Chinese Continental Scientific Drilling Project Research Articles

In Situ High-Pressure Raman Spectroscopic, Single-Crystal X-ray Diffraction, and FTIR Investigations of Rutile and TiO2II

In ultrahigh-pressure (UHP) metamorphic rocks, rutile is an important accessory mineral. Its high-pressure polymorph TiO2II can be a significant indicator of pressure in the diamond stability field. In the present study, in situ high-pressure Raman spectroscopic measurements of natural rutile in UHP eclogite from the main hole of the Chinese Continental Scientific Drilling Project (CCSD) have been conducted up to ~16 GPa. Rutile and recovered TiO2II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy. The results indicate that (1) the phase transition from rutile to baddeleyite-type TiO2 terminates at about 16 GPa under compression at ambient temperature; (2) the metastable TiO2II in the exhumated UHP rocks formed during deep continental subduction can be characterized by a highly distorted octahedral site in the crystal structure. X-ray powder diffraction analyses (with Cu Kα radiation) at ambient conditions are sufficient for identifying the lamellae of TiO2II within natural rutile based on the angles (2θ) of two strong peaks at 25.5° and 31.5°; (3) rutile and recovered TiO2II in the continental slabs can contain certain amounts of water during deep subduction and exhumation. The estimated water contents of rutile in the present study range from 1590 to 1780 ppm of H2O by weight. In the crystal structure of TiO2II, hydrogen can be incorporated close to the long O-O edges (>2.5143 Å) of the TiO6 octahedra. Further studies on the pressure–temperature stability of hydroxyls in rutile and TiO2II may help to understand the transportation and release of water in subducted continental slabs.

Discing behavior and mechanism of cores extracted from Songke-2 well at depths below 4,500 m

Understanding in-situ stresses is the foundation for investigating deep rock mechanics. In practice, the fracture morphology of a disced core can often reflect its in-situ environment and disturbance-induced fracture process to some extent. In this study, three-dimensional dynamic scanning technology is used to reconstruct the fracture morphology of the disced cores extracted from Songke-2 as well as part of the Chinese Continental Scientific Drilling Project . The roughness of various regions of the fracture surfaces of the cores is investigated. Additionally, a parametric system is developed for the quantitative study of the integrity coefficient of the disced rock samples. Furthermore, the mechanisms and characteristics of the fractures along the stress paths are investigated. According to the findings, when shallow cores undergo discing, they have notable saddle-shaped coarse fractures, which may be related to the properties of the cores at these depths. Deep cores demonstrate discing behavior, but the generated discs are relatively thick. Deep core fracture surfaces are mostly smooth and straight, possibly propagating from the outside to the inside. Under excavation disturbance in the hydrostatic state, the extreme values of the maximum tensile stress at the core stubs show a saddle-shaped distribution. In general, the results of this study can serve as a theoretical foundation for the quantitative assessment of the intactness of disced cores. Furthermore, because disc patterns and characteristics are associated with stress states, this approach can be used to evaluate in-situ stress environments.

Detection of the deep biosphere in metamorphic rocks from the Chinese continental scientific drilling.

It is generally accepted that there is a vast, well-populated biosphere in the subsurface, but the depth limit of the terrestrial biosphere has yet to be determined, largely because of the lack of access to the subsurface. Here as part of the Chinese Continental Scientific Drilling (CCSD) project in eastern China, we acquired continuous rock cores and endeavored to probe the depth limit of the biosphere and the depth-dependent distribution of microorganisms at a geologically unique site, that is, a convergent plate boundary. Microbiological analyses of ultra-high-pressure metamorphic rock cores taken from the ground surface to 5,158-meter reveal that microbial distribution was continuous up to a depth of ~4,850m, where temperature was estimated to be ~137°C. The metabolic state of these organisms at such great depth remains to be determined. Microbial abundance, ranging from 103 to 108 cells/g, was also related to porosity, but not to the depth and rock composition. In addition, microbial diversity systematically decreased with depth. Our results support the notion that temperature is a key factor in determining the lower limit of the biosphere in the continental subsurface.

