rock forming mineral and petrogenetic indicator in the mantle. However, the behavior of the Lu-Hf system at mantle conditions is not yet well-understood, and stringent sample size requirements and issues relating to melt infiltration typically impede obtaining reliable garnet ages for single xenoliths. In this study, we tested the capabilities of modern Lu-Hf methods to date garnet in kimberlite-hosted mantle xenoliths that record mantle melting, metasomatism and shearing. The samples are from the Siberian, Kaapvaal and Slave cratons, and include garnet-bearing olivine pyroxenites, lherzolites and websterites, as well as orthopyroxenite with exsolved garnet, and include both sheared and granular lithologies. The analyses yielded low-dispersion Lu-Hf isochrons for single xenoliths with age uncertainties down to 0.3 % and ages up to 3.0 Ga. Garnet in equilibrium with clinopyroxene in coarse lherzolite and websterite samples records Proterozoic ages, whereas high-Cr garnet from coarse harzburgite and orthopyroxenite samples lacking clinopyroxene yields Archean ages. The age data do not reflect 'normal' volume diffusion-controlled chronometric closure but instead indicate strong age retention barring events of fluid or melt infiltration. Only sheared samples yielded Lu-Hf ages close to the age of kimberlite eruption. These ages highlight that deformation and metasomatism are ongoing within otherwise stable cratonic roots. Initial Hf isotope compositions obtained from the Lu-Hf isochrons approximate chondritic values in the Archean and become gradually more superchondritic with time. These results indicate that the cratonic mantle, as well as the melts that metasomatized it since the Archean, derive from a moderately depleted mantle reservoir that has existed since at least 3 Ga. Together, the new Lu-Hf data provide a new temporal framework for the petrological development of the SCLM and establish Lu-Hf garnet geochronology as prime geochronometer for dating mantle processes.

Raman spectroscopy, especially Raman imaging, has become a popular technique for fluid inclusion studies. Raman imaging is particularly useful for the study of tiny inclusions appearing in high or ultrahigh pressure (UHP) metamorphic minerals. The small size of daughter phases, as well as the presence of liquid or gas phases, precludes the application of microprobe analysis for unexposed inclusions. Nontransparent inclusions, usually assigned as "opaque", "ore," or "black" accessory minerals, are rarely studied by Raman spectroscopy due to the high absorbance of laser energy by black materials, their unstable behavior at the laser beam, and low Raman signal. Despite these difficulties, a number of documented examples show that multiphase inclusions in these metamorphic rocks may persist. These inclusions provide valuable information on the composition of fluids in deeply subducted environments. For the first time, we demonstrate that "opaque" or "black" inclusions in UHP rocks are multiphase. They contain CO2 + CH4 gas bubbles +/- calcite and graphite with different degrees of crystallinity. These multiphase fluid inclusions coexist side by side with "classical" inclusions which also contain CO2 and CH4 gases, as well as liquid water, but no graphite. Our findings demonstrate that Raman imaging of "classical" and "black" fluid inclusions and subsequent data treatment by different techniques may bring important information about their composition.

The conditions of the formation of K-cymrite in volatile-rich pelite and partially devolatilized mica quartz–muscovite–chlorite schist were experimentally investigated at pressures of 5.5, 6.3, and 7.8 GPa and temperatures ranging from 900 to 1090°C corresponding to hot subduction geotherm. Experimental samples at these P–T conditions formed assemblage of solid phases (Grt + Coe + Phe + Cpx + Ky, with accessory Po + Ru + Zrn ± Mnz) and water-enriched supercritical fluid–melt. Analysis of the obtained data indicates that the stability of phengite and its potential replacement by K-cymrite depends on the P–T conditions and the amount of volatiles in the metasediment. In samples of volatile-rich pelite and mica schist at 5.5 GPa and 900°C, as well as at 6.3 GPa and 1000°C, phengite remains stable in equilibrium with 3–13 wt

Rare-Earth Elements (REE) are key geochemical tracers of crust-mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce-Pr-Nd relative to La and Sm. The presence of 0.005 to 0.008 wt % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm-Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure-temperature conditions of 1025 degrees C to 1080 degrees C and 3.6 to 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227-0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 +/- 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites, as well as the differences ancient (Archean/Proterozoic) and Phanerozoic subduction processes.

