U-Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid-Atlantic Ridge
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Rioux_et_al-2015-Journal_of_Geophysical_Research%3A_Solid_Earth.pdf
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Author(s)
Jöns, Niels
Lissenberg, C. Johan
Bach, Wolfgang
Kylander-Clark, Andrew
Hacker, Bradley
Rioux, Matthew
Bowring, Samuel A.
Dudas, Francis O.
Date Issued
April 2015
Journal
Journal of Geophysical Research: Solid Earth
Publisher
American Geophysical Union (AGU)
Citation
Rioux, Matthew et al. “U-Pb Dating of Interspersed Gabbroic Magmatism and Hydrothermal Metamorphism During Lower Crustal Accretion, Vema Lithospheric Section, Mid-Atlantic Ridge.” Journal of Geophysical Research: Solid Earth 120, 4 (April 2015): 2093–2118 © 2015 The Authors
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Final published version
Abstract
New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected ²⁰⁶Pb/²³⁸U dates of 13.528±0.101 to 13.353±0.057Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1-20μm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected ²⁰⁶Pb/²³⁸U dates for the inclusion-rich zircons range from 13.598±0.012 to 13.503±0.018Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction,206Pb/238U dates range from 13.341±0.162 to 12.993±0.055Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges. Key words: zircon; xenotime; hydrothermal; Mid-Atlantic Ridge; Vema; mid-ocean ridge
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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DOI of Published Version
https://doi.org/10.1002/2014JB011668