Scientific Drilling ‐ to Construct the Telescopes that Inserting to the Earth Interior

AbstractMankind live in the earth for countless years, but until now; people do not really understand the connotation of the Earth. We know that the earth composition including the lithosphere, the asthenosphere, mantle and core. Of course, the lithosphere supports all the life on Earth. For a long time, geoscientists trying to use all kind of methods such as geological, geophysical and geochemical methods to detect and study the earth, but the knowledge about earth are mostly indirect. Through the direct observation to the lithosphere, people can understand and recognize the plate movement of ocean and the mainland, crustal stress, earthquakes, volcanic processes, deep resources, the origins of life, global climate change and biodiversity. They are all the basis of a series of geosciences problems (Su and Yang, 2010).Geological specimens, especially the true samples from deep of the Earth, are the most directly study subjects for geologists. But the only way to access the true samples from deep of the earth is drilling. The most directly relevant evidence always originated from the deep of the earth, such as core, cuttings, fluid samples and other physical samples.Continental scientific drilling has been demonstrated which is an efficient technique for directly obtaining information from the Earth's surface to the deep crust, and is acknowledged as “to build a telescope inserting to the interior of the Earth”, as well as “a key for opening the door of the Earth”. Over the last four decades, continental scientific drilling has achieved great success in enhancing our knowledge of the Earth, and in providing information on mineral resources, large engineering projects and global change. SinoProbe‐05 is a new scientific drilling venture, which builds on the success of the Chinese Continental Scientific Drilling Project (CCSD), and is similar to the current major scientific drilling project on the Wenchuan earthquake fault. SinoProbe‐05 will focus on 6 critical tectonic and mineral resource regions, including the Jinchuan Cu‐Ni sulphide deposits in Gansu, the Luobusa chromite deposits in Tibet, the Tengchong volcano‐thermal tectonic zone in Yunnan, the Yudu‐Ganxian polymetallic deposits in South China, the Tongling polymetallic deposit and the Luzong volcanic basin and mineral deposit district in Anhui. As of the end of 2013, all of these pilot holes have been completed, all of them have achieved the desired scientific objectives.The construction of another ICDP project, Songke No.2 well, has come to an end. Current well depth is 5929m. Drilling throughout the Cretaceous strata is just around the corner (The design well depth is 6400m.). This will be the first complete Cretaceous stratigraphic profile in the world.The deep exploration project which will be stared soon will build a large number of different depths of scientific drilling holes. The deepest hole depth will reach to 13000m. We believe that the construction of these scientific coring drilling holes will provide geologists with a lot of real core samples. These cores can meet the needs for different geoscience research areas. No doubt, the research results based on these cores will promote China's geological science research to a new height, of course; will also contribute to the progress of the world's earth science. This is also a good opportunity to promote China's drilling technology.So, we know that no advanced drilling technology, no enough high quality samples from the deep of the Earth, the in‐depth studies for geosciences will be restricted of course (Zhang et al., 2013).

A reassessment of the stress and natural fracture orientations from analysis of image logs in the Chinese Continental Scientific Drilling Program borehole at Donghai county, Jiangsu province, China

The Chinese Continental Scientific Drilling (CCSD) project has drilled a 5100 m deep research borehole in the Sulu ultra-high pressure (UHP) metamorphic belt, eastern China. The UHP metamorphic belt is thought to be a product of continent-continent collision and has consequently experienced very intensive structural deformation. Based on a more complete well log data set of CCSD borehole, we can have a more detailed and reliable study on the structure features of borehole wall and other rock physical properties than a previous study published in 2009. Abundant data related to borehole breakouts (BOs), drilling induced tensile fractures (DITFs) and natural fractures were collected from the image logs. The BO and DITF data indicate that the average direction of the maximum horizontal principal stress (SH) of the CCSD borehole site is about 79.2° which is consistent with the convergent direction (E-W) of the Pacific Sea Plate with respect to the Eurasian Plate. Analysis of DITFs indicated that in the case of the CCSD borehole, axial drilling induced tensile fractures (ADITFs) occur occasionally in the upper section (0–2300 m) of the borehole with low dipping angle (0–10°), while transverse drilling induced tensile fractures (TDITFs) occur significantly in the lower section (3800–5000 m) with high dipping angle (10–30°). The natural fracture distribution at depth in the metamorphic rocks of the CCSD borehole indicates that (1) the failure strength of rocks and borehole depth are two factors that affects natural fracture frequency, (2) most of the dip azimuth of natural fractures is consistent with the dip azimuth of foliations observed in the core, (3) the development of most of the natural fractures probably was dominated by the development of foliations, and both the natural fractures and foliations developed in response to the subduction and exhumation of the Sulu terrene.

