Showing papers by "Yong-Fei Zheng published in 2023"

Serpentinization and deserpentinization of the mantle wedge at a convergent plate margin: evidence of orogenic peridotites from a composite oceanic-continental subduction zone

Abstract: Serpentinites of the mantle wedge were identified from a composite oceanic-continental subduction zone that occurs as the Hong’an orogen in east-central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modelling as well as zircon U-Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, LREE and HFSE in the whole-rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive εHf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong’an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U-Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high to ultrahigh-pressure metamorphic rocks in the Hong’an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative εHf(t) values of -16.0 to -2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during Triassic continental collision. In this context, the mantle wedge peridotites were firstly metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage of fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision.

Progressively elevated geothermal gradients for Miocene metamorphism in the Himalayan orogen

Abstract: The property of regional metamorphism at convergent plate margins is substantially related to the change of geothermal gradients, resulting in different types of crustal reworking through metamorphic dehydration and partial melting. The relationship between crustal metamorphism and thermal evolution of collisional orogens can thus be deciphered by investigating the change of metamorphic thermobaric ratios in both space and time. This is illustrated by a combined study of field observation, petrographic observation, whole-rock and mineral major and trace elements, laser ablation−inductively coupled plasma−mass spectrometry zircon and monazite U-Th-Pb isotopes and trace elements, and phase equilibrium modeling of granitic gneisses from the Cona area, southern Tibet, in the Himalayan orogen. The results reveal variations in spatial level, protolith age, petrological composition, and metamorphic pressure−temperature−time path along an N-S transect across the Higher Himalayas. On this basis, three types of granitic gneisses were identified for the Miocene metamorphism. Type I gneiss was formed at 16.2−14.7 Ma under upper amphibolite facies conditions of 720−735 °C and 0.77−0.82 GPa, Type II gneisses were formed at 18.3−14.3 Ma under granulite facies conditions of 765−795 °C and 0.99−1.06 GPa, and Type III gneisses were formed at 21.8−13.0 Ma under high-pressure granulite facies conditions of 850−875 °C and 1.40−1.45 GPa. These rocks underwent anatectic metamorphism at lower to upper crustal depths from 22 to 13 Ma, and their metamorphic thermobaric ratios increase from 586−803 °C/GPa at 22−18 Ma to 878−967 °C/GPa and finally to 1071 °C/GPa at 15−13 Ma. This increase corresponds to a progressive increase in metamorphic geothermal gradients from 16.1−22.0 °C/km to 24.1−26.5 °C/km and finally to 29.4 °C/km. Therefore, the crustal rocks in the Himalayan orogen experienced the three types of regional metamorphism at the three different geothermal gradients in the Miocene. The progressively elevated geothermal gradients are attributed to continental rifting due to asthenospheric upwelling consequential to thinning of the lithospheric mantle in the collisional orogen. This has important implications for the tectonic evolution of collisional orogens in the post-collisional stage.

Transmissive Mode Laser Micro-Ablation Performance of Ammonium Dinitramide-Based Liquid Propellant for Laser Micro-Thruster

Abstract: The transmissive mode laser micro-ablation performance of near-infrared (NIR) dye-optimized ammonium dinitramide (ADN)-based liquid propellant was investigated in laser plasma propulsion using a pulse YAG laser with 5 ns pulse width and 1064 nm wavelength. Miniature fiber optic near-infrared spectrometer, differential scanning calorimeter (DSC) and high-speed camera were used to study laser energy deposition, thermal analysis of ADN-based liquid propellants and the flow field evolution process, respectively. Experimental results indicate that two important factors, laser energy deposition efficiency and heat release from energetic liquid propellants, obviously affect the ablation performance. The results showed that the best ablation effect of 0.4 mL ADN solution dissolved in 0.6 mL dye solution (40%-AAD) liquid propellant was obtained with the ADN liquid propellant content increasing in the combustion chamber. Furthermore, adding 2% ammonium perchlorate (AP) solid powder gave rise to variations in the ablation volume and energetic properties of propellants, which enhanced the propellant enthalpy variable and burn rate. Based on the AP optimized laser ablation, the optimal single-pulse impulse (I)~9.8 μN·s, specific impulse (Isp)~234.9 s, impulse coupling coefficient (Cm)~62.43 dyne/W and energy factor (η)~71.2% were obtained in 200 µm scale combustion chamber. This work would enable further improvements in the small volume and high integration of liquid propellant laser micro-thruster.

Enhancement of the 123.6 nm light intensity in a krypton lamp by optical pumping

Abstract: In this work, we report a method to significantly increase the intensity of vacuum ultraviolet (VUV) light at 123.6 nm in a krypton lamp. Additional VUV photons were obtained by quenching metastable atoms via optically pumping the 4p5 (2P°3/2)5s 2[3/2]°J=2–4p5 (2P°3/2)5p 2[5/2]J=2 transition at 810.7 nm. Compared with static gas conditions, the spectral data indicated a remarkable intensity enhancement of 123.6 nm VUV light by flowing gas without optical pumping. Under the 810.7 nm optical pumping, the VUV light intensity at 123.6 nm increases with the increase of laser powers and beam diameters, which mainly takes place in the discharge area. Most notably, a 37% enhancement on 123.6 nm VUV light intensity was achieved with a longitudinal 9 mm beam diameter, 906 mW of 810.7 nm laser pumping at flowing gas mode, which is of great significance for all-optical atom trap trace analysis.