Rheology and Deformation Processes of Fine-grained Quartz Aggregate
Authors: Ghosh Subhajit , Stünitz Holger , Raimbourg Hugues , Précigout Jacques 
Published: February 2023

Grain size is a critical parameter for viscous deformation processes and controls the rheological behaviour (weakening and strain localizations in mylonites) of polycrystalline aggregates. Recently, &#160;Ghosh et al., (2022) used Tana quartzite (~200 &#956;m) to develop a new grain-size-insensitive flow law (stress exponent, n = 2; activation energy, Q = 110 kj/mol). The n = 2 is interpreted to indicate the serial operation of grain interior (dislocation glide) and grain boundary (dissolution-precipitation, DPC + Grain boundary sliding, GBS) accommodation processes. It is shown that all the previous results obtained from coarse-grained (&#8764;100 - 200 &#956;m) quartz aggregates (high total strain, significant recrystallization) plot within a factor of &#8764;5 times the strain rate predicted by this flow law. Moreover, a number of earlier studies with fine-grained (3.6 to 12 &#956;m) quartz reported an n-value range of 1.7 to 2.5 (Kronenberg and Tullis 1984; Fukuda et al., 2018), similar to the values obtained from coarse-grained samples. A deviation from the grain-size-insensitive creep-dominated process is expected at those grain-size ranges and poses the question: how much weakening is possible by grain-size-reduction within the continental crust?Using a Griggs-type apparatus, we deformed fine-grained (~ 3-4 &#956;m) as-is and 0.1 wt.% H2O-added novaculites, representing a fully recrystallized shear zone material, at the same condition as coarse-grained Tana quartzite (~200 &#956;m). The as-is novaculite is ~3.5 times stronger than the H2O-added novaculite. A similar strength difference (~4 times) was also observed between the as-is and H2O-added Tana quartzite.&#160;The H2O-added novaculite is ~4 times weaker than the H2O-added Tana.&#160;The as-is novaculite is ~4.5 times weaker than the as-is Tana quartzite. Thus, the addition of 0.1 wt.% H2O causes a similar order of weakening as the smaller grain size; the as-is novaculite has a similar strength as the data predicted from the H2O-added Tana flow law. The maximum strength difference (more than an order of magnitude) arises between the end-member samples i.e., the as-is Tana and H2O-added novaculite, which indicate the combined effect of grain size and H2O. The n (1.91 to 2.19) and Q (118 kj/mol) values measured from novaculites are similar to the Tana and the Tana flow law can express all the novaculite data within the error associated with the Griggs-type apparatus, only by manipulating the A-value of the flow law. Therefore, the n and Q values are largely grain-size-insensitive, while the strength differences due to grain size and added-H2O can be expressed by varying the A-value. Further, the similarity in the n and Q values indicates that the same deformation mechanisms (i.e., dislocation creep accommodated by grain boundary processes) operate in both materials. Microstructural analysis of the novaculite reveals the development of a core-shell structure of individual grains due to grain boundary migration (GBM). A combination of deformation mechanisms (dislocation glide, GBM, GBS) are mutually dependent and interact to achieve the bulk sample strain. The variations in A-value may reflect the efficiency of the GBS mechanism depending on grain size and H2O content. Finally, the implications of these results on crustal strength will be discussed.