Hao, Lu-LuNan, Xiao-YunKerr, Andrew C.Li, Si-Qi...
11页
查看更多>>摘要：In recent years, the melange model has been increasingly considered as an important way to transfer slab components to arc sources in modern subduction zones. This model differs from the classic slab fluid/melt metasomatism model in that it invokes physical mixing of bulk sediment, altered oceanic crust (AOC), serpentinite, and mantle wedge at the slab-mantle interface. However, due to the lack of subducted sediment compositions, the melange model has not been applied to any significant extent in paleo-subduction zones. The lack of evidence for bulk AOC and serpentinite components in melange sources also hinders our understanding of any melange origin for arc-type magmas. Here, we report MgBa-Sr-Nd isotope compositions of the early Paleozoic Fushui mafic rocks in the Qinling orogen of central China, to trace their origin and constrain slab component transfer. The Fushui mafic rocks show typical arc-type geochemical features and enriched Sr-Nd isotope compositions, indicating the likely contribution of subducted sediments. They have low Rb/Ba ratios (<0.1), and most samples show delta Ba-138/134 values of -0.38 to +0.10 parts per thousand, similar to those of sediments (-0.2 to +0.1 parts per thousand) but lower than those of MORBs (+0.03 to +0.14 parts per thousand), indicating that these low delta Ba-138/134 values are most likely derived from sediment components. One Fushui sample has a high delta Ba-138/134 of +0.31 parts per thousand, similar to that of AOC (up to +0.4 parts per thousand). This high delta Ba-138/134 is not correlated with fluid input; instead, it results from the contribution of bulk AOC. The Fushui rocks exhibit variable delta Mg-26 values (-0.23 to -0.11 parts per thousand), slightly higher than those of MORBs. This most likely reflects 26Mg-enriched subducted serpentinite components in their source. Our results not only identify the variable slab components (sediment, AOC, and serpentinite) in the arc source, but also suggest that these slab components may be transferred to their arc source by the melange process. This study therefore provides solid evidence for the generation of arc magmas by melange processes in paleo-subduction zones, which confirms an important role for the melange model in slab material transport. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：Nitrogen, carbon, hydrogen and sulfur are essential elements for life and comprise about 1% of terrestrial planet masses. These elements dominate planetary surfaces due to their volatile nature, but the Earth's interior also constitutes a major C-H-N-S reservoir. Resolving the origin of the surficial versus deep volatile reservoirs requires the past 4.5 Giga-years of mantle outgassing and ingassing processes to be reconstructed, involving many unknowns. As an alternative, we propose to define the primordial distribution of volatiles resulting from degassing of the Earth's magma ocean (MO). The equilibrium partitioning of C-H-O-N-S elements between the MO and its atmosphere is calculated by means of solubility laws, extrapolated to high temperatures and over a large range of redox conditions. Depending on the redox conditions, the amount of volatiles, and the size of the MO considered, we show that the last MO episode may have degassed 40-220 bar atmospheres, whereas hundreds to thousands of ppm of C-H-O-N-S can be retained in the magma. Two contrasting scenarios are investigated: reduced vs. oxidized MO. For reduced cases (<IW-2), an H-C +/- N-rich atmosphere can be formed, whereas the atmosphere under oxidizing conditions (>IW + 2) would be dry and C-N-S-rich. An intermediate redox state produces a C-N atmosphere. In many cases, the present-day surficial abundances (atmosphere + ocean + crust) of C and N, the most volatile elements, are very close to the calculated primordial MO - atmosphere distribution. This probably means that lithospheric recycling and post-magma ocean degassing only moderately alter the surficial abundances of these elements. Sulfur, in contrast, must have been mostly outgassed by post-MO events. Changes in redox conditions during magma ocean degassing played a first order role in the composition of the primordial atmosphere of planets. We suggest that the more oxidized conditions on Venus due to H-loss may have played a role in the growth of a dry MO atmosphere on this planet compared to an H-bearing one on Earth. To verify these first order assertions, constraints on volatile behaviorunder extreme magma ocean conditions and upon magma ocean solidification are urgently needed. (C) 2021 Elsevier B.V. All rights reserved.
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