The bromine and chlorine isotopic composition of the mantle as revealed by deep geothermal fluids

Pinti, Daniele L.; Shouakar-Stash, Orfan; Clara Castro, M.; Lopez-Hernández, Aida; Hall, Chris M.; Rocher, Océane; Shibata, Tomo et Ramírez-Montes, Miguel (2020). « The bromine and chlorine isotopic composition of the mantle as revealed by deep geothermal fluids ». Geochimica et Cosmochimica Acta, 276, pp. 14-30.

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Chlorine (Cl) and bromine (Br) are rare elements when considering the whole Earth. However, being highly volatile elements, their delivery and retention processes during planetary accretion and chemical differentiation provide important clues about the formation of the Earth. Variations in Cl and Br isotopic systems (37 Cl/35 Cl or d37 Cl and81 Br/79 Br or d81 Br) among terrestrial reservoirs could trace these processes. While the Br isotopic value of the mantle remains entirely unknown, the measurement of mantle Cl isotopic values is a controversial subject, with measured d37 Cl values ranging from À3 to 0‰ in midocean-ridge basalts (MORBs) and from À2 to +3‰ in oceanic island basalts (OIBs). Here, we report newly-determined d81 Br and d37 Cl values, together with noble gas 3 He/4 He (R) ratios, measured in geothermal fluids from production wells of three Mexican fields: Cerro Prieto, Las Tres Vı´rgenes, and Los Azufres. Relationships between3 He/4 He ratios and both d37 Cl and d81 Br suggest that geothermal fluid volatiles have three distinct sources: (1) a local crustal source, enriched in radiogenic4 He (R = 1.7–1.9Ra, where Ra is the atmospheric 3 He/4 He ratio), and halogens from brines with d37 Cl and d81 Br of +0.1 and +0.3‰ respectively; (2) the mantle wedge, with 3 He/4 He ratios of 6–6.5Ra, typical of arc volcanism, and d37 Cl and d81 Br of À0.4 and À1.0‰ respectively, typical (for Cl) of fluids derived from the dehydration of serpentinite in the subducting slab; and (3) a mantle source, with 3 He/4 He ratios of 7.7–8.2Ra, typical of MORBs, and d37 Cl and d81 Br of +0.9 and +0.7‰ respectively. These results suggest that the primitive mantle Cl isotopic composition was positive – possibly !+3‰, as measured in some OIBs – and inherited during the Moon forming impact. The progressive subduction of isotopically lighter halogens over the last 2–3 Ga could have progressively lowered this initial value to those currently measured in the depleted mantle beneath Mexico. It is speculated that the different isotopic values measured in mantle rocks and fluids could reflect the heterogeneous regassing of subducted halogens and the inefficient homogenization of recycled material in the MORB source, as suggested in other studies by the heterogeneous isotopic compositions of the heavier Ar and Xe of the convective mantle.

Type: Article de revue scientifique
Mots-clés ou Sujets: Bromine, Chlorine, mantle, halogen cycle, Moon-forming impact
Unité d'appartenance: Centres institutionnels > Centre de recherche en géochimie et géodynamique (GEOTOP)
Déposé par: Daniele Luigi Pinti
Date de dépôt: 20 mai 2020 07:56
Dernière modification: 20 mai 2020 07:56
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