scholarly journals The142Nd/144Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present

2020 ◽  
Vol 117 (26) ◽  
pp. 14738-14744 ◽  
Author(s):  
Eugenia Hyung ◽  
Stein B. Jacobsen
Keyword(s):  
Plate Tectonics ◽  
Early Earth ◽  
Solar System Formation ◽  
Crustal Rocks ◽  
Stirring Rate ◽  
Mid Ocean Ridge ◽  
Stirring Time ◽  
Ocean Ridge ◽  
Island Basalt ◽  
4.1 Ga

Early silicate differentiation events for the terrestrial planets can be traced with the short-lived146Sm-142Nd system (∼100-My half-life). Resulting early Earth-produced142Nd/144Nd variations are an excellent tracer of the rate of mantle mixing and thus a potential tracer of plate tectonics through time. Evidence for early silicate differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by142Nd/144Nd measurements of rocks that show both higher and lower (±20 ppm) values than the present-day mantle, demonstrating major silicate Earth differentiation within the first 100 My of solar system formation. We have obtained an external 2σ uncertainty at 1.7 ppm for142Nd/144Nd measurements to constrain its homogeneity/heterogeneity in the mantle for the last 2 Ga. We report that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 Ga are within 1.7 ppm of the average Earth142Nd/144Nd value. Considering mafic and ultramafic compositions, we use a mantle-mixing model to show that this trend is consistent with a mantle stirring time of about 400 My since the early Hadean. Such a fast mantle stirring rate supports the notion that Earth’s thermal and chemical evolution is likely to have been largely regulated by plate tectonics for most of its history. Some young rocks have142Nd/144Nd signatures marginally resolved (∼3 ppm), suggesting that the entire mantle is not equally well homogenized and that some silicate mantle signatures from an early differentiated mantle (>4.1 Ga ago) are preserved in the modern mantle.

Fluids in Submarine Mid-Ocean Ridge Hydrothermal Settings

Elements ◽  
2020 ◽  
Vol 16 (6) ◽  
pp. 389-394
Author(s):  
Esther M. Schwarzenbach ◽  
Matthew Steele-MacInnis
Keyword(s):  
Heat Budget ◽  
Oceanic Lithosphere ◽  
Life Forms ◽  
Hydrothermal Systems ◽  
Hydrothermal Fluids ◽  
Early Earth ◽  
Geologic Time ◽  
Mid Ocean Ridge ◽  
Time Changes ◽  
Ocean Ridge

Seawater interaction with the oceanic lithosphere crucially impacts on global geochemical cycles, controls ocean chemistry over geologic time, changes the petrophysical properties of the oceanic lithosphere, and regulates the global heat budget. Extensive seawater circulation is expressed near oceanic ridges by the venting of hydrothermal fluids through chimney structures. These vent fluids vary greatly in chemistry, from the metal-rich, acidic fluids that emanate from “black smokers” at temperatures up to 400 °C to the metal-poor, highly alkaline and reducing fluids that issue from the carbonate–brucite chimneys of ultramafic-hosted systems at temperatures below 110 °C. Mid-ocean ridge hydrothermal systems not only generate signifi-cant metal resources but also host unique life forms that may be similar to those of early Earth.

Origin of the Indus ophiolite linked to the mantle transition zone (410–660 km)

2021 ◽  
Author(s):  
Souvik Das ◽  
Asish R. Basu
Keyword(s):  
Transition Zone ◽  
Plate Tectonics ◽  
Tectonic Setting ◽  
Ultrahigh Pressure ◽  
Historical Contingency ◽  
Mantle Transition Zone ◽  
Contingency Model ◽  
Mid Ocean Ridge ◽  
Consistent Change ◽  
Ocean Ridge

