scholarly journals Origin of kimberlite from the base of the upper mantle

2021 ◽  
Author(s):  
Wei-Dong Sun ◽  
Lipeng Zhang ◽  
Rui Li ◽  
Guozhi Xie ◽  
Lu Liu ◽  
...  
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Lower Mantle ◽  
Explosive Eruption ◽  
Ferric Iron ◽  
Mantle Peridotite ◽  
Explosive Eruptions ◽  
Stable Phase ◽  
Excess Carbon

Abstract Kimberlite is characterized by explosive eruption powered by excess carbon dioxides (CO2)1 and water2. Given that diamond is the dominant stable phase of carbon in the upper mantle3, it is obscure where does the excess CO2 in kimberlite has come from. Here we show that ferric iron oxidizes diamond at 1900K, 20GPa and 2000K, 25GPa, forming CO2. The lower mantle is dominated by bridgmanite, which is rich in ferric iron4. Bridgmanite decomposes once it is brought to the upper mantle, releasing extra ferric iron. Therefore, the oxidation of diamond may have been popularly occurring at the base of the upper mantle, forming CO2-rich carbonated domains that are the main source of kimberlite. The rising kimberlitic magma reaches the lithosphere mantle of thick cratons before it crosses the solidus line of mantle peridotite, and thus keeps its volatile-rich nature that drives explosive eruptions. When the lithospheric mantle is thinner than ~140 km, kimberlite changes into much less explosive magmas due to partial melting of mantle peridotite, and, consequently, entrained diamond is mostly oxidized during the magma’s slower ascension.

scholarly journals Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

Minerals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 209
Author(s):  
Shoji Arai
Keyword(s):  
Partial Melting ◽  
Magma Chamber ◽  
Upper Mantle ◽  
Mantle Peridotite ◽  
Common Origin ◽  
Layered Intrusions ◽  
Magma Chambers ◽  
Genetic Link ◽  
Podiform Chromitites ◽  
Crustal Magma

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.

scholarly journals Speculations on the Generation and Movement of Komatiites

2020 ◽  
Vol 61 (7) ◽  
Author(s):  
Dan McKenzie
Keyword(s):  
Boundary Layer ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Thermal Boundary Layer ◽  
Solidus Temperature ◽  
Stable Phase ◽  
Average Temperature ◽  
Low Concentrations ◽  
Greenstone Belts ◽  
Archean Greenstone Belts

Abstract The discovery of komatiites, first in South Africa and then in many other Archean greenstone belts, with MgO concentrations of 20–30% and eruption temperatures of more than ∼1600 °C, showed that some parts of the mantle were hotter in the Archean than they are now. Since their discovery there have been many speculative proposals as to how such magmas can form. At present melt is produced by mantle upwelling, because the solidus temperature gradient of the mantle is steeper than that of isentropic decompression gradient at depths of less than 300 km. In contrast, in the lower half of the upper mantle the solidus gradient is shallower than the isentropic gradient, and, therefore, isentropic upwelling cannot generate melt. At the base of the upper mantle limited melting can occur, either in the thermal boundary layer at the base of the upper mantle, or in the upper part of the lower mantle where the solidus gradient is steeper than the isentropic gradient. In both cases melting can occur at depths of more than 600 km, where Ca perovskite, CaPv, is a stable phase on the solidus. A surprising feature of the partitioning between melt and solid CaPv is that most trace elements are compatible in the solid. Partitioning into CaPv can, therefore, account for the low concentrations of such elements in komatiites. The temperatures required to generate such magmas in plumes need be no more than ∼50 °C above those of Phanerozoic plumes. The presence of komatiites in the Archean, therefore, requires plume temperatures in the first half of the Earth’s history to have been somewhat hotter than they are now, but does not constrain the average temperature of the Archean upper mantle.

In-situ reaction-replacement origin of hornblendites in the Early Cretaceous Laiyuan complex, North China Craton, and implications for its tectono-magmatic evolution

2021 ◽  
Author(s):  
Jia Chang ◽  
Andreas Audétat ◽  
Jian-Wei Li
Keyword(s):  
Partial Melting ◽  
North China Craton ◽  
Lithospheric Mantle ◽  
Lower Crust ◽  
North China ◽  
Ultramafic Rocks ◽  
Magmatic Evolution ◽  
Mafic Magmas ◽  
Felsic Rocks

Abstract Two suites of amphibole-rich mafic‒ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive-volcanic complex (North China Craton) are studied here by detailed petrography, mineral- and melt inclusion chemistry, and thermobarometry to demonstrate an in-situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr-Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic‒ultramafic rocks occur mainly as amphibole-rich mafic‒ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in-situ crystallization of hydrous mafic magmas that experienced gravitational settling of early-crystallized olivine and clinopyroxene at low pressures of 0.10‒0.20 GPa (∼4‒8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55‒75 vol% hornblende, 10‒20 vol% orthopyroxene and 3‒10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0.97‒1.25 GPa (∼37‒47 km crustal depth). Mass balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic‒ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥ 20‒30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic-ultramafic intrusions formed during ∼132‒138 Ma, which overlaps with the timespan of ∼126‒145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110‒125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was likely triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

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