Gescherte Peridotite: Deformation, Metasomatose und beginnende Destabilisierung von Kratonen
Zusammenfassung der Projektergebnisse
In this project, we set out to understand how cratonic mantle blocks can become destabilized by investigating the relationships between deformation processes in the mantle lithosphere and geochemical changes driven by metasomatic interactions. Two suites of natural sheared peridotite xenoliths from Kimberley (14 samples), RSA and Lesotho (12 samples) were the subject of this study, which employed a combination of modern methods. The Kimberley peridotites exhibit intense deformation textures, ranging from porphyroclastic to fluidal mosaic. Thermobarometric analysis of the porphyroclasts yield conditions of T=840–1000°C and P=3–4.5 GPa, indicating an origin in the mid-lithosphere and classification as low-T sheared peridotites (Heckel et al. 2022). Trace element concentrations in the porphyroclasts reveal different types of pre-deformation metasomatic events, including (carbonatitic) fluid and kimberlitic melt metasomatism. Cpx and Opx trace element signatures imply interaction with PIC or MARID-related metasomatic agents, which also caused variable degrees of oxidation. “Young” metasomatism during deformation is recorded in the neoblasts with Ti-enrichment being the most pronounced effect. A special protocol was developed for the electron microprobe to permit reliable trace element analysis of Ti, Cr, Al, Na and P in olivine. These data document further metasomatism and heating up to 1200°C (i.e. during deformation). Crystallographic preferred orientations (CPO) of olivine neoblasts (determined by EBSD) are consistent with elevated concentrations of water or melt during deformation. Multiple fabrics often occurring in a single sample imply either specific (hydrous) deformation conditions or overprinting of an older fabric by a younger event. Deformation under hydrous conditions implies hydrolytic weakening of the cratonic, lithospheric mantle. The sheared peridotites from Lesotho can be divided into three groups, based primarily on the Ca content of olivine and opx, but also on texture (Heckel et al. 2023a). Group I resembles the low-T sheared peridotites from Kimberley. Group II samples contain olivine and opx with elevated Ca concentrations and are considered moderate-T sheared peridotites (T=1200-1250°C at 5.0–5.5 GPa). Olivine and opx in Group III samples have the highest Ca contents and register the highest P-T conditions of all samples studied: 6.0±0.5 GPa and 1400±50°C. Measured Fe3+/∑Fe of the garnet porphyroclasts yield ∆logƒO2 values of ~FMQ- 2, (i.e. diamond stability field). Metasomatism during deformation is revealed by complex chemical changes recorded in olivine neoblasts. The Cpx in moderate-T and high-T sheared peridotites have compositions similar to Cr-rich and Cr-poor clinopyroxene megacrysts found in southern African kimberlites, suggesting a genetic link with megacryst-forming magmas. Olivine neoblasts exhibit six different CPO fabrics, most of which indicate the presence of a hydrous metasomatic agent during deformation (Heckel et al. 2023a). The H2O contents of olivine, opx and cpx porphyroclasts were directly measured by FTIR and SIMS. Minerals in the low-T sheared peridotites indicate OH-equilibrium, while disequilibrium between olivine and the pyroxenes is evident in the high-T, Group III samples (Heckel et al, in prep). Disequilibrium is most likely caused by changes in the H2O content of olivine, with metasomatism believed to be the most important factor. Measured H2O concentrations tend to higher or lower values than those predicted by the CPO fabrics (Heckel et al., 2023a), implying that textural re-equilibration is slower than H migration and enabling the evolution of metasomatic and deformation events to be investigated. Sheared peridotites are considered to reflect a ‘metasomatism-deformation-cycle’. Old metasomatic events cause rheological weakening, forming weak zones in the lithosphere that are more likely to get affected by subsequent metasomatism and deformation. After a period of extensive metasomatism the peridotites anneal to become coarse-grained again, albeit mechanically weakened. The low-T sheared peridotites formed at mid-lithospheric depths adjacent to melts conduits. The medium-T and high-T sheared peridotites represent wall rock deformation intimately related to intrusion of proto-kimberlite magma at greater depths, down to the lithosphere-asthenosphere boundary. The multiplication of structurally weak volatile-enriched zones means that at times of high magmatic activity, there will be accelerated physical erosion of the deeper lithospheric mantle, resulting in thinning and destabilization of cratonic roots (Heckel et al., 2023a).
Projektbezogene Publikationen (Auswahl)
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‘Cold’ sheared peridotites from Kimberley (Kaapvaal craton, SA): shortlived deformation connected with transient heating and metasomatism. 3rd European Mineralogical Conference (Cracow, Poland).
Heckel C.; Woodland A. B.; Seitz H.-M.; Linckens J. & Gibson S. A.
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Sheared peridotites from Kimberley (South Africa): Connecting deformation and metasomatism? DMG - Section meeting geochemistry and petrology (Frankfurt, Germany)
Heckel C.; Woodland A. B.; Linckens J. & Gibson S. A.
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Sheared peridotites from Kimberley (South Africa): Ti-enrichment of olivine neoblasts through a 1200°C hot metasomatic agent. Goldschmidt Conference 2021 (Lyon, France).
Heckel C.; Woodland AB, Linckens J. & Gibson SA
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Different types of sheared peridotites from Lesotho (Kaapvaal craton). Goldschmidt Conference 2022 (Hawai’i, USA)
Heckel C.; Woodland A. B.; Linckens J. & Gibson S. A.
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Metasomatism-deformationcylce: Connecting deformation and metasomatism in the lithospheric, cratonic mantle. DMG - Section meeting geochemistry and petrology (Mainz, Germany)
Heckel C.; Woodland A. B.; Linckens J.; Gibson S. A. & Seitz H.-M.
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Metasomatism-deformationcylce: Connecting deformation and metasomatism in the lithospheric, cratonic mantle. Earth Mantle Workshop 2022 (Toulouse, France)
Heckel C.; Woodland A. B.; Linckens J.; Gibson S. A. & Seitz H.-M.
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Sheared Peridotites from Kimberley (Kaapvaal Craton, RSA): Record of Multiple Metasomatic Events Accompanied with Deformation. Journal of Petrology, 63(10).
Heckel, Catharina; Woodland, Alan B.; Linckens, Jolien; Gibson, Sally A. & Seitz, Hans-Michael
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Sheared peridotites from the cratonic mantle beneath Lesotho (Kaapvaal craton): Investigating deformation and related metasomatic processes from mid-lithospheric depth to the LAB. GeoMinKöln (Cologne, Germany)
Heckel C.; Woodland A. B.; Linckens J.; Gibson S. A. & Seitz H.-M.
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Low-T sheared peridotites from the Kaapvaal craton: Record of hydrous metasomatism during deformation. Goldschmidt Conference 2023 (Lyon, France)
Heckel C.; Withers A. C.; Woodland A. B.; Gibson S. A. & Ludwig T.
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Sheared Peridotites from Northern Lesotho: Metasomatism-Induced Deformation and Craton Destabilization. Journal of Petrology, 64(10).
Heckel, Catharina; Woodland, Alan B.; Linckens, Jolien; Gibson, Sally A. & Seitz, Hans-Michael
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Sheared Peridotites: Linking Deformation and Metasomatism contributing to the onset of Craton destabilization. Doctoral thesis. Goethe Universität Frankfurt, June 2023, pp 331.
Heckel, C.
