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Stable isotopes (N, H, O, and B) in subduction environment

Vincent Busigny Lab. Géochimie des Isotopes Stables & Lab. Géochimie et Cosmochimie IPG Paris, France. Stable isotopes (N, H, O, and B) in subduction environment. Progressive devolatilization in subduction environment. Fluids released from the slab: --> induce earthquake, volcanism

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Stable isotopes (N, H, O, and B) in subduction environment

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  1. Vincent Busigny Lab. Géochimie des Isotopes Stables & Lab. Géochimie et Cosmochimie IPG Paris, France Stable isotopes (N, H, O, and B) in subduction environment

  2. Progressive devolatilization in subduction environment Fluids released from the slab: --> induce earthquake, volcanism + recycling from Earth surface to the deep mantle Modified after Schmidt and Poli (1998)

  3. Tracing the fate of volatile-bearing phases in subduction zone Experimental petrology (piston cylinder, multianvil and diamond anvil press) Theoretical petrology (thermodynamic model)

  4. Tracing the fate of volatile-bearing phases in subduction zone --> From natural samples: Arc volcanism (lava, fumeroles, hot springs) Metamorphic rocks In paleosubduction zone Karymsky Volcano, Kamchatka arc Photo from T. Fischer (1999) Eclogite

  5. Framework of this talk… 1 - Metamorphic rocks from paleosubduction zone - insight into devolatilization process (N, H) - oxygen isotope geothermometry 2 - Arc volcanism - boron isotope systematic

  6. 1 - Metamorphic rocks from paleosubduction zone - Insight into devolatilization process -

  7. Study of metamorphic rocks from paleosubduction zone --> Insight into the processes within subducting slab Require a sample set: - subducted to different depths - with similar protolith

  8. Study of metamorphic rocks from paleosubduction zone From petrology coupled with: - Major and trace element geochemistry Fluid mobile elements: Sr, Pb, LILE (K, Rb, Cs, Ba), U… Fluid immobile (“less mobile”) elements: Al, HFSE (Ti, Zr, Hf, Nb), Th, H-REE --> ratio “mobile/immobile” - Stable isotope geochemistry Traditional: C, H, O, N Non-traditional: Cl, B, Li --> isotopic composition (delta notation “”) ) > Fluid mobile elements )

  9. Stable isotope geochemistry: reminder… “Isotopic composition” (in ‰) = “isotope ratio” “Isotope fractionation” between A and B A-B = “fractionation factor” e.g. if A-B = 1.005, then A-B= A - B ~ 5‰

  10. Study of metamorphic rocks from paleosubduction zone Devolatilization process can be described as a Rayleigh distillation, by successive fluid production and extraction from the rock The basic sequence, which is repeated, is: 1 - Fluids are produced from partial mineral breakdown --> elements and isotopes are fractionated (depending on fluid/rock partitioning) 2 - Fluids are expelled from the rock If mineral phases breakdown is not seen by petrology, isotopic composition can trace it…

  11. Rayleigh distillation formalism Rayleigh distillation describes an exponential enrichment in the residual rock calculated as, •  = 0 - 1000.(F(-1)-1) 0 and  = isotopic composition of the rock before and after fluid loss from the slab F = fraction of the element under consideration remaining in the rock after fluid loss  = fractionation factor between fluid and rock

  12. Various lithology of the subducting oceanic lithosphere - Volatile and fluid-mobile elements are largely contained in sediments - Low-content in fresh mafic and ultramafic rocks but enriched by alteration

  13. Study of metasediments from paleosubduction zone Sediment contribution to subduction - Large amount of volatile and fluid-mobile elements - On first order, typical pelagic sediment: QUARTZ CARBONATE CLAY MINERAL QUARTZ CARBONATE PHENGITE Metamorphism (high pressure white mica) --> Fluid production depends on phengite stability Phengite = main OH-bearing mineral + carrying K, Rb, Cs, Li, B

