Atmospheric Ar and Ne returned from mantle depths to the Earth's surface by forearc recycling

Abstract

In subduction zones, sediments, hydrothermally altered lithosphere, fluids, and atmospheric gases are transported into the mantle, where ultrahigh-pressure (UHP) metamorphism takes place. However, the extent to which atmospheric noble gases are trapped in minerals crystallized during UHP metamorphism is unknown. We measured Ar and Ne trapped in phengite and omphacite from the youngest known UHP terrane on Earth to determine the composition of Ar and Ne returned from mantle depths to the surface by forearc recycling. An (40)Ar/(39)Ar age [7.93 ± 0.10 My (1σ)] for phengite is interpreted as the timing of crystallization at mantle depths and indicates that (40)Ar/(39)Ar phengite ages reliably record the timing of UHP metamorphism. Both phengite and omphacite yielded atmospheric (38)Ar/(36)Ar and (20)Ne/(22)Ne. Our study provides the first documentation, to our knowledge, of entrapment of atmospheric Ar and Ne in phengite and omphacite. Results indicate that a subduction barrier for atmospheric-derived noble gases does not exist at mantle depths associated with UHP metamorphism. We show that the crystallization age together with the isotopic composition of nonradiogenic noble gases trapped in minerals formed during subsolidus crystallization at mantle depths can be used to unambiguously assess forearc recycling of atmospheric noble gases. The flux of atmospheric noble gas entering the deep Earth through subduction and returning to the surface cannot be fully realized until the abundances of atmospheric noble gases trapped in exhumed UHP rocks are known.

Keywords: UHP metamorphism; atmosphere; geochronology; noble gas; subduction.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

(A and B) Photomicrographs of 89321 coesite eclogite from the Papua New Guinea UHP terrane. Cross-polarized light at 10× magnification. Abbreviations are garnet (Grt), omphacite (Omp), phengite (Ph), and rutile (Rt). Coesite (Coe) occurs as an inclusion in omphacite. (C) Peak pressure–temperature–time constraints for 89321 coesite eclogite (star). Thermobarometry constraints for coesite eclogite are indicated by dark gray boxes (22). Timing of peak metamorphism based on 40Ar/39Ar phengite (this study), Lu-Hf garnet (27), and U-Pb zircon (31) ages. Apatite fission track ages (0.6 Ma) from host gneiss at the coesite locality constrain the timing of exhumation to shallow crustal levels (27). Abbreviations for metamorphic facies are blueschist (BS), greenschist (GS), amphibolite (AM), eclogite (EC), and granulite (GR); mineral abbreviations are albite (Ab), jadeite (Jd), and quartz (Qtz). The light gray path shows the range in pressure–temperature conditions for the UHP terrane (33). The dashed blue line indicates subduction zone geotherm. The solidus for water-saturated crustal rocks and dehydration melting of phengite (36, 52) provide maximum temperature limits for the pressure–temperature–time path followed by coesite eclogite during exhumation to the surface.

Fig. 2.

Results of step heat experiment on phengite from coesite eclogite in the Papua New Guinea UHP terrane. The 40Ar/39Ar age spectrum is shown together with the inverse isochron plot (Inset). Data corrected for blanks, mass discrimination, and reactor-produced isotopes. Steps used to calculate the weighted mean age correspond to 84% of 39Ar released. The inverse isochron suggests mixing between radiogenic and atmospheric argon components. Additional discussion is in the text.

Fig. 3.

Argon abundances for irradiated phengite and omphacite from coesite eclogite vs. temperatures (°C) of laboratory outgassing. The comparison of phengite and omphacite values indicates that overall relatively higher abundances of Ar are outgassed from phengite and that most of Ar in phengite was released during laboratory heating at temperatures >1,200 °C. Radiogenic (40Ar*) is produced from radiogenic decay of 40K. 39ArK is produced during (n,p) irradiation of 39K. 38ArCl is produced by 37Cl(n,γ)38Cl(β)38Ar reaction during irradiation. 36Ar is the stable isotope of Ar, assumed to be primordial and/or atmospheric-derived. Irr. Ph, irradiated phengite; Irr. Omp, irradiated omphacite.

Fig. 4.

Argon isotopic compositions for both irradiated and natural (unirradiated) phengite and omphacite from coesite eclogite in the Papua New Guinea UHP terrane. Reference lines for atmospheric compositions (40Ar/36Ar and 38Ar/36Ar) are indicated (34). Results of step heat experiments on natural samples fall on a mixing line between atmospheric 38Ar/36Ar and 40Ar/36Ar ratios produced from a mixture of radiogenic and atmospheric components. Data for irradiated samples result from Ar-trapped component mixtures of atmospheric-, radiogenic-, and reactor-produced 38Ar. Errors are 1σ. For irradiated samples, reactor-produced 38Ar shifts 38Ar/36Ar ratios to values greater than the atmospheric ratio as indicated. Irr. Ph, irradiated phengite; Irr. Omp, irradiated omphacite; Omp, omphacite; Ph, phengite.

Fig. 5.

20Ne/22Ne and 38Ar/36Ar isotope data for step heat experiments on unirradiated (natural) omphacite and phengite. Data with large contributions from blank (>20% for Ar and >25% for Ne) are not included. Atmospheric values for 20Ne/22Ne and 38Ar/36Ar ratios (39) are indicated with horizontal and vertical lines, respectively. Results indicate the presence of trapped neon and argon, with compositions within error of the atmospheric values, in both omphacite and phengite. The hatched area indicates the range of 20Ne/22Ne values obtained for metamorphic diamonds from the Kokchetav massif (21). Neither mantle nor solar wind 20Ne/22Ne values were observed (44) in phengite and omphacite from the coesite eclogite studied. Omp, omphacite; Ph, phengite.

Fig. 6.

Plot of 20Ne/36Ar vs. 38Ar/36Ar for step heat experiments on unirradiated (natural) omphacite and phengite. Stars indicate MORB, air, and seawater values. Horizontal dashed lines indicate 20Ne/36Ar values for MORB, air, and seawater. Because the release of trapped Ne and Ar from a mineral at a given temperature is controlled by their relative diffusivities, use of an elemental ratio (i.e., 20Ne/36Ar) to decipher mixing components from step heat experiments is not always straightforward. (A) Four data points (two from phengite-1 and one each from phengite-2 and omphacite-1) have 20Ne/36Ar values between atmospheric and MORB values. Numbers other than these data points indicate extraction temperature (degrees Celsius). We did not detect 22Ne in three of the steps because of higher contribution of blank and/or higher analytical error. For the 600 °C extraction step for the phengite-1 sample, the 20Ne/22Ne is atmospheric, despite 20Ne/36Ar indicating the presence of an MORB component. (B) An enlarged section of A, with data showing 20Ne/36Ar ratios that range between atmospheric and seawater values. Lowest values (0.003) may derive from contributions from a sedimentary component (ref. and references therein). Omp, omphacite; Ph, phengite.