Environmental isotopes as tracers in geochemistry Environmental Isotopes

Environmental isotopes as tracers in geochemistry

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

Nucleosynthesis of the elements and isotopes + 1 H 2 H + b+ + v 2 H + 1 H 3 He + g 4 He + 4 He 8 Be + 4 He 12 C + g 12 C + 1 H 13 C + b+ + v + g 13 C + 1 H 14 N + g 14 N + 1 H 15 O + g 15 N + b+ + v 12 C + 4 He 16 O + 2 n 18 O. . . 1 H

H 2 O Oxygen Isotopes 18 O 16 O

8 protons + 10 neutrons 18 O 16 O 1 = 500

The Common Stable Environmental Isotopes Isotope Ratio 2 H 2 H/1 H 3 He/4 He 13 C/12 C 15 N/14 N 18 O/16 O 34 S/32 S 37 Cl/35 Cl % natural abundance 0. 015 0. 000138 1. 11 0. 367 0. 204 4. 21 24. 23 Reference (ratio) VSMOW (0. 00015575) Atmospheric He (1. 3 E– 6) VPDB (0. 011237) AIR N 2 (0. 003677) VSMOW (0. 0020052) VPDB (0. 0020672) CDT (0. 045005) SMOC (0. 324)

Delta - permil: d - ‰

Delta - permil: d - ‰ ‰ VSMOW

What is the relative enrichment or depletion of 18 O in crustal rocks (~0. 204%) relative to VSMOW

What is the relative enrichment or depletion of 18 O in crustal rocks (~0. 204%) relative to VSMOW

What is the relative enrichment or depletion of 18 O in crustal rocks (~0. 204%) relative to VSMOW

What is the relative enrichment or depletion of 18 O in crustal rocks (~0. 204%) relative to VSMOW = 17. 4‰ VSMOW crustal rocks are enriched in 18 O by 17. 4‰ or 1. 7% relative to the standard VSMOW

Isotope Ratio Mass Spectrometry

Distribution of isotopes in nature • Isotope fractionation during reaction • Rayleigh distillation during reservoir depletion • Mixing

Isotope fractionation, a

Physico-chemical fractionation

Physico-chemical fractionation

Physico-chemical fractionation

Physico-chemical fractionation

Isotope partitioning functions = symmetry value m = mass of isotope E = the energy state summed from the zero-point to energy of the dissociated molecule (J·mole– 1) k = Boltzmann constant (gas constant per molecule) = n · 1. 380658 · 10– 23 JK– 1 T = thermodynamic temperature K

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

For a water – vapor exchange at 25°C what is the d 18 O of vapor, where: • water d 18 Ow = 0. 0 ‰ VSMOW

For a water – vapor exchange at 25°C what is the d 18 O of vapor, where: • water d 18 Ow = 0. 0 ‰ VSMOW The fractionation factor (a) is: a 18 Ow-v = 1. 0093 The isotopic enrichment (e): and e 18 Ow-v = (a– 1) · 103 = 9. 3‰ e 18 Ov-w = – 9. 3‰

For a water – vapor exchange at 25°C what is the d 18 O of vapor, where: • water d 18 Ow = 0. 0 ‰ VSMOW e 18 Ow-v = (a– 1) · 103 = 9. 3‰ d 18 Ovapor = d 18 Owater – e 18 Owater-vapor = 0. 0 – 9. 3‰ = – 9. 3‰ • vapor d 18 Ov = – 9. 30‰ VSMOW

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 2 H and has a fractionation factor a 2 HH 2 O-H 2 = 3. 76 at 25°C.

Fractionation and Temperature lna. X-Y = a. T– 2 + b. T– 1 + c

Fractionation and Temperature

Fractionation - Other Systems