Crucial monitors of operation of deep Earth volatile cycles are the accumulated volatile masses in the exosphere, particularly when these are compared to outgassing fluxes and to the masses stored in ...the mantle. Further insight can be gained by examining the relative mantle/exosphere fractionations of the major (H2O, C, and N) volatiles. New estimates for the H2O and C contents of the convecting mantle (290 ± 80 and 110 ± 40 ppm, respectively) are derived from H2O/Ce and CO2/Ba ratios of oceanic basalts combined with convecting mantle Ce and Ba concentrations. Together with an earlier estimate of convecting mantle N (1.1 ± 0.55 ppm) and with surface inventories (including a new estimate for total exosphere C, 10.6±1.8×1022 g), these allow construction of exosphere-normalized mantle masses of major volatiles. Values for H2O, C, and N are, respectively 0.75 ± 0.2, 4.2 ± 2.0 and 0.7 ± 0.35, meaning subequal amounts of H2O and N are in the mantle and exosphere, but most of the bulk silicate Earth (BSE) C is in the mantle. Outfluxes of major volatiles from the mantle suggest exosphere replenishment times of 7.5 and 1 Ga for H2O and C. Previous estimates for outfluxes of N indicate a replenishment time of 80 Ga, but an alternative based on the C/N ratio of the depleted mantle is 16 Ga. Importantly, the normalized flux of C from the mantle to the exosphere is much greater than those for H2O and N, even though the exosphere C reservoir is by far the smallest of the three. This is owing to more efficient recycling of C relative to H2O and N, where “recycling” means return of materials from near-surface reservoirs to the convecting mantle, and/or to large surface reservoirs of H2O and N (but not C) inherited from Earth's early history. H2O/Ce, CO2/Nb, and CO2/Ba ratios are little-fractionated from one another during mantle melting, but the H2O/Ce ratio of the exosphere (1540 ± 360) is much greater than mantle ratios (200 ± 50), indicating that H2O is not recycled as efficiently as Ce or partial preservation of a large primordial ocean. In contrast, the exosphere CO2/Ba ratio (40 ± 14) is far smaller than the convecting mantle ratio (100 ± 20), indicating that C is recycled more efficiently than Ba. The small exosphere CO2/Ba ratio reflects substantial long-term Ba storage in the continental crust combined with significant recycling to the mantle of C. Monte Carlo simulations of C and Ba exchange between the interior and exterior reservoirs, taking into account uncertainties in total C and Ba in the BSE and in the exosphere, suggest that for 0–40% Ba recycling to the mantle, 35–80% of the C that has outgassed to the surface through time has been returned to the deep mantle. Some former surface C may be stored in the continental lithosphere, but observed xenolith C concentrations indicate that the lithosphere accommodates only a small fraction of C apparently returned to the mantle. Consideration of models in which the continental lithosphere stores substantial C has only small effect on the quantitative conclusions of this exercise.
•The mass of H2O, CO2, and N2 in the exosphere constrain deep Earth volatile cycles.•Understanding of deep Earth cycles of H2O, CO2, and N requires a comparative approach.•Exosphere CO2/Ba is less than that of the mantle, which points to recycling of carbon.
Parameterizations of experimental data constraining the influence of CO
2 and H
2O on silicate melting, combined with experimental data on the stability of carbonatite in the upper mantle, allow ...calculation of the stability of partial melts in the seismic low velocity zone (LVZ) beneath oceanic lithosphere. For mantle with volatile contents similar to the sources of mid-ocean ridge basalt (100
ppm H
2O, 60
ppm CO
2), small amounts of melt are thermodynamically stable throughout the LVZ. On a weight basis, CO
2 has less influence than H
2O in stabilizing near-solidus partial melts of peridotite, but stability of volatile-rich near-solidus melts is dominated by CO
2 rather than H
2O because CO
2 is less compatible than H
2O in peridotite. For mantle potential temperatures (MPT) ranging from 1300 to 1400
°C, CO
2-rich silicate melts are stable to depths of 130–180
km and carbonatite stable at greater depths and also in the shallow regions (70–100
km) beneath older lithosphere. Calculated melt fractions of carbonatite do not exceed 0.024% and silicate melt fractions are <0.1% beneath older (>40
Ma) lithosphere, except for a thin region at ∼120
km when the MPT approaches 1400
°C. These melt fractions are maxima if compaction is effective over the ∼100
Ma life of oceanic lithosphere. The regions of high melt fraction correspond broadly with the locus of low shear wave velocity imaged tomographically, though the melt fractions present may be too small to account for the seismic characteristics of the LVZ. Carbonatite cannot be present in large regions of the LVZ, particularly at intermediate depths (60–145
km) beneath young lithosphere (<40
Ma), and therefore cannot be responsible for the high electrical conductivity of this part of the LVZ, as has recently been proposed (
Gaillard et al., 2008). Horizontal lenses of melt beneath the lithosphere/asthenosphere boundary may be responsible for the sharp seismic G discontinuity that marks the lithosphere/asthenosphere boundary in oceanic domains, but if so they must consist of melt collected at that horizon by percolation of deeper magmas or melt concentrated in high aspect ratio enriched heterogeneities.
