第四節 氢穩定同位素 氢同位素的基本特征 测量方法 国际标准 分馏系数 常见应用

氢同位素特征 1H, 99.9844% 2H, 0.0156% (D deuterium， Urey 1931) 3H (tritium), radioactive, half-life = 12.26 yrs 2H/1H (D/H), 1.557*10-4 (Way et al., 1950) Oxidation states: H2O, H3O+, OH-, H2, CH4 H plays a major role in a wide variety of natural processes Dm/m is the largest, so does the largest a

Characteristic physical properties of H216O, D216O, and H216O

氢同位素范围

氢同位素测量方法 Equilibration of ml-sized samples with H2. Due to very large fractionation factor (0.2625 at 25oC), the measure H2 is complicated because of large depletion. Water is converted to H2 by passage over hot metals (U, Zn, Cr). Off-line Water is converted into pure H2 via combustion, reduction, and pyrolysis reactions in the presence of catalysts. On-line

Glass line, equilibrium method, H2 to IRMS (ml-size), ±1-3‰ EA-HT-IRMS, pyrolysis reaction (>1450oC), He carrier, 100-200ml, ±1-1.5‰ GB-IRMS, reduction and pyrolysis (Pt), He carrier, 100-500ml, ±1‰ TC/EA-IRMS, combustion, pyrolysis (>1450oC), He carrier, 10ml, ±1.5‰ GC-MS and GC-C-MS, H in organics H2+ (mass 2) and HD+ (mass 3) are measured by IRMS (4He+ tailing problem)

Standards V-SMOW Vienna Standard Mean Ocean Water 0‰ GISP Greenland Ice Sheet Precipitation -189.9‰ V-SLAP Vienna Standard Light Antarctic Precipitation -428‰ NBS-30 Biotite -65‰ Working standards

H fractionation factor = 1.074 At 25oC between Water and vapor

H isotope variations are due to phase transitions of water between vapor, liquid, and ice through evaporation/precipitation and/or boiling/condensation in the atmosphere, at the earth’s surface, and in the upper part of the crust. D water-ice = 21.2‰ During evaporation and condensation of water, H and O isotopes are fractionated in a similar fashion with a different magnitude, so that dD = 8d18O +10 in general

But the slope and intercept vary depending on the conditions of evaporation, vapor transport and precipitation. Global Meteoric Water Line

氧化还原反应

矿物与结晶水间的分馏 For H, pressure is a variable that must be taken into account in fluid-bearing systems.

Kinetic effect H isotope fractionation occurs during photosynthesis such that deuterium is depleted in the organically-bound hydrogen. The complexity of the various reactions during cellular metabolism involved in this fractionation makes the quantitative modeling on-going. (tree-ring) In salt solutions, isotopic fractionations can occur between water in the “hydration sphere” and free water, affected by charge and radius of salt ions.

Hydrosphere 大气降水

Influence of geographic and meteorological factors (Rainout, temperature, humidity) 纬度效应 高程效应 温度效应 雨量效应 dD = (8.17±0.06)d18O + (10.35±0.65)

Ice core (Temperature record) 南极-East Antarctic 北极-Greenland

Evaporation effect In an evaporative environment, enrichment of d18O is relatively larger than that of dD. This is because molecular diffusion adds a non-equilibrium fractionation term and the limited isotopic enrichment occurs as a consequence of molecular exchange with atmospheric vapor.

The seasonal variation of all meteoric water is strongly attenuated (混合作用) during transit and storage in the ground, but the attenuation varies with depth and with surface and bedrock geologic characteristics. Deep groundwaters show no seasonal variation and have an isotopic composition close to amount-weighted mean annual precipitation values. Evaporative losses before and during recharge shift the isotopic composition of groundwater towards higher d-values.

Formation water Formation waters are salines with salt contents ranging from ocean water to very dense Ca-Na-Cl brines. O and H isotopes are a powerful tool in the origin of subsurface waters. Although formation waters show a wide range in isotopic composition, waters within a sedimentary basin are usually isotopically distinct. Fluids most distant from the MWL tend to be the most saline. d value decrease with altitude. Water-rock interaction leads to d value shift.

Biosphere dD in plants

Upper Mantle/Magmatic system

Ore deposits and hydrothermal system