Introduction to Environmental Isotopes

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Environmental isotopes astracers in geochemistry

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Environmental Isotopes in Hydrogeology

• Introduction to the environmental isotopes
• Tracing the hydrologic cycle - 18O, 2H, 13C
• Tracing mineral systems - 18O, 13C, 34S

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Nucleosynthesis of the elements and isotopes
1H 1H ? 2H b v 2H 1H ? 3He g 4He 4He
? 8Be 8Be 4He ? 12C g 12C 1H ? 13C b v
g 13C 1H ? 14N g 14N 1H ? 15O g ? 15N
b v 12C 4He ? 16O 16O 2n ? 18O . . .
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(No Transcript)
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H2O
Oxygen
Isotopes
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8 protons 10 neutrons

18O
18O
1

16O
500
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The Common Stable Environmental Isotopes
Isotope Ratio natural Reference
(ratio) abundance 2H 2H/1H
0.015 VSMOW (0.00015575) 3He 3He/4He 0.000138
Atmospheric He (1.3E6) 13C 13C/12C 1.11 VPDB
(0.011237) 15N 15N/14N 0.367 AIR N2
(0.003677) 18O 18O/16O 0.204 VSMOW
(0.0020052) VPDB (0.0020672) 34S 34S/32S 4.21
CDT (0.045005) 37Cl 37Cl/35Cl 24.23 SMOC
(0.324)
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Delta - permil d -
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Delta - permil d -
VSMOW
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What is the relative enrichment or depletion of
18O in crustal rocks (0.204) relative to
VSMOW
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What is the relative enrichment or depletion of
18O in crustal rocks (0.204) relative to
VSMOW
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What is the relative enrichment or depletion of
18O in crustal rocks (0.204) relative to
VSMOW
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What is the relative enrichment or depletion of
18O in crustal rocks (0.204) relative to
VSMOW 17.4 VSMOW ?
crustal rocks are enriched in 18O by 17.4 or
1.7 relative to the standard VSMOW
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Isotope Ratio Mass Spectrometry
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Distribution of isotopes in nature

• Isotope fractionation during reaction
• Rayleigh distillation during reservoir depletion
• Mixing

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Isotope fractionation, a
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Physico-chemical fractionation
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Physico-chemical fractionation
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Physico-chemical fractionation
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Physico-chemical fractionation
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Isotope partitioning functions
? symmetry value m mass of isotope E the
energy state summed from the zero-point to
energy of the dissociated molecule (Jmole1) k
Boltzmann constant (gas constant per molecule)
n 1.380658 1023 JK1 T
thermodynamic temperature K
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Units

• Isotope Enrichment (e)
• Isotope difference in permil units between two
  reacting phases at equilibrium
• when a is small, then we can use

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• For a water vapor exchange at 25C
• what is the d18O of vapor, where
• water d18Ow 0.0 VSMOW

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• For a water vapor exchange at 25C
• what is the d18O of vapor, where
• water d18Ow 0.0 VSMOW
• The fractionation factor (a) is
• a18Ow-v 1.0093
• The isotopic enrichment (e)
• e18Ow-v (a1) 103 9.3
• and e18Ov-w 9.3

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• For a water vapor exchange at 25C
• what is the d18O of vapor, where
• water d18Ow 0.0 VSMOW
• e18Ow-v (a1) 103 9.3
• d18Ovapor d18Owater e18Owater-vapor
• 0.0 9.3
• 9.3
• vapor d18Ov 9.30 VSMOW

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• For most reactions in geochemistry
• d values are typically 50 to 50
• a values are close to 1 (0.98 to 1.02)
• e values are typically 20 to 20
• Except for some extreme reactions and light
  isotopes . . .
• e.g. hydrogen gas produced from water is strongly
  depleted in 2H and has a fractionation factor
  a2HH2O-H2 3.76 at 25C.

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Fractionation and Temperature
lnaX-Y aT2 bT1 c
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Fractionation and Temperature
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Fractionation - Other Systems