Y. Ogasawara

Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan

Abstract Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdykol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20-60 mm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm-1 with a weak band at about 271 cm-1. To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO5-CaSi2O5. This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4-1.8 wt% K2O and over 1000 ppm H2O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.

Discovery of coesite from Indus Suture Zone (ISZ), Ladakh, India: Evidence for deep subduction

Coesite, the high-pressure polymorph of quartz has been identified for the first time in the Tso-Morari Crystalline Complex, Ladakh (India) in the Himalayan belt. The preservation of coesite grains as inclusions in garnet within eclogite boudins indicates the existence of UHP metamorphism in this continental collision setting. The coesite was identified optically and its presence confirmed by its characteristic Raman bands. Both coesite and polycrys-talline quartz inclusions exhibit prominent radial fractures in their host garnet. The silica inclusions (monomineralic coesite, monomineralic quartz and bimineralic quartz+coesite) are associated with various textural features and well-developed chemical zonation within the garnet, which show the prograde nature of the UHP metamorphism. Preliminary P-T estimates suggest that the coesite growth took place at pressure > 28 kbar (> 90 km depth) and temperatures > 640°C. Significantly, the coesite inclusions are interpreted as having suffered decompression during exhumation without changing to quartz, most likely due to the rapid uplift. This finding also indicates deep subduction of the Indian plate beneath the Asian continent. The occurrence of coesite and subduction of the Indian plate was essentially governed by a low geothermal gradient which occurred prior to rapid exhumation. This was vital for generating the coherent picture of metamorphism and exposure of UHP rocks.

Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks

The dolomite-dissociation textures documented here in rocks from the Kokchetav ultrahigh-pressure massif suggest that the experimentally expected dolomite dissociation happened in the subducted slabs represented by these rocks. Two reactions, magnesite 5 C 1 MgO 1 O2, and majoritic garnet 1 MgO 1 H2O 5 garnet 1 clinochlore, recorded in carbonate inclusions and the host majoritic garnet are responsible for generation of graphite and clinochlore during the exhumation. The dolomite dissociation indicates that carbonate materials were subducted to depths of .250 km below Earth’s surface. Such deep subduction evidently brings abundant carbon and carbonate into deep Earth.

Carbonate-Bearing UHPM Rocks from the Tso-Morari Region, Ladakh, India: Petrological Implications

Evidence of ultrahigh-pressure metamorphism (UHPM) of subducted Indian continental crust in the form of carbonate-bearing coesite eclogite is preserved in the Tso-Morari Crystalline Complex (TMC) in eastern Ladakh, India. These eclogites, which occur as boudins in kyanite/sillimanite—grade rocks of the Puga Formation, contain essential mineral assemblages (garnet, clinopyroxeneomphacite, phengite, rutile, epidote-zoisite/clinozoisite and quartz), as well as coesite, talc, kyanite, magnesite, aragonite, dolomite, and Mg-calcite. Coesite, magnesite, and dolomite occur as inclusions in zoned garnet. The carbonate-bearing coesite eclogite underwent three stages of metamorphism—prograde, peak, and retrograde. The prograde assemblage is characterized by the presence of magnesite and a SiO2 polymorph, which is stable throughout the metamorphic process from the prograde to retrograde stage. At ultrahigh-pressure (27 kbar) and a temperature of 650°C, quartz transforms to coesite. Peak metamorphism was characterized by the development of coesite in garnet coexisting with high-Si phengite, clinopyroxene, magnesite, aragonite, dolomite, zoisite/clinozoisite, kyanite, and talc at a pressure of >39 kbar and temperature of >750°C. This is in good agreement with the estimated peak pressure and temperature judging from the composition of phengite, jadeite barometry, and garnet-clinopyroxene, garnet-phengite thermometry. Enstatite formed with talc and kyanite at a pressure of >31 kbar and temperature of 750°C. With a subsequent decrease in pressure, retrogression is constrained by the development of chlorite and chloritoid, which surround the garnet at a minimum pressure of 4-5 kbar and temperature of <500°C. Mineral assemblages in the carbonate-bearing coesite eclogite reveal that prograde metamorphism started with greenschist-facies conditions and reached the ultrahigh-pressure eclogite facies, passing through the intermediate blueschist facies. During UHP metamorphism, pressure abruptly doubled with a slight change of temperature, defining a geothermal gradient of 6–7°C/km. The UHP material was brought back to the surface along a path by rapid and almost isothermal exhumation.

Phlogopite and Coesite Exsolution from Super-Silicic Clinopyroxene

We document the exsolution of phlogopite and coesite/quartz from pre-existing super-silicic clinopyroxene in dolomite marble from the Kokchetav massif, northern Kazakhstan. The exsolution texture was formed by clinopyroxene decomposition through the reaction (3enstatite + 2KalSi2O6)cpx = phlogopite + 4coesite. Phlogopite exsolution must have occurred at pressures less than 8.0 GPa (at 1000°C), where the garnet + super-silicic clinopyroxene + phlogopite assemblage was stable, based on experimentally defined phase relations. Observations described in this report suggest that the precursor clinopyroxene was stable at pressures higher than 8 GPa (>1000°C), implying that the dolomite marble was subducted to mantle depths greater than 240 km. Such pro-found subduction could be an important mechanism to transport abundant H2O and potassium into the deep Earth.

