Experimental determination of CO2 content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans

Megan S. Duncan, Rajdeep Dasgupta, Kyusei Tsuno

Abstract

Knowledge of the carbon carrying capacity of peridotite melt at reducing conditions is critical to constrain the mantle budget and planet-scale distribution of carbon set at early stage of differentiation. Yet, neither measurements of CO2 content in reduced peridotite melt nor a reliable model to extrapolate the known solubility of CO2 in basaltic (mafic) melt to solubility in peridotitic (ultramafic) melt exist. There are several reasons for this gap; one reason is due to the unknown relative contributions of individual network modifying cations, such as Ca2+ versus Mg2+, on carbonate dissolution particularly at reducing conditions. Here we conducted high pressure, temperature experiments to estimate the CO2 contents in silicate melts at graphite saturation over a compositional range from natural basalts toward peridotite at a fixed pressure (P) of 1.0 GPa, temperature (T) of 1600 °C, and oxygen fugacity (logfO2 ~ IW+1.6). We also conducted experiments to determine the relative effects of variable Ca and Mg contents in mafic compositions on the dissolution of carbonate. Carbon in quenched glasses was measured and characterized using Fourier transform infrared spectroscopy (FTIR) and Raman Spectroscopy and was found to be dissolved as carbonate (CO32-). The FTIR spectra showed CO32- doublets that shifted systematically with the MgO and CaO content of silicate melts. Using our data and previous work we constructed a new composition-based model to determine the CO2 content of ultramafic (peridotitic) melt representative of an early Earth, magma ocean composition at graphite saturation. Our data and model suggest that the dissolved CO2 content of in reduced, peridotite melt is significantly higher than that of basaltic melt at shallow magma ocean conditions; however, the difference in C content between the basaltic and peridotitic melts may diminish with depth as the more depolymerized peridotite melt is more compressible. Using our model of CO2 content at graphite saturation as a function of PT-fO2-melt composition, we predict that a superliquidus shallow magma ocean should degas CO2. Whereas if the increase of fO2 with depth is weak, a magma ocean may ingas a modest amount of carbon during crystallization. Further, using the carbon content of peridotite melt at logfO2 of IW and the knowledge of C content of Fe-rich alloy melt, we also consider the core-mantle partitioning of carbon, showing that DCmetal/peridotite of a shallow magma ocean is generally higher than previously estimated.

 

Duncan, M. S., Dasgupta, R., Tsuno, K. (2017). Experimental determination of CO2 content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans. Earth and Planetary Science Letters 466, 115-128. doi:10.1016/j.epsl.2017.03.008

JGR-ES: Morphodynamic modeling of fluvial channel fill and avulsion time scales during early Holocene transgression, as substantiated by the incised valley stratigraphy of the Trinity River, Texas

Kaitlin E. Moran , Jeffrey A. Nittrouer, Mauricio M. Perillo, Jorge Lorenzo-Trueba, and John B. Anderson

Journal of Geophysical Research: Earth Surface (2016) doi:10.1002/2015JF003778

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges to explore the effects of backwater-induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of early Holocene sea-level rise (4.3 mm yr1 ), the Trinity fluvial-deltaic system progrades 13 m yr1 , followed by backstepping of 27 m yr1 . Avulsion is reached at the position of maximum sediment deposition (located 108 km upstream of the outlet) after 3,548 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment between the channel and floodplain influences the timing and location of avulsion over millennial time scales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems.

CO2 content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

James Eguchi, Rajdeep Dasgupta

DOI: 10.1007/s00410-017-1330-8

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550 °C, and fO2 of FMQ −3.2 to FMQ −2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln(K0) = −21.79 ± 0.04, ΔV0 = 32.91 ± 0.65 cm3mol−1, ΔH0 = 107 ± 21 kJ mol−1, and dissolution of CO2 as CO32−: ln(K0) = −21.38 ± 0.08, ΔV0 = 30.66 ± 1.33 cm3 mol−1, ΔH0 = 42 ± 37 kJ mol−1, where K0 is the equilibrium constant at some reference pressure and temperature, ΔV0 is the volume change of reaction, and ΔH0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.