Fifteen years of the Chinese Continental Scientific Drilling Program

Abstract. Continental scientific drilling can be regarded as a telescope into the Earth's interior because it provides process insight and uncompromised samples of rocks, fluids, and even sampled from the deep biosphere from the Earth's surface to great depths. As one of the three founding members of the International Continental Scientific Drilling Program (ICDP), ICDP China has made great achievements in many scientific drilling-related research fields. Based on the ICDP participation it attracted global attention of scientists and set up not only the Chinese Continental Scientific Drilling (CCSD) Program in 2001 but also a growing number of ambitious drilling projects in the country. The 5158 m deep borehole of the CCSD project at Donghai County in the Sulu ultrahigh-pressure metamorphic terrain demonstrates that large amounts of crustal rocks of the South China Block have been subducted to at least 120 km, followed by rapid uplift. After successful completion of drilling at Donghai, several continental scientific drilling projects were conducted with funding of the Chinese government and partially with support of ICDP, resulting in a total drilling depth of more than 35 000 m. These projects encompass the Continental Environmental Scientific Drilling Program of China, the Scientific Drilling Project of Wenchuan Earthquake Fault Zone, the Continental Scientific Drilling Project of Cretaceous Songliao Basin, and the Program of Selected Continental Scientific Drilling and Experiments. On the occasion of the 20th anniversary of the ICDP and the 15th anniversary of the CCSD Program, this paper reviews the history and major progress of the CCSD Program.

Noble gas composition and 40Ar/39Ar age in eclogites from the main hole of the Chinese Continental Scientific Drilling project

We present the first comprehensive noble gas study on eclogites. The four eclogite samples were recovered during the Chinese Continental Scientific Drilling and are from two distinct profile depth sections differing in their degree of interaction with meteoric water, based on their δ 18O-values (surface related and of mantle-type). Hence, noble gas analyses offer the potential to further discriminate between shallow (meteoric) and deep (mantle) fluid sources. Noble gas compositions reveal typical crustal fluid compositions, characterized by a variable mixture of atmospheric gases with significant contributions of nucleogenic neon, radiogenic 4He*, radiogenic 40Ar*, fissiogenic 131–136Xe, and presumably bariogenic 131Xe, but no significant addition of mantle gases. This signature can be also considered to represent one endmember component of eclogitic diamonds. Concentrations of non-radiogenic noble gases are rather low, with depletion of light relative to the heavier noble gases. Eclogites from lower depth which experienced a higher degree of interaction with meteoric water also showed higher contributions of atmospheric gas compared with eclogites recovered from greater depth. This is interpreted to result from interaction with high-salinity fluids during ultrahigh pressure (UHP). It demonstrates that the atmospheric noble gas abundance is a proxy for interaction with surface related fluids. 40Ar/39Ar (inverse) isochron ages of two phengite separates (241.2 ± 0.4 Ma and 275.0 ± 1.8 Ma, 1σ-errors) predate the main phase of UHP metamorphism (ca. 220 Ma). Biotite yields an integrated age of about 1100 Ma. These age values are interpreted to reflect the likely addition of excess 40Ar without any chronological meaning.

