Stability and structures of hydrous minerals in the transition zone of the Martian mantle
Final Report Abstract
This project aimed at experimentally determining the stability and structure of hydrous mantle silicates and the water storage capacity of the Martian interior, which is more iron-rich than Earth´s mantle. The major results achieved in this study are as follows: A) The effect of iron on the stability of dense hydrous Mg-Fe silicates The dense hydrous Mg-Fe silicates, phase D and superhydrous B show stabilities up to 1300°C at 23 GPa (phase D, MgFeSiO4 + 9.5 wt% H2O bulk composition) and for the simple Martian mantle composition up to 1450°C at 20.5 GPa (phase D and superhydrous B), which represents in both systems of this study a higher thermal stability of DHMS than previously reported. This suggests that phase D and superhydrous B may be relevant dense hydrous Mg-Fe silicates in iron-rich mantles of planetary systems. B) The effect of iron on the water content of ringwoodite The results of this study show that the water storage capacities of iron-rich ringwoodites of about 0.4-0.7 wt% H2O are considerably reduced compared to pure Mg-ringwoodite. Thus, the ringwoodite samples show an inverse correlation of iron- and water content, implying that the water storage capacity of ringwoodite decreases towards the Feendmember. The magnesium-site octahedra represent the favoured protonation site in ferroan ringwoodites corresponding to the Mg2+ = 2H+ water substitution mechanism. In addition, ferric iron diminishes the water content of ringwoodites due to the reduction of potential protonation sites. This is caused by the creation of Mg-site vacancies by the oxidation of iron and the occupation of octahedral sites by Fe-atoms, which are probably not involved in water substitution mechanism. These results indicate that less water can be stored in nominally anhydrous mantle silicates of iron-rich planetary mantles. C) The effect of iron on the compressibility of ferroan ringwoodite The physical properties of ringwoodite such as density and compressibility determine the characteristics of the lower part of the mantle transition zone e.g. compressional wave velocities. Thus, the compressibility of hydrous ferroan ringwoodite, which is more relevant for Mars, was determined using single crystal x-ray diffraction method. The bulk modulus of hydrous ferroan ringwoodites obtained in this study are very similar (KT0 = 186.5(9) and KT0 = 184.1(7) GPa for sample 3854 and 4218 respectively) and have values close to those reported for dry Mg- and Fe-endmember ringwoodite. This would suggest therefore that the oxygen closed-packing of the spinel structure plays a major role in determining the compressibility of ferroan ringwoodites. D) The water storage potential of a hydrous Martian mantle The water storage capacity of iron-rich nominally anhydrous minerals is required to understand the dynamics of planetary interiors. The water storage capacity of the Martian interior is mainly determined by the amount of water stored in olivine (~0.3 wt% H2O), wadsleyite (~0.6 wt% H2O) and ringwoodite (~1.1 wt% H2O). This implies that the water storage capacity of the Martian mantle increases from upper mantle to lower transition zone. The mineralogical structure (hydrous mantle) of planet Mars according to the areotherm of Fei and Bertka (2005) is composed of upper mantle and transition zone. A lower mantle consisting of perovskite is probably absent in the Martian interior. Therefore, the entire upper mantle and transition zone account for the water content of the Martian mantle, which would reach a concentration of 0.1 % of the Mars’ mass under saturated conditions.
Publications
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(2007) Stability of hydrous ringwoodite in the Martian mantle. Geochimica et Cosmochimica Acta 71(15), A306, supplement
Ganskow, G., Langenhorst, F., Pollok, K., Frost, D., Keppler, H.
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(2010) Effect of iron on the compressibility of hydrous ringwoodite. American Mineralogist 95(5-6), 747-753
Ganskow, G., Boffa-Ballaran, T., Langenhorst, F.