ABSTRACT The southeast Ladakh (India) area displays one of the best-preserved ophiolite sections in this planet, in places up to 10 km thick, along the southern bank of the Indus River. Recently, in situ, ultrahigh-pressure (UHP) mineralogical evidence from the mantle transition zone (MTZ; ∼410–660 km) with diamond and reduced fluids were discovered from two peridotite bodies in the basal mantle part of this Indus ophiolite. Ultrahigh-pressure phases were also found by early workers from podiform chromitites of another coeval Neo-Tethyan ophiolite in southern Tibet. However, the MTZ phases in the Indus ophiolite are found in silicate peridotites, but not in metallic chromitites, and the peridotitic UHP phases show systematic and contiguous phase transitions from the MTZ to shallower depth, unlike the discrete UHP inclusions, all in Tibetan chromitites. We observe consistent change in oxygen fugacity (fO2) and fluid composition from (C-H + H2) to (CO2 + H2O) in the upwelling peridotitic mantle, causing melting to produce mid-ocean-ridge basalt (MORB). At shallow depths (<100 km) the free water stabilizes into hydrous phases, such as pargasitic amphibole, capable of storing water and preventing melting. Our discoveries provide unique insights into deep sub-oceanic-mantle processes, and link deep-mantle upwelling and MORB genesis. Moreover, the tectonic setting of Neo-Tethyan ophiolites has been a difficult problem since the birth of the plate-tectonics concept. This problem for the origin of ophiolites in mid-ocean-ridge versus supra-subduction zone settings clearly confused the findings from Indus ophiolites. However, in this contribution, we provide arguments in favor of mid-ocean-ridge origin for Indus ophiolite. In addition, we venture to revisit the “historical contingency” model of E.M. Moores and others for Neo-Tethyan ophiolite genesis based on the available evidence and have found that our new results strongly support the “historical contingency” model.

scholarly journals Venus’ light slab hinders its development of planetary-scale subduction and habitability

2022 ◽  
Author(s):  
Junxing Chen ◽  
Hehe Jiang ◽  
Ming Tang ◽  
Jihua Hao ◽  
Meng Tian ◽  
...  
Keyword(s):  
Plate Tectonics ◽  
Carbon Sink ◽  
Global Scale ◽  
Climate Feedback ◽  
Early Earth ◽  
Crustal Rocks ◽  
Planetary Scale ◽  
Habitable Planet ◽  
Missing Carbon Sink

Abstract Terrestrial planets Venus and Earth have similar sizes, masses, and bulk compositions, but only Earth developed planetary-scale plate tectonics. Plate tectonics generates weatherable fresh rocks and transfers surface carbon back to Earth’s interior, which provides a long-term climate feedback, serving as a thermostat to keep Earth a habitable planet. Yet Venus shares a few common features with early Earth, such as stagnant-lid tectonics and the possible early development of a liquid ocean. Given all these similarities with early Earth, why would Venus fail to develop global-scale plate tectonics? In this study, we explore solutions to this problem by examining Venus’ slab densities under hypothesized subduction-zone conditions. Our petrologic simulations show that eclogite facies may be reached at greater depths on Venus than on Earth, and Venus’ slab densities are consistently lower than Earth’s. We suggest that the lack of sufficient density contrast between the high-pressure metamorphosed slab and mantle rocks may have impeded self-sustaining subduction. Although plume-induced crustal downwelling exists on Venus, the dipping of Venus’ crustal rocks to mantle depth fails to transition into subduction tectonics. As a consequence, the supply of fresh silicate rocks to the surface has been limited. This missing carbon sink eventually diverged the evolution of Venus’ surface environment from that of Earth.