Common Pb isotope mapping of UHP metamorphic zones in Dabie orogen, Central China: Implication for Pb isotopic structure of subducted continental crust

We report Pb isotopic compositions for feldspars separated from 57 orthogneisses and 2 paragneisses from three exhumed UHPM slices representing the North Dabie zone, the Central Dabie zone and the South Dabie zone of the Dabie orogen, central-east China. The feldspars from the gneisses were recrystallized during Triassic continental subduction and UHP metamorphism. Precursors of the orthogneisses are products of Neoproterozoic bimodal magmatic events, those in north Dabie zone emplaced into the lower crust and those in central and south Dabie zones into middle or upper crust, respectively. On a 207Pb/204Pb vs. 206Pb/204Pb diagram, almost all orthogneisses data lie to the left of the 0.23Ga paleogeochron and plot along the model mantle evolution curve with the major portion of the data plotting below it. On a 208Pb/204Pb vs. 206Pb/204Pb diagram the most of data of north Dabie zone extend in elongate arrays along the lower crustal curve and others extend between the lower crustal curve to near the mantle evolution curve for the plumbotectonics model. This pattern demonstrates that the Pb isotopic evolution of the feldspars essentially ended at 0.23Ga and the orthogneiss protoliths were principally dominated by reworking of ancient lower crust with some addition of juvenile mantle in the Neoproterozoic rifting tectonic zone. According to geological evolution history of the locally Dabie orogen, a four-stage Pb isotope evolution model including a long time evolution between 2.0 and 0.8Ga with a lower crust type U/Pb ratio (μ=5–6) suggests that magmatic emplacement levels of the protoliths of the orthogneisses in the Dabie orogen at 0.8Ga also play an important role in the Pb evolution of the exhumed UHPM slices, corresponding to their respective Pb characters at ca. 0.8–0.23Ga. For example, north Dabie zone requires low μ values (3.4–9.6), while central and south Dabie zones require high μ values (10.9–17.2). On the other hand, Pb isotopic mixing between north and central or south Dabie zones during retro-grade metamorphism enhanced by the extensive magmatism in the Cretaceous has also been observed in the 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams. A combined study of common Pb isotopic compositions of Dabie orthogneisses and Sulu UHPM rocks from the Chinese Continental Scientific Drilling project demonstrates that a slab marked by extremely unradiogenic Pb observed in the main hole was absent in the Dabie orogen. However, occurrence of some Mesozoic granitoids with such unradiogenic character in the Dabie orogen suggests that their source may be a buried unradiogenic unit underlying below north Dabie zone. This case study clearly shows that whether the position of the Dabie data relative to the orogen curve of the plumbotectonic model is helpful in understanding the Pb isotopic structure and evolution of subducted continental crust.

Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie-Sulu classic ultrahigh-pressure metamorphic terrane, East-Central China

The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high- P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T -time path, suggesting exhumation rates of 5–10 km/Myr. A similar P-T -time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that Delta; 18 O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.

