Stable isotopes N, H, O, and B

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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
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Progressive devolatilization in subduction
environment

• Fluids released from the slab
• --gt induce earthquake, volcanism
• recycling from Earth surface to the deep
  mantle

Modified after Schmidt and Poli (1998)
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Tracing the fate of volatile-bearing phases in
subduction zone
Experimental petrology (piston cylinder,
multianvil and diamond anvil press)
Theoretical petrology (thermodynamic model)
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Tracing the fate of volatile-bearing phases in
subduction zone
--gt From natural samples
Arc volcanism (lava, fumeroles, hot springs)
Metamorphic rocks In paleosubduction zone
Karymsky Volcano, Kamchatka arc Photo from T.
Fischer (1999)
Eclogite
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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
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1 - Metamorphic rocks from paleosubduction
zone - Insight into devolatilization process -
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Study of metamorphic rocks from paleosubduction
zone
--gt Insight into the processes within subducting
slab
Require a sample set - subducted to different
depths - with similar protolith
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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 --gt ratio mobile/immobile -
Stable isotope geochemistry Traditional C, H,
O, N Non-traditional Cl, B, Li --gt isotopic
composition (delta notation ?)
) gt Fluid mobile elements )
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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
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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 --gt
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
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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

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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
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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)
--gt Fluid production depends on phengite
stability Phengite main OH-bearing mineral
carrying K, Rb, Cs, Li, B
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• Nitrogen in subducted sediments

• - 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
• --gt N isotopes can be used to trace phengite
  destabilization

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• Nitrogen geochemistry

• - 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

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• Samples Schistes Lustrés metasediments (Western
  Alps, Europe)

• Protoliths mixing of clay minerals, calcite
  and quartz in variable proportions

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• Geological background about Alpine metamorphic
  rocks

Plaque européenne
Plaque africaine
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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)

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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)

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• N isotopic composition in the Schistes Lustrés
  metasediments

With increasing metamorphic conditions, d15N
range remains constant ---gt N was preserved
during subduction
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• Relationships between N and K, Rb, Cs in the
  Schistes Lustrés

Devolatilization Fractionation because DNH4 ?
DCs ? DK ? DRb
Variations of concentration protoliths
inheritance No fractionation these
fluid-mobile elements were not
devolatilized during metamorphism
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Comparison between Schistes Lustrés and Catalina
Schists
GLOSS would be here (Plank Langmuir, 1998)
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Comparison between Schistes Lustrés and Catalina
Schists
Schistes Lustrés if any, very small fluid
loss... (phengite preservation)
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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 gtgt K gt Rb (Volfinger, 1976
Melzer and Wunder, 2000 Zack et al., 2001)
Catalina Schists (California) data from Bebout et
al. (1999)
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• Comparison of the Schistes Lustrés with previous
  studies

• Catalina Schists
• California -
• (Bebout and Fogel, 1992)

• Erzgebirge Schists
• Germany -
• (Mingram and Bräuer, 2001)

• Schistes Lustrés
• Western Alps -
• (Busigny et al., 2003)

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• Nitrogen behavior in subducted sediments a
  strong geothermal control

- Erzgebirge and Catalina Schists high geotherms
(13 and 20C/km, respectively) - Schistes
Lustrés low geotherm (8C/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)

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• Nitrogen behavior in subducted sediments a
  strong geothermal control

- Erzgebirge and Catalina Schists high geotherms
(13 and 20C/km, respectively) - Schistes
Lustrés low geotherm (8C/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)

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• Nitrogen behavior in subducted sediments a
  strong geothermal control

Metamorphic diamonds from Kokchetav massif
(Kazakhstan) ---gt very high N (gt 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)

• Kokchetav metasediments
• (Zhang et al., 1997)

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• Nitrogen behavior in subducted sediments a
  strong geothermal control

Fumeroles and hot springs from Central American
Arc ---gt 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)
---gt 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.

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• Hydrogen isotope composition in the Schistes
  Lustrés metasediments

With increasing metamorphic conditions, dD range
remains constant ---gt H2O was preserved during
subduction
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• Conclusions on metamorphic rocks from
  paleosubduction zones

• 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
• --gt 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)
• --gt This is in agreement with N and H
  isotopes geochemistry
• 5 - Sedimentary N and H2O are likely massively
  recycled to the deep mantle

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1 - Metamorphic rocks from paleosubduction
zone - Oxygen isotope geothermometry -
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• Temperature dependence of isotopic fractionation

?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 gt 500C, ?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)
(Suzuoki and Epstein, GCA1976)
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• Temperature dependence of isotopic fractionation

?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 gt 500C, ?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)
(Suzuoki and Epstein, GCA1976)
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• Temperature dependence of oxygen isotopic
  fractionation

Consider an isotopic equilibrium between quartz
(taken as a reference mineral) and several other
minerals (X) within a single rock sample,
A plot of ?18OQtz-X - A(Qtz,X) versus
B(Qtz,X) gives a linear correlation, with a
slope of 106/T2 ISOTHERM DIAGRAM
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• Isotherm diagram from O isotopes in an eclogite

Eclogite from Aarsheim, Selje (Norway) ?18OQtz
9.9 ?18OOmp 7.3 ?18OGt 6.7 ?18OBi
7.1 ?18ORu 4.9
--gt the slope is 106/T2
(data from Agrinier et al., 1985)
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• Take Home Message

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