Lead- and strontium-isotope geochemistry of the Karmutsen Formation, Vancouver Island, British Columbia

1989 ◽  
Vol 26 (5) ◽  
pp. 908-919 ◽  
Author(s):  
Anne Andrew ◽  
Colin I. Godwin
Keyword(s):  
Isotope Geochemistry ◽  
Island Arcs ◽  
Flood Basalts ◽  
Continental Flood Basalts ◽  
Ocean Island ◽  
Mid Ocean Ridge ◽  
Isotopic Mixing ◽  
Ocean Ridge ◽  
Island Basalt ◽  
Basalt Type

Lead-isotope whole-rock ratios of the Triassic flood basalts of the Karmutsen Formation are heterogeneous, with 206Pb/2MPb = 18.72–21.51, 207Pb/204Pb = 15.56–15.77, and 208Pb/204Pb = 38.16–40.15. Whole-rock lead initial ratios are coincident with galena analyses having 206Pb/204Pb = 18.1–19.0, 207Pb/204Pb = 15.53–15.60, and 208Pb/204Pb = 38.2–38.6. Lead, uranium, and thorium concentrations are greater than for mid-ocean-ridge basalts, less than for island arcs, but similar to those in ocean-island and continental flood basalts. Isotopic similarities exist between the Columbia River – Chilcotin continental flood basalts and the Karmutsen Formation, suggesting similar origins for these basalts. The heterogeneity in whole-rock ratios can be explained in terms of isotopic mixing between a lead-rich average crustal lead type and an ocean-island basalt-type mantle source. Thus the Karmutsen Formation displays evidence for sediment contamination of the mantle.

scholarly journals Investigating influences on the Pb pseudo-isochron using three-dimensional mantle convection models with a continental reservoir

2021 ◽  
Author(s):  
James Panton ◽  
J. Davies ◽  
Tim Elliott ◽  
Morten Andersen ◽  
Donald Porcelli ◽  
...  
Keyword(s):  
Mantle Convection ◽  
Plate Tectonics ◽  
Three Dimensional ◽  
Isotope Ratios ◽  
Pb Isotope ◽  
Ocean Island Basalts ◽  
Mid Ocean Ridge ◽  
Subducted Oceanic Crust ◽  
Ocean Ridge ◽  
Relative Change

For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.

scholarly journals Geoquímica dos diques máficos de Brumado, porção sudeste do Bloco Gavião, Bahia, Brasil

2019 ◽  
Vol 19 (3) ◽  
pp. 237-252
Author(s):  
Lílian Mercês Pereira Varjão ◽  
Angela Beatriz De Menezes Leal
Keyword(s):  
Ocean Island Basalt ◽  
Ocean Island ◽  
Ridge Basalt ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Island Basalt

No sudoeste do estado da Bahia, entre as cidades de Brumado e Caetité e mais a norte próximo às cidades de Tanque Novo, Paramirim, Botuporã, Macaúbas, Novo Horizonte e Ibitiara, ocorrem diques máficos formados em ambiente intraplaca. São toleítos continentais apresentando valores de índice de diferenciação em magnésio (mg#) entre 0,25 e 0,45, indicativos de líquidos magmáticos evoluídos. Plagioclásio e augita predominaram no processo magmático evolutivo, cujos diagramas não mostram alterações significativas por processos secundários. Os diques máficos originaram-se de fonte pouco heterogênea tipo Enriched Mid-Ocean Ridge Basalt (E-MORB), provavelmente com leve contribuição de fluidos tipo Ocean Island Basalt (OIB). Os dados geoquímicos sugerem influência significativa da crosta continental inferior na origem da fonte mantélica, não havendo sinais de importante contribuição sedimentar.

scholarly journals Runaway climate cooling of ocean planets in the habitable zone: a consequence of seafloor weathering enhanced by melting of high-pressure ice

2019 ◽  
Vol 488 (2) ◽  
pp. 1580-1596 ◽  
Author(s):  
A Nakayama ◽  
T Kodama ◽  
M Ikoma ◽  
Y Abe
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Plate Tectonics ◽  
Atmospheric Co2 ◽  
Terrestrial Planets ◽  
Habitable Zone ◽  
Cold Climates ◽  
Ice Melting ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