P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths

Research Article| July 01, 2012 P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths Shengsi Sun; Shengsi Sun 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada Search for other works by this author on: GSW Google Scholar Shaocheng Ji; Shaocheng Ji * 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada2State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Search for other works by this author on: GSW Google Scholar Qian Wang; Qian Wang * 3State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Search for other works by this author on: GSW Google Scholar Matthew Salisbury; Matthew Salisbury 4Geological Survey of Canada-Atlantic, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Search for other works by this author on: GSW Google Scholar Hartmut Kern Hartmut Kern 5Institut für Geowissenschaften, Universität Kiel, Kiel 24098, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Shengsi Sun 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada Shaocheng Ji * 1Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada2State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Qian Wang * 3State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Matthew Salisbury 4Geological Survey of Canada-Atlantic, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Hartmut Kern 5Institut für Geowissenschaften, Universität Kiel, Kiel 24098, Germany *E-mail: sji@polymtl.ca; wangqian@gig.ac.cn. Publisher: Geological Society of America First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (7): 651–654. https://doi.org/10.1130/G33045.1 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shengsi Sun, Shaocheng Ji, Qian Wang, Matthew Salisbury, Hartmut Kern; P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths. Geology 2012;; 40 (7): 651–654. doi: https://doi.org/10.1130/G33045.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We compared the averages of P-wave velocities (Vp) for four lithologies (felsic gneiss, eclogite, retrograde eclogite, and amphibolite) collected from core material sampled by the Chinese Continental Scientific Drilling Project (CCSD) at depths to 5158 m, and from surface outcrops in the Dabie-Sulu (China) ultrahigh-pressure metamorphic terrane. The velocities were measured at pressures to 800 MPa for 69 CCSD samples and 145 surface samples. The Vp-pressure relationship is described by a four-parameter exponential equation, allowing the determination of the crack-closing pressure (Pc), the intrinsic pressure derivative of velocity (D), the intrinsic velocity of the crack-free compacted rock at zero pressure (V0), and the maximum velocity increase due to crack closure (B0). The shape of the nonlinear segment of the velocity-pressure curve is controlled by a decay parameter (k). The CCSD core samples are characterized by significantly higher V0, B0, and k values and lower D and Pc compared to their surface analogues. This suggests that extrapolation of the surface rock data to depth may result in substantial underestimates in seismic velocities, and that the intrinsic pressure derivatives obtained from the Vp-pressure relations for the core samples are more suitable for the determination of in situ velocities at great depth than those derived from measurements on rocks exposed at the surface. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic velocities and anisotropy of core samples from the Chinese Continental Scientific Drilling borehole in the Sulu UHP terrane, eastern China

A detailed study of seismic properties (P and S wave velocities, hysteresis, anisotropy and shear wave splitting) has been carried out on a unique suite of deep borehole core samples from the Chinese Continental Scientific Drilling (CCSD) project, which penetrated 5158 m into the Sulu ultrahigh‐pressure (UHP) metamorphic terrane (China). Seismic velocities of the deep core samples are more and less sensitive to pressure in the low pressure (<200–300 MPa) nonlinear and high pressure (>200–300 MPa) linear regimes, respectively, than samples from the surface. The comparison suggests that the high pressure data from the core samples are much more reliable for extrapolation to deeper crust than the data from surface analogs that have been subjected to long histories of weathering and alteration along intergranular and transgranular cracks. The significant increases in the pressure sensitivity of seismic velocities for the core samples in the nonlinear regime indicate that drilling‐induced and stress‐relief microcracks with small aspect ratios are fresh and clean without secondary mineral in‐fillings, and are thus easy to close completely under the applied hydrostatic pressure conditions of the laboratory. The data also elucidate that the velocity‐pressure data can successfully provide important hints about the preferred orientation of microcracks that causes P wave velocity anisotropy and shear wave splitting in cracked rocks, and that the effect of compression on the Vp/Vsratios is negligible for crack‐free compacted rocks. The seismic velocities of equivalent isotropic (fabric‐free) and crack‐free crystalline aggregates calculated from room pressure single crystal elastic constants using the Voigt average are in good agreement with the laboratory data at ∼200 MPa. Comparison of the seismic reflection image from the vicinity of the borehole with the normal‐incidence reflection coefficient profile computed from the laboratory‐measured velocities and densities infers that the seismic reflections originate from mafic (eclogite and retrograde eclogite) or ultramafic units within dominantly felsic rocks.

The time of eclogite formation in the ultrahigh pressure rocks of the Sulu terrane: Constraints from Lu–Hf garnet geochronology

Eclogites from the main borehole of the Chinese Continental Scientific Drilling project yield highly precise Lu–Hf garnet–clinopyroxene ages of 216.9±1.2Ma (four samples) and 220.5±2.7Ma (one sample). The spatial distribution of the rare earth elements in garnet is consistent with the preservation of primary growth zoning, unmodified by diffusion, which supports the interpretation that the Lu–Hf ages date the time of formation of garnet, the major rock forming mineral in the eclogites. The preservation of primary REE-zoning, despite peak metamorphic temperatures around 800–850°C, indicates that the Lu–Hf chronometer is perfectly suitable to date garnet-forming reactions in high grade rocks. The range of Lu–Hf ages for eclogites in the Dabie–Sulu UHP terrane point to episodic rather than continuous growth of garnets and thus punctuated metamorphism during the collision of the North China Block and the Yangtze Block. The U–Pb ages and Hf-isotope systematics of zircon grains from one eclogite sample imply a protracted geologic history of the eclogite precursors that started around 2Ga and culminated in the UHP metamorphism around 220Ma.

