Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

Stacia M. Gordon; Christopher L. Kirkland; Steven M. Reddy; Hannah J. Blatchford; Donna L. Whitney; Christian Teyssier; Noreen J. Evans; Bradley J. McDonald

doi:10.1016/j.epsl.2021.116810

Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

Stacia M. Gordon, Christopher L. Kirkland, Steven M. Reddy, Hannah J. Blatchford, Donna L. Whitney, Christian Teyssier, Noreen J. Evans, Bradley J. McDonald

Earth and Environmental Sciences-Twin Cities

Research output: Contribution to journal › Article › peer-review

28 Scopus citations

Abstract

Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.

Original language	English (US)
Article number	116810
Journal	Earth and Planetary Science Letters
Volume	560
DOIs	https://doi.org/10.1016/j.epsl.2021.116810
State	Published - Apr 15 2021

Bibliographical note

Funding Information:
We thank Malcolm Roberts for his assistance with the EPMA analyses. This manuscript greatly benefited from comments by Chloe Bonamici, Josh Garber, Zachary Michels, Mark Pearce, Frank Spear, and Mike Stearns. We also acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, the University of Western Australia, a facility funded by the University, State and Commonwealth Governments . Analysis in the GeoHistory Facility, JdLC was enabled by AuScope ( http://auscope.org.au ) and the Australian Government via the National Collaborative Research Infrastructure Strategy (NCRIS).

Funding Information:
We thank Malcolm Roberts for his assistance with the EPMA analyses. This manuscript greatly benefited from comments by Chloe Bonamici, Josh Garber, Zachary Michels, Mark Pearce, Frank Spear, and Mike Stearns. We also acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, the University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Analysis in the GeoHistory Facility, JdLC was enabled by AuScope (http://auscope.org.au) and the Australian Government via the National Collaborative Research Infrastructure Strategy (NCRIS).This work was supported by the National Science Foundation (grants EAR-1624546, EAR-1827198 and EAR-1062187 to Gordon, EAR-1040980 to Whitney, and EAR-1827220 to Teyssier) and the Australian Research Council (grant DP160104637 and LE130100053 to Reddy).

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

UHP terrane
deformation
petrochronology
titanite
trace elements
u-pb geochronology

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title = "Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements",

abstract = "Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.",

keywords = "UHP terrane, deformation, petrochronology, titanite, trace elements, u-pb geochronology",

author = "Gordon, {Stacia M.} and Kirkland, {Christopher L.} and Reddy, {Steven M.} and Blatchford, {Hannah J.} and Whitney, {Donna L.} and Christian Teyssier and Evans, {Noreen J.} and McDonald, {Bradley J.}",

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N2 - Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.

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Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

Abstract

Bibliographical note

Keywords

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