The Experts below are selected from a list of 4869 Experts worldwide ranked by ideXlab platform
Lifei Zhang - One of the best experts on this subject based on the ideXlab platform.
-
Water in Coesite: incorporation mechanism and operation condition, solubility and P-T dependence, and contribution to water transport and Coesite preservation
Geoscience Frontiers, 2021Co-Authors: Wei Yan, Lifei Zhang, Yanyao Zhang, Weidong Sun, Christina Yan Wang, Xi LiuAbstract:Abstract A series of Coesite, coexisting with or without a liquid phase, was synthesized in the nominal system SiO2–H2O at 800–1450 °C and 5 GPa. Micro-Raman spectroscopy was used to identify the crystalline phase, electron microprobe and LA-ICP-MS were employed to quantify some major and trace elements, and unpolarized FTIR spectroscopy was applied to probe the different types of hydrogen defects, explore water-incorporation mechanisms and quantify water contents. Trace amounts of Al and B were detected in the Coesite. Combining our results with the results in the literatures, we have found no positive correlation between the Al contents and the “Al”-based hydrogen concentrations, suggesting that previously proposed hydrogen-incorporation mechanism H+ + Al3+ ↔ Si4+ does not function in Coesite. In contrast, we have confirmed the positive correlation between the B contents and the B-based hydrogen concentrations. The hydrogen-incorporation mechanism H+ + B3+ ↔ Si4+ readily takes place in Coesite at different P-T conditions, and significantly increases the water content at both liquid-saturated and liquid-undersaturated conditions. For the SiO2–H2O system, we have found that type-I hydrogarnet substitution plays a dictating role in incorporating water into Coesite at liquid-saturated condition, type-II hydrogarnet substitution contributes significantly at nearly dry condition, and both operate at conditions in between. The water solubility of Coesite, as dictated by the type-I hydrogarnet substitution, positively correlates with both P and T, c H 2 O = −105(30) + 5.2(32) × P + 0.112(26) × T, with c H 2 O in wt ppm, P in GPa and T in °C. Due to its low water solubility and small fraction in subducted slabs, Coesite may contribute insignificantly to the vertical water transport in subduction zones. Furthermore, the water solubility of any Coesite in exhuming ultra-high pressure metamorphic rocks should be virtually zero as Coesite becomes metastable. With an adequately fast water-diffusion rate, this metastable Coesite should be completely dry, which may have been the key factor to the partial preservation of most natural Coe. As a byproduct, a new IR experimental protocol for accurate water determination in optically anisotropic nominally anhydrous minerals has been found. Aided with the empirical method of Paterson (1982) it employs multiple unpolarized IR spectra, collected from randomly-orientated mineral grains, to approximate both total integrated absorbance and total integrated molar absorption coefficient. Its success relies on a high-level orientation randomness in the IR analyses.
-
petrology and u pb zircon dating of Coesite bearing metapelite from the kebuerte valley western tianshan china
Journal of Asian Earth Sciences, 2013Co-Authors: Xin Yang, Lifei Zhang, Zuolin Tian, Thomas BaderAbstract:Abstract This paper deals with the petrology and U–Pb dating of Coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved Coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + Coesite + phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP Coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the Coesite-bearing metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of Coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.
-
Coesite in the eclogite and schist of the atantayi valley southwestern tianshan china
Chinese Science Bulletin, 2012Co-Authors: Zeng Lü, Lifei ZhangAbstract:Coesite is an indicator mineral of ultra-high-pressure metamorphism. Since Coesite was reported in the Habutengsu Valley, we have also found it in eclogite and schist from the Atantayi Valley in the southwestern Tianshan, China. Petrographic and micro-Raman analyses were carried out for the Atantayi metamorphic rocks and Coesite was recognized in the predominant rock types, i.e. schist and eclogite, from three sections. The Coesite-bearing schist consists mainly of garnet, Na-Ca amphibole, quartz, white mica and albite; the Coesite-bearing eclogite is mainly composed of omphacite, garnet, glaucophane and zoisite. The Coesite occurs as various mineral inclusions within porphyroblastic garnet. Findings of Coesite in eclogite and associated schist indicate not only the regional in situ formation of the Atantayi ultra-high-pressure eclogite, but also the large areal extent of ultra-high-pressure metamorphism in southwestern Tianshan, extending up to 10 km north-south and 60–80 km east-west.
