Program Gibbs

ReNTRs and ReERs

What are ReNTRs and ReERs?

• ReNTRs are retrograde net transfer reactions
• ReERs are retrograde exchange reactions
• These reactions operate to change the composition of minerals from those at the metamorphic peak. This makes it a challenge to the petrologist to decipher peak metamorphic P-T conditions using thermobarometry.
• ReNTRs can easily shift compositions so that temperatures calculated from thermobarometry are higher than the metamorphic peak
• ReERs typically shift compositions so that temperatures calculated from thermobarometry are lower than the metamorphic peak
• Operating together, ReNTRs and ReERS can, in some instances, result in compositions that, when used for thermometry, yield precisely the peak temperature, but the compositions are in no way representative of the metamorphic peak compositions.

Program Gibbs and ReNTRs and ReERs

Program Gibbs can be used to calculate the effect of ReNTRs and ReERs and thus examine the extent to which these retrograde reactions have affected peak thermobarometry. Gibbs can also be used to construct P-T contour diagrams so that the P-T paths over which a particular ReNTR will operate can be examined.

The following links are to pages that discuss specific examples of ReNTRs and ReERs

• ReNTRs in high grade migmatites from the Valhalla complex
• ReNTRs in a Himalayan metapelite
• ReNTRs in staurolite zone schists

References

• Florence, F. P. and Spear, F. S. (1993) Influences of reaction history and chemical diffusion on P-T calculations for staurolite schists from the Littleton Formation, northwestern New Hampshire. American Mineralogist, 78, 345-359.
• Kohn, M. J. and Spear, F. S. (2000) Retrograde Net Transfer Reaction Insurance for P-T Estimates. Geology, 28, 1127-1130.
• Robinson, P. (1991) The eye of the petrographer, the mind of the petrologist. American Mineralogist, 76, 1781-1810.
• Robinson, P. R., Hollocher, K. T., Tracy, R. J. and Dietsch, C. W. (1982) High grade Acadian regional metamorphism in south-central Massachusetts. In Joesten, R. and Quarrier, S., Ed., NEIGC 74th Annual Meeting, p. 289-340. 5, State Geological and Natural History Survey of Connecticut, Storrs, Connecticut.
• Spear, F. S. (1991) On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling. Journal of Metamorphic Geology, 9, 379-388.
• Spear, F. S. (1992) Thermobarometry and P-T paths from granulite facies rocks: an introduction. Journal of Precambrian Research, 55, 201-207.
• Spear, F. S. and Florence, F. P. (1992) Thermobarometry in granulites: Pitfalls and new approaches. Journal of Precambrian Research, 55, 209-241.
• Spear, F. S. and Parrish, R. (1996) Petrology and petrologic cooling rates of the Valhalla Complex, British Columbia, Canada. Journal of Petrology, 37, 733-765.

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