Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis

Gauthiez‑Putallaz, Laure; Rubatto, Daniela; Hermann, Jörg (2016). Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis. Contributions to mineralogy and petrology, 171(2), pp. 1-20. Springer 10.1007/s00410-015-1226-4

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Keywords High-pressure fluids · Whiteschists · U–Pb
dating · Oxygen isotopes · Ion microprobe · Metasomatism
Introduction
The subduction of crustal material to mantle depths and its
chemical modification during burial and exhumation contribute
to element recycling in the mantle and the formation
of new crust through arc magmatism. Crustal rocks that
Abstract The Dora-Maira whiteschists derive from metasomatically
altered granites that experienced ultrahighpressure
metamorphism at ~750 °C and 40 kbar during
the Alpine orogeny. In order to investigate the P–T–time–
fluid evolution of the whiteschists, we obtained U–Pb ages
from zircon and monazite and combined those with trace
element composition and oxygen isotopes of the accessory
minerals and coexisting garnet. Zircon cores are the
only remnants of the granitic protolith and still preserve a
Permian age, magmatic trace element compositions and
δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and
Si-poor whiteschist compositions shows that there are two
main fluid pulses during prograde subduction between 20
and 40 kbar. In Si-poor samples, the breakdown of chlorite
to garnet + fluid occurs at ~22 kbar. A first zircon
rim directly overgrowing the cores has inclusions of prograde
phlogopite and HREE-enriched patterns indicating
zircon growth at the onset of garnet formation. A second
main fluid pulse is documented close to peak metamorphic
conditions in both Si-rich and Si-poor whiteschist when
talc + kyanite react to garnet + coesite + fluid. A second
metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single
metamorphic overgrowth capturing phengite and talc inclusions
was observed in the Si-rich whiteschists. Garnet rims,
zircon rims and monazite are in chemical and isotopic equilibrium
for oxygen, demonstrating that they all formed at
peak metamorphism at 35 Ma as constrained by the age of
monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma).
The prograde zircon rim in Si-poor whiteschists has an
age that is within error indistinguishable from the age of
peak metamorphic conditions, consistent with a minimum
rate of subduction of 2 cm/year for the Dora-Maira
unit. Oxygen isotope values for zircon rims, monazite and
garnet are equal within error at 6.4 ± 0.4 ‰, which is in
line with closed-system equilibrium fractionation during
prograde to peak temperatures. The resulting equilibrium
Δ18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The
in situ oxygen isotope data argue against an externally
derived input of fluids into the whiteschists. Instead, fluidassisted
zircon and monazite recrystallisation can be linked
to internal dehydration reactions during prograde subduction.
We propose that the major metasomatic event affecting
the granite protolith was related to hydrothermal seafloor
alteration post-dating Jurassic rifting, well before the
onset of Alpine subduction.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Rubatto, Daniela and Hermann, Jörg

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0010-7999

Publisher:

Springer

Language:

English

Submitter:

Daniela Rubatto

Date Deposited:

19 May 2016 10:21

Last Modified:

22 Feb 2017 20:49

Publisher DOI:

10.1007/s00410-015-1226-4

BORIS DOI:

10.7892/boris.81289

URI:

https://boris.unibe.ch/id/eprint/81289

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