Carbon is one of the most important elements of our planet, and 90% of it resides in the Earth's interior. Hauri (Figure 1.1) of the Carnegie Institution of Science and Louise Kellogg of the University of California at Davis, whose intellectual curiosity, scientific excellence, and good humor were inspiring and will always be remembered.
Introduction
On the other hand, if the C abundance and C/X ratios of the BSE differ from these known building blocks, one must investigate whether any post-delivery processes could have altered these quantities such that the BSE LEVE geochemistry could be established. The current understanding of the fractionation of C and other LEVEs during the nucleation process will also be used to place constraints on the C budget of the nucleus.
Constraints on the Compositions of Terrestrial Building Blocks .1 Constraints from Isotopes of Refractory Elements
Indeed, such fractionation processes may reconcile some of the observed differences between LEVE BSE and chondrite isotopic compositions.60 For example, the carbon isotopic compositions of many CI chondritic materials are markedly lighter (δ13C, about –15 to –7‰50) than the average carbon isotopic composition of the Earth's mantle (δ13C, about –5‰61). ,62). The figure also lists the main geochemical information used to estimate the BSE concentrations of the elements in question in the original studies.
C and Other Volatiles: Abundances, Ratios, and Forms in Various Classes of Meteorites and Comparison with the BSE
The C/N ratio of BSE to that of building blocks can help trace the history of C accumulation and differentiation on the early Earth. The C/S ratio of BSE is higher than that of C-depleted E-chondrites, but is similar to or lower than that of C-rich carbonaceous chondrites (Figure 2.4c).
Establishing LEVE Budgets of the BSE After Core Formation?
Assuming a similar mantle C store as discussed earlier, the C/H ratio of the BSE is estimated to be BSE. The C/H ratio of BSE is lower than that of all classes of chondritic meteorites (Figure 2.4). N is also incompatible in S-rich melts of Fe alloys,18 hence the S/N ratio of the BSE would also decrease after segregation of the sulphide melt.
Establishing the Volatile Budget of the BSE through Equilibrium Accretion and MO Differentiation
DalloyC =silicate and C solubility in silicate melts is influenced by the incorporation mechanism of C into the silicate melt structure. C decreases with increases in Si content in the alloy or indirect decreases infO2 of the alloy-silicate systems.
C and Other LEVE Budgets of the BSE: A Memory of Multistage Accretion and Core Formation Process with Partial Equilibrium?
This does not mean that these processes did not affect the volatile BSE inventory, and it is plausible that all these processes acted together. This would occur as the cores of these embryos reach the C solubility limit and expel graphite or diamond into the upper silicate fraction (Figure 1). Therefore, a merger of the silicate reservoirs of such embryos with the proto-silicate Earth reservoir could cause the BSE LEVE inventory.
Carbon as a Light Element in the Core
Badro et al.160 also reached a similar conclusion that to simultaneously satisfy the geochemical and geophysical constraints, the Earth's core composition cannot contain C as one of the major light elements. Therefore, all the constraints so far point to Earth's core being the largest C reservoir, yet C is a minor light element in the core.
Conclusion
By coupling S and C delivery in the final 13% of accretion, Wood et al.60 showed that to obtain the known Mo and W abundances of the mantle, the bulk C content of the core must be <1 wt%. Despite the agreement in the arguments advanced by many recent studies that require the accretion of one or more large planetary embryos with little post-impact equilibration of the impactor's core with the Earth's mantle/MO, differences remain in their details.
Limits of Knowledge and Unknowns
Allègre, C., Manhès, G., & Lewin, É., Chemical composition of the Earth and volatility controls on planetary genetics. Planet Earth. Willbold, M., Elliott, T., & Moorbath, S., Tungsten isotopic composition of the Earth's mantle before the final bombardment. Nature.
Introduction
Constraints on Carbon versus Other Light Elements in Earth’s Core .1 Constraints from Phase Relations of Iron – Light Element Systems
If the eutectic composition is less than 1 wt%, a binary Fe–C composition with 5% density contrast between coexisting solid and liquid is unlikely. The Fe–S binary exhibits eutectic behavior between Fe and FeS at 1 bar and the sulfur content of the eutectic decreases with pressure (Figure 3.3).
Implications of Carbon as a Major Light Element in the Core
The presence of other light elements such as carbon, oxygen and/or sulfur is required to lower its crystallization temperature. While the alloying effect of oxygen on the eutectic point of the Fe-S system was found to be minor, 113 impact experiments at 100–200 GPa estimated that the presence of 8 wt%.
