James Connolly

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Connolly

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James

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0000-0001-5060-4281

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Publications 1 - 10 of 136

Incorporating self-consistently calculated mineral physics into thermochemical mantle convection simulations in a 3-D spherical shell and its influence on seismic anomalies in Earth's mantle
Nakagawa, Takashi; Tackley, Paul J.; Deschamps, Frédéric; et al. (2009)
Geochemistry, Geophysics, Geosystems
Phase assemblages of mantle rocks calculated from the ratios of five oxides (CaO-FeO-MgO-Al₂O₃-SiO₂) by free energy minimization were used to calculate the material properties density, thermal expansivity, specific heat capacity, and seismic velocity as a function of temperature, pressure, and composition, which were incorporated into a numerical thermochemical mantle convection model in a 3-D spherical shell. The advantage of using such an approach is that thermodynamic parameters are included implicitly and self-consistently, obviating the need for ad hoc parameterizations of phase transitions which can be complex in regions such as the transition zone particularly if compositional variations are taken into account. Convective planforms for isochemical and thermochemical cases are, however, not much different from those computed using our previous, simple parameterized reference state, which means that our previous results are robust in this respect. The spectrum and amplitude of seismic velocity anomalies obtained using the self-consistently calculated material properties are more “realistic” than those obtained when seismic velocity is linearly dependent on temperature and composition because elastic properties are dependent on phase relationship of mantle minerals, in other words, pressure and temperature. In all cases, the spectra are dominated by long wavelengths (spherical harmonic degree 1 to 2), similar or even longer wavelength than seismic tomographic models of Earth, which is probably due to self-consistent plate tectonics and depth-dependent viscosity. In conclusion, this combined approach of mantle convection and self-consistently calculated mineral physics is a powerful and useful technique for predicting thermal-chemical-phase structures in Earth's mantle. However, because of uncertainties in various parameters, there are still some shortcomings in the treatment of the postperovskite phase transition. Additionally, transport properties such as thermal conductivity and viscosity are not calculated by this treatment and are thus subject to the usual uncertainties.
Melting of siderite to 20GPa and thermodynamic properties of FeCO3-melt
Kang, Nathan; Schmidt, Max; Poli, Stefan; et al. (2015)
Chemical Geology
Thermodynamic Geodynamics: The Power of Volume Change
Regenauer-Lieb, Klaus; Morra, Gabriele; Ruepke, Lars; et al. (2005)
EOS
An experimental study of the role of shear deformation on partial melting of a synthetic metapelite
Tumarkina, Elizaveta; Misra, Santanu; Burlini, Luigi; et al. (2011)
Tectonophysics
Mineral reactions and melt generation in Metapelites: Insights from torsion experiments and thermodynamical modelling
Tumarkina, Elizaveta; Misra, Santanu; Burlini, Luigi; et al. (2009)
Geochimica et Cosmochimica Acta
Mapping the Earth's thermochemical and anisotropic structure using global surface wave data
Khan, Amir; Boschi, Lapo; Connolly, James (2011)
Journal of Geophysical Research: Solid Earth
We have inverted global fundamental mode and higher-order Love and Rayleigh wave dispersion data jointly, to find global maps of temperature, composition, and radial seismic anisotropy of the Earth's mantle as well as their uncertainties via a stochastic sampling-based approach. We apply a self-consistent thermodynamic method to systematically compute phase equilibria and physical properties (P and S wave velocity, density) that depend only on composition (in the Na2-CaO-FeO-MgO-Al2O3-SiO2 model system), pressure, and temperature. Our 3-D maps are defined horizontally by 27 different tectonic regions and vertically by a number of layers. We find thermochemical differences between oceans and continents to extend down to ∼250 km depth, with continents and cratons appearing chemically depleted (high magnesium number (Mg #) and Mg/Si ratio) and colder (>100°C) relative to oceans, while young oceanic lithosphere is hotter than its intermediate age and old counterparts. We find what appears to be strong radial S wave anisotropy in the upper mantle down to ∼200 km, while there seems to be little evidence for shear anisotropy at greater depths. At and beneath the transition zone, 3-D heterogeneity is likely uncorrelated with surface tectonics; as a result, our tectonics-based parameterization is tenuous. Despite this weakness, constraints on the gross average thermochemical and anisotropic structure to ∼1300 km depth can be inferred, which appear to indicate that the compositions of the upper (low Mg# and high Mg/Si ratio) and lower mantle (high Mg# and low Mg/Si ratio) might possibly be distinct.