The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the ...rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03–0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of ∼200–330 km in the upper mantle.
We document compositions of minerals and melts from 3 GPa partial melting experiments on two carbonate-bearing natural lherzolite bulk compositions (PERC: MixKLB-1 + 2·5 wt% CO2; PERC3: MixKLB-1 + 1 ...wt% CO2) and discuss the compositions of partial melts in relation to the genesis of alkalic to highly alkalic ocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C; PERC3: ∼1050°C) carbonatitic partial melts with <10 wt% SiO2 and ∼40 wt% CO2 evolve continuously to carbonated silicate melts with >25 wt% SiO2 and <25 wt% CO2 between 1325 and 1350°C in the presence of residual olivine, orthopyroxene, clinopyroxene, and garnet. The first appearance of CO2-bearing silicate melt at 3 GPa is ∼150°C cooler than the solidus of CO2-free peridotite. The compositions of carbonated silicate partial melts between 1350 and 1600°C vary in the range of ∼28–46 wt% SiO2, 1·6–0·5 wt% TiO2, 12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and 42–48 wt% SiO2, 1·9–0·5 wt% TiO2, ∼10·5–8·4 wt% FeO*, and ∼15–26 wt% MgO for PERC3. The CaO/Al2O3 weight ratio of silicate melts ranges from 2·7 to 1·1 for PERC and from 1·7 to 1·0 for PERC3. The SiO2 contents of carbonated silicate melts in equilibrium with residual peridotite diminish significantly with increasing dissolved CO2 in the melt, whereas the CaO contents increase markedly. Equilibrium constants for Fe*–Mg exchange between carbonated silicate liquid and olivine span a range similar to those for CO2-free liquids at 3 GPa, but diminish slightly with increasing dissolved CO2 in the melt. The carbonated silicate partial melts of PERC3 at <20% melting and partial melts of PERC at ∼15–33% melting have SiO2 and Al2O3 contents, and CaO/Al2O3 values, similar to those of melilititic to basanitic alkali OIB, but compared with the natural lavas they are more enriched in CaO and they lack the strong enrichments in TiO2 characteristic of highly alkalic OIB. If a primitive mantle source is assumed, the TiO2 contents of alkalic OIB, combined with bulk peridotite/melt partition coefficients of TiO2 determined in this study and in volatile-free studies of peridotite partial melting, can be used to estimate that melilitites, nephelinites, and basanites from oceanic islands are produced from 0–6% partial melting. The SiO2 and CaO contents of such small-degree partial melts of peridotite with small amounts of total CO2 can be estimated from the SiO2–CO2 and CaO–CO2 correlations observed in our higher-degree partial melting experiments. These suggest that many compositional features of highly alkalic OIB may be produced by ∼1–5% partial melting of a fertile peridotite source with 0·1–0·25 wt% CO2. Owing to very deep solidi of carbonated mantle lithologies, generation of carbonated silicate melts in OIB source regions probably happens by reaction between peridotite and/or eclogite and migrating carbonatitic melts produced at greater depths.
