AN IMPROVED ESTIMATE OF THE DIAMOND – GRAPHITE TRANSITION

The transition from diamond to graphite is a key equilibrium for interpreting ultrahigh pressure metamorphic rocks. Despite widespread interest, there remain significant systematic differences between experimental determinations of P and T and thermodynamic calculations of the transition. It is not possible to construct a thermodynamically consistent transition curve through the nominal P and T of the experimental brackets. A new curve that is consistent with expanded experimental brackets and the best thermophysical data is steeper than the transition proposed by Kennedy and Kennedy (K&K,1976).

Several thermodynamic treatments suggest a steeper P-T slope than the experiments. K&K provided a preferred linear equation describing the experiments in the interval 1400 K – 1900 K, which commonly has been used to illustrate the transition pressure at lower and higher T. Most thermodynamic data yield equilibrium pressures lower than the experiments by as much as 2 kbar, and, at lower temperatures, calculated pressures are systematically lower by as much as 5 kbar. Calculated pressures at 3000 K vary from more than 8 kbar above to almost 20 kbar below the position of the transition extrapolated from the experiments.

Ambiguities in both the experimental and thermophysical data contribute to the observed discrepancies. Experimental brackets were expanded by ± 25 K and ± 0.5 kbar in order to account for uncertainties in the correction of thermocouple temperatures for the effect of pressure by K&K (1976). Differences in the heat capacity functions used to extrapolate to high T account for the large range of calculated P at high T. The best agreement between calorimetric data and the entropy and enthalpy of the transition extracted from the experiments requires good estimates of the T and P dependence of molar volumes, including T dependence of the bulk modulus.

A transition curve in close agreement with the calorimetric entropy and enthalpy of transition that is also consistent with the expanded experimental brackets passes through 16.3 kbar, 298 K; 34.2 kbar, 1000 K; 63.2 kbar, 2000 K; and 96.3 kbar, 3000 K.

Kennedy, C.S. and Kennedy, G.C., 1976. The equilibrium boundary between graphite and diamond. Journal of Geophysical Research, 81, 2467-2470.