Liu
Chuan-Zhou
Liu
Chuan-Zhou
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ArticleTectonic controls on block rotation and sheeted sill emplacement in the Xigaze Ophiolite (Tibet): the construction mode of slow-spreading and ultraslow-spreading oceanic crusts(American Geophysical Union, 2021-01-28) Liu, Tong ; Dick, Henry J. B. ; Liu, Chuan-Zhou ; Wu, Fu-Yuan ; Ji, Wen-Bin ; Zhang, Chang ; Zhang, Wei-Qi ; Zhang, Zhen-Yu ; Lin, Yin-Zheng ; Zhang, ZhenThe internal structure of oceanic crusts is not well understood due to the limitation of deep drilling. However, that of ophiolites, i.e., on-land ancient analogs of oceanic lithosphere, could be precisely mapped and measured. The Xigaze ophiolite in Tibet has been regarded as “peculiar”, due to the sheeted sill complex in its upper crust, and non-sheeted diabase sills/dikes crosscutting its mantle and lower crust, which are geometrically different from the primarily vertical sheeted dike complex. Based on extensive field observations, here we present petrological and geochemical data for the Xigaze ophiolite to decipher the origin of sheeted sill complex and its implications for the construction of oceanic crusts. Diabases in the Xigaze ophiolite could be subdivided into sheeted sills, Group 1 non-sheeted dikes, and Group 2 non-sheeted sills, based on their orientations. These diabases cut other lithologies, and hence belong to the latest-stage products. Based on petrological, geochemical, and structural data, we highlight the important role of detachment fault in the generation of sheeted and non-sheeted sills. During the formation of oceanic crust, large block exhumation, multi-stage rotations, and foundering are argued here as key mechanisms for the generation of Xigaze sheeted and non-sheeted dikes/sills, all of which are in the evolution of detachment fault systems. These processes are also not uncommon for asymmetrical segments at modern slow-spreading and ultraslow-spreading ridges, but are rare at symmetrical segments. Due to the evolution of detachment fault, the internal structures of (ultra)slow-spreading ridges are more complex than those at fast-spreading ridges.
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ArticleArchean cratonic mantle recycled at a mid-ocean ridge(American Association for the Advancement of Science, 2022-04-15) Liu, Chuan-Zhou ; Dick, Henry J. B. ; Mitchell, Ross N. ; Wei, Wu ; Zhang, Zhen-Yu ; Hofmann, Albrecht W. ; Yang, Jian-Feng ; Li, YangBasalts and mantle peridotites of mid-ocean ridges are thought to sample Earth’s upper mantle. Osmium isotopes of abyssal peridotites uniquely preserve melt extraction events throughout Earth history, but existing records only indicate ages up to ~2 billion years (Ga) ago. Thus, the memory of the suspected large volumes of mantle lithosphere that existed in Archean time (>2.5 Ga) has apparently been lost somehow. We report abyssal peridotites with melt-depletion ages up to 2.8 Ga, documented by extremely unradiogenic 187Os/188Os ratios (to as low as 0.1095) and refractory major elements that compositionally resemble the deep keels of Archean cratons. These oceanic rocks were thus derived from the once-extensive Archean continental keels that have been dislodged and recycled back into the mantle, the feasibility of which we confirm with numerical modeling. This unexpected connection between young oceanic and ancient continental lithosphere indicates an underappreciated degree of compositional recycling over time.
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ArticleTrans-lithospheric ascent processes of the deep-rooted magma plumbing system underneath the ultraslow-spreading SW Indian Ridge(American Geophysical Union, 2024-01-23) Ma, Ben ; Liu, Ping‐Ping ; Dick, Henry J. B. ; Zhou, Mei-Fu ; Chen, Qiong ; Liu, Chuan-ZhouProcesses of magma generation and transportation in global mid-ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end-member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow-spreading ridges. Here we present detailed mineralogical studies of high-Mg and low-Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High-Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high-An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low-Mg basalts contain low-Mg# Ol and low-An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low-Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high-Mg and low-Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high-Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low-Mg melts may have evolved from the high-Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow-spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.