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Introduction: Since the introduction of high
precision negative thermal ionization mass
spectrometry for Re and Os isotopic measurements
in 1991[1,2,3], two problems persisted in obtaining
an absolute chronology with the Re-Os system. The
first problem centers on the precision and accuracy
of Os spike calibration. The main difficulties arise
from the (1) absence of stoichiometric Os metal or
salts of high purity to serve as primary standards and
(2) the lack of isotopic equilibrium between Os of
different oxidation states and of different chemical
forms in the spike and standard. Re-Os  isochrons
have been determined to per-mil range precision in
both slope and initial [4,5,6]. But because the
accuracy of Os spike is currently limited to 0.5 to
1.6%, the isochron may be horizontally off by
~1.6%. This will have a significant effect of the
calculated initial Os isotopic composition. The
second problem is that the half-life of
187
Re is only
known to ~3% based on the direct measurement [7].
Any age calculated from the slope of an isochron
has an uncertainty of ~3%. The uncertainty of the
187
Re half-life ultimately depends on the accuracy of
the Os spike calibration. It is thus clear that this is
an area that needs further improvement, so that the
Re-Os chronometer as an absolute dating tool can be
applied to its fullest potential.