Behaviour of wet quartzite: deformation experiments revisited
- 1Charles University in Prague, Institute of Petrology and Structural Geology, Prague, Czechia (pongracp@natur.cuni.cz)
- 2Department of Geosciences, University of Tromsø
- 3Institute of Earth Sciences, University of Orléans
Since quartz is among the most abundant minerals in continental crust and one of the first to show plasticity with increasing pressure and temperature, understanding its mechanical behavior is crucial for estimates on crustal strength and modeling of geodynamic processes. Since discovery of significantly lower mechanical strength of quartz as a consequence of H2O presence in the crystal (Griggs & Blacic, 1965), remarkable amount of work has been done in order to improve knowledge about processes and mechanisms responsible for so called H2O weakening effect. As the weakening effect depends on molecular H2O, it is a disequilibrium weakening process that is difficult to incorporate into existing flaw laws.
In order to estimate mechanical behavior of quartz in presence of H2O, we performed deformation experiments in solid-medium Griggs-type apparatus in coaxial setting under controlled laboratory conditions using very pure natural quartzite from Tana quarry in northern Norway. Behavior of as-is and 0.1 wt% H2O added samples was studied in 1) eight shortening experiments at 900 °C, 1 GPa and constant strain rate of 10-6 s-1 reaching 5% and 30% strain, 2) six strain rate stepping experiments covering 10-5, 10-6 and 10-7 s-1, 3) two temperature stepping experiments covering 750, 850 and 950 °C and 4) one hot-pressing experiment maintaining the starting experimental conditions for 14 hours.
There is a negligible strength difference between the as-is and H2O added samples. Both H2O added and as-is strain rate steeping experiments had shown surprisingly low stress exponent, with the highest value of 2.26. Temperature stepping experiments gave activation energy values of 177 kJ/mol and 198 kJ/mol. In all studied samples, strain increases towards the sample centers exhibiting grain size decrease from initial 250 – 300 µm. Three principal deformation mechanisms contributing to the bulk strain were identified: 1) crystal plasticity of original grains manifested by flattening, undulatory extinction, and development of subgrains, 2) cracking of original grains demonstrated by fluid inclusion trails and minor grain offset and 3) dynamic recrystallization via subgrain rotation recrystallization indicated by misorientation analysis from EBSD data. FTIR spectroscopy was applied to evaluate H2O speciation, quantity and distribution. Regardless of added H2O, most of deformed original grains showed relative H2O concentration between 0 and 400 H/106Si, implying significant decrease of H2O content from original 600 to 2000 H/106Si measured in undeformed grains. Average H2O concentration in grain boundaries showed 750 H/106Si for as-is samples and 1300 H/106Si for H2O added. Plasticity is most visible in CL-images, as well as higher degree of grain fragmentation and crack density in samples with added H2O. Ubiquitous presence of fluid along the grain boundaries, demonstrated by FTIR results, may have facilitated sliding along grain boundaries which, in turn, could explain the low stress exponent derived from strain rate stepping experiments.
REFERENCES:
Griggs, D. T. & Blacic, J. D. (1965): Quartz: Anomalous Weakness of Synthetic Crystals. Science 147(3655):292-95.
How to cite: Pongrac, P., Jeřábek, P., Stünitz, H., Raimbourg, H., Nègre, L., and Précigout, J.: Behaviour of wet quartzite: deformation experiments revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13172, https://doi.org/10.5194/egusphere-egu2020-13172, 2020
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Hi Petar,
super interesting results. Looking at the newly formed (grown?) grains in your slide 12, I'm wondering what impact their presence would have on grains size - differential stress relations. I'd assume, that those newly grown grains will not follow the same size - stress relation as recrystallizing grains. How easy is it to incorporate those by mistake as recrystallized grains (e.g. in the light microscope) during grain size analysis? Do you think this is a common feature in that type of experiments or somehow special to your experiments?
Another question: Strongly deformed grains are more dry (in terms of fluid inclusions, I guess?) than less deformed grains. Do you have any suggestion on the mechanism of fluid extraction?
Cheers,
Rüdiger
Hi Rüdiger,
thank you very much for your comment and very interesting questions! Regarding fragmented and overgrown grains, your point is very good. These grains would for sure increase the average recrystallized grain size values in the samples, so for the piezometry purposes it is very important to exclude them from calculations. One way how they can be recognized in domains of newly formed grains is mis-to-mean relation – since they are basically cracked from the parent grains, which are expected to have higher level of internal lattice distortion compared to recrystallized grains, they still preserve this original internal misorientation. Of course, there is some level of uncertainty if mis-to-mean of particular cracked grains is very low. Another helpful way to distinguish them from recrystallized grains is different luminescence visible on CL maps. With optical microscope, it is possible to distinguish them only if they show angular shapes and if they are bigger than average recrystallized grains size. So far, I believe that this mechanism of cracking and growth of small fragmented grains increases subsequently with H2O addition, but this is something that needs to be investigated in more details.
In terms of strongly deformed grains, according to my results, they are evidently ‘’losing’’ H2O as they are being deformed, and probably releasing it into the boundary space. Your question about the mechanism responsible for this is very interesting. One of my guesses at this stage of research is that the fluid might migrate through the micro-cracks network in the grains (which afterwards could be healed) and be extracted due to stress concentration differences. But this is definitely another point that needs further investigation. Any suggestion from you about it would be very welcome!
Cheers,
Petar