An atypical variety of iowaite with an extremely low content of divalent cations (Mg + Fe2+)/Fe3+ of 2.75 : 1 is identified in a late hydrothermal mineral assemblage of the Udachnaya kimberlite pipe, Yakutia. Its crystal structure is studied using a single crystal: polytype 3R, space group R3̅m , a = 3.1213(3) Å, с = 23.621(4) Å, V = 199.30(5) Å3; R1 = 0.027. Some crystals are characterized by zonation indicating a natural ion exchange, the leading schemes of which are 2Cl– → SO _4^2 - and 2Cl– → 2SO _4^2 - + Ca2+. This is the first reliable case of natural anion exchange in minerals of the hydrotalcite supergroup. The anion exchange reaction occurs with an increase of interlayer distance from 8.0 to 11.2 Å. The composition of the mineral and the character of its zonation bear information on the evolution of late hydrothermal processes.

Understanding calcite genesis in ultrahigh-pressure crustal rocks is a key to the reconstruction of the evolution of ultrahigh-pressure metacarbonate rocks. Here, we present new data and a new model on the genesis and the P-T conditions of the formation of calcite found in the ultrahigh-pressure calc-silicate rocks from the Kokchetav massif. In the studied sample aragonite inclusions coexist with Type A calcite inclusions (previously interpreted as mineral inclusions) and the inclusions of Type B calcite (previously interpreted as derived from the crystallization of carbonatitic melt) in cores of garnet porphyroblasts. The most Mg-rich calcite from Type A inclusions coexisting with aragonite inclusions in one garnet growth zone shows X-Ca = 0.935 implying their crystallization during a retrograde metamorphic stage at P similar to 2.3 GPa and T similar to 870 degrees C along the P-T path. Type A calcite and aragonite inclusions were also found coexisting in one growth zone with K-bearing clinopyroxene inclusion (omega[K2O] = 0.5 wt.%). Such a high K2O-content in clinopyroxene testify that the pressure of inclusion capture exceeded 3.5 GPa, which contradicts the P-T conditions estimated by X-Ca in magnesian calcite. Thus, Type A calcite inclusions were initially captured as an aggregate of aragonite+ magnesian calcite at ultrahigh pressure metamorphic stage (P >= 3.5 GPa, T = 900-1,000 degrees C) and then re-equilibrated at lower conditions (P <= 2.3 GPa and T <= 870 degrees C). The trace element composition of aragonite and Type A and Type B calcite from inclusions was also studied to clarify calcite genesis in these inclusions. Aragonite shows high LREE (5-57 ppm) and Sr-content (600-800 ppm). Calcite from Type A inclusions shows low LREE (2.9-19.8 ppm) and Sr-content (490-670 ppm). Calcite from Type B inclusions forms two groups according to the LREE and Sr content distribution (Type B1 and Type B2). Trace element distribution in Type B1 calcite is identical to that of Type A calcite, while Type B2 calcite shows high LREE (6.8-64.9 ppm) concentrations along with low Sr-content (180-340 ppm). Type A and Type B1 calcite is interpreted to have been re-equilibrated. Type B2 calcite inclusions crystallized from the hydrous carbonatitic melt.

It is generally accepted that aragonite crystals of biogenic origin are characterized by significantly higher twin densities compared to samples formed during geological processes. Based on our single crystal X-ray diffraction (SCXRD) and transmission electron microscopy (TEM) study of aragonite crystals from various localities, we show that in geological aragonites, the twin densities are comparable to those of the samples from crossed lamellar zones of molluscs shells. The high twin density is consistent with performed calculations, according to which the Gibbs free energy of twin-free aragonite is close to that of periodically twinned aragonite structure. In some cases, high twin densities result in the appearance of diffuse scattering in SCXRD patterns. The obtained TEM and optical micrographs show that besides the twin boundaries (TBs) of growth origin, there are also TBs and especially stacking faults that were likely formed as the result of local strain compensation. SCXRD patterns of the samples from Tazouta, in addition to diffuse scattering lines, show Debye arcs in the a ∗ b ∗ plane. These Debye arcs are present only on one side of the Bragg reflections and have an azimuthal extent of nearly 30°, making the whole symmetry of the diffraction pattern distinctly chiral, which has not yet been reported for aragonite. By analogy with biogenic calcite crystals, we associate these arcs with the presence of misoriented subgrains formed as a result of crystal twisting during growth.