Petrology and geochemistry of UHP-metamorphosed ultramafic–mafic rocks from the main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH), China: Fluid/melt-rock interaction: Mafic–ultramafic complex from CCSD-MH

Mafic–ultramafic rocks recovered between 600 and 690 m in the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) Project in the Sulu terrane consist of garnet peridotite, garnet pyroxenite, garnet clinopyroxenite and eclogite. All of the rocks have undergone ultrahigh pressure (UHP) metamorphism with peak conditions of at least ∼750–850 °C/3.5–4.0 (GPa). The protoliths were mainly lherzolite, websterite, minor clinopyroxenite and harzburgite, with major and trace element compositions characteristic of crustal layered intrusions. The low Mg# of the whole rocks and mafic minerals, layered textures, and adjacent lithology pairs of garnet peridotite - garnet pyroxenite and garnet pyroxenite–eclogite pair probably reflect fractional crystallization of the protolith magma. Whole-rock MgO and FeO abundances decrease and Al 2O 3, CaO and SiO 2 increase toward the mafic endmembers. Extensive metasomatism and metamorphism influenced REE and trace element patterns. At least three episodes of fluid/melt metasomatism are recognized: a partial melting event that depleted K and Rb during the UHP metamorphism; a retrograde metasomatism that occurred during exhumation, leading to the formation of serpentine + phlogopite veins; and a late subgreenschistfacies hydrothermal event that formed carbonates and calc–silicate minerals. The protoliths of the garnet-bearing mafic–ultramafic rocks of the CCSD-MH are interpreted as crustal cumulates, which were subducted during collision between the Yangtze and Sino-Korean Cratons in the Triassic. During subduction, the original ultramafic rocks of the CCSD-MH were transformed to garnet peridotites and pyroxenites.

Mineral hydrogen isotopes and water contents in ultrahigh-pressure metabasite and metagranite: Constraints on fluid flow during continental subduction-zone metamorphism

Systematic measurements of hydrogen isotopes and total water contents were carried out for metamorphic minerals from ultrahigh-pressure (UHP) eclogite, amphibolite, schist and granitic gneiss in continuous core segments from the main hole of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. Together with previous studies of both outcrop and drilling samples, the results yield a dataset that is the most extensive for hydrous and nominally anhydrous minerals from UHP metamorphic rocks (especially for UHP metagranite). This provides insights into the property and scale of fluid flow during continental subduction-zone metamorphism. Hydrous and nominally anhydrous minerals show large variations in δD values from −155 to −20‰, consistent with the incorporation of meteoric water into protoliths of UHP metaigneous rocks by high-T hydrothermal alteration. Both equilibrium and disequilibrium H isotope fractionations occur between coexisting hydrous and nominally anhydrous minerals, indicating the differential effects of retrograde metamorphism on the H isotope systems of UHP minerals during the exhumation of deeply subducted continental crust. Besides the structural OH, nominally anhydrous minerals (NAMs) also contain large amounts of water in the form of molecular H2O that is depleted in D relative to the structural OH. There are negative correlations between total water contents and δD values for garnet, omphacite and rutile in the eclogite, indicating a preferential loss of the D-poor molecular water from the NAMs by diffusion relative to the structural OH during the decompression exhumation. As a consequence, symplectites after the mafic minerals are depleted in D relative to their precursors. On the other hand, amphibole and plagioclase in the amphibolite have δD values similar to, or greater than, garnet and omphacite from the adjacent eclogite. This suggests that the retrograde fluid is enriched in D relative to the NAMs of eclogite and thus derived from the decomposition of D-rich hydrous minerals. In either case, the retrograde fluid is internally buffered in stable isotope compositions. Nevertheless, the exsolution of molecular water and structural hydroxyl from the NAMs is responsible for D-poor fluid, whereas the decomposition of hydrous minerals is responsible for the D-rich fluid. The systematic measurements of water contents for various minerals from the different UHP lithologies demonstrate significant differences in total water contents between eclogite and gneiss. This may be a basic cause for their different behaviors of fluid activity during the exhumation.

