Radiating cracks around chromite inclusions in olivine: constraints on P-Thistories based on the thermoelastic properties of minerals

Radiating tensional cracks around chromite inclusions in forsterite-rich ol ivine porphyroclasts have been observed in the protomylonitic rim of a post -Variscan shear zone in the Finero peridotite complex (Ivrea Zone, northern Italy). These radial fractures developed during exhumation of the peridoti te complex as a result of differences in the thermoelastic behavior of the chromite and the olivine. The formation of radial cracks in the system chromite-olivine was calculate d taking into account the contrast in thermal expansion, isothermal compres sibility and shear modulus of both phases for retrograde P-T conditions enc ountered during exhumation. The pressure range of investigation was 0.7 GPa to 3.0 GPa at temperatures up to 1200 degrees C, representing average uppe r mantle P-T conditions. Olivine fracturing depended on the inclusion entra pment temperature and pressure the slope of the retrograde P-T path was les s important. Numerical calculations yielded a set of isocrack-lines with a negative delta P/delta T in a P-T grid. The position of the whole set of is ocrack-lines on the P-T plane depended on the inclusion entrapment pressure and temperature. The calculated model can be used to estimate temperature and pressure appropriate to the following cases: (1) if independent evidenc e allows the determination of P-T conditions during crack formation, then t he inclusion entrapment conditions can be calculated, (2) if the inclusion entrapment conditions and the retrograde path are known, the P-T conditions of crack formation can be determined. For the Finero peridotite complex the dynamic recrystallization of olivine and the growth of antigorite + tremolite during a late metamorphism Limited the lowermost P-T conditions for fracturing to 0.35 GPa and 550 degrees C. These values correspond to minimum inclusion entrapment conditions of P ap proximate to 0.7 GPa and T greater than or equal to 1000 degrees C. The fol lowing exhumation history for the Finero peridotite complex is suggested: o livine and chromite crystallized at depths from 24 km to 30 km at temperatu res from 1000 to 1200 degrees C. During exhumation, high temperature sheari ng started under granulite facies P-T conditions at around 19 km depth at t emperatures from 775 to 940 degrees C. Further decrease of P and T produced the opening of radial cracks around chromite inclusions in olivine at more than 10 km depth and T greater than or equal to 550 degrees C depending on the inclusion entrapment conditions.