Diamond crystals with numerous mineral inclusions from the Guanamo placer deposit (Venezuela) were studied in detail. The inclusions in the studied diamonds are represented by a set of minerals typical of eclogite paragenesis indicators—omphacite, garnet, kyanite, coesite and rutile. Besides single mineral inclusions, multiphase inclusions were also identified. Mineral associations of multiphase inclusions adjacent within less than 100 microns of each other within the same growth zone can vary considerably. For example, in one of the diamond crystals examined, multiphase inclusions are represented by the following associations: sanidine-dolomite-anatase and magnetite-rutile-dolomite-apatite-kokchetavite-graphite. Note that this is the first discovery of kokchetavite as an inclusion in kimberlite diamond crystals. Until now, this hexagonal polymorphic modification of KAlSi3O8 has only been diagnosed in minerals from rocks of the continental crust that were formed under conditions of high or ultra-high pressure metamorphism. Thus, the subduction of continental crustal material into the mantle and its important role in the metasomatic alteration of upper mantle rocks in Venezuela can be inferred from the presence of kokchetavite as inclusions in kimberlite diamond crystals.

This paper reports investigation on zircons from quartz syenite, alkaline and foid syenite, as well as metasomatic rock from the fenitization zone hosted by the Burpala massif. It is performed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), cathodoluminescence (CL), laser ablation inductively coupled plasma mass spectrometry (LA ICP MS), and Raman spectroscopy. Generally, all zircons from igneous rocks show rhythmic, crystal growth zoning or sector zoning (type I) except for some zircons from alkaline syenites (type II) showing patchy zoning. They systematically contain pores or cavities. The REE patterns of magmatic zircons share similar features: e.g. depletion of LREE ((Yb/La)N up to 35000), large positive Ce (Ce/Ce* 6–427) and small negative Eu (Eu/Eu* 0.37–0.93) anomalies. Zircons crystallized from quartz syenites at 830±30 °C at the early stage of rock formation, while zircons from alkaline and foid syenites crystallized at the later stage of rock formation (680–750 °C). Meanwhile, crystallization of zircons with rhythmic zoning (type I) occurs at later magmatic stage, while the formation zircons of type II is probably related to the separation of the highly fluorinated aqueous fluid from the residual melt.Zircons from fenites have a bipyramidal habit holding a heterogeneous mosaic core and a homogeneous (or rhythmic zoning) rim. The cores of zircon show flat REE patterns without significant anomalies, while the rims are characterized by noticeable fractionation of REE ((Yb/La)N 85–615) and show a positive Ce anomaly (Ce/Ce* 4–18). The Raman spectra of the cores show a higher degree of crystallinity than the rims, and their flat REE spectra are probably related to the contamination by micro inclusions. The discordant U-Pb age of 295±3 Ma was obtained for zircon rims, which is consistent with the age of formation of igneous rocks of the Burpala massif (298–291 Ma). The latter supports the syngenetic origin of metasomatic ore mineralization with the main stage of massif formation.

Compressibility and pressure-induced structural evolution of kokchetavite, the hexagonal polymorph of KAlSi3O8, has been studied up to 11.8 GPa using synchrotron single-crystal X-ray diffraction. Two phase transitions were observed at pressures of similar to 0.3 and 10.4 GPa. Kokchetavite-I (as-synthesized, P6/mcc) transforms into kokchetavite-II with the P6c2 space group. Kokchetavite-II -> kokchetavite-III phase transition at similar to 10.4 GPa is accompanied by a change of symmetry to probably orthorhombic. After pressure release, kokchetavite reverts to the initial single-crystal state with P6/mcc space group. A second-order Birch-Murnaghan equation of state was calculated for phase kokchetavite-II with coefficients V-0 = 1486(3) angstrom(3), K-0 = 59(2) GPa.