To determine partitioning of C between upper mantle silicate minerals and basaltic melts, we executed 26 experiments between 0.8 and 3 GPa and 1250–1500 °C which yielded 37 mineral/glass pairs ...suitable for C analysis by secondary ion mass spectrometry (SIMS). To enhance detection limits, experiments were conducted with 13C-enriched bulk compositions. Independent measurements of 13C and 12C in coexisting phases produced two C partition coefficients for each mineral pair and allowed assessment of the approach to equilibrium during each experiment. Concentrations of C in olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and garnet (gt) range from 0.2 to 3.5 ppm, and resulting C partition coefficients for ol/melt, opx/melt, cpx/melt and gt/melt are, respectively, 0.0007±0.0004 (n=2), 0.0003±0.0002 (n=45), 0.0005±0.0004 (n=17) and 0.0001±0.00007 (n=5). The effective partition coefficient of C during partial melting of peridotite is 0.00055±0.00025, and therefore C is significantly more incompatible than Nb, slightly more compatible than Ba, and, among refractory trace elements, most similar in behavior to U or Th. Experiments also yielded partition coefficients for F and H between minerals and melts. Combining new and previous values of DFmineral/melt yields bulk DFperidotite/melt=0.011±0.002, which suggests that F behaves similarly to La during partial melting of peridotite. Values of DHpyx/melt correlate with tetrahedral Al along a trend consistent with previously published determinations.
Small-degree partial melting of the mantle results in considerable CO2/Nb fractionation, which is likely the cause of high CO2/Nb evident in some Nb-rich oceanic basalts. CO2/Ba is much less easily fractionated, with incompatible-element-enriched partial melts having lower CO2/Ba than less enriched basalts. Comparison of calculated behavior of CO2, Nb, and Ba to systematics of oceanic basalts suggests that depleted (DMM-like) sources have 75±25 ppm CO2 (CO2/Nb=505±168, CO2/Ba=133±44), whereas enriched sources of intraplate basalts similar in concentrations to primitive mantle have 600±200 ppm CO2. If all mantle reservoirs are expressed in the current inventory of oceanic basalts for which nearly undegassed CO2 concentrations are available, then we estimate the likely range of mantle C concentrations to be 1.4–4.8×1023 grams of C, or 1.5–5.2 times the mass of the current C surface reservoir. Depending on the assumed Ba and Nb contents of average oceanic crust, resulting ridge fluxes of C range from 7.2×1013 to 2.9×1014 g/yr.
•We present experimental partitioning of C between basalt and mantle minerals.•Peridotite/melt partitioning of C=0.00055±0.00025.•C is less compatible than Nb, more compatible than Ba, and similar to U and Th.•The MORB source contains 75±25 ppm CO2, and OIB sources have 600±200 ppm CO2.
In this study, we present a detailed, statistical analysis of black hole growth and the evolution of active galactic nuclei (AGN) using cosmological hydrodynamic simulations run down to z = 0. The ...simulations self-consistently follow radiative cooling, star formation, metal enrichment, black hole growth and associated feedback processes from both Type II/Ia supernovae and AGN. We consider two simulation runs, one with a large comoving volume of (500 Mpc)3 and one with a smaller volume of (68 Mpc)3 but with a factor of almost 20 higher mass resolution. We compare the predicted statistical properties of AGN with results from large observational surveys. Consistently with previous results, our simulations can widely match observed black hole properties of the local Universe. Furthermore, our simulations can successfully reproduce the evolution of the bolometric AGN luminosity function for both the low-luminosity and the high-luminosity end up to z = 3.0, only at z = 1.5–2.5, the low-luminosity end is overestimated by up to 1 dex. In addition, the smaller but higher resolution run is able to match the observational data of the low bolometric luminosity end at higher redshifts z = 3–4. We also perform a direct comparison with the observed soft and hard X-ray luminosity functions of AGN, including an empirical correction for a torus-level obscuration, and find a similarly good agreement. These results nicely demonstrate that the observed ‘antihierarchical’ trend in the AGN number density evolution (i.e. the number densities of luminous AGN peak at higher redshifts than those of faint AGN) is self-consistently predicted by our simulations. Implications of this downsizing behaviour on active black holes, their masses and Eddington ratios are discussed. Overall, the downsizing behaviour in the AGN number density as a function of redshift can be mainly attributed to the evolution of the gas density in the resolved vicinity of a (massive) black hole (which is depleted with evolving time as a consequence of star formation and AGN feedback).
The origin of volatiles in the Earth's mantle Hier‐Majumder, Saswata; Hirschmann, Marc M.
