Disorientation control on trace element segregation in fluid-affected low-angle boundaries in olivine

Tacchetto Tommaso1,2, Reddy Steven1,2, Saxey David2, Fougerouse Denis 1,2, Rickard William2, Clark Chris1

1School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Geoscience Atom Probe, Curtin University, Perth, Australia

The interfaces between minerals (grain boundaries s.l.) play a critical role in controlling the rheological behaviour of rocks and the ability of fluids to penetrate them in the deep crust. However, the role of chemical segregation in controlling the behaviour of mineral interfaces remains largely unexplored. In this work, we combined electron backscattered diffraction (EBSD) and atom probe tomography (APT) to assess the relationship between deformation-related low-angle boundaries in naturally-deformed olivine and the degree of trace element segregation to those boundaries. The sample studied comes from the Bergen Arcs (Norway), where high-grade, dry, metamorphic rocks of the lower crust have been overprinted by fluid-present high-pressure metamorphism during the Caledonian tectonic subduction between 430 and 410 Ma. EBSD orientation mapping of deformed olivine is used to characterise the slip systems associated with deformation and the misorientation relationships within different parts of the microstructure. APT has then been used to systematically target grain boundaries of different misorientation angle (up to 8°). 

The analysed boundaries formed by sub-grain rotation recrystallisation associated with {100}<001> slip system developed during the fluid-catalysed metamorphism. APT data show that olivine trace elements segregated to the low-angle boundaries during this process. Boundaries with < 2° degrees show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at increased misorientation (> 2°), the interface becomes more ordered with dislocation geometries defined by linear concentrations of trace elements, and which are consistent with the EBSD data. These boundaries show a systematic correlation of increasing trace element segregation with misorientation angle. In particular, elements are generally more enriched at higher degrees of distortion, where variations are mostly significant for Ca (from 0.07 up to 0.6 at%) and Cl (up to 0.3 at%). Elements that are segregated to the low-angle boundaries (Ca, Al, Ti, P, Mn, Fe, Na, Mg and Co) are here interpreted to be captured and accumulated by dislocations as they migrate to the sub-grain boundary interfaces. However, the exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect a small but significant contribution of an external fluid source during the fluid-related deformation. In particular, since the occurrence of H in olivine is strongly attributed to Ti defects, the segregation of Ti to grain boundaries is consistent with the detected enrichment of hydrogen in the low-angle interfaces.

The observed compositional segregation of trace elements to low-angle boundaries have significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. Furthermore, the nanoscale correlation between heterogeneous distribution of elements like Ti and the diffusion of H along boundary interfaces within olivine might have the potential to yield important implication for the understanding of the hydrogen distribution in the upper mantle and its consequences for the rheological properties of mantle-rocks during deformation.


Biography

Tommaso completed his MSc in Geology at the Univesity of Padova (Italy) in 2017 and awarded in 2017 of a Temporary Research Fellowship. He is now completing his 3rd year of PhD at Curtin University focused on the investigation of metamorphic processes in the precence of fluids at the nanoscale.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.