The Activity of Chromite in Multicomponent Spinels: An Experimental Study with Implications for the Metamorphic History of Equilibrated Ordinary Chondrites

An experimental technique for evaluating the activity of chromite in multicomponent spinels was developed by equilibrating the spinel of interest with a Pt-alloy under a controlled temperature and oxygen fugacity. The thermodynamic properties of the ternary Pt-Fe-Cr system was evaluated, such that activities of Cr and Fe in an equilibrated Pt-alloy can be used to calculate the activity of chromite in the spinel of interest. The ternary activity model formulation used is based on the characterization of each bounding binary system, Pt-Fe, Pt-Cr, Fe-Cr, with the addition of ternary interaction terms. The Pt-Fe and Pt-Cr systems are described as asymmetric regular solutions with interaction parameters of W_(PtFe)=-138.0±3.3, W_(FePt)=-90.8±24.0, and W_(PtCr)=-129.1±1.2, W_(CrPt)=-80.9±4.4, and D_(PtCr)=+94.4±2.5 kJ/mol (1σ), respectively. Combined with literature thermodynamic properties for the Fe-Cr system, the ternary interaction parameters in the Pt-Fe-Cr system were found to be C_(Cr)=0, C_(Pt)=+ll5.7, and C_(Fe)=-68.6 kJ/mol. Using this technique, the metamorphic history of equilibrated ordinary chondrites was evaluated by examining the compositions and textures of olivines, pyroxenes, spinels, and alloys. Equilibrium temperatures based on Fe-Mg exchange between olivine and spinel exhibit a range of 680-796°C in H, L, and LL ordinary chondrites spanning petrographic type 4 to 6. Type 4 chondrites in all groups record variable temperatures that are lower than or equal to those of types 5 and 6 chondrites, implying decoupling of metamorphic temperature from petrographic type. Cooling rates near 800°C were found to be 1-3 K/Ma, slow enough to allow continous re-equilibration of spinel grains from peak metamorphic temperatures to the olivine-spinel equilibration temperature. The temperature-oxygen fugacity relationships in equilibrated H chondrites were constrained from spinel-alloy and olivine-pyroxene-alloy phase assemblages based on the mineral compositions and activity-composition models. Log_(10)f_(02) values based on the assemblage olivine-pyroxene-alloy are -1.75±0.02 log units below Iron-Wüstite (IW) buffer, regardless of petrographic type. The log_(10)f_(02) values calculated based on the spinel-alloy coexistence are at least ~1.5 log units more oxidizing than those based on the olivine-pyroxene-alloy if olivine-spinel equilibration temperatures (728-820°C) are assumed. This probably indicates that closure for spinel-alloy equilibria occurred under retrograde conditions at temperatures below 700°C.</p

The activity of chromite in multicomponent spinels: Implications for T-fO_2 conditions of equilibrated H chondrites

Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt-alloys with spinel at known temperature and fO_2. Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T-fO_2 conditions for the H-group chondrites Avanhandava (H4), Allegan (H5), and Guareña (H6). Log10fO2 values based on the assemblage of olivine + orthopyroxene + metal are 2.19–2.56 log units below the iron-wüstite (IW) buffer for any equilibration temperature between 740 and 990 °C, regardless of petrographic type. Only lower limits on fO_2 could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (≤3 ppb). Log10fO_2 values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine-spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ∼625 °C for Allegan and Guareña and below ∼660 °C for Avanhandava

