The absence of lithium isotope fractionation during basalt differentiation:
New measurements by multi-collector sector ICP-MS
Paul B. Tomascak *
Fouad Tera
Department of Terrestrial Magnetism, Carnegie Institution of Washington,
5241 Broad Branch Rd. NW, Washington, DC 20015
Rosalind T. Helz
U.S. Geological Survey,
Reston, VA 20192
Richard J. Walker
Department of Geology, University of Maryland,
College Park, MD 20742
Geochimica et Cosmochimica Acta, (1999)
v. 63 (6), p. 907-910
We report measurements of the isotopic composition of lithium in basalts
using a multi-collector magnetic sector plasma-source mass spectrometer
(MC-ICP-MS). This is the first application of this analytical technique to
Li isotope determination. External precision of multiple replicate and
duplicate measurements for a variety of sample types averages ±1.1permil
(2s population). The method allows for the rapid (~ 8 min/sample) analysis
of small samples (~ 40 ng Li) relative to commonly employed thermal
ionization methods. The technique has been applied to a suite of samples
from Kilauea Iki lava lake, Hawaii. The samples range from olivine-rich
cumulitic lava to SiO2- and K2O-enriched differentiated liquids, and have
d7Li (per mil deviation of sample 7Li/6Li relative to the L-SVEC standard)
of +3.0 to +4.8. The data indicate a lack of per mil-level Li isotope
fractionation as a result of crystal-liquid fractionation at temperatures
greater than 1050oC. This conclusion has been tacitly assumed but never
demonstrated, and is important to the interpretation of Li isotope results
from such geochemically complex environments as island arcs.