Junfeng Zhang

Nanoarchitectured materials composed of fullerene-like spheroids and disordered graphene layers with tunable mechanical properties

Type-II glass-like carbon is a widely used material with a unique combination of properties including low density, high strength, extreme impermeability to gas and liquid and resistance to chemical corrosion. It can be considered as a carbon-based nanoarchitectured material, consisting of a disordered multilayer graphene matrix encasing numerous randomly distributed nanosized fullerene-like spheroids. Here we show that under both hydrostatic compression and triaxial deformation, this high-strength material is highly compressible and exhibits a superelastic ability to recover from large strains. Under hydrostatic compression, bulk, shear and Youngs moduli decrease anomalously with pressure, reaching minima around 1-2u2009GPa, where Poissons ratio approaches zero, and then revert to normal behaviour with positive pressure dependences. Controlling the concentration, size and shape of fullerene-like spheroids with tailored topological connectivity to graphene layers is expected to yield exceptional and tunable mechanical properties, similar to mechanical metamaterials, with potentially wide applications.

Temperature of prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane: Constraints from Ti-in-zircon thermometer

In order to constrain temperature during subduction and subsequent exhumation of felsic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai (威海) region in the Sulu (苏鲁) ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompressional partial melting during early stage exhumation and subsequent fractional crystallization of primary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the protolith of the migmatitic gneiss is Mid-Neoproterozoic (∼780 Ma) magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. The estimated growth temperatures (625–717 °C) of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eclogite-facies condition during subduction. Zircon recrystallized rims (228±2 Ma) in the Pl-rich leucosome layers provide the lower limit of the decompresssional partial melting time during exhumation. The ages from 228±2 to 219±2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 °C, and the temperatures of new growth zircon grains (219±2 Ma) in Kfs-rich vein are between 769 and 529 °C. The estimated temper atures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indicating that a process of fractional crystallization of the partial melt during exhumation.

Zircon Geochronological Evidence for Participation of the North China Craton in the Protolith of Migmatite of the North Dabie Terrane

The plate affiliation of the North Dabie terrane (NDT) has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals participation of crustal rocks of the North China craton (NCC) in the protolith in addition to the more common crustal rocks of the Yangtze craton (YC). The evidence of an NCC affinity for protolith of migmatite in the NDT is the ∼2.5 Ga (2 486±14 and 2 406±26 Ma) magamtic age and ∼1.8 Ga (1 717±79 Ma) metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7–0.8 Ga (e.g., 787±12 Ma) magamtic zircon age. Mid-Neoproterozoic magmatic age (0.7–0.8 Ga) is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite samples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.

Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80u2009km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet.The triggering mechanism of deep seismicity in Tibet remains unclear. Here the authors use experiments to show that granulite when deformed becomes brittle as it passes into the ecologite stability field developing macroscopic riedel fault zones thus providing an explanation for deep seismicity in Southern Tibet.