Normalized Homologous Temperature of Olivine: Implications for the Effect of Iron Content on Mantle Deforamtio

The flow laws and fabric transition of olivine under different thermal-mechanical conditions are generally obtained from HP-HT experiments on San Carlos olivine (Fo90), which hampers our understanding of the effect of iron content on the mantle flow and deforamtion. Because the melting temperature is related with the strength of the bonds binding a crystalline material together, the normalized homologous temperature T/Tm(P), is defined by the ratio between the absolute temperature of olivine and its melting point Tm at certain pressure P. Using the forsterite-fayalite melting loop at room pressure, the generalized mean is adopted to predict the dependence of Tm on pressure and iron content of olivine up to 7.5 GPa. Using T/Tm(P), we can evaluate the integreted effect of temperature and pressure on the deformation mechanisms of olivine. A new olivine fabric diagram is set up as a function T/Tm(P) and water content. The result indicates that the A-type olivine fabric becomes dominant at T/Tm(P) > 0.72, while the B- and C-type olivine fabrics are favorable at T/Tm(P) < 0.7, no matter their water content. Partial melting will produce resudies with higher Fo number in olivine, and consequently, a more rigid mantle with higher Tm. The relatively smaller T/Tm(P) of olivine beneath cratons can explain the long-term stability of the continental roots. In addition, a 2D profile of olivine T/Tm(P) for the oceanic lithosphere shows good correlation between T/Tm(P) and the distribution of earthquakes. Therefore T/Tm(P) of olivine can be used as a parameter to model the rheology of the upper manlte, especailly when taking into account of the compostional change in different tectonic processes.