-
Coesite inclusions in garnet from eclogitic rocks in western Tianshan, northwest China: Convincing proof of UHP metamorphism
American Mineralogist, 2008Co-Authors: Lifei Zhang, Kurt BucherAbstract:Coesite inclusions in garnet have been recognized in eclogitic rocks from western Tianshan, northwest China. The Coesite grains exhibit distinct radial cracks in host porphyroblastic garnet; some Coesite relics are well preserved, whereas others are partially replaced by quartz. Coesite has been identified optically and then confirmed by in situ Raman spectroscopy, showing the characteristic band at 522 cm -1 and subsidiary bands at 428, 326, 271, 178, 151, and 121 cm -1 . The eclogitic rocks contain garnet, omphacite, and Na-Ca-amphibole, and they are rich in white mica (>30%) and graphite. Peak conditions of 570-630 °C and 2.7-3.3 GPa are constrained by garnet-clinopyroxene geothermometry and the occurrence of Coesite. The presence of Coesite and widespread quartz inclusions in garnet with radial cracks indicative of former Coesite in these unique graphitic rocks confirms the previous suggestion of the UHP terrane for the western Tianshan, China.
-
Relict Coesite exsolution in omphacite from Western Tianshan eclogites, China
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, Juhn G. LiouAbstract:Exsolution rods of relict Coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for Coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in omphacite. Lamellae of Coesite were apparently exsolved from supersilicic omphacite at P 5.0 GPa and the transformation from Coesite to quartz occurred during retrograde metamorphism.
J G Liou - One of the best experts on this subject based on the ideXlab platform.
-
factors in the preservation of Coesite the importance of fluid infiltration
American Mineralogist, 2005Co-Authors: Jed L. Mosenfelder, Joseph R Smyth, Hans-peter Schertl, J G LiouAbstract:The survival of Coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of Coesite. We used infrared spectroscopy to measure water contents of Coesite and Coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in Coesite are below the detection limit of ~100 ppm H2O. The silica phases surrounding Coesite, however, show varying amounts of H2O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing Coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (<100 ppm H2O, red-violet CL); “mosaic” quartz, which is actually chalcedony (up to 0.4 wt% H2O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% H2O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing Coesite in other UHP samples studied contains on the order of 500 ppm H2O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of Coesite.
-
u pb shrimp ages recorded in the Coesite bearing zircon domains of paragneisses in the southwestern sulu terrane eastern china new interpretation
American Mineralogist, 2005Co-Authors: Fulai Liu, J G LiouAbstract:Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSD-PP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low- P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low- P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to Coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of Coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to Coesite. The 227 ± 9 Ma (the weighted mean age) obtained from Coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for Coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.
-
relict Coesite exsolution in omphacite from western tianshan eclogites china
American Mineralogist, 2005Co-Authors: Lifei Zhang, J G Liou, Shuguang Song, Yongliang Ai, Xuping LiAbstract:Exsolution rods of relict Coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for Coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in omphacite. Lamellae of Coesite were apparently exsolved from supersilicic omphacite at P 5.0 GPa and the transformation from Coesite to quartz occurred during retrograde metamorphism.
-
ultrahigh pressure mineral inclusions in zircons from gneissic core samples of the chinese continental scientific drilling site in eastern china
European Journal of Mineralogy, 2002Co-Authors: Liu Fulai, Xu Zhiqin, Shigenori Maruyama, Ikuo Katayama, J G Liou, Hideki Masago, Yang JingsuiAbstract:The Chinese drillhole CCSD-PP2 penetrated to a depth of 1028.28 m near Donghai in the southwestern Sulu terrane. Core samples consist mainly of para- and orthogneisses with minor intercalated layers of amphibolite and eclogite. Coesite and Coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified using Raman spectroscopy and electron microprobe analysis as inclusions in zircon separates from para- and orthogneiss samples. Matrix minerals in the Coesite-bearing para- and orthogneiss samples are amphibolite-facies assemblages of quartz + feldspar + epidote + biotite ± amphibole ± garnet ± titanite. In most cases, zircons, however, preserve multi-stage assemblages in different domains; the parageneses and compositions of these included minerals yielded P-T paths consistent with those deduced previously from eclogite and garnet peridotite outcrops in Donghai. In generally, quartz inclusions were identified in zircon cores and rims, whereas Coesite and other UHP mineral inclusions occur in the mantles (intermediate zircon growth zone between core and rim) of the same zircons. The occurrence of Coesite inclusions in zircon indicates that eclogites together with their country rock gneisses experienced UHP metamorphism and retrogression. Coesite inclusions are ubiquitous in paragneiss and orthogneiss core samples in CCSD-PP2 drillhole and in outcrop samples from the southwestern Sulu terrane, indicating that Coesite-bearing lithologies are widespread in the Sulu terrane, requiring subduction of a large terrane to mantle depths. Cathodoluminescence images of zircons from gneissic rocks display distinct zoning from core to rim, that can be genetically correlated with inherited (especially in the orthogneisses) or prograde, UHP, and retrograde mineral inclusion assemblages. These images reveal irregular zoning patterns and various thickness of cores, mantles, and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith and metamorphic age dating.
