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Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation.

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Title: Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation.
Authors: Corvò, Stefania1,2 (AUTHOR) stefania.corvo@unipv.it, Maino, Matteo1,2 (AUTHOR), Piazolo, Sandra3 (AUTHOR), Kylander-Clark, Andrew R.C.4 (AUTHOR), Orlando, Andrea5 (AUTHOR), Seno, Silvio1 (AUTHOR), Langone, Antonio1,2 (AUTHOR) antonio.langone@unipv.it
Source: Earth & Planetary Science Letters. Oct2023, Vol. 620, pN.PAG-N.PAG. 1p.
Subjects: Sphene, Substitution reactions, Crystal grain boundaries, Dislocation density, Shear zones, Uranium-lead dating
Geographic Terms: Italy
Abstract: Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a post-Variscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cm-scale layered rocks showing compositional variation consisting of alternating 'amphibole-rich' (i.e., amphibolites) and 'clinopyroxene/plagioclase-rich' domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs. • Petrochronological and microstructural study on titanite from mylonitic rocks. • Titanite records heterogeneous features as function of microdomain composition. • Deformation microstructures exert significant control on petrochronological record. • Fluid availability governed the development of certain microstructural features. • Lithologies deforming in the presence of fluids are best for dating deformation. [ABSTRACT FROM AUTHOR]
Copyright of Earth & Planetary Science Letters is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation.
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  Data: Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a post-Variscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cm-scale layered rocks showing compositional variation consisting of alternating 'amphibole-rich' (i.e., amphibolites) and 'clinopyroxene/plagioclase-rich' domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs. • Petrochronological and microstructural study on titanite from mylonitic rocks. • Titanite records heterogeneous features as function of microdomain composition. • Deformation microstructures exert significant control on petrochronological record. • Fluid availability governed the development of certain microstructural features. • Lithologies deforming in the presence of fluids are best for dating deformation. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Earth & Planetary Science Letters is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
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    Identifiers:
      – Type: doi
        Value: 10.1016/j.epsl.2023.118349
    Languages:
      – Code: eng
        Text: English
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      – SubjectFull: Sphene
        Type: general
      – SubjectFull: Substitution reactions
        Type: general
      – SubjectFull: Crystal grain boundaries
        Type: general
      – SubjectFull: Dislocation density
        Type: general
      – SubjectFull: Shear zones
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      – SubjectFull: Uranium-lead dating
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      – SubjectFull: Italy
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      – TitleFull: Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation.
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            NameFull: Corvò, Stefania
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              Text: Oct2023
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