Carbon in the Core Over Time
Ongoing carbon sequestration of the core may be a result of subduction of the hydrothermally altered oceanic lithosphere, transporting carbonates and organic matter into the deep Earth. While CaCO3in slabs may have been preserved under reducing lower mantle conditions, the MgCO3 component could have been destabilized by metallic ferrous diamonds or iron carbides. 120 Slab-derived Fe-C mixtures are expected to partially melt in the D´´ layer. 111 The melt may have accumulated near the CMB core over time and the episode 3 runoff.
Conclusion
The presence of 1 wt% carbon in the outer core provides a good match for VP and is consistent with the coexistence of a molten iron alloy with solid silicate at the CMB. At least 1–2 wt% sulfur is likely present in the outer core and would help explain its density deficit and the largely molten state of the core.
Limits to Knowledge and Unknowns
Bazhanova, ZG, Oganov, AR, & Gianola, O., Fe-C and Fe-H systems under pressure from the Earth's inner core. Komabayashi, T., Thermodynamics of melting relationships in the system Fe-FeO at high pressure: implications for oxygen in the Earth's core.
Introduction
These estimates can then be used to determine the amount of carbon in the present-day Earth's mantle by simulating possible reactions within simplified mantle mineral compositions that identify the roles of pressure, temperature and bulk chemistry on carbon speciation. Answers to these questions reveal how carbon has evolved in the mantle over time and where it may reside in the present-day mantle.
The Abundance, Speciation, and Extraction of Carbon from the Upper Mantle over Time
The mobilization of this primordial carbon, probably in the form of elemental carbon in Fe(Ni) and CH4 alloys in the magma ocean, as suggested by Li et al.,12 is related to the evolution of the redox state of the mantle over time. Given that redox melting—today and also in the early mantle—requires reduction of Fe2O3 (i.e.
The Stability of Reduced and Oxidized Forms of Carbon in the Upper Mantle
These models, supported by melting experiments, show that the generation of CO2-rich basalts up to ~6 wt% in the melt, such as those from Atlantic popping rocks37, requires the contribution of Figure 4.2 LogfO2 (normalized to the FMQ buffer) determined for peridotite and eclogetic xenolites using oxy-thermobarometry for spinel/garnet peridotite and eclogite.18,26,27The blue curve is thefO 2 calculated for Eq. 4.2) along a 40 mWm-2 cratonic geothermal that defines the stability field between diamond (or graphite) and solid (liquid) carbonate within peridotite rocks. 4.1).18The yellow arrow indicates the proposed oxidation state of a convection mantle contaminated by variable volumes of submerged carbonated lithologies.37DCDD/G = dolomite-coesite-diopside (diamond/graphite).
The Redox State and Speciation of C in the Transition Zone and Lower Mantle
The latter question connects to the possibility that the fO2 of the transition zone and lower mantle can be buffered by an influx of oxidized carbon in the form of solid carbonate, but questions remain about which carbonates these could be. The speciation of carbon at conditions from the upper to the lower mantle, whether in the form of diamond or carbonate (either liquid or solid), has been the focus of investigations over the past decades.
Seismic Detectability of Reduced and Oxidized Carbon in Earth’s Mantle The presence of carbon within the mantle may be detectable using seismic data. A recent
They observed a dramatic softening of the C11, C33, C12, and C13 modules and stiffening of the C44 and C14 modules across the spin transition in the mixed spin state. As a result, FeCO3 is unlikely to occur in the lower mantle at depths greater than ~1200 km.
Conclusion
Based on their work, mixed spin-state ferromagnesite is expected to exhibit anomalous elasticity in the middle lower mantle, including a negative Poisson's ratio and a drastically reduced compressional wave velocity (VP). Carbon has low solubility in mantle minerals; 4-7 therefore it occurs primarily in the form of gas (red circles), liquids or melts (orange circles) and accessory solid phases (green circles), including diamonds (octahedra).
Limits to Knowledge and Unknowns
The deep carbon cycle and melting in the Earth's interior.Earth and Planetary Science Letters (Frontiers. CaCO3-III and CaCO3-VI, high-pressure polymorphs of calcite: possible host structures for carbon in the Earth's mantle.Earth and Planetary Science Letters.
Introduction
Unlike lithospheric diamonds, super deep diamonds are not as easy to classify as eclogitic or peridotite. Much of the research described in this chapter focuses on superdeep diamonds, as the study of superdeep diamonds is most relevant to the deep carbon cycle.
