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Abstract
AbstractNew 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 (11°N). 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 ± 0.057 Ma. 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 <1–20 µm 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 ± 0.018 Ma 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 ± 0.055 Ma 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.
Highlights
Geologic evidence indicates that hydrothermal circulation plays an important role in cooling the lower crust at slow-spreading mid-ocean ridges
Detailed studies of deep drill cores into lower crustal gabbros exposed by detachment faults (Atlantis Massif, Mid-Atlantic Ridge (MAR), and Atlantis Bank, Southwest Indian Ridge (SWIR)) have documented multiple generations of hydrothermal metamorphism, including high-temperature amphibolite-facies metamorphism attributed to fluid infiltration at or near the ridge axis [Nozaka and Fryer, 2011; Robinson et al, 1991; Stakes et al, 1991; Vanko and Stakes, 1991] and greenschist-facies and lower grade metamorphism related to cooling and exhumation [Bach et al, 2001; Beard et al, 2009; Delacour et al, 2008; Frost et al, 2008; Nozaka and Fryer, 2011; Nozaka et al, 2008; Robinson et al, 1991; Stakes et al, 1991; Vanko and Stakes, 1991]
Low Ti amphiboles of cummingtonitic to gruneritic composition (XMg = Mg/(Mg + Fe) = 0.45–0.75; Si = 7.5–7.92 per formula unit (p.f.u.), formula based on 23 oxygen atoms; Table S1 in the supporting information) preserve evidence of an early phase of high- temperature hydrothermal metamorphism (Figure 2a)
Summary
Geologic evidence indicates that hydrothermal circulation plays an important role in cooling the lower crust at slow-spreading mid-ocean ridges. Detailed studies of deep drill cores into lower crustal gabbros exposed by detachment faults (Atlantis Massif, Mid-Atlantic Ridge (MAR), and Atlantis Bank, Southwest Indian Ridge (SWIR)) have documented multiple generations of hydrothermal metamorphism, including high-temperature amphibolite-facies metamorphism attributed to fluid infiltration at or near the ridge axis [Nozaka and Fryer, 2011; Robinson et al, 1991; Stakes et al, 1991; Vanko and Stakes, 1991] and greenschist-facies and lower grade metamorphism related to cooling and exhumation [Bach et al, 2001; Beard et al, 2009; Delacour et al, 2008; Frost et al, 2008; Nozaka and Fryer, 2011; Nozaka et al, 2008; Robinson et al, 1991; Stakes et al, 1991; Vanko and Stakes, 1991]. If the overall cooling history of the crust is repeatedly perturbed by successive magmatic intrusions, gabbros in the lower crust could be affected by multiple generations of hydrothermal metamorphism [Gillis et al, 1993]
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