Thermodynamic properties of the Pt-Fe system

We determined activity-composition relationships for the Pt-Fe system by equilibrating Fe-oxides with Pt-Fe alloys at temperatures in the range of 1200–1400 °C and oxygen fugacities from 1.6 to 7.7 log units above the iron-wüstite (IW) buffer. The system is characterized by strong negative deviations from ideality throughout the investigated temperature range (e.g., γ^(alloy)_Fe <0.02 for X^(alloy)_Fe <0.3). Our data are consistent with an asymmetric regular solution of the form: RTln γ^(alloy)_Fe = [W_(G1)+2(W_(G2)-W_(G1))X^(alloy)_Fe](X^(alloy)_(Pt))^2 where W_(G1) = –138.0 ± 3.3 kJ/mol and W_(G2) = –90.8 ± 24.0 kJ/mol (1σ). Based on experiments at 1200–1400 °C, variations in the activity coefficients at a given composition are consistent with ln γ^(alloy)_Fe(T_1)/ln γ^(alloy)_Fe(T_2)=T_2/T_1. The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from: log_(10)f_(O_2) = log{exp[lna^(liq)_(Fe2SiO4) - lna^(liq)_Si)2 - 2 lna^(alloy)_(Fe) -(-ΔG^(0)_(r)/RT)]} where ΔG^(0)_(r) is the standard state free energy for the reaction 2Fe^(alloy) + O_2 + SiO^(liq)_2 = Fe_(2)SiO^(liq)_4. We obtained values of a^(alloy)_(Fe) from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO_2 and Fe_(2)SiO_4 components in the liquid and ΔG^(0)_(r). We provide sample calculations showing how to predict the optimum Fe concentrations for pre-saturation of Pt-bearing containers to reduce Fe loss from the charge during experiments on magmatic liquids at high temperatures and pressures from 1 atm to 40 kbar

Experimental determination of the activity of chromite in multicomponent spinels

We determined activity–composition relationships in Pt-Cr and Pt-Fe-Cr alloys at 1300°C experimentally and used the results to constrain the thermodynamic properties of chromite–picrochromite spinels. The Pt-Cr binary is characterized by strong negative deviations from ideality throughout the investigated composition range and the activity–composition relationship can be fit by a four-suffix asymmetric regular solution with three binary interaction parameters. The ternary alloy was modeled as a four-suffix asymmetric regular solution; the three ternary interaction parameters in this model were constrained by combining interaction parameters for the three bounding binaries taken from this and previous work with results for a set of experiments in which the activity of Cr in Pt-Fe-Cr-alloys was fixed by coexisting Cr_(2)O_3 at known fO_2. The free energy of formation of FeCr_(2)O_4 at 1300°C was determined using the activities of Fe and Cr in Pt-alloys in equilibrium with oxide mixes of FeCr_(2)O_4 and Cr_(2)O_3. The free energy of formation of chromite from Fe+Cr_(2)O_3+O_2 is −202.7 ± 0.4 kJ/mol (1σ), indistinguishable from literature values. The corresponding free energy of formation of FeCr_(2)O_4 from the elements is −923.5 ± 2.1 kJ/mol (1σ), and the enthalpy of formation at 298 K is −1438 kJ/mol. The activity–composition relationship for the chromite component in (Fe,Mg)Cr_(2)O_4 solid solutions was determined from a set of experiments in which Pt-alloys were equilibrated with spinel + Cr_(2)O_3. (Fe,Mg)Cr_(2)O_4 spinels are nearly ideal at 1300°C; modeling our data with a one-site symmetric regular solution yields an interaction parameter of +2.14 ± 0.62 kJ/mol (1σ), similar to values based on data from the literature

The activity of chromite in multicomponent spinels: Implications for T-fO2 conditions of equilibrated H chondrites

Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt-alloys with spinel at known temperature and fO2. Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T-fO2 conditions for the Hgroup chondrites Avanhandava (H4), Allegan (H5), and Guarea (H6). Log10fO2 values based on the assemblage of olivine + orthopyroxene + metal are 2.19-2.56 log units below the iron-wstite (IW) buffer for any equilibration temperature between 740 and 990 degrees C, regardless of petrographic type. Only lower limits on fO2 could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (3 ppb). Log10fO2 values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine-spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ~625 degrees C for Allegan and Guarea and below ~660 degrees C for Avanhandava.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202