V44A-01
Detachment and Exhumation of Ultra-high-pressure Rocks During Continental Subduction
In many Phanerozoic collisional orogens, ultra-high-pressure (UHP) rocks are inferred to have been formed and exhumed during the transition from oceanic subduction to continental collision. We use fully coupled, thermal- mechanical, upper-mantle-scale models to investigate how detachment and exhumation of UHP material in the subduction channel are affected by the competition between down-channel shear traction and up-channel buoyancy forces. Our reference model involves constant velocity (5 cm/y) subduction of a weak continental margin and stronger continental interior, and includes strain weakening and reversible density changes accompanying metamorphic phase transformations. During subduction, decoupling of margin material initiates at depth along a shear zone that propagates upwards as subducted material progressively weakens, leading to diachronous detachment and exhumation of UHP and HP rocks. UHP exhumation begins first, as a buoyant plume that tunnels upwards along the weak shear zone. HP material initially forms a stagnant nappe, which is then exhumed by, and folded over, the rising UHP plume. Model predictions are consistent with observed PTt paths and timing constraints from natural examples. The results can be interpreted in terms of a lubrication theory of channel flow in which the exhumation number, E, expresses the relative contributions of Poiseuille (up-channel) and Couette (down-channel) components. The value of E depends on the pressure gradient (dominated by density contrasts), channel thickness, effective viscosity, and subduction velocity, and varies with both time and distance down the channel. Although buoyancy is the main driving force for exhumation, viscous strain weakening plays a critical role by first reducing the downward Couette drag and then promoting upward Poiseuille flow of low-viscosity UHP material. In natural systems, other weakening mechanisms may also play a role. Wide- continental-margin models produce a low-viscosity channel that tunnels up to lower crustal levels, suggesting a link between subducted margin properties and exhumation style, with implications for UHP exhumation in different parts of the Himalayan-Tibetan system.
V44A-02 INVITED
Numerical modeling of mantle wedge processes and exhumation of UHP mantle in subduction zones
The upwelling of subduction generated partially molten rocks is potentially a mechanism for the exhumation of UHP rocks through the mantle wedge. We investigated this processes using a 2-D coupled petrological- thermomechanical model that incorporates slab dehydration and water transport as well as partial melting of mantle and crustal rocks. This approach allows us to study the dynamics of mantle wedge processes including evolution of partially molten plumes and their interaction with surrounding dry mantle. To study the internal structure of the plumes we used ultra-high resolution numerical simulations with 10 billion active markers to detail the internal structure of natural plumes originating from the slab. The plumes consist of partially molten hydrated peridotite, dry solid mantle and subducted oceanic crust, which may comprise up to 12 volume % of the plume. As the plumes grow and mature these materials mix chaotically resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges. The recent discovery of garnet bearing peridotites in the subduction zone of the Great Antilles in Hispaniola has raised questions about the process that leads to their exhumation. To evaluate whether upwelling plumes are a plausible exhumation mechanism we investigated the dynamics of subduction of slow spreading ridges. The results show that subduction of strongly serpentinized oceanic plate causes strong dehydration of the slab and leads to a rheological weakening of the interface between subducting and overriding plate. This weakening triggers trench retreat and massive asthenospheric upwelling into the gap between the two plates. The P-T paths computed for this scenario can explain exhumation of UHP (4GPa) rocks.
