The kinetics of organic maturation in the Torres del Paine contact aureole and lessons for RSCM thermometry

Raman Spectroscopy of Carbonaceous Matter (RSCM) thermometry is a widely utilized method for estimating peak metamorphic temperatures based on the crystallinity and composition of carbonaceous matter (CM). In this study, we apply RSCM thermometry to samples from the contact aureole of the Torres del Paine Intrusive Complex (TPIC) and compare these temperature estimates with constraints from phase petrology and thermal modeling. While Raman spectra reveal a systematic increase in CM crystallinity toward the intrusion, peak temperatures estimated with RSCM are consistently lower than the modeled temperatures in the outer and middle part of the aureole, while both approaches give consistent temperatures close to the contact.

2-D thermal models suggest that the heating of the metasediments occurred over 2,000–10,000 years, while cooling occurs over some tens of thousands of years, depending on proximity to the intrusion, as well as the relative position of the sample with respect the intrusion (roof, side, or below). A review of contact metamorphic studies conducted around different sizes of intrusions underscores the critical role of the heating pulse duration.  Hence, RSCM thermometry needs to take into account the thermal history, as was similarly shown for sedimentary basins and Anchizonal metamorphism (Sweeney and Burnham, 1990).

To address this limitation, we extend a first-order kinetic model originally developed for vitrinite reflectance by Sweeney and Burnham (1990) to describe CM crystallinity evolution by considering a total set of 34 parallel reactions that includes 14 new reactions with higher activation energies to account for the reactions that dominate maturation at higher temperatures, responsible for transforming amorphous CM into graphite. Our new kinetic model was calibrated by assuming that the maturations described by the thermometer for regional metamorphism were obtained by maintaining the temperature at the peak metamorphic temperature for at least 1Myr.  Using this approach we find a better fit for maximum temperatures obtained in the Torres del Paine contact aureole. Our results highlight the importance of kinetic effects for RSCM thermometry. Hence, we strongly advise against the use of the RSCM thermometer for short-lived thermal pulses, unless an approximate knowledge of the temperature-time history is known. 

References

Sweeney, J.J. and Burnham, A.K. (1990) ‘Evaluation of a Simple Model of Vitrinite Reflectance Based on Chemical Kinetics’, AAPG Bulletin, 74(10), pp. 1559–1570. Available at: https://doi.org/10.1306/0C9B251F-1710-11D7-8645000102C1865D.