A long section of serpentinized depleted mantle peridotite
C. Johan LissenbergA. M. McCaigSusan Q. LangPeter BlumNatsue AbeWilliam J. BrazeltonRémi ColtatJeremy R. DeansKristin DickersonMarguerite GodardBarbara E. JohnFrieder KleinRebecca KuehnKuan-Yu LinHaiyang LiuEthan LopesToshio NozakaAndrew J. ParsonsVamdev PathakMark K. ReaganJordyn RobareIvan P. SavovEsther M. SchwarzenbachO.J. SissmannGordon SouthamFengping WangC.G. WheatLesley AndersonSarah Treadwell
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The upper mantle is critical for our understanding of terrestrial magmatism, crust formation, and element cycling between Earth's solid interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution have been primarily inferred by surface sampling and indirect methods. We recovered a long (1268-meter) section of serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. We find depleted compositions with notable variations in mantle mineralogy controlled by melt flow. Dunite zones have predominantly intermediate dips, in contrast to the originally steep mantle fabrics, indicative of oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the full depth of the core and is overprinted by oxidation in the upper 200 meters. Alteration patterns are consistent with vent fluid composition in the nearby Lost City hydrothermal field.Keywords:
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The mantle’s compositional structure reflects the thermochemical evolution of Earth. Yet, even the radial average composition of the mantle remains debated. Here, we analyze a global dataset of shear and compressional waves reflecting off the 410- and 660-km discontinuities that is 10 times larger than any previous studies. Our array analysis retrieves globally averaged amplitude-distance trends in SS and PP precursor reflectivity from which we infer relative wavespeed and density contrasts and associated mantle composition. Our results are best matched by a basalt-enriched mantle transition zone, with higher basalt fractions near 660 (~40%) than 410 (~18–31%). These are consistent with mantle-convection/plate-recycling simulations, which predict that basaltic crust accumulates in the mantle transition zone, with basalt fractions peaking near the 660. Basalt segregation in the mantle transition zone also implies that the overall mantle is more silica enriched than the often-assumed pyrolitic mantle reference composition.
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Abstract Low‐δ 26 Mg basalts are commonly interpreted to represent melts derived from carbonated mantle sources. The mantle domain feeding low‐δ 26 Mg Cenozoic basalts in eastern China overlaps the so‐called Big Mantle Wedge (BMW) above the stagnant Pacific slab in the mantle transition zone, which indicates that the BMW is an important carbon reservoir generated by the slab. However, Mg isotopic composition in the nearby mantle beyond the BMW and, thus, the spatial extent of carbonated components in the mantle beneath eastern Asia have not yet been extensively characterized. Therefore, it remains largely unconstrained if additional or alternative carbon reservoirs exist. Here we carried out a geochemical study on Cenozoic Huihe nephelinites, which crop out ~500 km west of the present‐day BMW. These rocks are characterized by negative K, Zr, Hf, and Ti anomalies, high Zr/Hf, Ca/Al ratios, and low δ 26 Mg values, which suggest that they are derived from a carbonated mantle source. The composition of the nephelinites demonstrates that low δ 26 Mg mantle components exist at significant distances from the present‐day BMW, which highlights that in addition to the stagnant Pacific slab, other oceanic slab(s) also contribute(s) carbonate‐bearing crustal materials to the mantle sources of Cenozoic volcanism in eastern Asia.
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To quantify the alteration processes of mantle from geophysical data, an understanding of the relationship between alteration and the physical properties of mantle peridotite is essential. In this study, we employed independent component analysis (ICA) to evaluate variations in the physical properties of altered peridotites collected by the Oman Drilling Project, to understand the alteration processes of mantle peridotite in the Samail ophiolite. We analyzed multivariate physical properties (density, porosity, P-wave velocity, electrical resistivity, permeability, magnetic susceptibility, and color reflectance) that had been measured on core samples. The ICA results show that the observed variations in physical properties can be explained broadly by four independent components. Through their relationships with physical properties and comparisons with petrological and geochemical data from previous studies, we infer the four independent components to represent distinct alteration processes: the early and late stages of serpentinization, magnetite formation, and near-surface carbonation. These processes develop differently during the overall process of alteration, and they influenced each physical property in different ways. Our results demonstrate that ICA can separate the effects of multiple processes of alteration on various physical properties of the altered peridotites, which previously had been difficult to quantify.
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