A geophysical perspective on the bulk composition of Mars
Khan, Amir; Liebske, Christian; Rozel, Antoine; et al. (2018)
Journal of Geophysical Research: Planets
We invert the Martian tidal response and mean mass and moment of inertia for chemical composition, thermal state, and interior structure. The inversion combines phase equilibrium computations with a laboratory-based viscoelastic dissipation model. The rheological model, which is based on measurements of anhydrous and melt-free olivine, is both temperature and grain size sensitive and imposes strong constraints on interior structure. The bottom of the lithosphere, defined as the location where the conductive geotherm meets the mantle adiabat, occurs deep within the upper mantle (∼200–400 km depth) resulting in apparent upper mantle low-velocity zones. Assuming an Fe-FeS core, our results indicate (1) a mantle with a Mg# (molar Mg/Mg+Fe) of ∼0.75 in agreement with earlier geochemical estimates based on analysis of Martian meteorites; (2) absence of bridgmanite- and ferropericlase-dominated basal layer; (3) core compositions (15–18.5 wt% S), core radii (1,730–1,840 km), and core-mantle boundary temperatures (1620–1690°C) that, together with the eutectic-like core compositions, suggest that the core is liquid; and (4) bulk Martian compositions with a Fe/Si (weight ratio) of 1.66–1.81. We show that the inversion results can be used in tandem with geodynamic simulations to identify plausible geodynamic scenarios and parameters. Specifically, we find that the inversion results are largely reproducible by stagnant lid convection models for a range of initial viscosities (∼10¹⁸–10²⁰ Pa s) and radioactive element partitioning between crust and mantle around 0.01–0.1. The geodynamic models predict a mean surface heat flow between 15 and 25 mW/m².
On mantle chemical and thermal heterogeneities and anisotropy as mapped by inversion of global surface wave data
Khan, A.; Boschi, L.; Connolly, James (2009)
Journal of Geophysical Research: Solid Earth
We invert global observations of fundamental and higher-order Love and Rayleigh surface wave dispersion data jointly at selected locations for 1-D radial profiles of Earth's mantle composition, thermal state, and anisotropic structure using a stochastic sampling algorithm. Considering mantle compositions as equilibrium assemblages of basalt and harzburgite, we employ a self-consistent thermodynamic method to compute their phase equilibria and bulk physical properties (P, S wave velocity and density). Combining these with locally varying anisotropy profiles, we determine anisotropic P and S wave velocities to calculate dispersion curves for comparison with observations. Models fitting data within uncertainties provide us with a range of profiles of composition, temperature, and anisotropy. This methodology presents an important complement to conventional seismic tomography methods. Our results indicate radial and lateral gradients in basalt fraction, with basalt depletion in the upper and enrichment of the upper part of the lower mantle, in agreement with results from geodynamical calculations, melting processes at mid-ocean ridges, and subduction of chemically stratified lithosphere. Compared with preliminary reference Earth model (PREM) and seismic tomography models, our velocity models are generally faster in the upper transition zone (TZ) and slower in the lower TZ, implying a steeper velocity gradient. While less dense than PREM, density gradients in the TZ are also steeper. Mantle geotherms are generally adiabatic in the TZ, whereas in the upper part of the lower mantle, stronger lateral variations are observed. The retrieved anisotropy structure agrees with previous studies indicating positive as well as laterally varying upper mantle anisotropy, while there is little evidence for anisotropy in and below the TZ.
