Tracing fluids during medium to ultra-high pressure metamorphism: insights by combined in situ oxygen isotopes and trace element analysis

Abstract

Fluids are an essential component of tectonic and metamorphic processes such as subduction and crustal anatexis. Fluids are elusive to trace as they commonly escape high-pressure rocks. This study uses oxygen isotopes to identify fluid influxes in metamorphic rocks and tie them to geologic events, measuring δ18O in situ by ion microprobe in garnet, zircon, apatite, monazite and lawsonite. New method developments are presented for δ18O analyses by Sensitive High Resolution Ion MicroProbe: (i) in apatite, a precision of 0.2‰ (1σ) is achieved; diffusion modelling shows that apatite is expected to preserve oxygen isotope signatures from 400-450°C and below; (ii) a matrix correction scheme is derived for monazite oxygen isotope measurement, allowing a precision of 0.35‰ (1σ); (iii) rutile oxygen isotope measurements yield major orientation effects. Fluid pulses generated by prograde dehydration reactions are investigated in the ultra-high-pressure Dora Maira whiteschists, Italy. Rare-earth-element abundances indicate prograde monazite and zircon growth (pre-garnet 34.5±0.7 Ma, 6.4‰ and syn-garnet at 34.9±0.4 Ma, 6.2 to 6.7‰), which are linked to dehydration reactions using thermodynamic modelling. This allows drawing a P-T-time-fluid path that implies that (i) prograde subduction from 25 to 45 kbar occurred within a couple of My (minimum burial rate of 2 cm/yr); (ii) high-pressure fluids were of internal origin and metasomatism likely have occurred at a rifting phase before subduction. Four phases of fluid circulations are identified in the high-pressure low-temperature lawsonite eclogites and blueschists of the Tavşanlı zone, Turkey. The Halilbağı unit is an oceanic complex containing various sediments and serpentinite together with 222±5 Ma MORB and 123±3 Ma OIB lithologies (zircon U-Pb, whole-rock major and trace elements). The sequence was thoroughly altered and mechanically mixed at the seafloor and in the accretionary prism, leading to overall high whole-rock δ18O of 11.0-17.0‰ for metabasites. Garnet, apatite and lawsonite are zoned in δ18O in samples across the unit, with contrasts of 7‰ in a MORB eclogite (garnet core: 6.3‰, rim: 13‰) and 3‰ in an impure quartzite (apatite core: 19.5‰, rim: 17‰). Petrographic and trace element evidence allow identifying localised prograde-peak fluid influx, and homogenisation of oxygen isotopes and Sr/Pb at the start of retrogression by pervasive fluid circulation across the unit. Heavy δ18O signatures (WR: 11.8 to 13.6‰) were measured in Eoarchaean metasediments from the Isua supracrustal belt, Greenland. The sources of the sediments were mantle-derived boninites (mafic component) and andesites (felsic component, detrital/volcanic zircons dated at 3709 Ma,δ18O 5.3‰). Three garnet growth zones record high δ18O (9 to 10‰), in equilibrium with the whole-rock. Rare-earth-element and petrographic evidence allow identifying a higher-pressure signature in the high- δ18O garnet, which can be linked to a 3690-3660 Ma tectonic event. The elevated δ18O signature in the metasediments thus originated from surficial processes (e.g. weathering) before 3690Ma. Melting of such heavy-δ18O amphibolite-facies sediments could represent a source for early Archean high-δ18O magmas and zircons. The combination of oxygen isotope and trace element microanalysis in zoned minerals proves a powerful tool for uncovering multistage minor and major fluid infiltration events in metamorphic rocks.