  14. - N initially derives from organic matter • - N is then trapped as ammonium ion, NH4+ • NH4+ and Rb+, Cs+ are substituted for K+ in K-bearing minerals: • 1- in oceanic sediments: potassic clay mineral (illite) • 2- in metasediments: phengite (HP white mica) • The fate of N in subduction should • depend on phengite stability • --> N isotopes can be used to trace phengite destabilization Nitrogen in subducted sediments

  15. - Two stable isotopes: 14N (~99,64 %) • 15N (~ 0,36 %) • Occurrence in all terrestrial reservoirs (mantle, crust, hydrosphere, biosphere, atmosphere) • As gaseous, dissolved or solid, and with various oxidation states (N2, NO3-, NO2-, NH3, NH4+, NOx …) • Nitrogen isotopic composition is expressed in  unit (in ‰): • where the standard is atmosphere Nitrogen geochemistry

  16. Samples: Schistes Lustrés metasediments (Western Alps, Europe) • Protoliths ~ mixing of clay minerals, calcite and quartz in variable proportions

  17. Plaque européenne Plaque africaine Geological background about Alpine metamorphic rocks

  18. Devolatilization proxies • Nitrogen isotopic composition: 14N is preferentially released in fluids relative to 15N • N loss during metamorphism = [N] decreases and d15N increases • (Haendel et al., 1986; Bebout & Fogel, 1992) • Coupling NH4 and K, Rb, Cs: fluid-mobile elements having different partition coefficients between phengite and fluid (i.e. DNH4 ≠ DCs ≠ DK ≠ DRb). • (Melzer & Wunder, 2000; Zack et al., 2001)

  19. Devolatilization proxies 1000 ln min-water = min-water = min - water • Hydrogen isotopic composition • 2D is preferentially released in fluids relative to 1H • Water loss during metamorphism = [H2O] decreases and dD increases • (Suzuoki and Epstein, GCA1976)

  20. N isotopic composition in the Schistes Lustrés metasediments With increasing metamorphic conditions, d15N range remains constant ---> N was preserved during subduction

  21. Devolatilization = Fractionation because DNH4 ≠ DCs ≠ DK ≠ DRb Variations of concentration = protoliths inheritance No fractionation = these fluid-mobile elements were not devolatilized during metamorphism Relationships between N and K, Rb, Cs in the Schistes Lustrés

  22. Comparison between Schistes Lustrés and Catalina Schists GLOSS would be here (Plank & Langmuir, 1998)

  23. Comparison between Schistes Lustrés and Catalina Schists Schistes Lustrés: if any, very small fluid loss... (phengite preservation)

  24. Comparison between Schistes Lustrés and Catalina Schists Catalina Schists: strong devolatilization process (phengite dehydration) Devolatilization processes are characterized by strong Cs loss. D (fluid/phengite) Cs >> K > Rb (Volfinger, 1976; Melzer and Wunder, 2000; Zack et al., 2001) Catalina Schists (California) data from Bebout et al. (1999)

  25. Catalina Schists • California - • (Bebout and Fogel, 1992) • Erzgebirge Schists • Germany - • (Mingram and Bräuer, 2001) • Schistes Lustrés • Western Alps - • (Busigny et al., 2003) Comparison of the Schistes Lustrés with previous studies

  26. - Erzgebirge and Catalina Schists: high geotherms (13 and 20°C/km, respectively) - Schistes Lustrés: low geotherm (8°C/km) Geotherms : • “Cool” and “warm” current subductions (Peacock, 1999) • Schistes Lustrés • (Agard et al., 2001) • Catalina Schists • (Grove and Bebout, 1995) • Erzgebirge Schists • (Mingram et Bräuer, 2001) Nitrogen behavior in subducted sediments: a strong geothermal control

  27. - Erzgebirge and Catalina Schists: high geotherms (13 and 20°C/km, respectively) - Schistes Lustrés: low geotherm (8°C/km) Geotherms : • “Cool” and “warm” current subductions (Peacock, 1999) • Schistes Lustrés • (Agard et al., 2001) • Catalina Schists • (Grove and Bebout, 1995) • Erzgebirge Schists • (Mingram et Bräuer, 2001) Nitrogen behavior in subducted sediments: a strong geothermal control