-
occurrences of intergranular Coesite in ultrahigh p rocks from the sulu region eastern china implications for lack of fluid during exhumation
American Mineralogist, 1996Co-Authors: J G Liou, Ru Y ZhangAbstract:Several Coesite grains along garnet-omphacite grain boundaries were discovered in eclogites from Yangkou Beach of the Sulu region, eastern China. These grains exhibit variable extents of conversion of Coesite to palisade- through mosaic- to granoblastic-quartz aggregates. The rare occurrence of intergranular and matrix Coesite in ultrahigh-pressure (UHP) crustal rocks is related to the fast rate of exhumation and the low amount of fluid infiltration required during retrograde recrystallization. Sluggish reactions due to the lack of fluid in both UHP and retrograde metamorphism resulted in the occurrence of intergranular Coesite in these eclogites, the preservation of gabbroic texture and relict igneous minerals, and the metastable persistence of low-P assemblages in the Coesite-stability field.
E. Bruce Watson - One of the best experts on this subject based on the ideXlab platform.
-
An experimentally calibrated thermobarometric solubility model for titanium in Coesite (TitaniC)
Contributions to Mineralogy and Petrology, 2019Co-Authors: Zach R. Osborne, Jay B. Thomas, William O. Nachlas, Suzanne L. Baldwin, Megan E. Holycross, Frank S. Spear, E. Bruce WatsonAbstract:Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in Coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the Coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of Coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that Coesite crystals are relatively homogenous. The Ti concentrations of Coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006 . https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010 . https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti^4+ substitutes for Si^4+ on fourfold tetrahedral sites in Coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in Coesite by using a least-squares method to fit Ti concentrations in Coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{Coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ R T ln X TiO 2 Coesite = - 55.068 + 0.00195 × T ( K ) - 1.234 × P ( kbar ) + R T ln a TiO 2 rutile , where R is the gas constant 8.3145 × 10^−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{Coesite}}$$ X TiO 2 Coesite is the mole fraction of TiO_2 in Coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ a TiO 2 rutile is the activity of TiO_2 in the system referenced to rutile. Ti-in-Coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-Coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.
-
An experimentally calibrated thermobarometric solubility model for titanium in Coesite (TitaniC)
Contributions to Mineralogy and Petrology, 2019Co-Authors: Zach R. Osborne, Jay B. Thomas, William O. Nachlas, Suzanne L. Baldwin, Megan E. Holycross, Frank S. Spear, E. Bruce WatsonAbstract:Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in Coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the Coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of Coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that Coesite crystals are relatively homogenous. The Ti concentrations of Coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in Coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in Coesite by using a least-squares method to fit Ti concentrations in Coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{Coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{Coesite}}$$ is the mole fraction of TiO2 in Coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ is the activity of TiO2 in the system referenced to rutile. Ti-in-Coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-Coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.
Juhn G. Liou - One of the best experts on this subject based on the ideXlab platform.
-
Factors in the preservation of Coesite: The importance of fluid infiltration
American Mineralogist, 2005Co-Authors: Jed L. Mosenfelder, Joseph R Smyth, Hans-peter Schertl, Juhn G. LiouAbstract:The survival of Coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of Coesite. We used infrared spectroscopy to measure water contents of Coesite and Coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in Coesite are below the detection limit of ~100 ppm H2O. The silica phases surrounding Coesite, however, show varying amounts of H2O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing Coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (
-
Relict Coesite exsolution in omphacite from Western Tianshan eclogites, China
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, Juhn G. LiouAbstract:Exsolution rods of relict Coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for Coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in omphacite. Lamellae of Coesite were apparently exsolved from supersilicic omphacite at P 5.0 GPa and the transformation from Coesite to quartz occurred during retrograde metamorphism.