Compressibility and structural evolution of K-cymrite, hexagonal high-pressure KAlSi3O8·H2O, has been studied up to 18 GPa using synchrotron single crystal X-ray diffraction in Ne pressure medium. K-cymrite retains its original symmetry P6/mmm up to a pressure of 7.3 GPa. As the pressure increases from 7.3 to 8.5 GPa, the weak satellite reflections appear on diffraction patterns and remain up to maximum applied pressure of 18 GPa indicating incommensurate modulation. However, main reflections can be still indexed in hexagonal cell and structure successfully solved in initial P6/mmm group. After pressure release, K-cymrite reverts to initial non-modulated single-crystal state. The parameters of third-order Birch-Murnaghan equation of state for K-cymrite are V0 = 190.45(12) ų, K0 = 56.5(7) GPa and K0’ = 3.2(12), with bulk modulus notably deviating from earlier result (K0 = 45(2) GPa and K0’ = 1.3(10)) obtained in vaseline media.

Selected crystals of natural K-bearing tourmalines, extracted from a quartzofeldspathic rock from the Kumdy-Kol microdiamond deposit (an ultrahigh-pressure region of Kokchetav Massif, northern Kazakhstan), were characterized using a scanning electron microscope, an electron microprobe and single-crystal X-ray diffraction to investigate the impact of K uptake on the tourmaline structure. All the studied crystals belong to the maruyamaite–oxy-dravite/dravite compositional field, with K contents ranging from 0.03 to 0.47 apfu (atoms per formula unit), and contain a minor fluor-uvite component that increases towards oxy-dravite and dravite. The compositional variability of our samples can be expressed as a sequence of substitutions ranging from maruyamaite to oxy-dravite, dravite and fluor-uvite (or vice versa). Specifically, the substitutions that lead from maruyamaite to oxy-dravite to dravite are (1) XK + AlTOT + O1O ↔ XNa + MgTOT + O1O and (2) XNa + MgTOT + O1O ↔ XNa + MgTOT + O1OH, respectively. Conversely, the substitutions that lead from oxy-dravite to dravite to fluor-uvite are (1) XNa + MgTOT + O1O ↔ XNa + MgTOT + O1OH and (2) XNa + MgTOT + O1OH ↔ XCa + MgTOT + O1F, respectively. By analysing the difference between the bond valence sum and mean formal charge at the X site, we show that an increase in the K content (K > 0.21 apfu) results in the compression of X–O bonds (overbonded cation). Conversely, lower K contents lead to the stretching of the bonds (underbonded cation). Compared to the K-dominant analogues with ZFeO6 povondraite-type framework, K-bearing tourmalines with a smaller ZAlO6 framework such as maruyamaite should only be stable at higher-pressure conditions, as pressure is necessary to squeeze the relatively large K cation into the tighter X cavity. In both cases, the essential condition for the formation of K-dominant tourmalines is the extremely high K activity in the crystallization fluid. The K-tourmaline from the Kokchetav Massif may have crystallized under high-pressure (HP) conditions, with an upper limit between 3.5–7 GPa, during retrograde metamorphism following the ultrahigh-pressure (UHP) metamorphic peak.

40 Ar/ 39 Ar analyses were made on maruyamaite (potassium-dominant tourmaline) from tourmaline-quartz-feldspar rocks discovered within the Kumdy-Kol microdiamond deposit (Kokchetav massif, North Kazakhstan). Turmaline yielded well-defined 40 Ar/ 39 Ar plateau age spectra whose values coincide within the error - ages of 502.3±8.0, 502.2±8.0, 506.0±8.0 Ma. These ages are much younger than the age of 530±2 Ma determined for high-pressure metamorphism by different methods. Thus, the formation of tourmaline-rich rocks of the Kumdy-Kol deposit cannot be associated with the high-pressure metamorphic events, and, therefore, testifies in favor of the low-pressure nature of maruyamaite. Based on the coincidence of age data for tourmaline crystals with different potassium contents, it can be concluded that the K/Ar system in tourmaline can be used for dating metasomatic and metamorphic processes.