Ultrahigh-pressure and Retrograde Metamorphic Ages for Paleozoic Protolith of Paragneiss in the Main Drill Hole of the Chinese Continental Scientific Drilling Project (CCSD-MH), SW Sulu UHP Terrane

Abstract Laser Raman spectroscopy and cathodoluminescence (CL) images show that most zircon crystals separated from paragneiss in the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu terrane, contain low-pressure mineral-bearing detrital cores, coesite-bearing mantles and quartz-bearing or mineral inclusion-free rims. SHRIMP U-Pb dating on these zoned zircons yield three discrete and meaningful age groups. The detrital cores yield a large age span from 659 to 313 Ma, indicating the protolith age for the analyzed paragneiss is Paleozoic rather than Proterozoic. The coesite-bearing mantles yield a weighted mean age of 228 ± 5 Ma for the UHP event. The quartz-bearing outmost rims yield a weighted mean age of 213 ± 6 Ma for the retrogressive event related to the regional amphibolite facies metamorphism in the Sulu UHP terrane. Combined with previous SHRIMP U-Pb dating results from orthogneiss in CCSD-MH, it is suggested that both Neoproterozoic granitic protolith and Paleozoic sedimentary rocks were subducted to mantle depths in the Late Triassic. About 15 million years later, the Sulu UHP metamorphic rocks were exhumed to mid-crustal levels and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is about 6.7 km/Ma. Such a fast exhumation suggests that the Sulu UHP paragneiss and orthogneiss returned towards the surface as a dominant part of a buoyant sliver, caused as a consequence of slab breakoff.

Channelized fluids in subducted continental crust: constraints from δD–δ18O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD = −97‰ to −69‰). Quartz δ18O values range from −2.5‰ to 9.6‰; fluid inclusions hosted in quartz have correspondingly low δ18O values of −11.66‰ to 0.93‰ (T h = 171.2∼318.8°C). The low δD and δ18O isotopic data indicate that protoliths of some CCSD HP-UHP metamorphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ18O of the quartz veins and fluid inclusions vary greatly with the drillhole depth. Lower δ18O values occur at depths of ∼900–1000 m and ∼2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and silicates and is much more abundant in the quartz veins and silicate minerals than is hydrogen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composition. Therefore, different δD and δ18O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation. Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure–temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P–T conditions within the stability field of lawsonite, dehydration and associated high-Si fluid release may have occurred as hydrous minerals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the surface, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.

In situ LA-SF-ICP-MS U-Pb dating of metasomatic zircon growth during retrogression of UHP eclogites, Sulu deep drilling hole, China

Thermobarometry and in situ U–Pb dating of zircon in texturally and petrologically defined positions are used to assess exhumation accompanied by fluid-enhanced retrogression of ultra high pressure (UH P ) eclogite core samples from the Chinese Continental Scientific Drilling (CCSD) project. Eclogites from the CCSD project show discrete retrograde zones that can be attributed to exhumation processes. The retrogression from UH P eclogite to H P eclogite, amphibolite and finally greenschist facies conditions is fluid-induced. The occurrence of significant barium concentrations in two mica generations points to an external fluid source during retrogression. This Ba and F-bearing fluid mobilized zirconium from the eclogitic host, which was preferentially precipitated as overgrowths on pre-existing grains in the retrograde zone. Thermodynamic modelling with progressive addition of H 2 O to the bulk composition simulates the petrological development of the fluid-influenced retrograde zone. Growth of zircon precipitated from the fluid occurred between 730°–740 °C at 3.46 GPa and 700 °C at 1.4 GPa. LA-SF-ICPMS analyses point to a local onset of metasomatic zircon growth at 233.1 ± 5.3 Ma ongoing to 207.4 ± 1.6 Ma that is spatially limited to the retrograde zone.