V44A-03
Exhumation processes in oceanic and continental context
Despite the growing amount of data on surface horizontal displacement, the vertical movements of the lithosphere and exhumation processes at convergent plate boundaries are still poorly known. Petrological and geochronological data on High-Pressure to Ultra High Pressure Low-temperature metamorphic rocks provide invaluable constraints on the behaviour of convergent zone. On one hand, the development of in situ datings coupled with more and more precise and continuous pressure-temperature estimates allow the trajectory of subducted rocks to be followed in the 2D thermal-depth (T-Z) field. On the other hand, thermo-mechanical numerical model allow the trajectory of subducted rocks to be followed in the 4D X-Z- T-deformation space. The combination of worldwide natural data with numerical model emphasizes the following salient results: - Whatever their origin (continental or oceanic), the exhumation of HP to UHP rocks is related to convergent processes. - Exhumation of solid rocks requires the weakening of the subduction channel by occurrence of hydrated sediments, hydrated peridotites or local melting. - The driving forces responsible for exhumation are a combination of buoyancy (dominant for continental rocks exhumation) and channel flow coupled with underplating (dominant for oceanic rocks exhumation) - Exhumation velocities are independent of plate velocities: i) low velocity (< 5 mm/yr) exhumation of HP-LT metasediments (P< 2.5 GPa, T < 600°C) is a long-lasting process, which occurs in an accretionary wedge environment; ii) low to intermediate velocity (5 < v < 30 mm/yr) exhumation of HP to UHP (< 3 GPa < 650°C) oceanic rocks is a long-standing (Caribbean) or a transient processes (Alps), which occurs within the serpentinite subduction channel; iii) high velocity (up to 40 mm/yr) of UHP (up to 6 GPa, 900°C) continental unit occurred in the mantle wedge combining both astenospheric return flow and buoyancy forces.
V44A-04
P, T conditions of the CCSD (Chinese Continental Scientific Drilling) eclogites; importance of ferric estimation based on Mössbauer and micro-XANES analysis
Recent UHP experiments in C-O-H fluid-bearing MORB system have revealed that phase change and fluid composition depend on oxygen fugacity (e.g. Molina and Poli, 2000). If oxygen fugacities represented by the equilibrium NNO (Ni-NiO) or FMQ (fayalie-magnetite-quartz) are assumed to be the average condition of UHP metamorphism, then the phase assemblages of UHP rocks are expected to have graphite/diamond only, graphite/diamond +carbonates, or carbonates only depending on the bulk compositions (Poli and Fumagalli, 2004). CCSD (Chinese Continental Scientific Drilling) samples are probably the best for identification of C- species in UHP rocks. We investigated nine eclogites from various depths (170 to 2000 m). Under the microscope, the eclogites contain garnet, clinopyroxene. quartz and rutile with or without phengite, graphite, apatite, zircon, pyrite, talc, and K-feldspar. Graphite is always recognized with pyrite, suggesting oxygen fugacity was low (NNO) at UHP stage. The presence of graphite suggests that the eclogites released H2O-rich (CO2- poor) fluids and melt at UHP stage. The studied eclogites contain CaEs component in clinopyroxene. Therefore, Fe3+ content can not be calculated based on EPMA analysis. We estimated Fe3+/Fe2+ based on Mossbauer and micro-XANES analysis. With ignoring the Fe3+, P-T conditions based on the assemblage of phengite-garnet-cpx- (coesite) can be estimated as P=3-5 GPa, and T = 850–950 oC. However, the Fe3+ estimation from the clinopyroxenes based on Mössbauer and micro-XANES analysis corrects the P, T condition as 3-4 GPa, and 650 -780 oC. The geothermobarometry based on the kyanite-garnet-cpx- phengite-coesite assemblage (Ravna and Terry, 2004) is reliable because temperature estimation is independent from Fe3+ content in clinopyroxene. The estimation from one eclogite gives P=3.4 GPa, and T=750oC, quite consistent with the above estimation. The thermometry based on Zr concentration from rutile also gives estimated as 600 to 730 oC. Ref. M & Poli (2002) EPSL, 176, 295; P & F (2004) EMU notes in miner. vol. 5. 307; R & T (2004) JMG, 22, 579; .Z & K (1996) Petrology and Structural Geology 7, Kluwer Academic Press, 49.
V44A-05
Thermal structure of the Sulu UHP terrane in the CCSD-main hole from 100 to 300 m constrained by Zr concentration in eclogitic rutile
The core rocks recovered by the main hole (5100 m deep) of the Chinese Continental Scientific Drilling (CCSD) project in the southern Sulu UHP terrane, China are composed of eclogites, various gneisses and minor peridotite cumulates, and were experienced subduction-zone UHP metamorphism. Eclogites are mainly restricted to the upper 100-2000 m; among them, Phn-bearing eclogite is most common, Ky-bearing, and Zo/Ep- bearing eclogites are minor. Rutile, zircon and apatite occur as accessory phases in all eclogite cores. In order constrain thermal structure of the CCSD-MH zirconium concentration in rutile coexisting with zircon and quartz from 22 eclogite cores (each 16-45 points) from 101 to 2774 m were analyzed by electron microprobe. The Zr content of rutile ranges from 30 to 1,000 ppm; most are about 100-400 mm. The Zr-in-rutile geothermometer yields the maximum and average temperatures at 645-875°C and 595-680°C by the calibration of Zack et al. (2004), and 594-755°C and 570-623°C by the calibration of Watson et al. (2006), respectively. The maximum temperature of 700-800°C obtained occurs at 100-530 m and >1600 m, which are very close to the temperature calculated at 3 GPa by Cpx-Grt thermometers. In a shear zone of 870- 1113 m the maximum-T show a small range of 660-690°C, slightly lower than those of eclogites at the top part of 100-530 m, probably due to retrograde metamorphism caused by strong deformation. If the influence of pressure on the Zr-substitution in rutile is considered (Tomkins et al., 2007), the maximum temperature at > 30 kbar would be changed. In general, the Zr-in-rutile thermometer yields slightly low-T in comparison with those of using the Fe-Mg portioning between garnet and clinopyroxene, and only the maximum temperatures may record the temperature of peak stage metamorphism. As the Sulu UHP eclogites and their associated gneissic rocks have been subjected to long duration of 20-40 Ma UHP metamorphism and amphibolite-facites retrograde overprint, the difference of the obtained temperatures from different sample or individual sample may record the different stage of growth and equlibration of minerals.
V44A-06
Metamorphic Evolution, Mineral Chemistry and Thermobarometry of Ultrahigh-Pressure Eclogites From the North Qaidam Metamorphic Belt, Western China
The North Qaidam UHP belt is composed of discontinuous exposures over 500 km of leucocratic gneisses containing mafic eclogite and local garnet-peridotite that experienced peak metamorphism 430-590 Ma. Although coesite has been reported in one locality, it remains uncertain whether all mafic eclogites experienced UHP conditions, and the P-T paths of exhumation are not well constrained. To address this issue a detailed study of paragenesis and petrology of mafic eclogite from the Luliang Shan was conducted. Four stages of mineral growth with can be identified: Stage I, anhydrous eclogite facies mineral assemblage; Stage II, early amphibole- eclogite facies assemblage; Stage III, amphibole-eclogite facies assemblage and Stage IV, epidote-amphibolite facies assemblage. Stage I records the high pressures of 25+/-2 kbar at 590+/-25 degC. Stage II minerals record growth during decompression through 24-18 kbar and associated increase in temperature to 650+/-25 degC. Stage III minerals record P-T conditions of 605+/-25 degC and 16+/-2 kbar. Stage IV minerals temperatures of 415-510 degC, with maximum pressure constrained by the epidote-amphibolite assemblage. Peak P-T conditions are within error of the coesite-quartz equilibria. The resulting P-T path is better constrained through decompression than previously published paths for the Qaidam belt revealing that the exhumation path is clockwise with slight heating during decompression. The identification of Stages II, III and IV assemblage allows construction of a more detailed retrograde P-T path, revealing that the Luliang Shan eclogites experienced heating during exhumation. Temperature at peak pressure was significantly lower than at other North Qaidam eclogite localities. The data also suggest that geothermal gradients varied significantly along strike in the belt, both at the maximum depth of lithospheric subduction and at middle crustal depths to which the eclogite-bearing lithologies were exhumed.