How habitable zones and super-Earths lead us astray

William B. Moore, A. Lenardic, A. M. Jellinek, C. L. Johnson, C. Goldblatt and R. D. Lorenz

“The English language … becomes ugly and inaccurate because our thoughts are foolish, but the slovenliness of our language makes it easier for us to have foolish thoughts.” – George Orwell

The field of exoplanet research is currently experiencing a period of integration among researchers from the planetary exploration, astrobiology, and astronomical observation communities (among others). In addition, exoplanet discoveries generate strong interest from the public and a need for frequent communication of scientific discoveries to non-scientists. Together, these factors have put significant pressure on the field to develop an easily digested scientific shorthand for frequently used concepts. While useful, such terminology is often imprecise and can ultimately mislead the very audience it was designed to reach.

Nature Astronomy Volume 1, Feb 2, 2017 DOI: 10.1038/s41550-017-0043

http://rdcu.be/o3yL

Episodic nature of continental arc activity since 750 Ma: A global compilation

Episodic nature of continental arc activity since 750 Ma: A global compilation

Wenrong Cao, Cin-Ty Lee, Jade Star Lackey

Earth and Planetary Science Letters (2017) 461:85-95

http://dx.doi.org/10.1016/j.epsl.2016.12.044

Continental arcs have been recently hypothesized to outflux large amounts of CO2 compared to island arcs so that global flare-ups in continental arc magmatism might drive long-term greenhouse events. Quantitative testing of this hypothesis, however, has been limited by the lack of detailed studies on the spatial distribution of continental arcs through time. Here, we compile a worldwide database of geological maps and associated literature to delineate the surface exposure of granitoid plutons, allowing reconstruction of how the surface area addition rate of granitoids and the length of continental arcs have varied since 750 Ma. These results were integrated into an ArcGIS framework and plate reconstruction models. We find that the spatial extent of continental arcs is episodic with time and broadly matches the detrital zircon age record. Most vigorous arc magmatism occurred during the 670–480 Ma and the 250–50 Ma when major greenhouse events are recognized. Low continental arc activity characterized most of the Cryogenian, middle–late Paleozoic, and Cenozoic when climate was cold. Our results indicate that plate tectonics is not steady, with fluctuations in the nature of subduction zones possibly related in time to the assembly and dispersal of continents. Our results corroborate the hypothesis that variations in continental arc activity may play a first order role in driving long-term climate change. The dataset presented here provides a quantitative basis for upscaling continental arc processes to explore their effects on mountain building, climate, and crustal growth on a global scale.

The relative abundances of resolved 12CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases

E. D. Young, I. E. Kohl, B. Sherwood Lollar, G. Etiope, D. Rumble III, S. Li (李姝宁), M. A. Haghnegahdar, E. A. Schauble, K. A. McCain, D. I. Foustoukos, C. Sutclife, O. Warr, C. J. Ballentine, T. C. Onstott, H.  Hosgormez, A. Neubeck, J. M. Marques, I. Pérez-Rodríguez, A. R. Rowe, D. E. LaRowe, C. Magnabosco, Laurence Y. Yeung, Jeanine L. Ash, L. T. Bryndzia. Geochim. Cosmochim. Acta (2017).

doi: 10.1016/j.gca.2016.12.041

Abstract

We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ12CH2D2 and Δ13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Δ12CH2D2 and Δ13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis versus biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.

The fate of sulfide during decompression melting of peridotite – implications for sulfur inventory of the MORB-source depleted upper mantle

Shuo Ding and Rajdeep Dasgupta

Abstract: Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg#>60) with S and Cu mostly concentrated in 800–1000 ppm and 80–120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100–200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280–1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼10–15% melting with bulk S of 100–150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of View the MathML sourceand initial Cu in the mantle can all affect the Cu concentration of primitive MORBs, 100–200 ppm S in the MORB source mantle can satisfy both S and Cu geochemistry of partial melts parental to ocean floor basalts.

