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

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

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

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

Thermal evolution of an ancient subduction interface revealed by Lu–Hf garnet geochronology, Halilbagi Complex (Anatolia)

The thermal structure of subduction zones exerts a major influence on deep-seated mechanical and chemical processes controlling arc magmatism, seismicity, and global element cycles. Accretionary complexes exposed inland may comprise tectonic blocks with contrasting pressure–temperature (P–T) histories, making it possible to investigate the dynamics and thermal evolution of former subduction interfaces. With this aim, we present new Lu–Hf geochronological results for mafic rocks of the Halilbagi Complex (Anatolia) that evolved along different thermal gradients. Samples include a lawsonite–epidote blueschist, a lawsonite–epidote eclogite, and an epidote eclogite (all with counter-clockwise P–T paths), a prograde lawsonite blueschist with a “hairpin”-type P–T path, and a garnet amphibolite from the overlying sub-ophiolitic metamorphic sole. Equilibrium phase diagrams suggest that the garnet amphibolite formed at ~0.6–0.7 GPa and 800–850 °C, whereas the prograde lawsonite blueschist records burial from 2.1 GPa and 420 °C to 2.6 GPa and 520 °C. Well-defined Lu–Hf isochrons were obtained for the epidote eclogite (92.38 ± 0.22 Ma) and the lawsonite–epidote blueschist (90.19 ± 0.54 Ma), suggesting rapid garnet growth. The lawsonite–epidote eclogite (87.30 ± 0.39 Ma) and the prograde lawsonite blueschist (ca. 86 Ma) are younger, whereas the garnet amphibolite (104.5 ± 3.5 Ma) is older. Our data reveal a consistent trend of progressively decreasing geothermal gradient from granulite-facies conditions at ~104 Ma to the epidote-eclogite facies around 92 Ma, and the lawsonite blueschist–facies between 90 and 86 Ma. Three Lu–Hf garnet dates (between 92 and 87 Ma) weighted toward the growth of post-peak rims (as indicated by Lu distribution in garnet) suggest that the HP/LT rocks were exhumed continuously and not episodically. We infer that HP/LT metamorphic rocks within the Halilbagi Complex were subjected to continuous return flow, with “warm” rocks being exhumed during the tectonic burial of “cold” ones. Our results, combined with regional geological constraints, allow us to speculate that subduction started at a transform fault near a mid-oceanic spreading centre. Following its formation, this ancient subduction interface evolved thermally over more than 15 Myr, most likely as a result of heat dissipation rather than crustal underplating

Origins of high δ18O in 3.7-3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks

2020 Elsevier B.V. Elevated δ18O is used as a marker for the presence of continents and surficial alteration in the Eoarchean and Hadean. This study establishes a timeline for δ18O enrichment in Eoarchean metasedimentary rocks of the Isua supracrustal belt in Greenland. The source-rocks for the protolith of these metasedimentary rocks are mafic to intermediate magmatic rocks of ≥3709 ± 4 Ma, based on the age of zircons found in volcanogenic layers. The δ18O of +5.4 ± 0.4‰ of the zircon crystals indicate that the sources had a primary mantle-derived signature. However, garnet in two metasediments yields higher δ18O values of +8.7 to +9.7‰, in equilibrium with a whole-rock of +11 to +12‰ at 500-600 °C. This requires that the mafic protolith was weathered at surficial conditions, in agreement with previous conclusions based on major element geochemistry. The garnet grains with high δ18O record four growth zones, assigned to I) arc-building thermal metamorphism, II-III) terrane assembly at medium to high-pressure conditions, estimated to occur at 3660-3690 Ma and IV) late-Archaean overprint likely at ca. 2690 Ma. This shows that material with originally mantle-like δ18O was altered at low temperature (near-surface) to generate elevated oxygen isotope signatures and then recycled to middle-crustal conditions within 10-50 million years of crystallization in the Eoarchean. We propose that melting of such rocks could produce the zircon crystals with high δ18O that are found in the detrital and magmatic record in the Archean

Measurement of in-situ oxygen isotope ratios in monazite by SHRIMP ion microprobe: Standards, protocols and implications

Monazite forms at sub-solidus conditions in a variety of metamorphic rocks and has been proven to be reactive to fluids, and thus is a potential monitor of fluid-rock interaction. As monazite can preserve multiple growth zones, in order to explore the potential of monazite as a fluid tracer in metamorphic conditions, microbeam analysis is required. We performed oxygen isotope analysis of monazite using the SHRIMP ion microprobe and newly characterized standards, for which we obtained laser fluorination δ18O values (USGS-44069 monazite 7.67±0.26‰ and Itambé monazite 0.46±0.20‰). Reproducibility of δ18O ion microprobe analyses for USGS-44069 and Itambé monazites is in the order of 0.4-0.6‰, standard deviation at 95%c.l., similar to what is routinely obtained for silicates. This reproducibility is comparable to that of the analyses of experimental P-rich glasses, which are assumed to be homogeneous and free of geological imperfections. The variable composition of natural monazite has the potential to produce matrix effects during ion microprobe measurements. Monazite grains from the Malagasy syenite (Madagascar) and the Dora Maira whiteschists (Italy) display a scatter in δ18O values that show a negative correlation with Th content and is related to the huttonite [ThSiO4] and cheralite [CaTh(PO4)2] substitutions in monazite. The matrix effect on oxygen isotope measurements can be significant and is estimated to produce a shift in δ18O of circa -0.85 or -1.9‰ for every 10wt.% Th introduced by the huttonite and cheralite components, respectively. Corrections for this matrix effect are proposed on the basis of the natural samples investigated.Oxygen isotope fractionation factors for monazites of different compositions, cheralite and huttonite were calculated with the increment method. The results suggest that the substitution of trivalent LREE by tetravalent Ce and Th results in consistent enrichment of 18O in the monazites, whereas the substitution of tetravalent Th by divalent Ca results in the depletion of 18O in cheralite-rich monazite.Monazites from high-grade metasediments (Mount-Stafford, Central Australia) preserve inherited cores, but are homogeneous in oxygen composition. This suggests that diffusion may efficiently erase the oxygen isotope signature in monazite that experienced ~. 800. °C metamorphism

The Diamantina monazite: a new low-Th reference material for microanalysis

Most monazite reference materials (RMs) for in situ U-Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix-matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low-U and low-Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA-(Q, SF, MC)-ICP-MS and SIMS) are within uncertainty of the ID-TIMS data. U-Pb LA-(Q, MC)-ICP-MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of −18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng