Abstract: Rheological studies at high pressure and temperature using in-situ X-ray diffraction and imaging have made significant progresses in recent years, thanks to a combination of recent developments in several areas: (1) advances in synchrotron X-ray techniques, (2) advances in deformation devices and the abilities to control pressure, temperature, stress, strain and strain rates, (3) theoretical and computational advances in stress determination based on powder and single crystal diffraction, (4) theoretical and computational advances in modeling of grain-level micromechanics based on elasto-plastic and visco-plastic self-consistent formulations. In this article, we briefly introduce the experimental techniques and theoretical background for in-situ high pressure, high temperature rheological studies, and then review recent studies of rheological properties of major mantle materials. Some currently encountered issues have prompted developments in single-crystal quasi-Laue diffraction for complete stress tensor determination and textural evolution of poly-phased composites based on X-ray microtomography. Future prospects are discussed.
Abstract: A deformation experiment of ringwoodite with a strain of 9% was achieved at 20 GPa and 1 700 K and at a strain rate of 3×10−5 s−1 using a deformation-DIA (D-DIA) apparatus and a multi-anvil 6-6 (MA 6-6) assembly. The crystallographic orientations of the deformed sample were successfully analyzed by the electron backscatter diffraction (EBSD) method, although any notable lattice-preferred orientation (LPO) was not observed presumably due to the insufficient strain in the present experiment. In this study, the deformation experiment on ringwoodite succeeded at P-T conditions consistent with the lower part of the mantle transition zone and at a controlled strain rate for the first time. The present study extended the pressure range of deformation experiments in the D-DIA apparatus from 16 GPa in our earlier study to 20 GPa at 1 700 K. The successful extension of the pressure range demonstrates potential importance of the D-DIA apparatus in studying rheological properties of minerals under the P-T conditions of the whole mantle transition zone.
Abstract: Technical developments for simple shear deformation experiments at high pressures were made. The newly designed cell assembly can be compressed by deformation-DIA apparatuses with the MA 6-6 system, which consists of six second-stage tungsten carbide anvils (with a truncated edge length of 5 mm) and the anvil guide. Deformation of samples was barely observed during the compression process, showing that the shear strain of the deformed samples can be measured by the rotation of a strain marker. Simple shear deformation experiments on anhydrous and hydrous olivine aggregates were conducted under upper mantle conditions (pressures of 5.2–7.6 GPa and temperatures of 1 473–1 573 K), and sample deformation with a shear strain of γ=0.8−1.2 was successfully achieved at a shear strain rate of 4.0×10−5−7.5×10−5 s−1. The present study extended the pressure range of simple shear deformation experiments in the deformation-DIA apparatus from 3 GPa in an early study to 7.6 GPa at high temperatures.
Abstract: To provide constraints on the rheological properties of garnet, we have experimentally investigated the creep behavior of garnet at high pressures and temperatures using a deformation-DIA. Samples were cold-pressed from a garnet powder and deformed at constant displacement rates ranging from 1.1×10−5 to 2.6×10−5 s−1 at high temperatures (1 273–1 473 K) and high pressures (2.4–4.1 GPa). Differential stress and pressure were measured using X-ray diffraction techniques based on the elastic strain of various lattice planes as a function of orientation with respect to the applied stress field. The plastic strain of a deforming sample was monitored in-situ through a series of radiographs. Our results provide a measure of the dependence of creep rate of garnet on the temperature with an activation energy of ~280 kJ/mol and on pressure with an activation volume of ~10×10−6 m3/mol. The flow behavior of garnet quantified by this study provides the basis for modeling geodynamic processes occurring within subducted lithosphere.
Abstract: To investigate the role of grain boundary sliding during dislocation creep of dunite, a series of deformation experiments were carried out under anhydrous conditions on fine-grained (~15 μm) samples synthesized from powdered San Carlos olivine and powdered San Carlos olivine+1.5 vol.% MORB. Triaxial compressive creep tests were conducted at a temperature of 1 473 K and confining pressures of 200 and 400 MPa using a high-resolution, gas-medium deformation apparatus. Each sample was deformed at several levels of differential stress between 100 and 250 MPa to yield strain rates in the range of 10−6 to 10−4 s−1. Under these conditions, the dominant creep mechanism involves the motion of dislocations, largely on the easy slip system (010), accommodated by grain boundary sliding (gbs). This grain size-sensitive creep regime is characterized by a stress exponent ofn=3.4±0.2 and a grain size exponent of p=2.0±0.2. The activation volume for this gbs-accommodated dislocation creep regime is V*=(26±3)×10−6 m2·mol−1. Comparison of our flow law for gbs-accommodated dislocation creep with those for diffusion creep and for dislocation creep reveals that the present flow law is important for the flow of mantle rocks with grain sizes of < 100 μm at differential stresses > 20 MPa. Hence, gbs-accommodated dislocation creep is likely to be an important deformation mechanism in deep-rooted, highly localized shear zones in the lithospheric upper mantle.
Abstract: Lattice preferred orientation (LPO) and seismic anisotropy of orthopyroxene (enstatite) in mantle xenoliths from Spitsbergen, Svalbard, near the Arctic, are studied. LPOs of enstatite were determined using electron backscattered diffraction (EBSD). We found four types of LPOs of orthopyroxene and defined them as type-AC, -AB, -BC, and -ABC. Type-AC LPO of orthopyroxene is defined as (100) plane aligned subparallel to foliation and  axis aligned subparallel to lineation. Type-AB LPO is defined as (100) plane aligned subparallel to foliation and  axis aligned subparallel to lineation. Type-BC LPO is defined as (010) plane aligned subparallel to foliation and  axis aligned subparallel to lineation. Type-ABC LPO is defined as both (100) and (010) planes aligned subparallel to foliation with a girdle distribution of both  and  axes normal to lineation and  axis aligned subparallel to lineation. We report for the first time the type-AB, -BC, and -ABC LPO of orthopyroxene. We found that the LPO pattern has a correlation with the content of orthopyroxene in the specimen. Nicolet 6700 FTIR (Fourier transformation infrared) study of enstatite showed that type-AC LPO was observed mostly in the samples of enstatite with low water content. It is found that the strength of the LPO of enstatite decreases with increasing water content and has a correlation with the strength of the LPO of olivine: the stronger the LPO of enstatite, the stronger the LPO of olivine. Seismic anisotropy of enstatite was smaller than that of olivine in the same specimen.
Abstract: We report here lattice preferred orientations (LPOs) and seismic properties of eclogites from the Sulu (苏鲁) UHP terrane. Our results show strong fabrics in omphacite and amphibole, and approximately random fabrics in garnet with or without strong shape preferred orientations (SPOs). Dislocation creep is likely to be responsible for the observed omphacite fabrics that vary with geometry and orientation of finite strain ellipsoid. Weak garnet LPOs suggest that garnet did not accommodate plastic strain or was not deformed by dislocation creep with a dominant slip system. The calculated seismic properties of eclogites and their component minerals show a strong correlation with their LPOs. Seismic anisotropies are mostly induced by omphacite component in fresh eclogites and by amphibole component in retrograded eclogites, respectively. Retrogression of omphacite to amphibole and quartz will increase seismic anisotropies but decreases seismic velocities of eclogite. Garnet component increases the seismic velocities but decreases seismic anisotropies of eclogite. Comparison of the calculated and the measured seismic properties of eclogites suggests that both methods resolve comparable results with some discrepancies. Compositional layering can play a very important role in determining the seismic properties of eclogites in addition to LPO.
Abstract: During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite. The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP-MS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475–509, 420–457, and 485–514 Ma, respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20–80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these regions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle depth of stishovite stability field (> 9 GPa) is occasional or universal; (2) the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field (> 300 km); (3) the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in metamorphism of the HP-UHP rocks in the West and Middle Central China orogen, but also in continental deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction.
Abstract: The Sulu (苏鲁)-Dabie (大别) orogen in East-Central China formed during the subduction and collision of the Yangtze block with the North China block in Early Mesozoic (240–225 Ma). Constraints on the nature and derivation of eclogites, which are a significant component of the orogen, can provide useful information about subduction-zone metamorphism and crust-mantle interaction. The U-Pb ages, Hf-isotope ratios and trace-element compositions of zircons in eclogites from the Jiangzhuang (蒋庄) (Jiangsu (江苏) Province) and Rongcheng (荣成) (Shandong (山东) Province) areas indicate that the protoliths of the eclogites derived from ultramafic-mafic complexes or mafic intrusion in the subducted continental lithosphere. The upper intercept age of 852±10 Ma and high ɛHf (up to 14.7) of the Neoproterozoic zircons in a Jiangzhuang sample indicate that the protoliths represent products of the Neoproterozoic addition of juvenile materials to the older (i.e., Paleo-Mesoproterozoic) continental crust. The zircon ages of eclogites from both localities mainly record the Triassic (230–220 Ma) metamorphism, consistent with the formation of the Sulu orogen in Early Mesozoic. The lower intercept age of 316±4 Ma in a Jiangzhuang sample suggests that thermal activity relating to the paleo-Tethyan in Late Carboniferous also affected the eclogitic protolith.
Abstract: The study of δ13CPDB (Pee Dee Belemnite) and nitrogen contents in 1 to 5-μm-diameter microdiamonds included in garnets from the quartz-feldspathic gneisses (Erzgebirge, Germany) was performed in situ with the Nano-scale Secondary Ion Mass Spectrometer. The results revealed that there were two stages of diamond crystallization from a C-O-H supecritical fluid rich in biogenic carbon and diverse minor elements of crustal origin. The δ13CPDB of the Erzgebirge diamond of the first stage falls in the range −17‰ to 19‰, with an average value of −17.8‰; the average content of nitrogen is 820 ppm. Diamonds of the second stage are characterized by δ13CPDB=−21.5‰ to −25.5‰, with an average value of −23.24‰; the average nitrogen content is non-homogeneously scattered from 740 ppm to 3 370 ppm among 6 diamonds situated in garnets within the same polished rock slide. Both diamond of the first stage and diamond of the second stage carbon reservoirs belong to biogenic matter, therefore confirming deep subduction of the continental crust sediments and their subsequent exhumation during the Variscan orogeny.
Abstract: This study carried out a comprehensive review of mantle-derived garnet peridotites on the basis of their garnet-spinel compositional relationships. The P-T estimates of the garnet peridotites of both orogenic and xenolithic derivations confirm previously established two garnet-spinel transition (GST) zones. Results of natural samples and experimental studies of all the garnet peridotites plot below the first GST at low P (< 2 GPa). The second GST at high P (6-10 GPa), obtained from the experimental system (MgO-Cr2O3-SiO2), was firstly evidenced from natural samples in this study and may imply a challenge to our understanding of Cr-rich and Al-poor mineral phase transition under ultrahigh P condition. Based on the data and our estimates, we propose the existence of another GST within the garnet peridotite discontinuous zone and a potential spinel-phase peridotite zone in the upper mantle.
Abstract: Two types of melt pockets, closed melt pocket (CMP) and open melt pocket (OMP), are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling (秦岭), Central China. The Haoti (好梯) CMPs have a mineral assemblage of olivine+ clinopyroxene+amphibole+K-feldspar, whereas the Baiguan (白关) CMPs are composed of olivine+clinopyroxene+ilmenite+carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, clinopyroxene, glass, low modal abundances of amphibole, K-feldspar (Kfs), ilmenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, olivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Clinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-situ decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.
Abstract: Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton (NCC), were studied to reveal the Mesozoic crust-mantle interaction. These xenoliths are characterized by low SiO2 (37.7 wt.%-46.0 wt.%) and high Al2O3 (10.8 wt.%-17.9 wt.%) contents. Their Mg# (60-75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their low SiO2 contents. They have low rare-earth element (REE) contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex REE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode (generally > 30%), these features suggest that they are high-pressure metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic compositions (143Nd/144Nd=0.511 763-0.512 173, 87Sr/86Sr=0.705 34-0.706 99) and fall in the trend defined by the > 110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83-134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-rich granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatic underplating and mixing with the pre-existing lower crust (i.e., AFC process). However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted (a fossil oceanic slab) or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal eclogite, although it cannot be excluded that some of the melts were derived from depleted mantle peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogenesis as the > 110 Ma Mesozoic basalts from the NCC.
Abstract: This article reports the petrography and mineral chemistry of dunite xenoliths and olivine xenocrysts entrained by the Early Cretaceous Xi'anli (西安里) hornblende (Hb)-gabbros from the southern Taihang (太行) Mountains, with the aim of constraining the nature of the Mesozoic lithospheric mantle in Central China. Rounded dunite xenoliths are 1-3 cm3 in size and display porphyroclastic, tabular, and protogranular textures. Chromite with Cr#=60-89 is common in the xenoliths. Olivine xenocrysts of 4-6 mm in size are also found in the Hb-gabbros. Orthopyroxene reaction rims are commonly observed around olivine xenocrysts or between dunite xenoliths and host rocks. The porphyroclastic olivines within dunite xenoliths and olivine xenocrysts have kink bands and Mg#=83-94. The Mg# of olivine cores and rims are 89-94 (average, 90) and 83-86 (average, 84.4), respectively. The CaO contents of all olivines from the xenoliths and xenocrysts are less than 0.1 wt.%, suggesting a lithospheric mantle origin. The Ca content (214 ppm-818 ppm) and Ti content (15 ppm-137 ppm) in the xenoliths and xenocrysts are similar to those of olivines from the dunite xenoliths, but are much higher than those of olivines from harzburgite and lherzolite xenoliths in the Fushan (符山) intrusion. This finding implies that the xenoliths and xenocrysts may have originated from harzburgites or lherzolites that were intensively modified by silica-rich melts. This result, combined with high Mg # (92-94) of olivine cores from the dunite xenoliths and xenocrysts, indicates that these olivine xenocrysts and dunite xenoliths could represent the residue of ancient (Archean or Paleoproterozoic) lithospheric mantle and might have experienced the same intensive modification by silica-rich melts as the host magma, resulting in enrichment in MgO and SiO2.
Abstract: Application of reliable thermobarometer on garnet-bearing mantle xenoliths and granulite xenoliths entrained by Cenozoic basalts in eastern China reveals two main types of geotherm. The first type, as exampled by Hannuoba (汉诺坝), Mingxi (明溪) and probably Northeast China, is characterized by constant slope of data in the P-T space. The second type, as exampled by the high geotherms of Nüshan (女山) and probably Xinchang (新昌), is characterized by variable slopes, with the samples with pressure < 2 MPa defining a slow slope, whereas the samples with pressure > 2 MPa define a virtually vertical slope. The different slopes in the second type of geotherm may correspond to different heat transfer mechanisms, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle. This observed transition in thermal transfer mechanism is consistent with theoretical modeling. The two types of geotherm are not mutually exclusive, because the second type may characterize the thermal state of whole lithospheric section including both mechanical boundary layer (MBL) and thermal boundary layer (TBL), while the first type may only depict the MBL. The variable geotherms for different regions are indicative of a heterogeneous lithospheric structure in eastern China. (a) Eastern North China craton (NCC) is characterized by a second-type geotherm, corresponding to a thin lithosphere (~70 km). Comparison of the equilibrium temperatures of spinel peridotites with this geotherm constrains the depth to Moho in eastern North China craton to be 30 km. In contrast, western NCC (Hannuoba: the first-type geotherm) possesses a relatively low thermal gradient, indicative of a thick lithosphere (> 90–100 km) and a thick crust-mantle transition zone. The dramatic change in crustal and mantle structure across the DTGL (Daxing'anling (大兴安岭)-Taihangshan (太行山) gravity lineament) is consistent with recent seismic studies. (b) There is a decrease in thermal gradient and in lithospheric thickness from the coast (Xinchang: the second-type geotherm) to the inland (Mingxi: the first-type geotherm) in South China (from ~80 km to > 90 km), which is collaborated with westward variation in basalt geochemistry. (c) The weak convex-upward pattern of the geotherm in Qilin (麒麟) and Leizhou (雷州) Peninsula is peculiar, probably reflecting a transitional feature between conductive and advective heat transfer. It may result from impregnation of mantle plume on the base of the lithosphere. These new results not only provide a basic framework for the ongoing 4-D lithosphere mapping project in eastern China, but also yield important implications for deep processes that operated over the past.
Abstract: The Alpe Arami garnet peridotite of the Southern Swiss Alps is associated with eclogites and included within quartzofeldspathic gneisses. Controversy has swirled around the depth of origin of this massif since the 1970s when application of the newly-developed technique of thermobarometry suggested a depth of last equilibration of greater than 120 km. Such controversy accelerated in 1996 when we reported microstructural evidence of extensive precipitation of ilmenite and spinel from olivine and proposed a much greater depth of origin. Subsequent experiments showed that it was possible to dissolve the observed amount of TiO2 in olivine, but only at depths in excess of 300 km, agreeing with the earlier proposal. In 1999 we added new, independent, evidence concerning exsolution of high-pressure clinoenstatite from diopside that in-and-of-itself required a depth of origin in excess of 250 km. Subsequently, we also added evidence from the surrounding eclogites of very high pressures and experimental evidence that the pyroxenes included in the amoeboid garnets of this rock had exsolved from a majoritic parent at perhaps even greater pressures. In refutation of the first two of these observations, suggestions were made that (i) we had made a serious error in our estimate of how much ilmenite was present in olivine (and therefore how much TiO2 had been dissolved in olivine); (ii) the ilmenite had not been exsolved from olivine but former titanian clinohumite had been present and broke down to yield the ilmenite; (iii) the pyroxene exsolved from diopside had been high-temperature clinoenstatite. In all three of these cases, the alternatives offered were claimed to be accommodated at low pressures. Here we review the essence of this controversy and show that the only scenario that can explain all of the data is the one that we originally proposed; indeed, the more recent data have strongly supported that interpretation and pushed the minimum origin of the massif to depths approaching 400 km.
Abstract: Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archean ophiolite sequence in the North China craton, in the Dongwanzi (东湾子)-Zunhua (遵化) structural belt and correlatives in the Wutaishan (五台山) area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into cumulate ultramafic rocks including serpentinized dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultramafic rocks interpreted as depleted mantle harzburgite tectonites. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophiolite makes it one of the world's oldest recognized, laterally-extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5–2.4 Ga cross-cutting younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite, and podiform-chromite bearing dunite occur as tectonic blocks (tens to hundreds of meters long) in a biotite-gneiss and BIF matrix, intruded by tonalite and granodiorite, in the Zunhua structural belt. Blocks in this metamorphosed Archean ophiolitic mélange preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The ophiolite-related mélange marks a suture zone across the North China craton, traced for more than 1 600 km along the central orogenic belt. Many of the chromitite bodies are localized in dunite envelopes within harzburgite tectonite, and have characteristic nodular and orbicular chromite textures, known elsewhere only from ophiolites. The chromites have variable but high chrome numbers (Cr/(Cr+Al)=0.74−0.93) and elevated P, also characteristic of suprasubduction zone ophiolites. The high chrome numbers, coupled with TiO2 < 0.2 wt.% and V2O5 < 0.1 wt.% indicate high degrees of partial melting from a very depleted mantle source and primitive melt for the chromite. A Re-Os isochron from the chromites indicates an age of 2.6 Ga, showing that they are the same age as the Dongwanzi ophiolite. The range in initial Os isotopic compositions in the chromites in these ophiolitic blocks is small and well within the range seen in modern ophiolites. The ultramafic and ophiolitic blocks in the Zunhua mélange are therefore interpreted as dismembered and strongly deformed parts of the Dongwanzi ophiolite. We suggest the name "Dongwanzi-Zunhua ophiolite belt" for these rocks. Geochemical and structural features of the Dongwanzi ophiolite suggest that it formed in a forearc environment and was incorporated in an accretionary prism soon after it formed. Neoarchean and Paleoproterozoic (2.50 and 1.90 Ga) high-pressure granulites form a belt more than 700 km long along the western side of the central orogenic belt. Several Neoarchean sedimentary basins consisting of conglomerate, greywacke, and shale are located along the eastern side of the central orogenic belt, and are interpreted as remnants of a foreland basin. The three belts record the Neoarchean subduction and collision between an arc terrane and eastern blocks of the North China craton in the Neoarchean, and further deformation and metamorphism in the Paleoproterozoic related to collisions on the northern margin of the already amalgamated North China craton.
Abstract: Simultaneous ultrasonic elastic wave velocity and in situ synchrotron X-ray measurements on grossular garnet were carried out up to 17 GPa and 1 650 K. P- and S-wave velocities and bulk and shear modulus showed linear pressure and temperature dependence. These data yielded a pressure derivative of the bulk modulus of 4.42(7) and a shear modulus of 1.27(3), which are in good agreement with those of garnets with variable chemical compositions. Temperature dependence of the bulk modulus of grossular (−1.36×10−2 GPa/K) is also similar to that of other garnets, while the temperature dependence of the shear modulus of grossular (−1.11×10−2 GPa/K) is higher than those of magnesium end-member garnets and pyrolitic garnet.
Abstract: Ultrasonic interferometry was utilized in conjunction with synchrotron-based X-ray diffraction and X-radiographic imaging to determine the compressional and shear wave velocities and unit-cell volumes of pyrite (FeS2) at room temperature and pressures up to 9.6 GPa. Fitting all of the experimental volume and velocity data to third-order finite-strain equations yielded the adiabatic zero-pressure bulk and shear moduli and their first pressure derivatives: KS0=138.9(7) GPa, G0=112.3(3) GPa, (∂KS0/∂P) T=KS0′=6.0(1), (∂G0/∂P) T=G0′=3.0(< 1), where the numbers in parentheses represent the 1σ uncertainty in the last significant digit. These results are in good agreement with several previous static compression studies on this material but differ quite strongly from the results obtained via first principles calculations. This study presents the first direct measurement of the bulk shear properties of this material.
Abstract: Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20–160 GPa and temperatures of 2 500–6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature (Tm) of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary (CMB) of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD (molecular dynamics) simulations. For liquid silica, the O-O coordination number is found to be 12 along the Tm and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10−9–3.3×10−9 m2/s, and the viscosity is 0.02–0.03 Pa·s along the Tm. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.
Abstract: In-situ Raman spectroscopic measurements of lawsonite in the OH and H2O stretching region were conducted up to approximately 2.5 GPa and 400 ℃ with a hydrothermal diamondanvil cell. In the experimental design at room temperature, the ratio of band-area and the width at half maximum of the decomposed Raman band shows discontinuities at 0.8 GPa, and correspondingly, the 695-cm-1 peak in the low-wavenumber region was split into two peaks. In the experiments at high temperature, the splitting of the 695-cm-1 peak was induced at 2.4 GPa and 250 ℃. These findings demonstrate the existence of discontinuities in the pressure response of the strength of the hydrogen bonds, which suggests a possible change of the hydrogen-bonded structure in lawsonite under these conditions.