Thermodynamic constraints on devolatilization of variably altered oceanic ultramafic rocks during subduction: Implications for subarc mantle oxidation
Peng, Weigang; Evans, Katy A.; Connolly, James; et al. (2025)
Earth and Planetary Science Letters
Fluids released by devolatilization of subducted serpentinites at subarc depths trigger partial melting of the overlying mantle wedge and contribute to arc magmatism. The subarc mantle is more oxidized relative to the oceanic mantle, but the potential role of fluids derived from serpentinites during subduction in this oxidation remains contentious. Here, we compile bulk compositions of variably altered oceanic ultramafic rocks at slow- to ultraslow-spreading mid-ocean ridges worldwide, including partially and completely serpentinized ultramafic rocks, carbonate-bearing serpentinites (referred to as ophicarbonates), and talc-altered serpentinites. Using thermodynamic modeling, we quantify the oxygen fugacity (fO₂) of fluids produced during breakdown of antigorite, chlorite, and talc, which are the major water carriers in these lithologies under subarc conditions, along typical subduction geotherms. Results show that the redox states of the rocks prior to subduction play an important role in the fO₂ of the deep-released fluids and that the subduction geotherms play a minor role. Partially and completely serpentinized ultramafic rocks and ophicarbonates with initial Fe³⁺/Fe$_{total}$ ratios of 0.45, 0.84, and 0.78 generate fluids with increasing fO₂ to 2.4–2.8, 3.7–4.0, and 3.0–3.4 log units above the fayalite–magnetite–quartz (FMQ) buffer, respectively, during antigorite dehydration, which remains almost constant during chlorite dehydration. These calculations, combined with previous experimental and modeling results, suggest that oxidized fluids are liberated through antigorite and chlorite breakdown in subducted serpentinites, and the fluid fO₂ may be positively linked to the initial bulk Fe³⁺/Fe$_{total}$ ratios. Inconsistency occurs between the modeling results and the sample-based study, given that the transformation of S-bearing phases in natural rocks is likely more complicated. In contrast, talc-altered serpentinites have relatively low Fe³⁺/Fe$_{total}$ ratios and total Fe contents, and fluids characterized by decreasing fO₂ to ∼0.5 log units below the FMQ buffer can be produced during primary devolatilization. Quantification of fluid-mediated mass transfer indicates that dehydration of antigorite and chlorite in partially serpentinized ultramafic rocks in subducted oceanic slabs can oxidize the subarc mantle on typical subduction timescales, particularly along the cold to intermediate geotherms.
Modeling open system metamorphic decarbonation of subducting slabs
Gorman, P. J.; Kerrick, Derrill M.; Connolly, James (2006)
Geochemistry, Geophysics, Geosystems
Fluids derived from the devolatilization of subducting slabs play a critical role in the melting of the mantle wedge and global geochemical cycles. However, in spite of evidence for the existence and mobility of an aqueous fluid phase during subduction metamorphism, the effect of this fluid on decarbonation reactions in subducting lithologies remains largely unquantified. In this study we present results from thermodynamic modeling of metamorphic devolatilization of subducted lithologies for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach in which fluid flow is an intrinsic component of the chemical model offers an alternative to closed system models of subduction zone decarbonation. In general, our models simulating pervasive fluid flow in subducting lithologies predict CO₂ fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H₂O-rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO₂ is returned to the deep mantle at convergent margins, particularly in cool and intermediate subduction zones. Our results demonstrate that for most subduction zones, a significant proportion of the CO₂ derived from the slab is lost beneath the fore arc, and therefore CO₂ flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab-derived CO₂ flux for individual margins. Nevertheless, our predicted global slab-derived CO₂ flux from convergent margins of 0.35–3.12 × 10¹² mols CO₂/yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab-derived CO₂ flux is significantly less than atmospheric CO₂ drawdown by chemical weathering, significant CO₂ emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.

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James Connolly

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