Caution on the use of NBS 30 biotite for hydrogen-isotope measurements with on-line high-temperature conversion
Haiping Qi1, Tyler B Coplen, Gerard A Olack
1U.S. Geological Survey, 431 National Center, Reston, VA, 20192, USA.
Rationale:
The supply of NBS 30 biotite is nearly exhausted. During measurements of NBS 30 and potential replacements, reproducible δ(2)HVSMOW-SLAP values could not be obtained by three laboratories using high-temperature conversion (HTC) systems. The cause of this issue has been investigated using the silver-tube technique for hydrogen-isotope measurements of water.
Methods:
The δ(2)HVSMOW-SLAP values of NBS 30 biotite, other biotites, muscovites, and kaolinite with different particle sizes, along with IAEA-CH-7 polyethylene, and reference waters and NBS 22 oil that were sealed in silver-tube segments, were measured. The effect of absorbed water on mineral surfaces was investigated with waters both enriched and depleted in (2)H. The quantitative conversion of hydrogen from biotite into gaseous hydrogen as a function of mass and particle size was also investigated.
Results:
The δ(2)HVSMOW-SLAP values of NBS 30 obtained by three laboratories were as much as 21 ‰ too high compared with the accepted value of -65.7 ‰, determined by conventional off-line measurements. The experiments showed a strong correlation between grain size and the δ(2)HVSMOW-SLAP value of NBS 30 biotite, but not of biotites with lower iron content. The δ(2)HVSMOW-SLAP values of NBS 30 as a function of particle size show a clear trend toward -65.7 ‰ with finer grain size.
Conclusions:
Determination of the δ(2)HVSMOW-SLAP values of hydrous minerals and of NBS 30 biotite by on-line HTC systems coupled to isotope-ratio mass spectrometers may be unreliable because hydrogen in this biotite may not be converted quantitatively into molecular hydrogen. Extreme caution in the use and interpretation of δ(2)HVSMOW-SLAP on-line measurements of hydrous minerals is recommended.
