Project

Discipline

kinetics of mineral reactions; contact metamorphism; thermodynamics; thermal modelling

Lead
Lay summary
The Torres del Paine is a beautifully exposed granite and gabbro intrusion. The current project looks at the contact metamorphism, which resulted from the intrusion of the hot granitic and mafic magmas, 12.45-12.6 Ma years ago. We have established the geometry of the intrusion, the extend of the contact aureole, and we have obtained precision dates for the intrusive rocks to establish the sequence of heat pulses. This information is now being used to model the 3-D thermal evolution of the contact aureole and to model mineral reaction kinetics. Rocks from the roof, the floor and the side of the intrusion have been collected and phase petrology and mineral chemistry will be determined. This will permit to calibrate the kinetic models proposed for mineral reactions. Research is done by the PhD student Robert Bodner, who finishes his PhD by the end of this project.

Direct link to Lay Summary

Last update: 21.02.2013

Self-organised

Number	Title	Start	Funding scheme

I request a total of 10 months of salary to complete the work started on contact metamorphic reaction kinetics by Robert Bodner. The work proposed is the thermal modelling of the Torres del Paine intrusion, as well as the calibration of the kinetic parameters, which determine the crystallization kinetics of cordierite in this shallow contact aureole. The Torres del Paine intrusion and its contact aureole (Patagonia, Chile) were studied in the last grant period, since in this unique field laboratory it is possible to study the contact metamorphism in the host rocks at the top and bottom, as well on the side of the intrusion. So far we have established (1) the succession and number of magma pulses: there are 3 granitic and 3 mafic intrusions in the Torres del Paine; (2) the intrusive geometry for these principle stages; (3) the age of all but 1 pulse has been dated using high precision U/Pb TIMS dating of zircons. We have sampled the host rock on the side, top and bottom of the intrusion, and obtained crystal size distributions, established the cordierite forming reactions, and started on thermodynamic modelling of these reactions.We have shown that the granite intruded in at least three pulses over a time interval of about 80’000 - 100’000 years. While mafic magmatism started at the same time as the earliest granitic intrusion (ca. 12.6 Ma), it outlasted granite magmatism by 50’000 years. Initial stable isotope studies indicate that no significant amount of fluid flow occurred during cooling of the intrusion, though additional stable isotope data will be obtained. Conductive cooling seems to dominate. Crystal size distributions for 3 profiles above and one profile below the intrusion has been nearly completed, indicating a difference in grain size distribution, as expected by us initially. A nucleation and growth model has been adapted (following the approach of Foster and Dutrow, 2005)For the last 10 months of this grant period I propose to concentrate on the modelling of the intrusion history in 3-D. We will couple the obtained temperature-time history with forward modelling of diffusion limited and surface rate kinetics limited crystal growth, and to investigate the potential role of fluid production on development of pore pressure evolution using simple fluid flow models (e.g. Hanson 1995; Dutrow 2005). So far we have developed and adapted a nucleation and growth code in collaboration with C.T. Foster (Bodner et al., 2010), and a finite element model capable of simulating sequential intrusion of magma is currently being developed, so that in the remaining 10 months Robert Bodner can couple these codes and publish the results. As input we will use our age determinations (Michel et al., 2008; Leuthold et al., submitted), the established geometry of the intrusion (Leuthold et al., submitted), as well as the determined grain size distributions (Bodner et al., 2010). By varying the kinetic parameters, we will attempt to reproduce the grain size distributions above and below the intrusion. Implicitly we assume in this approach that the kinetics depend mainly on the thermal history. In parallel we will investigate if chemical parameters of the rocks are variable enough to significantly influence the CSD’s of our samples. Timing and Personal: This project asks for a 10 months salary for Robert Bodner, so that the research project can be successfully finished and the results published.