Planar features, Trace element mobilisation and recrystallization formed during lower crustal CO2 induced seismic deformation of olivine

Planar deformation features are a common feature in shock-deformed olivine, both experimentally in conditions corresponding to crustal shear zones [1] and impact structures e.g., [2] and in deep crustal shear zones [3, 4].  Hence, the identification of different planes associated with the shock deformation is essential to access the stress levels during deformation, important feature during studies of earthquake deformation.  A combination of optical and EBSD data combined to infer which of the possible crystallographic planes and EPMA to study trace elements to investigate planar deformation features and grain size reduction in olivine. Samples originate from the Reinfjord Ultramafic Complex, exposing lower crustal earthquakes induced by with CO₂ bearing magmatic volatiles causing reaction facilitated grainsize reduction and weakening [3, 4].  First, calculated plane traces are compared with the observed plane traces in the free open source Matlab ® toolbox MTEX  [5], then the dip and dip direction of the observations of planes in the optical microscope.  Our results demonstrate: 1) That several planes are active during high stress deformation of lower crustal olivine rich rocks. 2) Some planes develop recrystallization features, whereas others develop later and do not develop recrystallization features. 3) Our results shows that these new olivine grains are a mix of grains with an orientation relationship with the host grains and grains that are far of the orientation of the host grain. 3) Further investigation using trace element mapping shows that P (Phosphorous) is a marker of fluid involvement in the recrystallization. P is mobilized preferably along grain boundaries and sub-grain boundaries involving twist, shown by zones of local P enrichment.

By looking at several grains we found that the developed fractures highly depend on the orientation of the host grain with respect to the external stress field.  Using the demonstrated methodology, it should be possible to map out the relative abundance of planar deformation features along different crystallographic planes in high stress deformed olivine and other transparent silicates.  The method can be refined by calculation of the exact thickness of the sample using interference colours calculated using the code published by [6] now available in MTEX.  This will enable the calculation of exact plane inclinations extracted from multifocal optical images that can be compared with crystallographic planes calculated in MTEX from the EBSD data. Further combination of trace elements reveals that fluid mobilisation is involved in the recrystallization process.

 

 

[1]   Druiventak A, Trepmann C A, Renner J and Hanke K  2011  Earth Planet. Sci. Lett. 311 199‑211

 [2]  Stöffler D, Keil K and Edward R D S  1991  Geochim. Cosmochim. Acta 55 3845-3867

 [3]  Ryan E J, et al.  2021  Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes deep earthquakes.  J. Struct. Geol. 2022

[4]       Sørensen, B.E., et al., In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening. Earth and Planetary Science Letters, 2019.

[5] Bachmann F, Hielscher R and Schaeben H  2010  Solid State Phenomena 160 63-68

[6] Sørensen B E  2013  Eur. J. Mineral. 25 5-10