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Equations of state, phase relations, and oxygen fugacity of the Ru-RuO (sub 2) buffer at high pressures and temperatures

Katherine Armstrong, Nicki C. Siersch, Tiziana Boffa Ballaran, Daniel J. Frost, Tony Yu and Yanbin Wang
Equations of state, phase relations, and oxygen fugacity of the Ru-RuO (sub 2) buffer at high pressures and temperatures
American Mineralogist (March 2020) 105 (3): 333-343


Experimental studies and measurements of inclusions in diamonds show that ferric iron components are increasingly stabilized with depth in the mantle. To determine the thermodynamic stability of such components, their concentration needs to be measured at known oxygen fugacities. The metal-oxide pair Ru and RuO (sub 2) are ideal as an internal oxygen fugacity buffer in high-pressure experiments. Both phases remain solid to high temperatures and react minimally with silicates, only exchanging oxygen. To calculate oxygen fugacities at high pressure and temperature, however, requires information on the phase relations and equation of state properties of the solid phases. We have made in situ synchrotron X-ray diffraction measurements in a multi-anvil press on mixtures of Ru and RuO (sub 2) to 19.4 GPa and 1473 K with which we have determined phase relations of the RuO (sub 2) phases and derived thermal equations of state (EoS) parameters for both Ru and RuO (sub 2) . Rutile-structured RuO (sub 2) was found to undergo two phase transformations, first at approximately 7 GPa to an orthorhombic structure and then above 12 GPa to a cubic structure. The phase boundary of the cubic phase was constrained for the first time at high pressure and temperature. We have derived a continuous Gibbs free energy expression for the tetragonal and orthorhombic phases of RuO (sub 2) by fitting the second-order phase transition boundary and P-V-T data for both phases, using a model based on Landau theory. The transition between the orthorhombic and cubic phases was then used along with EoS terms derived for both phases to determine a Gibbs free energy expression for the cubic phase. We have used these data to calculate the oxygen fugacity of the Ru+O (sub 2) =RuO (sub 2) equilibrium, which we have parameterized as a single polynomial across the stability fields of all three phases of RuO (sub 2) . The expression is log (sub 10) f (sub O2) (Ru-RuO (sub 2) ) = (7.782-0.00996P+0.001932P (super 2) -3.76X10 (super -5P3) ) + (-13763+592P-3.955P (super 2) )/T + (-1.05X10 (super 6) -4622P)/T (super 2) , which should be valid from room pressure up to 25 GPa and 773-2500 K, with an estimated uncertainty of 0.2 log units. Our calculated f (sub O2) is shown to be up to 1 log unit lower than estimates that use previous expressions or ignore EoS terms.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 105
Serial Issue: 3
Title: Equations of state, phase relations, and oxygen fugacity of the Ru-RuO (sub 2) buffer at high pressures and temperatures
Affiliation: University of Bayreuth, Bayerisches Geoinstitut, Bayreuth, Germany
Pages: 333-343
Published: 202003
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 61
Accession Number: 2020-034457
Categories: General geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: University of Chicago, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2020
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