Sheared peridotites: linking deformation and metasomatism contributing to the onset of craton destabilization

• Applicants: Anthony Withers, Ph.D.; Professor Dr. Alan Butler Woodland
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429770270

Cratons represent pieces of the oldest continental crust that have a thick lithospheric mantle root (~180-250km). This root formed during the Archean (3.5-2.5 Ga) and its presence is directly attributable to the longevity of cratons. An important factor is a viscosity contrast between the root and the surrounding asthenosphere. One explanation for this contrast is that the base of the lithosphere is H2O-poor, and low OH contents in olivine and pyroxene make the peridotite rheologically stiffer (higher viscosity). This means that the viscosity contrast can be lowered by metasomatic activity that (re)-introduces volatiles and other chemical components, thereby leading to destabilization and potential breakup of cratonic blocks. Destabilization will be guided by focussing of shear stresses in shear zones. Such zones can facilitate and localize melt and fluid migration. Feedback mechanisms between metasomatism and deformation are not well known and require multidisciplinary studies to correlate geochemical modifications with deformation. "Sheared" peridotites have textures indicative of one or more episodes of strong deformation and reflect rheological behavior under upper mantle conditions. As sheared peridotites generally record the deepest depths for a given xenolith suite, such samples should serve as proxies for ductile shearing processes at or near the base of the cratonic lithosphere and are thus relevant to understanding how cratonic mantle may become destabilized.Our goal is to investigate the relationship between deformation processes in the mantle lithosphere and geochemical changes driven by metasomatism. Sheared peridotite xenoliths will be studied from Lesotho and Kimberley, S.A., that originated from different depths and temperatures, allowing the influence of these two parameters on textures and mineral geochemistry to be assessed. This is a multifaceted study involving: i) microstructural analysis, including grain size and lattice preferred orientation measurements on olivine and orthopyroxene via EBSD, ii) chemical characterization via EPMA, LA-ICP-MS and Mössbauer spectroscopy to determine intensive parameters (P,T,fO2) and assess metasomatic interactions, and iii) the measurement of OH concentrations in olivine and pyroxene using FTIR and SIMS. Characterization will be grain size dependent to detect changes due to deformation. We will compare trace element signatures in grains down to ~40-50 micrometres and OH concentrations will be analysed by SIMS in neoblastic grains down to ~30 micrometres. Reference materials are used to maintain consistency between FTIR and SIMS. The structural data will yield information on differential stress and deformation mechanism(s), allowing the viscosity to be calculated. These results will be assessed together with the geochemical data to identify the influence that metasomatism may have had on deformation behavior, and the implications these processes have for the destabilization of cratonic lithospheric roots.

• DFG Programme: Research Grants
• Co-Investigator: Jolien Linckens, Ph.D.