Metamorphic P–T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe–Mg partitioning thermometer vs. Zr‐in‐rutile thermometer

AbstractCore rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD‐MH) project, southern Sulu UHP terrane, east‐central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction‐zone UHP metamorphism. Coesite‐bearing eclogites are mainly present between the depths of 100–2000 m, but below 2000 m, mafic eclogites are rare. Selected elements (Zr, Nb, Cr, Fe, Si, Mg, Al & Ti) in rutile from 39 eclogite cores from 100 to 2774 m, and major elements of minerals from representative eclogites were analysed by electron microprobe. Zirconium and Nb concentrations of rutile cluster ∼100–400 and 200–700 ppm respectively. However, Zr and Nb contents in rutile from strongly retrograded eclogites show larger variations than those of fresh or less retrograded eclogites, implying that somehow fluid infiltration affected rutile chemistry during retrograde metamorphism. Zr contents in rutile inclusions in garnet and omphacite are slightly lower than those of the matrix rutile, suggesting that the rutile inclusions formed before or close to the peak temperature. The P–T conditions of the CCSD‐MH eclogites were estimated by both Fe–Mg exchange and Zr‐in‐rutile thermometers, as well as by the Grt–Cpx–Phn–Ky geothermobarometer. The maximum temperature range of 700–811 °C calculated at 40 kbar using the Zr‐in‐rutile thermometer is comparable with temperature estimates by the Fe–Mg exchange thermometer. The temperature estimates of eclogites in a ∼3000 m thick section define a continuous gradient, and do not show a distinct temperature gap, suggesting that the rocks from 100 to 3000 m depth might belong to a single, large‐scale UHP slab. These data combined with P–T calculations for CCSD‐MH peridotites yield a low geotherm (∼5 °C km−1) for the Triassic subduction zone between the Sino‐Korean and Yangtze cratons; it lies ∼30–35 mW m−2 conductive model geotherm.

Nb/Ta fractionation observed in eclogites from the Chinese Continental Scientific Drilling Project

This paper reports detailed analyses of Nb and Ta concentrations of 19 eclogite samples and their principal mineral constituents from the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD) and nearby outcrops. We observe highly fractionated and overall suprachondritic Nb/Ta values in minerals, e.g., rutile (4.8–87), titanite (12–62) and amphibole (2.0–67). Amphiboles in amphibolites (retrograded from eclogite) can be classified into two groups: a low Nb/Ta group that bears higher Al contents and is thus of higher pressure origin, and a high Nb/Ta, lower pressure group. The former group was likely formed during subduction; the latter may have formed during exhumation in the presence of rutile and titanite. The significant Nb/Ta fractionation in rutile and other minerals may reflect early dehydration of the subducted slab at shallow depths before the formation of rutile, which occurs at depths ≥50 km. The dehydration, with amphiboles existing as the main Nb–Ta-bearing phase, would lead to Nb/Ta fractionation, i.e., forming subchondritic Nb/Ta ratios in the released fluids and, complementarily, suprachondritic Nb/Ta ratios in the residual phases. While a large proportion of the fluids may escape from the slab to the mantle wedge, considerable amounts of the fluids can be retained in hydrous minerals within the descending slab, thus forming hydrated cold eclogites with subchondritic Nb/Ta characteristics. As subduction continues to depths over 50 km, rutile appears and consequently controls the Nb–Ta budget. In the presence of rutile, melting of the hydrated cold eclogites with very low Nb/Ta ratios would form magmas with negative Nb, Ta anomalies and subchondritic Nb/Ta. Further dehydration of the continuously descending slab results in even more fractionated Nb/Ta ratios in subsequently released fluids and residues, providing a feasible explanation for the large Nb/Ta variation observed in the modern arc magmas and residual eclogites.

Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at ∼650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.