Geochemistry, geophysics, geosystems : G3,
August 2017, 20170801, Letnik:
18, Številka:
8
Journal Article
Recenzirano
Odprti dostop
The Earth's deep interior contains significant reservoirs of volatiles such as H, C, and N. Due to the incompatible nature of these volatile species, it has been difficult to reconcile their storage ...in the residual mantle immediately following crystallization of the terrestrial magma ocean (MO). As the magma ocean freezes, it is commonly assumed that very small amounts of melt are retained in the residual mantle, limiting the trapped volatile concentration in the primordial mantle. In this article, we show that inefficient melt drainage out of the freezing front can retain large amounts of volatiles hosted in the trapped melt in the residual mantle while creating a thick early atmosphere. Using a two‐phase flow model, we demonstrate that compaction within the moving freezing front is inefficient over time scales characteristic of magma ocean solidification. We employ a scaling relation between the trapped melt fraction, the rate of compaction, and the rate of freezing in our magma ocean evolution model. For cosmochemically plausible fractions of volatiles delivered during the later stages of accretion, our calculations suggest that up to 77% of total H2O and 12% of CO2 could have been trapped in the mantle during magma ocean crystallization. The assumption of a constant trapped melt fraction underestimates the mass of volatiles in the residual mantle by more than an order of magnitude.
Plain Language Summary
The Earth's deep interior contains substantial amounts of volatile elements like C, H, and N. How these elements got sequestered in the Earth's interior has long been a topic of debate. It is generally assumed that most of these elements escaped the interior of the Earth during the first few hundred thousand years to create a primitive atmosphere, leaving the mantle reservoir nearly empty. In this work, we show that the key to this paradox involves the very early stages of crystallization of the mantle from a global magma ocean. Using numerical models, we show that the mantle stored substantially higher amounts of volatiles than previously thought, thanks to large quantities of melt trapped in the mantle due to rapid freezing of the magma ocean. Our models show that up to 77% of the total planetary budget of water and 12% of CO2 can be stored in the mantle due to this previously unaccounted process.
Key Points
Magma ocean freezing fronts retain more melt than previously thought
Melt thus retained in the residual mantle can dissolve substantial amount of volatiles
Following solidification of the magma ocean, the residual mantle can contain substantially larger amount of volatiles
We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar ...medium, comets, chondritic meteorites, and terrestrial planets; we include an updated estimate for the bulk silicate Earth (C/N = 49.0 ± 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macromolecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of â¼1â12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N bulk silicate Earth ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.
With the rapid pace at which exoplanets are being discovered, many efforts have now been dedicated to identifying which planets are expected to have the ingredients necessary for the development of life. In this work we explore the relative disposition of the essential elements carbon and nitrogen in each stage of star and planet formation, using the Earth and our solar system as grounding data. Our results suggest that planets like the Earth are readily supplied with these key elements, but their relative amounts on the surface and in the atmosphere will be highly variable.
Elastic recoil detection analysis (ERDA) was used to measure the H contents of 18 synthetic Fo90 olivines that had been hydrated to varying degrees in high pressure hydrothermal experiments. The ...infrared spectra of the olivines have been previously measured in the OH stretching region by Fourier transform infrared spectroscopy, and 16O1H secondary ion mass spectroscopy measurements have been made on the same samples. Mapping by ERDA, Rutherford backscattering and particle induced X-ray emission spectroscopies show that the synthetic olivines are homogeneous with respect to major element and H concentrations. Concentrations of H measured by ERDA vary between 270 and 2120ppm H2O. Measurements of OH/Si ratios by secondary ion mass spectroscopy and of total integrated area of OH stretching bands in principal absorption spectra by FTIR show excellent linear relationships to H concentrations determined by ERDA. The ERDA measurements are used to determine an infrared integral molar absorption coefficient of 45,200lmol−1cm−2 that can be used to determine H contents of olivines from high pressure experiments. The H content of Fo90 olivines (in wt. ppm H2O) is given by 0.119±0.006×total integrated absorbance per cm thickness.
► Olivines were synthesized in high pressure experiments. ► Their H contents were measured by elastic recoil detection analysis. ► Infrared OH stretching bands increase linearly with H content. ► The integral molar absorption coefficient for OH in synthetic olivine was determined.
The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. ...Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former.
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