Tracking growing garnet in the Earth's crust

Garnets have long been used as chemical tape recorders to understand how metamorphic rocks change and grow in the Earth’s crust. Garnet grows under specific pressure and temperature (P-T) conditions, often as other minerals in the rock dehydrate (lose water) as they are buried. The geological process that forced burial may be passive or active. Passive growth may occur because the rock was buried as sediments accumulated over it. Active growth may occur as mountains grow and fault motion forces rocks above the growing garnet.

Recent advances in thermodynamic modeling are offering exciting new possibilities for acquiring precise and reliable metamorphic records of the paths that the rocks that contain garnet followed as they were subjected to these passive or active events. In the past, we would analyze a garnet core and rim, obtain the P-T conditions for these analyses, and draw arrows that represented a path. Now, we can track the growth of the garnet for each compositional analyses across a garnet from the core to rim.

The approach is ultimately very simple. We only need two pieces of data: (1) the rock’s bulk composition, which we can obtain by analyzing whole chips of the rock for their chemical compositions and (2) the garnet composition from its core to rim. We use a thermodynamic modeling routine (Theriak-Domino) and the approach described by Moynihan & Pattison (2013) to generate the highest-resolution P-T paths possible from garnets as they grew in the crust.

Of course we have assumptions: the rocks should be closed systems and obey the laws of thermodynamics. We check to see what the P-T path we generate would predict what we observe in the garnet zoning. We obtain multiple paths from garnets from the same rock to see how well they share the same conditions and paths. We analyze garnet from the same geological units and check to see if their paths agree. We also use the traditional approaches to see how well our high-resolution paths match these independent approaches. We have even been comparing our paths to more recent thermodynamic approaches, such as the quartz-in-garnet barometer to see how well they agree.

Ultimately, it comes down to the field area. We want to know how mountains formed in the areas that we work. We use the high-resolution P-T paths to create models for the creation of the Himalayas and understand how rocks in western Turkey moved over geological time.

Collaborators

• Thomas Etzel, Dept. Geological Sciences, UT Austin

• Oscar Lovera, Dept. Earth, Planetary, and Space Sciences, UCLA

• Eric Kelly, formerly Dept. Geological Sciences, UT Austin

• Kyle Ashley, University of Pittsburgh

• Mark Harrison, Dept. Earth, Planetary, and Space Sciences, UCLA

• Ibrahim Cemen, University of Alabama

• Daniel Stöckli, The University of Texas at Austin

• Rebekah January, University of Texas at El Paso

Talk about this work: Development and Application of High-Resolution Garnet P-T-t paths to Himalayan Tectonics E.J. Catlos

Peer-reviewed publications regarding this work:

Catlos, EJ, Perez TJ, Lovera OM, Dubey CS, Schmitt AK., Etzel TM (2020). High‐resolution P‐T‐Time paths across Himalayan faults exposed along the Bhagirathi transect NW India: Implications for the construction of the Himalayan orogen and ongoing deformation. Geochemistry, Geophysics, Geosystems, 21, e2020GC009353, https://doi.org/10.1029/2020GC009353

Catlos EJ, Lovera OM, Kelly ED, Ashley KT, Harrison TM, Etzel T (2018) Modeling High-resolution Pressure-Temperature Paths across the Himalayan Main Central Thrust (central Nepal): Implications for the Dynamics of Collision. Tectonics, 37, 2363-2388.

Etzel, TM, Catlos EJ, Atakturk K, Kelly ED, Lovera OM, Cemen I, Diniz E, Stockli D (2019) Implications for thrust-related shortening punctuated by extension from P‐T paths and geochronology of garnet‐bearing schists. Tectonics 38 (6), 1974-1998.