Impact of Inconsistencies in Experimental Thermodynamic Data on Thermophysical Properties of Planetary Mantles

A new thermodynamic analysis has been performed on experimental thermophysical and phase diagram data of (Mg,Fe)_{2SiO_4} olivine, wadsleyite and ringwoodite solid solutions. The analysis demonstrates that large inconsistencies exist in the V- P- T data of wadsleyite and ringwoodite. It has been suggested in recent literature that a hydration effect is able to explain the large differences in volume measured by several independent groups of investigators [1,2]. However, this hydration effect does not explain the combination of a large measured volume associated with a large measured bulk modulus for a number of experimental V- P- T data sets [3-7]. We show the effects of the inconsistencies on the calculated phase diagram, bulk sound velocities and other thermodynamic properties. We have applied our thermodynamic analysis to iron rich compositions at pressure/temperature/iron content conditions representative for the mantles of Earth and Mars. For these conditions a strong compositional effect on thermodynamic properties in two- phase regions is observed from our thermodynamic model. This compositional effect is associated with the slopes of two- phase boundaries in pressure- composition and temperature- composition phase diagrams leading to a change up to 100% or more for specific thermodynamic properties thermal expansivity α , specific heat c_P and bulk modulus k_S. The amplitude of the anomalies increases with iron content larger than 10%. These anomalous two-phase zones, where olivine transforms to ringwoodite via wadsleyite, cover a pressure range of about 5 GPa. In the Earth's mantle transition zone these two-phase zones therefore occupy a depth range of some 150 km and the impact of these strong variations in α and c_P on mantle dynamics may be limited. Planet Mars with its weaker gravity field and reduced pressure gradient is an environment more susceptible to the impact of these two-phase compositional effects, even more so since the iron content of the Martian mantle is likely about twice as high as for Earth's mantle. We will show preliminary results of convection modeling including these effects for both Earth and Mars like conditions. Putting the variations in thermophysical properties in perspective we can say that at conditions prevailing in the Martian mantle, the V- P- T inconsistencies affect thermodynamic properties in two- phase regions by about 7%. This is substantially less than the mentioned variations up to 100%, due to the composition effect. On the other hand differences between properties computed with several modern thermodynamic databases amount to about 30%. [1] Smyth JR, Holl CM, Frost DJ, Jacobsen SD, Langenhorst F, McCammon CA (2003), Am. Miner. 88, 1402-1407. [2] Inoue T, Tanimoto Y, Irifune T, Suzuki T, Fukui H and Ohtaka O (2004), Physics Earth Planet. Int., in press. [3] Hazen RM Zhang J and Ko J (1990), Phys. Chem. Miner., 17, 416-419. [4] Hazen RM, Downs RT, Finger LW and Ko J (1993), Am. Miner. 78, 1320-1323. [5] Fei Y, Mao HK, Shu J, Parthasarathy, Bassett WA and Ko J (1992), J. Geophys. Res. 97, 4489-4495. [6] Meng Y et al., J. Geophys. Res. 98, 22199-22207. [7] Meng Y, Fei Y, Weidner DJ, Gwanmesia GD and Hu J (1994), Phys. Chem. Minerals 21, 407-412.