Shigenori Maruyama - One of the best experts on this subject based on the ideXlab platform.
-
Discovery of Coesite from Indus Suture Zone (ISZ), Ladakh, India: Evidence for deep subduction
European Journal of Mineralogy, 2004Co-Authors: Himanshu K. Sachan, Shigenori Maruyama, Barun K. Mukherjee, Yoshihide Ogasawara, Haruhito Ishida, Atsumi Muko, Nobuhiro YoshiokaAbstract: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.
-
ultrahigh pressure mineral inclusions in zircons from gneissic core samples of the chinese continental scientific drilling site in eastern china
European Journal of Mineralogy, 2002Co-Authors: Liu Fulai, Xu Zhiqin, Shigenori Maruyama, Ikuo Katayama, J G Liou, Hideki Masago, Yang JingsuiAbstract:The Chinese drillhole CCSD-PP2 penetrated to a depth of 1028.28 m near Donghai in the southwestern Sulu terrane. Core samples consist mainly of para- and orthogneisses with minor intercalated layers of amphibolite and eclogite. Coesite and Coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified using Raman spectroscopy and electron microprobe analysis as inclusions in zircon separates from para- and orthogneiss samples. Matrix minerals in the Coesite-bearing para- and orthogneiss samples are amphibolite-facies assemblages of quartz + feldspar + epidote + biotite ± amphibole ± garnet ± titanite. In most cases, zircons, however, preserve multi-stage assemblages in different domains; the parageneses and compositions of these included minerals yielded P-T paths consistent with those deduced previously from eclogite and garnet peridotite outcrops in Donghai. In generally, quartz inclusions were identified in zircon cores and rims, whereas Coesite and other UHP mineral inclusions occur in the mantles (intermediate zircon growth zone between core and rim) of the same zircons. The occurrence of Coesite inclusions in zircon indicates that eclogites together with their country rock gneisses experienced UHP metamorphism and retrogression. Coesite inclusions are ubiquitous in paragneiss and orthogneiss core samples in CCSD-PP2 drillhole and in outcrop samples from the southwestern Sulu terrane, indicating that Coesite-bearing lithologies are widespread in the Sulu terrane, requiring subduction of a large terrane to mantle depths. Cathodoluminescence images of zircons from gneissic rocks display distinct zoning from core to rim, that can be genetically correlated with inherited (especially in the orthogneisses) or prograde, UHP, and retrograde mineral inclusion assemblages. These images reveal irregular zoning patterns and various thickness of cores, mantles, and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith and metamorphic age dating.
-
Coesite exsolution from supersilicic titanite in uhp marble from the kokchetav massif northern kazakhstan
American Mineralogist, 2002Co-Authors: Yoshihide Ogasawara, Kyoka Fukasawa, Shigenori MaruyamaAbstract:Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdy-kol 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 μm 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.
-
Overpressures induced by Coesite-quartz transition in zircon
American Mineralogist, 2001Co-Authors: Jing-bo Liou, Bolin Cong, Shigenori MaruyamaAbstract:More than 120 Coesite and few polycrystalline quartz (Coesite pseudomorph) inclusions were identified by Raman microspectroscopy in zircons of various kinds from gneissic rocks located in the Dabieshan and Sulu ultrahigh-pressure (UHP) metamorphic terranes in eastern China. The Coesite inclusions have undergone, to various degrees, the Coesite-quartz transition. Raman spectra of Coesite and subsidiary quartz inclusions show various shifts, which are closely correlated to the extent of the transition. The Coesite-pseudomorph inclusions in zircon show the highest Raman shift. Calibrations based on the experimental work of Hemley (1987) yielded inconsistent present-day overpressures (0–24 kbar) in Coesite inclusions inside zircons. The availability of fluids, which resulted in the pervasive regional retrogression of the UHP gneisses during later exhumation stages, is regarded as the main factor controlling the extent of the Coesite-quartz transition. This study underscores the need for the elastic model applied by previous researchers to explain the preservation of the Coesite inclusions in rigid host minerals.