We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.
Diopside is one of the most important end-members of clinopyroxene, which is an abundant mineral in upper-mantle petrologic models. The amount of clinopyroxene in upper-mantle pyrolite can be ∼15 vol%, while pyroxenite can contain as high as ∼60 vol% clinopyroxene. Knowing the elastic properties of the upper-mantle diopside at high pressure-temperature conditions is essential for constraining the chemical composition and interpreting seismic observations of region. Here we have measured the single-crystal elasticity of Fe-enriched diopside (Di80Hd20, Di-diopside, and Hd-hedenbergite; also called Fe-enriched clinopyroxene) at high-pressure conditions up to 18.5 GPa by using in situ Brillouin light-scattering spectroscopy (BLS) and synchrotron X-ray diffraction in a diamond-anvil cell. Our experimental results were used in evaluating the effects of pressure and Fe substitution on the full single-crystal elastic moduli across the Di-Hd solid-solution series to better understand the seismic velocity profiles of the upper mantle. Using the third- or fourth-order Eulerian finite-strain equations of state to model the elasticity data, the derived aggregate adiabatic bulk and shear moduli (KS0, G0) at ambient conditions were determined to be 117(2) and 70(1) GPa, respectively. The first- and second-pressure derivatives of bulk and shear moduli at 300 K were (∂KS/∂P)T = 5.0(2), (∂2KS/∂P2)T = –0.12(4) GPa−1 and (∂G/∂P)T = 1.72(9), (∂2G/∂P2)T = –0.05(2) GPa−1, respectively. A comparison of our results with previous studies on end-member diopside and hedenbergite in the literatures shows systematic linear correlations between the Fe composition and single-crystal elastic moduli. An addition of 20 mol% Fe in diopside increases KS0 by ∼1.7% (∼2 GPa) and reduces G0 by ∼4.1% (∼3 GPa), but has a negligible effect on the pressure derivatives of the bulk and shear moduli within experimental uncertainties. In addition, our modeling results show that substitution of 20 mol% Fe in diopside can reduce VP and VS by ∼1.8% and ∼3.5%, respectively, along both an expected normal mantle geotherm and a representative cold subducted slab geotherm. Furthermore, the modeling results show that the VP and VS profiles of Fe-enriched pyroxenite along the cold subducted slab geotherm are ∼3.2% and ∼2.5% lower than AK135 model at 400 km depth, respectively. Finally, we propose that the presence of Fe-enriched pyroxenite (including Fe-enriched clinopyroxene, Fe-enriched orthopyroxene, and Feenriched olivine), can be an effective mechanism to cause low-velocity anomalies in the upper mantle regions atop the 410 km discontinuity at cold subudcted slab conditions.
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 RuO2 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 RuO2 to 19.4 GPa and 1473 K with which we have determined phase relations of the RuO2 phases and derived thermal equations of state (EoS) parameters for both Ru and RuO2. Rutile-structured RuO2 was found to undergo two phase transformations, first at ~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 RuO2 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 + O2 = RuO2 equilibrium, which we have parameterized as a single polynomial across the stability fields of all three phases of RuO2. The expression is log10fO2(Ru – RuO2) = (7.782 – 0.00996P + 0.001932P2 – 3.76 × 10–5P3) + (–13 763 + 592P – 3.955P2)/T + (–1.05 × 106 – 4622P)/T2, 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 fO2 is shown to be up to 1 log unit lower than estimates that use previous expressions or ignore EoS terms.