tivity on the carmel faul

3.3.2 Single sample dating
Single sample dating was done using equation [1] according to Broecker and
Kaufman (1965):
[1] [ 230 Th/ 234 U] = [ 238 U/ 234 U]*(1-e -λ230t ) + {1-[ 238 U/ 234 U]}*{λ 230 /(λ 230 -λ 234 )}*[1-e (-λ230-λ234)t ]
As mentioned above (ch. 1.3- Dating of speleothems), this equation is based on the
assumption that no initial 230 Th is present in the sample. Any initial 230 Th is always
accompanied by a much larger amount of 232 Th, therefore 232 Th is routinely monitored
during analysis, and samples containing less than an acceptable threshold of 230 Th/ 232 Th
can be identified (Hellstrom, 2007). According to Kaufman et al. (1998) a sample is
considered to have a high detrital content, which requires a correction for the contribution
of 230 Th by the decay of 232 Th, if this ratio is less than ~30, since for larger ratios the
correction becomes negligible compared to the analytical uncertainty. Richards and
Dorale (2003) and Li et al. (1989) argued that thresholds of between 100 and 300 for the
case of mass spectrometric dating, due to the high-precision analysis of initial 230 Th.
Hellstrom (2007) argued that even a value of 230 Th/ 232 Th=300 requires a correction for
the derital fraction.
Considering these higher values, most ages obtained for Denya Cave seismite samples
need to be corrected for initial 230 Th, using as a correction factor the detrital molar ratio of
232 Th/ 238 U. The most common correction factor for crust value rocks is 3.8 (Wedepohl,
1995). Even higher values of 4.2 were determined (e.g. Kuperman, 2005). Haase-
Schramm et al. (2003) reported a single sample correction factor for Lake Lisan
aragonites detritus of 232 Th/ 238 U atomic ratio=0.85 (i.e. 232 Th/ 238 U detrital molar ratio of
0.83=0.27 ac<strong>tivity</strong> ratio*3.05). They also suggested that some initial 230 Th found in those
samples was contributed by hydrogenous Th. In a carbonate terrain the correction factor
was calculated by Kaufman et al. (1998), using an isochron method, yielding a value of
1.8±0.25=detrital molar ratio (Soreq cave). In their study Kaufman et al. (1998) showed
that all the Th in the speleothems from Soreq cave was associated with detrital material.
In their method the slopes of isochrons in a plot of ( 232 Th/ 234 U) vs. ( 230 Th/ 234 U) were used
in order to calculate the detrital molar ratio of 232 Th/ 238 U. Those isochron lines are
essentially mixing lines between a clean sample (low detrital component) and the DEM
within the lamina, since stalagmites often have observable differences in the quantity of
detrital material along growth layers (Richards and Dorale, 2003). Following Kaufman et
al. (1998) isochron slopes and their intersects with the Y axis ( 230 Th/ 234 U) enables a
calculation of a correction factor as follows:
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