V44A-07
Diamond Nucleation from Amorphous Carbon and Graphite with COH Fluids: an in Situ High Pressure and Temperature Laser-Heated Diamond Anvil Cell Experimental Study
Microdiamonds from orogenic belts contain nanometer size fluid inclusions suggesting diamond formation from supercritical COH fluids. Previous studies have shown that diamonds synthesized from high pressure and temperature experiments with supercritical COH fluids are characterized by skeletal morphology and solid oxide inclusions. However, mechanism and kinetics of graphite/carbon-to-diamond transformation promoted by COH fluids at high pressure and high temperature conditions are not well understood. Here we report in situ observations of diamond nucleation from COH fluids in laser-heated diamond anvil cell. Our experimental starting materials were amorphous carbon (impurity < 2ppm) and graphite (99.9% pure). Oxalic acid dihydrate (COOH)2·2H2O) was added to amorphous carbon and glucose (C6H12O6) was added to both amorphous carbon and graphite. The organic compounds (3 wt.%) provide CO2- and CH4-rich fluid environments respectively during their breakdown at high pressure and temperature. The mixtures were kept at temperature of 1400-1700 °C and pressure of 8-10 GPa for 10-30 minutes. Experiments show that only nanocrystals of diamond were nucleated from amorphous carbon in CO2-rich fluid environment. The fastest rate of diamond nucleation and growth of ~15 micron size crystals was found in the mixture of amorphous carbon with glucose (CH4-rich environment), whereas only nanocrystalline nuclei were produced in the mixture of graphite with glucose. We have also established that under anhydrous conditions, no diamond nucleation occurred in pure graphite, and only nanocrystals of diamond were observed in the amorphous carbon starting material at temperatures 1700-1900 °C. Our results revealed that the kinetics of diamond nucleation depend on the ¡°precursor": diamond nucleates and grows faster from amorphous carbon than from graphite in the presence of COH fluid; in our anhydrous experiments diamond nucleates only from amorphous carbon. These results demonstrate: (1) diamond nucleation from anhydrous graphite starting material is hampered because of the well-known kinetic barrier that requires a huge amount of energy to convert sp2-bonding to sp3-bonding, whereas there is no difficulty for diamond nucleation in amorphous carbon material; (2) the critical role of supercritical COH fluid for promoting the graphite-to-diamond transformation.
V44A-08
Textural relationship and compositions of ilmenite-corundum exsolutions in rutile from kimberlitic kyanite eclogite xenoliths: microstructural evidence using EBSD
Rutile is one of the most common accessory minerals in high pressure (HP) and ultrahigh-pressure (UHP) rocks of crustal and mantle origin. Among those rocks eclogites are very abundant in metamorphic belts and as xenoliths in kimberlite pipes. Some of these xenoliths contain coesite or diamond. Eclogite xenolith rutiles contain high abundance of minor elements reaching up to 0.8 wt.% Al2O3, 2.5 wt.% Fe2O3, 1.4 wt.% Nb2O5, 0.45 wt.% ZrO2 and are mostly heterogeneous with widely varying Al, Fe and Mg contents. These heterogeneities are caused by the presence of closely associated sigmoidal oriented lamellae of ilmenite and corundum which have been detected for the first time in rutile from Roberts Victor mine eclogite xenolith (Sobolev, Yefimova, 2000, Intern. Geol. Rev., v. 42, p. 758-767). We report here on the wide occurrence of such lamellae in rutiles both from diamondiferous kyanite eclogites of Udachnaya mine, Siberia and more samples from Roberts Victor mine confirmed by EMPA. Electron backscatter diffraction (EBSD) measurements on two rutile grains from Roberts Victor mine kyanite eclogite confirm the presence of ilmenite plus corundum exsolutions. We found evidence for respective different crystallographic orientations related to the surrounded rutile host which will be demonstrated in detail in the present study.