  28. Metamorphic diamonds from Kokchetav massif (Kazakhstan) ---> very high [N] (> 2500 ppm) (De Corte et al., 1998, 1999) Diamonds of the Kokchetav metasediments Geotherms : • “Cool” and “warm” current subductions (Peacock, 1999) • Schistes Lustrés • (Agard et al., 2001) • Catalina Schists • (Grove and Bebout, 1995) • Erzgebirge Schists • (Mingram et Bräuer, 2001) Nitrogen behavior in subducted sediments: a strong geothermal control • Kokchetav metasediments • (Zhang et al., 1997)

  29. Fumeroles and hot springs from Central American Arc ---> subducted N = efficiently transferred back to the surface via arc volcanism i.e. no recycling of N to the deeper mantle (Fischer et al., 2002) ---> Geotherm in Central America is likely “warm”: (1) “Young” subducted oceanic crust (10-20 Ma), (2) Very high density of volcanoes, (3) Magnetic anomaly, (4) Occurrence of adakite in Costa Rica. Nitrogen behavior in subducted sediments: a strong geothermal control

  30. Hydrogen isotope composition in the Schistes Lustrés metasediments With increasing metamorphic conditions, dD range remains constant ---> H2O was preserved during subduction

  31. 1 - N and H2O can be preserved in “cool” subduction zone • 2 - N and H2O in subducted metasediments = controlled by geothermal gradient • 3 - N occurs as NH4+ substituting for K+ in phengite --> N isotopes can trace phengite preservation or destabilization in subduction • 4 - From HP experiments: phengites may be stable down to 250-300 km depth • (Schmidt, 1996; Domanik and Holloway, 2000) • --> This is in agreement with N and H isotopes geochemistry • 5 - Sedimentary N and H2O are likely massively recycled to the deep mantle Conclusions on metamorphic rocks from paleosubduction zones

  32. 1 - Metamorphic rocks from paleosubduction zone - Oxygen isotope geothermometry -

  33. I-J = 1000 ln I-J = I - J Isotopic fractionation (I-J)between two chemical species (I and J) decreases when T increases For temperature > 500°C, I-J = 1000.ln I-J = A + B.(106/T2) where A and B depend on the minerals I and J (Bottinga and Javoy, 1973, 1975; Javoy, 1977) Temperature dependence of isotopic fractionation (Suzuoki and Epstein, GCA1976)

  34. I-J = 1000 ln I-J = I - J Isotopic fractionation (I-J)between two chemical species (I and J) decreases when T increases For temperature > 500°C, I-J = 1000.ln I-J = A + B.(106/T2) where A and B depend on the minerals I and J (Bottinga and Javoy, 1973, 1975; Javoy, 1977) Temperature dependence of isotopic fractionation (Suzuoki and Epstein, GCA1976)

  35. Consider an isotopic equilibrium between quartz (taken as a reference mineral) and several other minerals (X) within a single rock sample, Temperature dependence of oxygen isotopic fractionation A plot of 18OQtz-X - A(Qtz,X) versus B(Qtz,X) gives a linear correlation, with a slope of 106/T2 = “ISOTHERM” DIAGRAM

  36. Eclogite from Aarsheim, Selje (Norway) 18OQtz = +9.9 ‰ 18OOmp = +7.3 ‰ 18OGt = +6.7 ‰ 18OBi = +7.1 ‰ 18ORu = +4.9 ‰ “Isotherm” diagram from O isotopes in an eclogite --> the slope is 106/T2 (data from Agrinier et al., 1985)

  37. 1 -Slab devolatilization (= volatile-bearing phase destabilization) can be traced in subduction zones from natural samples: - metamorphic rocks in paleosubduction zone - arc volcanism 2 - While petrological observations do not tell us how much of a mineral phase has been breakdown, stable isotopes can do this… 3 - All phases and isotopic evolution have to be related to the thermal structure of the subducting plate Take Home Message…

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