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The effect of iron on the stability, water content and compressibility of mantle silicates – Implications for a hydrous Martian interior
(2010)
- Summary This thesis addresses three different effects of Fe on the properties of nominally anhydrous transition minerals and dense hydrous Mg-silicates. Furthermore, the potential presence of dense hydrous Mg-Fe silicates (DHMS) in the Martian interior and the water storage potential of a hydrous Martian mantle are evaluated. 1. The effect of iron on the water content of ringwoodite The results show that the water contents of Fe-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 Fe- and water content. The Mg-site octahedra represent the favored protonation site in ferroan ringwoodites corresponding to the Mg2+=2H+ water substitution mechanism. In addition, Fe3+ 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 Fe 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 Fe-rich planetary mantles. 2. The effect of iron on the compressibility of hydrous ferroan ringwoodite The effect of Fe on the compressibility of ringwoodite is particularly important for the interpretation of the structure of Fe-rich planetary interiors such as the Martian mantle. Measurements at ambient conditions yield to unit-cell lattice parameters of a=8.1597(6) Å and V=543.28(13) ų (run 3854) and a=8.1384(3) Å and V=539.03(7) ų (run 4218). The P-V data were fitted with a second-order Birch-Murnaghan equation of state. The first pressure derivative of the bulk modulus, K´, was fixed to the value of 4 yielding to the following refined equation of state parameters: V0=543.32(7) ų and KT0=186.5(9) GPa (run 3854) and V0=539.01(5) ų and KT0=184.1(7) GPa (run 4218). Structural refinements indicate significant octahedral vacancies in sample 4218 due to the presence of 0.1 Fe3+ a.p.f.u. and the substitution of ~0.37 wt% H2O. The values of bulk modulus of (Mg,Fe)2SiO4-ringwoodites found in this study are very similar to that of the Fe-endmember ringwoodite, which suggests that the Fe substitution has little effect on the compressibility of ringwoodite. This also indicates that the close-packing of oxygens of the spinel structure is the major factor in determining its compressibility. This cannot be affected by the presence of up to 0.1 a.p.f.u. of vacancies. 3. The effect of iron on the stability of hydrous mantle silicates The transformation pressure of olivine to wadsleyite (simple Martian mantle composition) is lowered by 2 GPa. The shifts of stability fields toward lower pressures are possibly caused by the presence of Fe3+ and H2O. The shift of the olivine-wadsleyite transition would result in an extended upper mantle transition zone, compared to the anhydrous mantle, and consequently yield an increased water storage potential of the Martian mantle. The DHMS, phase D and superhydrous B (SHyB) 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 SHyB), which represents a higher stability of DHMS than previously reported. This suggests that phase D and SHyB are relevant DHMS in Fe-rich mantles of planetary systems. 4. The potential presence of dense hydrous Mg-Fe silicates in the Martian interior and the water storage potential of a hydrous Martian mantle This experimental study of mantle silicates indicates that the Martian mantle consists basically of upper mantle with olivine, garnet and pyroxene, as well as upper and lower transition zone build up by wadsleyite and ringwoodite together with pyroxene and majoritic-garnet, respectively. The water contents of wadsleyite with 0.6 wt% H2O and ringwoodite with 1.1 wt% H2O are reduced compared to the Mg-endmembers. The Martian transition zone, however, shows the largest water storage capacity since the upper mantle mineral olivine accommodates up to 0.3 wt% H2O. Pressure and temperature conditions at the core-mantle boundary are insufficient to reach the perovskite stability field, i.e. a lower Martian mantle cannot be expected based on the results of this study. These results imply that significant amounts of water can be stored in the Martian transition zone as well as the upper mantle. In addition, DHMS would be stable up to 1450°C at 20.5 GPa in a hydrous Martian mantle model. On the basis of thermal evolution models of the Martian mantle, and Fe partitioning data between mineral phases and melt it is discussed that DHMS may form in the Martian interior at P-T conditions corresponding to the lower Martian transition zone.
