Time Between 3 and 2 Ga: Transitional Events in the Earth’s History

2021 ◽  
Vol 62 (1) ◽  
pp. 25-43
Author(s):  
V.V. Yarmolyuk ◽  
M.I. Kuzmin ◽  
T.V. Donskaya ◽  
D.P. Gladkochub ◽  
A.B. Kotov
Keyword(s):  
Lower Mantle ◽  
Major Element ◽  
Time Span ◽  
Geologic History ◽  
Free Oxygen ◽  
Core Mantle Boundary ◽  
Global Geodynamics ◽  
O Isotope ◽  
Oceanic Basalts ◽  
Key Events

Abstract —The time span between 3 and 2 Ga in the geologic history encompassed a number of key events on the cooling Earth. The cooling interrupted heat transfer within and across the mantle, which caused changes in Earth’s major spheres and in the mechanisms of their interaction. The great thermal divergence at 2.5 Ga and differentiation into the depleted upper asthenospheric and primitive lower mantle affected the compositions of oceanic basalts. The lower mantle cooling recorded by a systematic decrease in the temperature of komatiite magma generation at the respective depths began at 2.5 Ga and was accompanied by increasing abundance of arc basalts and by changes in the behavior of the Sr, Nd, and O isotope systems. It was the time when the continental lithosphere consisting of subcontinental lithospheric mantle and crust began its rapid growth, while the crust became enriched in felsic material with high contents of lithophile elements. Magmatism of the 3–2 Ga time span acquired more diverse major-element chemistry, with calc-alkaline and alkaline lithologies like carbonatite and kimberlite. The dramatic changes were driven by subduction processes, whereby the crust became recycled in the mantle and the double layer (D”) formed at the core–mantle boundary. The events of the 3–2 Ga interval created prerequisites for redox changes on the surface and release of free oxygen into the atmosphere. In terms of global geodynamics, it was transition from stagnantlid tectonics to plate tectonic regime, which approached the present-day style about 2.0–1.8 Ga.

Core–mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics

2010 ◽  
Vol 289 (1-2) ◽  
pp. 232-241 ◽  
Author(s):  
Teresa Mae Lassak ◽  
Allen K. McNamara ◽  
Edward J. Garnero ◽  
Shijie Zhong
Keyword(s):  
Lower Mantle ◽  
Mantle Boundary ◽  
Core Mantle Boundary

scholarly journals Hydrous SiO2 in subducted oceanic crust and water transport to the core-mantle boundary

2020 ◽  
Author(s):  
Yanhao Lin ◽  
Qingyang Hu ◽  
Jing Yang ◽  
Yue Meng ◽  
Yukai Zhuang ◽  
...  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Mantle ◽  
Oceanic Lithosphere ◽  
Low Velocity ◽  
Water Storage Capacity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Subducted Oceanic Crust

Abstract Subduction of oceanic lithosphere transports surface water into the mantle where it can have remarkable effects, but how much can be cycled down into the deep mantle, and potentially to the core, remains ambiguous. Recent studies show that dense SiO2 in the form of stishovite, a major phase in subducted oceanic crust at depths greater than ~300 km, has the potential to host and carry water into the lower mantle. We investigate the hydration of stishovite and its higher-pressure polymorphs, CaCl2-type SiO2 and seifertite, in experiments at pressures of 44–152 GPa and temperatures of ~1380–3300 K. We quantify the water storage capacity of these dense SiO2 phases at high pressure and find that water stabilizes CaCl2-type SiO2 to pressures beyond the base of the mantle. We parametrize the P-T dependence of water capacity and model H2O storage in SiO2 along a lower mantle geotherm. Dehydration of slab mantle in cooler slabs in the transition zone can release fluids that hydrate stishovite in oceanic crust. Hydrous SiO2 phases are stable along a geotherm and progressively dehydrate with depth, potentially causing partial melting or silica enrichment in the lower mantle. Oceanic crust can transport ~0.2 wt% water to the core-mantle boundary region where, upon heating, it can initiate partial melting and react with the core to produce iron hydrides, providing plausible explanations for ultra-low velocity regions at the base of the mantle.

Origin, properties, and structure of breyite: The second most abundant mineral inclusion in super-deep diamonds

2021 ◽  
Vol 106 (1) ◽  
pp. 38-43
Author(s):  
Frank E. Brenker ◽  
Fabrizio Nestola ◽  
Lion Brenker ◽  
Luca Peruzzo ◽  
Jeffrey W. Harris
Keyword(s):  
Crystal Structure ◽  
Transition Zone ◽  
Lower Mantle ◽  
Major Element ◽  
Mineral Inclusion ◽  
Major Element Composition ◽  
Triclinic Space Group ◽  
Casio3 Perovskite ◽  
Almost All ◽  
Original Crystal

Abstract Earth's lower mantle most likely mainly consists of ferropericlase, bridgmanite, and a CaSiO3- phase in the perovskite structure. If separately trapped in diamonds, these phases can be transported to Earth's surface without reacting with the surrounding mantle. Although all inclusions will remain chemically pristine, only ferropericlase will stay in its original crystal structure, whereas in almost all cases bridgmanite and CaSiO3-perovskite will transform to their lower-pressure polymorphs. In the case of perovskite structured CaSiO3, the new structure that is formed is closely related to that of walstromite. This mineral is now approved by the IMA commission on new minerals and named breyite. The crystal structure is triclinic (space group: P1) with lattice parameters a0 = 6.6970(4) Å, b0 = 9.2986(7) Å, c0 = 6.6501(4) Å, α = 83.458(6)°, β = 76.226(6)°, γ = 69.581(7)°, and V = 376.72(4) Å. The major element composition found for the studied breyite is Ca3.01(2)Si2.98(2)O9. Breyite is the second most abundant mineral inclusion after ferropericlase in diamonds of super-deep origin. The occurrence of breyite has been widely presumed to be a strong indication of lower mantle (=670 km depth) or at least lower transition zone (=520 km depth) origin of both the host diamond and the inclusion suite. In this work, we demonstrate through different formation scenarios that the finding of breyite alone in a diamond is not a reliable indicator of the formation depth in the transition zone or in the lower mantle and that accompanying paragenetic phases such as ferropericlase together with MgSiO3 are needed.

Towards consistent seismological models of the core-mantle boundary landscape

2020 ◽  
Author(s):  
Paula Koelemeijer
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Seismic Velocity ◽  
Body Wave ◽  
P Wave ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Trade Offs ◽  
Multiple Data Sets

<p>The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints (Koelemeijer, 2020). I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics.</p><p>While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which partially helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models typically feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may be ruled out as possibility.</p><p>To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with both body-wave and normal-mode data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.</p><p><em>Koelemeijer, P. (2020), “Towards consistent seismological models of the core-mantle boundary landscape”. Book chapter in revision for AGU monograph "Mantle upwellings and their surface expressions", edited by Marquardt, Cottaar, Ballmer and Konter</em></p>

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