Physical Conditions of Diamond Formation .1 Measuring the Depth of Diamond Formation
The defects trapped in diamonds can be used to constrain estimates of the temperature that prevailed during a diamond's stay in the mantle and help constrain estimates of carbon's return path to the surface. If there was a constant temperature throughout the history of the diamond's stay in the lithosphere, then the ages of the two growth periods are 3.2 and 1.1 Ga.
Diamond-Forming Reactions, Mechanisms, and Fluids .1 Direct Observation of Reduced Mantle Volatiles in Lithospheric
These forms of isochemical diamond precipitation require liquids to remain relatively pure (i.e. gradual dilution of the liquid by addition of a melting component does not occur). 13C enrichments, while the single-fluid redox model predicts both 13C enrichments and depletions depending on whether the fluids are oxidized or reduced.
Sources of Carbon and Recycling of Volatiles .1 Atmospheric and Biotic Recycling of Sulfur into the Mantle
Modeling of the local covariations of δ13C–δ15N–N compositions within individual diamonds shows that they grew from parent fluids that include both oxidized (majority) and reduced (minority)107, highlighting the likely heterogeneity of the transition zone (Fig. 5.5c). During subduction, carbon and nitrogen appear to be effectively trapped in the oceanic lithosphere before being mobilized locally in the transition zone to form diamonds.
Mineral Inclusions and Diamond Types
Bureau et al.137 used observations of coexisting melt-fluid inclusions to infer the temperature at which the two phases begin to mix in the examined system. Carbonate basalt will melt in the deep upper mantle and transition zone, producing alkali-rich siliceous carbonate melts.
Limits to Knowledge and Questions for the Future
The depth of sublithospheric diamond formation and the redistribution of carbon in the deep mantle.Earth and Planetary Science Letters. Redox freezing and nucleation of diamond via magnetite formation in the Earth's mantle.Nature Communications.
Introduction
It is also often proposed that carbonate melts and carbonatite magmas are genetically linked to some CO2-bearing silicate melts (melts with >20 wt% SiO2 and dissolved, oxidized carbon, such as kimberlites, intraplate basalts, continental alkaline basalts, etc.) through processes such as carbonate-silicate fluid immiscibility, crystal fractionation, and oxidation of diamond or graphite. Genetic relationships between carbonate melts and CO2-containing silicate melts under appropriate conditions are also discussed in this chapter.
Constraints on Carbonate Stability in Earth’s Mantle
The melting of carbonaceous peridotite to form melts with high carbonate activities will therefore be confined to those regions of the upper peridotite mantle where the oxidation state is consistent with carbonate stability (i.e., where O2 is above the appropriate limiting response at given pressure and temperature). Fe measurements on primitive mid-ocean basalt (MORB) glasses25,26 coupled with high-pressure experimental and thermodynamic calibrations of relevant redox-controlling reactions.27-29 These studies indicate that carbonate stability in peridotite is likely to be generally limited to relatively shallow parts of the continental lithosphere (i.e. depths <~100 km; see Section 6.6.2 for more details).
Experimental Constraints on the Melting of Carbonate Peridotite in the Mantle
In the next section, we review the high-pressure experimental constraints on carbonate melting. In the following sections, we use experimental and other constraints to infer the existence and behavior of carbonate melts in different tectonic settings.
Carbonate Melts Associated with Subduction Zones
A significant proportion of the mass of carbon drawn into the mantle at subduction zones (anywhere between 20% and 100%) is recycled back to the surface by means of forearc degassing and arc magmatism,56 as discussed in detail in Chapter 10 of this volume. Upwelling of the hot core of the wedge may bring about retrograde melt-rock or fluid-rock reactions, locking some carbonate (+ H2O) phases into the mantle lithosphere and lower crust,56 but the most volatile flow of the slab is expected to be ultimately delivered via the upper fractionating arc and magmassing crust.
Melting of Subducted, Carbonated Sediment and Ocean Crust in the Deep Upper Mantle and Transition Zone
In the lower pressure range (e.g. <3–5 GPa), a carbonate phase is not always stable at the solidus, depending on the bulk CO2 and SiO2 content. At higher pressures of the deep upper mantle and transition zone, starting compositions again show remarkable control of melting behavior (Figure 6.2).
