Plate tectonics: A supercontinental boost

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.

A window for plate tectonics in terrestrial planet evolution?

Craig O’Neill, Adrian Lenardic, Matthew Weller, Louis Moresi, Steve Quenette, Siqi Zhang

Abstract: The tectonic regime of a planet depends critically on the contributions of basal and internal heating to the
planetary mantle, and how these evolve through time. We use viscoplastic mantle convection simulations, with evolving core–mantle boundary temperatures, and radiogenic heat decay, to explore how these factors affect tectonic regime over the lifetime of a planet. The simulations demonstrate (i) hot, mantle conditions, coming out of a magma ocean phase of evolution, can produce a ‘‘hot” stagnant-lid regime, whilst a cooler post magma ocean mantle may begin in a plate tectonic regime; (ii) planets may evolve from an initial hot stagnant-lid condition, through an episodic regime lasting 1–3 Gyr, into a plate-tectonic regime, and finally into a cold, senescent stagnant lid regime after 10 Gyr of evolution, as heat production and basal temperatures wane; and (iii) the thermal state of the post magma ocean mantle, which effectively sets the initial conditions for the sub-solidus mantle convection phase of planetary evolution, is one of the most sensitive parameters affecting planetary evolution – systems with exactly the same physical parameters may exhibit completely different tectonics depending on the initial state employed. Estimates of the early Earth’s temperatures suggest Earth may have begun in a hot stagnant lid mode, evolving into an episodic regime throughout most of the Archaean, before finally passing into a plate tectonic regime. The implication of these results is that, for many cases, plate tectonics may be a phase in planetary evolution between hot and cold stagnant states, rather than an end-member.

O’Neill, C., A. Lenardic, M. Weller, L. Moresi, and S. Quenette, A window for plate tectonics in terrestrial planet evolution, Phys. Planet. Int., 255, 80-92, 2016.

The importance of temporal stress variation and dynamic disequilibrium for the initiation of plate tectonics

V. Stamenkovic, T. Höink, and A. Lenardic

Abstract We use 1-D thermal history models and 3-D numerical experiments to study the impact
of dynamic thermal disequilibrium and large temporal variations of normal and shear stresses on the
initiation of plate tectonics. Previous models that explored plate tectonics initiation from a steady state,
single plate mode of convection concluded that normal stresses govern the initiation of plate tectonics,
which based on our 1-D model leads to plate yielding being more likely with increasing interior heat
and planet mass for a depth-dependent Byerlee yield stress. Using 3-D spherical shell mantle convection
models in an episodic regime allows us to explore larger temporal stress variations than can be addressed
by considering plate failure from a steady state stagnant lid configuration. The episodic models show that
an increase in convective mantle shear stress at the lithospheric base initiates plate failure, which leads
with our 1-D model to plate yielding being less likely with increasing interior heat and planet mass. In this
out-of-equilibrium and strongly time-dependent stress scenario, the onset of lithospheric overturn events
cannot be explained by boundary layer thickening and normal stresses alone. Our results indicate that in
order to understand the initiation of plate tectonics, one should consider the temporal variation of stresses
and dynamic disequilibrium.

Stamenkovic, V., T. Höink, and A. Lenardic (2016), The importance of temporal stress variation and dynamic disequilibrium for the initiation of plate tectonics, J. Geophys. Res. Planets, 121, doi:10.1002/2016JE004994.

The Solar System of Forking Paths: Bifurcations in Planetary Evolution and the Search for Life-Bearing Planets in Our Galaxy

A. Lenardic, J.W. Crowley, A.M. Jellinek, and M. Weller

Abstract. The presence of life, surface water, and plate tectonics makes Earth unique in our solar system. By contrast,
Venus, the planet closest to Earth in terms of bulk properties, is characterized by significantly higher surface temperature and pressure, a lack of surface water and life, and a different mode of tectonics (Grinspoon, 1997). These observations have motivated decades of studies aimed at the question of what leads to the differences between Earth and Venus. Modeling studies that address this question, as well as similar issues arising with comparisons of Earth and Mars, often assume that the profound differences between the current states of Earth and Venus reflect differences in planetary size, position to the Sun, and material parameters (e.g., strength of near-surface rock) (Fig. 1A). Models of this type are now guiding discussions related to the search for extrasolar planets that could support life. An alternative view is that the current states and tectonic regimes of Earth and Venus represent two equally possible solutions to the dynamical evolution of a planet with the same size, bulk composition, solar proximity, and material characteristics. Rather than being distinct consequences of physical or chemical differences, Earth and Venus may instead represent an inherent ‘‘bistability’’ in the dynamic state of terrestrial planets (Fig. 1B). Viewed this way, the present states of terrestrial planets can be most sensitive to potentially small variations in their geological or climatic histories, as opposed to bulk physical and chemical characteristics (i.e., bistability allows for planetary-scale bifurcations in time such that small, effectively random, fluctuations can cause the evolution paths of two planets with identical bulk properties to diverge over time). The potential for bistability and planetary-scale bifurcations in evolution paths is rarely addressed in studies of extrasolar planets but must ultimately inform the current vigorous search for habitable worlds. Indeed, the growing taxonomy of extrasolar planets may provide a ready means to characterize statistically and understand the potential for similar planetary bodies to evolve in distinct ways.