Mineral inclusions in zircon from diamond-bearing marble in the Kokchetav massif, northern Kazakhstan

In order to constrain the metamorphic evolution of deeply subducted impure marble from the Kokchetav massif, we investigated mineral inclusions in zircon, which is known to protect ultrahigh-pressure (UHP) phases from late-stage overprinting. Consequently, diamond, coesite, diopside, garnet, phlogopite, calcite, dolomite, graphite and apatite were identified as inclusion in the zircons. Silica phases were absent in the matrix, indicating they were completely consumed by the following prograde reaction: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Dolomite\ {+}\ 2\ SiO\_{2}\ =\ Diopside\ {+}\ 2\ CO\_{2}\) \end{document}. The relict of coesite inclusions in zircon indicates that prograde P-T trajectory crossed cut the quartz-coesite transition before the decarbonate reaction. Matrix diopsides contain abundant exsolved phengite lamellae, whereas the exsolution is absent in the inclusions in zircon. The diopside inclusions contain higher amounts of K2O (up to 0.56 wt%) and CaEskola component (up to 3.5 mol%) than those of the matrix (0.14 wt% and 2.1 mol%, respectively). Thus, phengite exsolution occurs in matrix pyroxene with low K2O and CaEskola components compared to inclusions in zircon. This observation indicates that decreasing K2O and CaEskola components during decompression resulted in the phengite exsolution in diopside. A trace of hydroxyl in phengite needles was perhaps initially incorporated within the precursor clinopyroxene under high-pressure conditions. The peak metamorphic P-T conditions of the impure marble are estimated to 60-80 kbar and 960-1050°C, derived from K2O solubility in diopside buffered by phlogopite and from the garnet-clinopyroxene geothermometer.

The Role of Fluid for Diamond-Free UHP Dolomitic Marble from the Kokchetav Massif

A diamond-free, dolomite-bearing UHP marble showing a banded structure occurs at Kumdy-kol in the Kokchetav Massif, northern Kazakhstan. Three centimeter-scale zones occur in this marble (sample no. Y665): zone A = Ti-clinohumite-bearing dolomitic marble; zone B = dolomite marble; and zone C = dolomitic marble lacking Ti-clinohumite and forsterite. The occurrence of phlogopite and garnet lamellae in diopside in zone C suggests UHP metamorphism. Large-area chemical mapping covering about two-thirds of the sample surface was conducted. Ti-K∝ X-ray images clearly indicate that TiO2 is concentrated in zone A. Ca-K∝ and Mg-K∝ images showed that zones A and C have similar modal compositions of carbonates, but they have strong contrasts in TiO2 contents and mineral paragenesis. Mineral assemblages in each zone were analyzed in terms of the CaO-(MgO, FeO)-TiO2-SiO2 compositional tetrahedron. In zone A, the aragonite-Ti-clinohumite tieline was stable under UHP conditions, whereas it was unstable in zones B and C. This indicates that XCO2 in zone A was lower than that in zones B and C. Infiltration of TiO2-bearing aqueous fluid is implied on the basis of paragenetic relations, distribution of Ti-bearing phases and chemical zonation in Ti-clinohumite in zone A. The lack of microdiamond in this dolomite-bearing carbonate rock suggests extremely low-XCO2 conditions, and implies the infiltration of an aqueous fluid during UHP metamorphism.

Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Abstract Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ 18 O VSMOW values as low as +9‰, δ 13 C VPDB =−9‰, and 87 Sr/ 86 Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of 13 C/ 12 C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.

Maruyamaite, K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O, a potassium-dominant tourmaline from the ultrahigh-pressure Kokchetav massif, northern Kazakhstan: Description and crystal structure

Abstract Maruyamaite, ideally K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O, was recently approved as the first K-dominant mineral-species of the tourmaline supergroup. It occurs in ultrahigh-pressure quartzofeldspathic gneisses of the Kumdy-Kol area of the Kokchetav Massif, northern Kazakhstan. Maruyamaite contains inclusions of microdiamonds, and probably crystallized near the peak pressure conditions of UHP metamorphism in the stability field of diamond. Crystals occur as anhedral to euhedral grains up to 2 mm across, embedded in a matrix of anhedral quartz and K-feldspar. Maruyamaite is pale brown to brown with a white to very pale-brown streak and has a vitreous luster. It is brittle and has a Mohs hardness of ∼7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.081 g/cm3. In plane-polarized transmitted light, it is pleochroic, O = darkish brown, E = pale brown. Maruyamaite is uniaxial negative, ω = 1.634, ε = 1.652, both ±0.002. It is rhombohedral, space group R3m, a = 15.955(1), c = 7.227(1) Å, V = 1593(3) Å3, Z = 3. The strongest 10 X-ray dif- fraction lines in the powder pattern are [d in Å(I)(hkl)]: 2.581(100)(051), 2.974(85)(1̄32), 3.995 (69)(2̄40), 4.237(59)(2̄31), 2.046(54)(1̄62), 3.498(42)(012), 1.923(36)(3̄72), 6.415(23)(1̄11), 1.595(22)(5̄.10.0), 5.002(21)(021), and 4.610(20)(030). The crystal structure of maruyamaite was refined to an R1 index of 1.58% using 1149 unique reflections measured with MoKα X-radiation. Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO2 36.37, Al2O3 31.50, TiO2 1.09, Cr2O3 0.04, Fe2O3 0.33, FeO 4.01, MgO 9.00, CaO 1.47, Na2O 0.60, K2O 2.54, F 0.30, B2O3(calc) 10.58, H2O(calc) 2.96, sum 100.67 wt%. The formula unit, calculated on the basis of 31 anions pfu with B = 3, OH = 3.24 apfu (derived from the crystal structure) and the site populations assigned to reflect the mean interatomic distances, is (K0.53Na0.19Ca0.26□0.02)ΣX=1.00(Mg1.19Fe0.552+Fe0.053+

Clinopyroxene phenocrysts (with green salite cores) in trachybasalts: implications for two magma chambers under the Kokchetav UHP massif, North Kazakhstan

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