Carbonate Melts and Kimberlites in the Cratonic Lithospheric Mantle The cratonic mantle lithosphere underlies ancient, continental blocks that have been
In the presence of metallic FeNi alloy, some reduced carbon will be accommodated as (Fe,Ni) carbides. The dashed white contours are approximate contours of O2 variation in the cratonic lithosphere based on studies of garnet peridotite xenoliths (references in the text).
Carbonate Melts beneath Ocean Islands in Intraplate Settings
This implies that in an upwelling mantle, partial melting of carbonate-bearing lithologies is initiated deeper than in the surrounding carbonate-free lithologies. The fluxed partial melting of eclogite and peridotite produces carbonated silicate melts in the upper mantle.
Carbonate Melts under Mid-ocean Ridges
A higher γSiO2 and an increased degree of polymerization of the silicate domain of the melt network also imply that saturation of orthopyroxene is preferred over saturation of olivine in the residue of melt-rock reaction. Thus, the stability field of orthopyroxene is improved over that of olivine.47,151–153 Cations such as Ca2+, Mg2+, Na+ and K+ also prefer to enter the melt structure to form carbonate complexes in the presence of CO2.
Crustally Emplaced Carbonatites
This would be consistent with more effective recycling of crustal material into the mantle via modern-style subduction processes, allowing the production of refertilized mantle domains that are the necessary sources of these magmas. Instead, these authors argue for the reactivation of lithospheric-scale structures or lineations due to far-field plate reorganization (e.g. due to the initiation of rifting or continental assembly182), which allows for low-scale melting of the fertile craton mantle kitsphere, as well as the production of instantaneous tectonic lithosphere, due to large rifting or tectonics. the surface.
Concluding Remarks
Limits to Knowledge and Unknowns
Carbonatite metasomatism in the northern Tanzanian mantle - petrographic and geochemical features. Earth Planet Sci Lett. Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical features.Earth Planet Sci Lett.
Introduction: Toward a Geophysical De fi nition of Incipient Melting and Mantle Metasomatism
This also requires addressing issues related to the fluid mechanics of melt advection in the mantle (i.e., how fast the incipient melt moves relative to conventional convection and plate velocities). This broad description is not a rule, as there are specific environments where such bright contrasts are not clearly observed, such as under cratons26 and in the enigmatic NoMelt region,27 a Pacific Ocean environment where no geodynamic perturbations seem to operate.
CO 2 -Rich Melts in the Mantle: Stability, Composition, and Structure .1 Partial Melting in the Presence of CO 2 and H 2 O: Incipient Melting
Following this broad picture of the initial melting process, we present here the range of melt compositions produced in different geodynamic environments. LAB exhibits a depth that varies with plate age.31 One can see that the degree of H2O-CO2 enrichment moderately affects the type of melt composition formed at lithospheric depth.
Physical Properties of CO 2 -Rich Melts in the Mantle
As expected, the influence of the H2O content on the density curve of the carbonated silicate melt is significant (compare the two curves with and without H2O). The magnitude of the viscosity change due to the inclusion of water in the melt is much smaller than that described for the transition from carbonate to basalt, but an important point is that the effect of water is independent of the melt chemistry: the addition of 1 wt.% H2O decreases melt viscosities by ∼0.1 log units for the entire melt.
Interconnection of CO 2 -Rich Melts in the Mantle
We must emphasize here that our interpretation differs from that of Minarik and Watson,70 who concluded that carbonate melt interconnectivity stops at 0.03–0.07 wt% added carbonate vol.% melt). There may be a gap at ∼0.07 vol.% melt, which requires corroborations, but this is not an interconnect failure, as bulk diffusivity enhancement is still observed up to 0.007 vol.%.
Mobility and Geophysical Imaging of Incipient Melts in the Upper Mantle .1 Melt Mobility as a Function of Melt Composition
In the most depleted mantle sources containing <100 ppm CO2, incipient melts are not mobile due to too small melt fractions (ie <0.02 vol.% carbonatite). Carbonatites (containing 40 wt% CO2) are labeled in equivalent ppm CO2 content in bulk rock.
Conclusions
The LAB is a concept that assumes that the Earth's upper mantle consists of two layers: the lower one, the asthenosphere, is adiabatic (convection controls heat transport); and the upper one, the lithosphere, is diffuse (diffusion spreads heat). These melting processes can be lithospheric or asthenospheric, and therefore geophysical discontinuities may not provide a picture of the LAB, but rather illuminate the dynamics of melting and melt transfers in the area of the LAB.
Limits to Knowledge and Unknowns