ASTROBIOLOGY Volume 16, Number 7, 2016ª Mary Ann Liebert, Inc.
DOI: 10.1089/ast.2015.1378

Climate-tectonic coupling: Variations in the mean, variations about the mean, and variations in mode

A. Lenardic, A. M. Jellinek, B. Foley, C. O’Neill, and W. B. Moore
Abstract Interactions among tectonics, volcanism, and surface weathering are critical to the long-term
climatic state of a terrestrial planet. Volcanism cycles greenhouse gasses into the atmosphere. Tectonics
creates weatherable topography, and weathering reactions draw greenhouse gasses out of the atmosphere.
Weathering depends on physical processes governed partly by surface temperature, which allows for the
potential that climate-tectonic coupling can buffer the surface conditions of a planet in a manner that allows
liquid water to exist over extended timescales (a condition that allows a planet to be habitable by life as we
know it). We discuss modeling efforts to explore the level to which climate-tectonic coupling can or cannot
regulate the surface temperature of a planet over geologic time. Thematically, we focus on how coupled
climate-tectonic systems respond to the following: (1) changes in the mean pace of tectonics and associated
variations in mantle melting and volcanism, (2) large-amplitude fluctuations about mean properties such as
mantle temperature and surface plate velocities, and (3) changes in tectonic mode.We consider models that
map the conditions under which plate tectonics can or cannot provide climate buffering as well as models
that explore the potential that alternate tectonic modes can provide a level of climate buffering that allows
liquid water to be present at a planet’s surface over geological timescales. We also discuss the possibility
that changes in the long-term climate state of a planet can feedback into the coupled system and initiate
changes in tectonic mode.

Lenardic, A., M. Jellinek, B. Foley, C. O’Neill, and W. B. Moore (2016),
Climate-tectonic coupling: Variations in the mean, variations about the mean, and variations in mode, J. Geophys. Res. Planets, 121, doi:10.1002/2016JE005089.

Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere

Yeung, L. Y., L. T. Murray, J. L. Ash, E. D. Young, K. A. Boering, E. L. Atlas, S. M. Schauffler, R. A. Lueb, R. L. Langenfelds, P. B. Krummel, L. P. Steele, and S. D. Eastham, “Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere,” J. Geophys. Res. Atmos. 121 (2016) doi: 10.1002/2016JD025455.

JGR Editor’s highlight:

Yeung et al report novel observations of the oxygen isotopes of O2 that provide information about the evolution of ozone in Earth’s atmosphere. The measurements span from near the surface to 33 km, and both a box model and 3D chemical transport model help indicate where and how isotopic signatures are reset. Such a proxy will be helpful to chemistry-climate models investigating the evolution of ozone over time, and as they have shown it can be interpreted using models without needing to incorporate the isotope tracers into the models themselves.

Abstract
The distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as Δ36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped-isotope budget of O2: Stratospheric air has higher Δ36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower Δ36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-Δ36 air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where Δ36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in Δ36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a ±10% range since 1978.

Effect of melt composition on crustal carbonate assimilation: Implications for the transition from calcite consumption to skarnification and associated CO2 degassing

Laura B. Carter, Rajdeep Dasgupta

Carter, L. B., and R. Dasgupta (2016), Effect of melt composition on crustal carbonate assimilation: Implications for the transition from calcite consumption to skarnification and associated CO2degassing, Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006444.

Abstract

Skarns are residue of relatively low-temperature magma-induced decarbonation in the crust largely associated with silicic plutons. Mafic magmatic intrusions are also capable of releasing excess CO2 due to carbonate assimilation. However, the effect of mafic to silicic melt evolution on the decarbonation processes, in addition to temperature controls on carbonate-intrusive magmatic systems, particularly at continental arcs, remains unclear. In this study, experiments performed in a piston cylinder apparatus at midcrustal depth (0.5 GPa) at supersolidus temperatures (900–1200°C) document calcite interaction with andesite and dacite melts at equilibrium under closed-system conditions at calcite saturation in a 1:1 melt-calcite ratio by weight. With increasing silica content in the starting melt, at similar melt fractions and identical pressure, assimilation decreases drastically (≤65% andesite-calcite to ≤18% dacite-calcite). In conjunction, the CaO/SiO2 ratio in melts resulting from calcite assimilation in andesitic starting material is >1, but ≤0.3 in those formed from dacite-calcite interaction. With increasing silica-content in the starting melt skarn mineralogy, particularly wollastonite, increases in modal abundance while diopsidic clinopyroxene decreases slightly. More CO2 is released with andesite-calcite reaction (≤2.9 × 1011 g/y) than with more skarn-like dacite-calcite interaction (≤8.1 × 1010g/y, at one volcano assuming respective calcite-free-superliquidus conditions and a magma flux of 1012 g/y). Our experimental results thus suggest that calcite assimilation in more mafic magmas may have first degassed a significant amount of crustal carbon before the melt evolves to more silicic compositions, producing skarn. Crustal decarbonation in long-lived magmatic systems may hence deliver significant albeit diminishing amounts of carbon to the atmosphere and contribute to long-term climate change.