Garnet and spinel in fertile and depleted mantle: Insights from thermodynamic modelling

We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth's upper mantle, using well-established free energy minimization techniques. The model is consistent with the available experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six different bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the garnet + spinel stability field is always broad at low temperatures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/m 2 ), garnet coexists with spinel across an interval of 10-15 km, at ca. 50-70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/m 2 geotherm), the width of the garnet-spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical characterization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# [i.e. Cr/(Cr + Al)mol] isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geobarometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections. © 2013 Springer-Verlag Berlin Heidelberg.