O-H_Stable_Isotopes

Fractionation of stable isotopes from one another

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Unformatted text preview: on the fractionation of stable isotopes from one another Wednesday, 15 August, 12 Stable Isotope Fractionation Isotopic Exchange Reactions (Equilibrium Fractionation) ½Si16O2 + H218O = ½Si18O2 + H216O Zn32S + Pb34S = Zn34S + Pb32S Wednesday, 15 August, 12 Stable Isotope Fractionation • • Degree = f(Δmass, temperature) • Insignificant for heavy radiogenic isotopes (e.g., 87Sr vs. 86Sr, 143Nd vs. 144Nd, 187Os vs. 188Os, 208Pb vs. 207Pb vs. 206Pb) Significant for light stable isotopes 2D vs. 1H 13C vs. 12C 15N vs. 14N 18O vs. 16O 34S vs. 32S Wednesday, 15 August, 12 Delta Function, δ • easier to measure ratios and easier to interpret differences, so values expressed as a relative difference function: ⎛ྎ Rx Rstd ⎞ྏ ⎛ྎ ⎞ྏ ⎛ྎ ⎞ྏ 3 Rx − Rstd 3 Rx δx = 10 ⎜ྎ − − 1⎟ྏ ⎟ྏ = 10 ⎜ྎ ⎟ྏ = 10 ⎜ྎ ⎝ྎ Rstd Rstd ⎠ྏ ⎝ྎ Rstd ⎠ྏ ⎝ྎ Rstd ⎠ྏ 3 • where Rx = ratio of heavy (rare) to light (abundant) isotope of element x • • expressed in parts per thousand or permil (‰) +ve values are isotopically heavier than standard, -ve values are isotopically lighter than standard Wednesday, 15 August, 12 Standards for Stable Isotopes • Measure isotope ratios on GCMS or SIMS using appropriate standards • Standards Element Standard Isotopic Ratio of Standard H C N O S V-SMOW V-PDB ATM V-SMOW CDT D/H = 155.76 x 10-6 13C/12C = 1123.72 x 10-5 N/A 18O/16O = 2005.20 x 10-6 34S/32S = 4500.45 x 10-4 V = Vienna standard, CDT = Canon Diablo troilite, PDB = Pedee Formation belemnite, SMOW = standard mean ocean water Wednesday, 15 August, 12 Delta Notation Wednesday, 15 August, 12 α and Δ Notation Where K = equilibrium constant and n = number of exchangeable isotopes ΔA-B = δA – δB ≈ 1000 lnαA-B Wednesday, 15 August, 12 Fractionation versus Temperature • For most anhydrous silicates, lnα...
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This document was uploaded on 03/06/2014 for the course ES 4502 at Memorial University.

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