 

Ding, S. & Dasgupta, R. (2017). The fate of sulfide during decompression melting of peridotite – implications for sulfur inventory of the MORB-source depleted upper mantle. Earth and Planetary Science Letters. doi:10.1016/j.epsl.2016.11.020

Scaling relationships and physics for mixed heating convection in planetary interiors: Isoviscous spherical shells

Matthew B. Weller, Adrian Lenardic, and William B. Moore

Abstract We use a suite of 3-D numerical experiments to test and expand 2-D planar isoviscous scaling
relationships of Moore (2008) for mixed heating convection in spherical geometry mantles over a range of
Rayleigh numbers (Ra). The internal temperature scaling of Moore (2008), when modified to account for
spherical geometry, matches our experimental results to a high degree of fit. The heat flux through the
boundary layers scale as a linear combination of internal (Q) and basal heating, and the modified theory
predictions match our experimental results. Our results indicate that boundary layer thickness and surface
heat flux are not controlled by a local boundary layer stability condition (in agreement with the results of Moore
(2008)) and are instead strongly influenced by boundary layer interactions. Subadiabatic mantle temperature
gradients, in spherical 3-D, are well described by a vertical velocity scaling based on discrete drips as opposed
to a scaling based on coherent sinking sheets, which was found to describe 2-D planar results. Root-meansquare
(RMS) velocities are asymptotic for both low Q and high Q, with a region of rapid adjustment between
asymptotes for moderate Q. RMS velocities are highest in the low Q asymptote and decrease as internal
heating is applied. The scaling laws derived by Moore (2008), and extended here, are robust and highlight the
importance of differing boundary layer processes acting over variable Q and moderate Ra.

Weller, M. B., A. Lenardic, and W. B. Moore (2016), Scaling relationships and physics for mixed heating convection in
planetary interiors: Isoviscous spherical shells, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2016JB013247.

The Energetics and Convective Vigor of Mixedmode Heating: Velocity Scalings and Implications for the Tectonics of Exoplanets

The energetics and convective vigor of mixed-mode heating: Velocity scalings and implications for the tectonics of exoplanets
Matthew B. Weller and Adrian Lenardic

Abstract The discovery of large terrestrial (~1 Earth mass (Me) to<10Me) extrasolar planets has prompted
a debate as to the likelihood of plate tectonics on these planets. Canonical models assume classic basal
heating scaling relationships remain valid for mixed heating systems with an appropriate internal temperature
shift. Those scalings predict a rapid increase of convective velocities (Vrms) with increasing Rayleigh numbers
(Ra) and non-dimensional heating rates (Q). To test this we conduct a sweep of 3-D numerical parameter space
for mixed heating convection in isoviscous spherical shells. Our results show that while Vrms increases with
increasing thermal Ra, it does so at a slower rate than predicted by bottom heated scaling relationships.
Further, the Vrms decreases asymptotically with increasing Q. These results show that independent of specific
rheologic assumptions (e.g., viscosity formulations, water effects, and lithosphere yielding), the differing
energetics of mixed and basally heated systems can explain the discrepancy between different modeling
groups. High-temperature, or young, planets with a large contribution from internal heating will operate in
different scaling regimes compared to cooler-temperature, or older, planets that may have a larger relative
contribution from basal heating. Thus, differences in predictions as to the likelihood of plate tectonics on
exoplanets may well result from different models being more appropriate to different times in the thermal
evolution of a terrestrial planet (as opposed to different rheologic assumptions as has often been assumed).

Weller, M. B., and A. Lenardic (2016), The energetics and convective vigor of mixed-mode heating: Velocity scalings
and implications for the tectonics of exoplanets, Geophys. Res. Lett., 43, doi:10.1002/2016GL069927.