Geobarometry, phase relations and elasticity of eclogite under conditions of Earth’s upper mantle
Eclogite rocks, composed mainly of garnet and clinopyroxene, form principally as a metamorphic product of oceanic crust as it undergoes subduction. The equilibrium between garnet and clinopyroxene is sensitive to temperature and pressure, therefore eclogitic outcrops and xenoliths can reveal important information on the chemical and mineralogical processes occurring during such episodes. This is particularly the case for lithospheric eclogitic xenoliths from Archean cratons, which can potential reveal information on ancient (> 1 Ga) subduction events. To obtain information on the depth of origin of eclogitic xenoliths the first project of this thesis was designed to establish a thermodynamically grounded geobarometer, which is based on the incorporation of tetrahedrally coordinated aluminum in clinopyroxene coexisting with garnet: 2/3 Ca3Al2Si3O12 + 1/3 Mg3Al2Si3O12 = CaAl2SiO6 + CaMgSi2O6 Grossular Pyrope CaTs Diopside The reaction was calibrated against high-pressure and high-temperature experiments carried out in the multi-anvil and piston-cylinder apparatus between pressures of 3 and 7 GPa and temperatures from 1200 to 1550 °C. Starting materials were hydrous and anhydrous synthetic mixtures of basaltic bulk compositions that yielded homogeneous bimineralic garnet-clinopyroxene phase assemblages. The experimental data set was expanded by employing results from previous experimental studies conducted in eclogitic systems, which widened the range of applicable conditions and compositions. The calibration reproduces experimental pressures of bimineralic eclogite assemblages, in addition to SiO2-saturated and kyanite-bearing eclogites, to within 0.4 GPa at the 95 % confidence interval. The barometer was then used to examine equilibration pressures recorded by natural mantle eclogites from various xenolith locations covering a wide pressure, temperature, and compositional range. The results seem to indicate that many eclogite xenoliths fall towards the hotter side of the range of geothermal temperatures displayed by peridotitic xenoliths from the same localities. The second project calibrates the composition of majoritic garnets, which contain excess silicon substituted onto the octahedrally coordinated garnet site, coexisting with clinopyroxene as a function of pressure, temperature and bulk composition. Majorite substitution for a given bulk composition increases with pressure, and its proportion can in principal be used as a geobarometer. Single majoritic garnet crystals are found as inclusions in diamonds, which are generally used to support a sublithospheric origin in the deeper upper mantle or transition zone. The chemical compositions of such inclusions indicate that they have formed from a number of different lithologies, including mafic, ultramafic and pyroxenitic. These inclusions give important insight into the environment where diamonds crystallize and the evolution of deep subducted crustal material. The empirical barometer studied here is based on the three major majoritic substitutions: 2Al3+ = Mg2+ + Si4+ (Mj), Mg2+ + Al3+ = Na1+ + Si4+ (NaSi), Mg2+ + Al3+ = Na1+ + Ti4+ (NaTi), and the secondary effect of chromium on the stability of Mj. The barometer was calibrated against experiments conducted in the multi-anvil apparatus between pressures of 6 and 16 GPa and temperatures of 1000 to 1400 °C. In order to expand the applicability to a wide range of compositions experiments were performed in three different mafic compositions and in one pyroxenitic composition. Moreover, existing experimental data in mafic and ultramafic systems, including mid-ocean ridge basalts, kimberlite, komatiite and peridotite bulk compositions were included in the calibration covering pressures from 6 to 20 GPa and temperatures from 900 to 2200 °C. Applying the geobarometer to natural majoritic diamond inclusions reveals clearly that eclogitic and pyroxenitic inclusions form dominantly at conditions near the top of the transition zone (300-400 km). Peridotitic inclusions, however, have formed generally at shallower depths ~200 km within the Earth’s upper mantle. This may reflect the differences expected for the oxygen fugacity between peridotitic and eclogitic/pyroxenitic rocks. If diamonds form through the reduction of carbonate or CO2 bearing melts then most peridotitic rocks would be already within the diamond stability field by depths of 200 km. At greater pressures carbon would remain immobilized as diamond and there is no mechanism by which new diamonds can form. Eclogitic rocks formed by the subduction of oceanic crust, however, should form an intrinsically more oxidized environment that remains within the carbonate stability field to much higher pressures. The diamond stability field would be eventually reached, however, due to either the effect of pressure on controlling Fe3+/Fe2+ equilibria or due to partial melting, which would preferentially remove ferric iron and lower the Fe3+/∑Fe of the residue, on which the oxygen fugacity is mainly dependent. In fact deep partial melting as slabs heat up may be the mechanism by which both the local oxygen fugacity is lowered, carbon is mobilized and pyroxenite rocks are formed, with the latter occurring through reaction with the surrounding peridotite. The third project was focused on the elasticity of garnet solid solutions formed from eclogitic compositions. Garnet is an important mineral because it is a major phase in the upper mantle and dominates mafic rocks at these conditions. The elastic behavior of garnet solid solutions plays a role in the interpretation of seismic data and is important for estimating the density contrast between subducting slabs and the surrounding mantle. High-precision single-crystal X-ray diffraction measurements at high pressure have been conducted on three different ternary garnet solid solutions with varying chemical composition in order to examine possible non-ideal contributions to the volume and compressibility of garnet solid solutions. Furthermore one experiment has been conducted at high pressures and high temperatures to examine the effect of temperature on the elasticity of complex garnet solid solutions. Experimental results reveal that the concentration of the almandine (Fe3Al2Si3O12) component has a significant effect on the elasticity, whereby 10 – 20 mole% of almandine is sufficient to overprint the previously reported minima of the bulk modulus along the pyrope grossular join. It has also been shown that minor compositional variations of Ca and Mg within the Mg Fe Ca garnet ternary are not resolvable within the analytical errors. Therefore, the two eclogitic samples have similar bulk moduli within the analytical uncertainties. In contrast to previous studies, no evidence was found that garnets have a K’ significantly different from 4. The high-temperature experiment revealed that the relatively small fraction of almandine in a solid solution increased the softening of garnet with temperature. Finally, the experimental volumes and calculated densities have been compared to the self-consistent thermodynamic model of Stixrude and Lithgow-Bertelloni (2005, 2011). The comparison clearly reveals that volume and elastic properties cannot be linearly interpolated as a function of composition. Moreover, it has been shown that the excess properties vary not only as a function of composition and pressure, but also as a function of temperature. The final project is focused on the partitioning of fluorine (F) between garnet, clinopyroxene, and silicate melt within eclogitic compositions. Fluorine is the most abundant halogen on Earth and plays an important role in the formation of ultrapotassic lithologies, i.e. lamproites that contain several wt.% F, in contrast to the average lithospheric mantle that contains only tens of µg/g F. The cycling and partitioning behavior of fluorine in the Earth’s mantle are not well understood. High-pressure experiments have been conducted in the multi-anvil apparatus to obtain mineral-melt partition coefficients between garnet, clinopyroxene, and coexisting silicate melt of fluorine in a mafic system under conditions of the Earth’s upper mantle. The results show that mafic crust can host significantly more fluorine than the surrounding ultramafic mantle, due to the much higher proportion of clinopyroxene and its high fluorine partition coefficient of D_F^(clinopyroxene/melt)= 0.057 – 0.074. Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting which deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.
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