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Leah Ziegler, Hongluo Zhang, Colin Jackson, Matt Jackson, Dave Stegman

Chemical consequences of perovskite fractionation from an ultramafic liquid with application to the dynamics of a basal magma ocean . Leah Ziegler, Hongluo Zhang, Colin Jackson, Matt Jackson, Dave Stegman. Team BMO:. What is a basal magma ocean?.

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Leah Ziegler, Hongluo Zhang, Colin Jackson, Matt Jackson, Dave Stegman

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  1. Chemical consequences of perovskite fractionation from an ultramafic liquid with application to the dynamics of a basal magma ocean Leah Ziegler, Hongluo Zhang, Colin Jackson, Matt Jackson, Dave Stegman Team BMO:

  2. What is a basal magma ocean? - a dense melt layer that forms at the bottom of the mantle – likely linked to a whole mantle magma ocean Labrosse et al., 2007

  3. Why a basal magma ocean? -poorly understood lower mantle structures: Garnero and McNamara, 2008 -missing geochemical reservoirs: Labrosse et al., 2007

  4. How would a basal magma ocean form? w/ Fe w/o Fe -Curvature of mantle liquidus can lead to crystallization initiating at mid-mantle depths -Density crossover between liquid and solid at mid-mantle depths is possible – depends on Kd-Fe Figures: Stixrude et al., 2009

  5. Key parameters for evaluating BMO: Density of liquid and solid with crystallization • f(Xliq, Xsol, XU+Th+K, time) Concentration of REE and heat producers (U+Th) associated with BMO products • Both Require: - major element partition coefficients (Kd) - trace element partition coefficients

  6. Step 1: Compile Kd database for Mg-Pv, Ca-Pv, & Fe-MgO -Database compiled at CIDER 2012 -Details of database: Total number of studies: 19 Experiments including -- Mg-Pv: 31 -- Ca-Pv: 10 – Fe-MgO:12 Pressure Range: 23 – 86 GPa Temperature Range: 2300 – 3500°C Database will be published with initial reporting of findings

  7. Effect of Al content on the solubility of Fe in MgPv & Kd-TE right: Frost and Langenhorst, 2002 left: Liebske et al., 2005 Importance of charge coupled substitutions

  8. Step 2: Parameterize Kds for Mg-Pv Models for Mg-Pv major elements: Predicted Observed Equal weight, multiple linear regression

  9. Step 2: Parameterize Kds for Mg-Pv Models for Mg-Pv trace elements: Predicted Observed Similar TE substitutions as pyroxene

  10. Step 3: Apply Kds for Mg-PvBMO crystallization Team BMO met in October at BU: - Refined Kd parameterizations • Developed stoichiometric Mg-Pv crystallization model • Quantified uncertainties in crystallization model predictions using montecarlo approach • Accounts for covariation in model parameters • Thank you Don Forsyth

  11. Model Results Liquid evolutionwith crystallization (batch): Bulk: McDonough & Sun BSE, 40 GPa Majors: cation mol fraction, Trace: PPM*

  12. Model Results KdFewith crystallization (BSE bulk, 40 GPa): Al-bearing Al-free Results from 78.5 GPa: Andrault et al., 2012 Al + Si causing offset between Nomura and Andrault ?

  13. Model Results Removal of heat producing elements and REE from liquid (batch): High Al in MgPv makes it significant reservoir for TEs Still working to incorporate full uncertainties into TE Kds

  14. Preliminary Findings -Al partitions equally between MgPV and liquid -REE+U+Th have similar substitutions into pyx and MgPv - Al AND Ca are important -Fe is a moderately incompatible element • Fe is strongly controlled by Al, but other effects appear significant (Siliq?)

  15. Current Directions Post-AGU Team BMO Meet up: - Develop new crystallization model • Weaver and Langmuir (1990) • Working with StephaneEscrig to implement his code - stoichiometry - self-consistent saturation of phases - Predict past Fe-MgO or Ca-Pv saturation - Couple chemical model to dynamical model

  16. Crystallization Model System of Equations: -Maintains stoichiometry on Si and Mg site

  17. Effect of Al content on Kd-Fe at high P & T Al-bearing Al-free Andraultet al., 2012

  18. Kds

  19. Solid Evolution

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