ABSTRACT Terrestrial planets covered globally with thick oceans (termed ocean planets) in the habitable zone were previously inferred to have extremely hot climates in most cases. This is because H2O high-pressure (HP) ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO2. Previous studies, however, ignored melting of the HP ice and horizontal variation in heat flux from oceanic crusts. Here, we examine whether high heat fluxes near the mid-ocean ridge melt the HP ice and thereby remove atmospheric CO2. We develop integrated climate models of an Earth-size ocean planet with plate tectonics for different ocean masses, which include the effects of HP ice melting, seafloor weathering, and the carbonate–silicate geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge is high enough to melt the ice, enabling seafloor weathering. In contrast to the previous theoretical prediction, we show that climates of terrestrial planets with massive oceans lapse into extremely cold ones (or snowball states) with CO2-poor atmospheres. Such extremely cold climates are achieved mainly because the HP ice melting fixes seafloor temperature at the melting temperature, thereby keeping a high weathering flux regardless of surface temperature. We estimate that ocean planets with oceans several tens of the Earth’s ocean mass no longer maintain temperate climates. These results suggest that terrestrial planets with extremely cold climates exist even in the habitable zone beyond the Solar system, given the frequency of water-rich planets predicted by planet formation theories.

scholarly journals Cr-spinel records metasomatism not petrogenesis of mantle rocks

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Hamed Gamal El Dien ◽  
Shoji Arai ◽  
Luc-Serge Doucet ◽  
Zheng-Xiang Li ◽  
Youngwoo Kil ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Plate Tectonics ◽  
Mantle Metasomatism ◽  
Mantle Melting ◽  
Ultramafic Rocks ◽  
Chromian Spinel ◽  
Core Composition ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Petrogenetic Indicator

Abstract Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes.

scholarly journals Decoding of Mantle Processes in the Mersin Ophiolite, Turkey, of End-Member Arc Type: Location of the Boninite Magma Generation

Minerals ◽  
2018 ◽  
Vol 8 (10) ◽  
pp. 464 ◽  
Author(s):  
Satoko Ishimaru ◽  
Yuji Saikawa ◽  
Makoto Miura ◽  
Osman Parlak ◽  
Shoji Arai
Keyword(s):  
Atomic Ratio ◽  
Mantle Melting ◽  
Ultramafic Rocks ◽  
Chromian Spinel ◽  
Crustal Rocks ◽  
Mantle Peridotites ◽  
Mid Ocean Ridge ◽  
Mantle Processes ◽  
Ocean Ridge ◽  
High Degree

The Mersin ophiolite, Turkey, is of typical arc type based on geochemistry of crustal rocks without any signs of mid-ocean ridge (MOR) affinity. We examined its ultramafic rocks to reveal sub-arc mantle processes. Mantle peridotites, poor in clinopyroxene (<1.0 vol.%), show high Fo content of olivine (90–92) and Cr# [=Cr/(Cr + Al) atomic ratio] (=0.62–0.77) of chromian spinel. NiO content of olivine is occasionally high (up to 0.5 wt.%) in the harzburgite. Moho-transition zone (MTZ) dunite is also highly depleted, i.e., spinel is high Cr# (0.78–0.89), clinopyroxene is poor in HREE, and olivine is high Fo (up to 92), but relatively low in NiO (0.1–0.4 wt.%). The harzburgite is residue after high-degree mantle melting, possibly assisted by slab-derived fluid. The high-Ni character of olivine suggests secondary metasomatic formation of olivine-replacing orthopyroxene although replacement textures are unclear. The MTZ dunite is of replacive origin, resulted from interaction between Mg-rich melt released from harzburgite diapir and another harzburgite at the diapir roof. The MTZ dunite is the very place that produced the boninitic and replacive dunite. The MTZ is thicker (>1 km) in Mersin than in MOR-related ophiolite (mostly < 500 m), and this is one of the features of arc-type ophiolite.

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