Pergamon
Geochimica et Cosmochimica Acta, Vol. 62, No. 23/24, pp. 3683–3688, 1998
Copyright © 1998 Elsevier Science Ltd
Printed in the USA. All rights reserved
0016-7037/98 $19.00
' .00
PII S0016-7037(98)00256-7
D
AVID
A. R
ICHARDS
, 1, * ,† S
IMON
H. B
OTTRELL
, 1 R
OBERT
A. C
LIFF
, 1 K
LAUS
S
TR
¨ , 1 and P
ETER
1 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
2 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
J. R
OWE
2
( Received July 31, 1997; accepted in revised form August 12, 1998)
Abstract —We demonstrate that U-Pb dating is a promising method for secondary carbonate materials of
Quaternary age and older by obtaining a low Pb ( # 10 ng g
206 Pb*/ 238 U age for a speleothem with high U ( !
10 !
g g
" 1 ) that is supported by an independent 230
" 1 ) and very
Th age. Thermal ionisation mass-spectrometry was used to determine the U and Pb isotopic ratios and concentrations for subsamples of a stalactite from
Winnats Head Cave, Peak District, UK. We obtained
206 Pb*/ 238
206 Pb/ 204
U age of 248 $ 10 ka, which is within error of the
Pb ratios up to 50, and determined a
207 Pb*/ 235 U age of 333 $ 79 ka and a-spectrometric U-Th age of % 255 ka. For samples of Tertiary and Quaternary age, the initial state of U-series disequilibrium is an important consideration and, as with most radiometric dating techniques, the mineral must have remained closed to U, Th, Pb, and all intermediate daughters. We show that dense calcite speleothems are ideal in this respect and that no loss of Rn has occurred. Unlike U-series disequilibrium methods, U-Pb dating has no upper limit and, hence, materials of Quaternary age older than 0.6 Ma can be analysed to investigate landscape development, paleoclimate, hominid evolution or hydrogeochemistry in carbonate terrains.
Copyright © 1998 Elsevier Science Ltd
1. INTRODUCTION
Valuable information related to such disparate fields of study as past climate, hydrogeochemistry, landscape development, and hominid evolution can be obtained from secondary calcite deposits such as speleothems and vein calcites. Numerous geochronological techniques have been applied successfully to secondary calcites to obtain information about environmental change for the last 0.6 Ma, but none has achieved adequate precision for older deposits of early Quaternary and Tertiary age. To remedy this situation, we investigated the feasibility of using U-Pb dating techniques that are more commonly used for much older deposits, indeed up to the age of the Earth.
Techniques utilising the U-series decay chain have provided accurate means of dating calcite deposits of Quaternary age.
Uranium-series disequilibrium methods have been by far the most commonly used, but have upper dating limits that preclude their application to early Quaternary and Tertiary deposits: High-precision thermal-ionisation mass-spectrometric
(TIMS) 230 Th234 U238 U techniques (Edwards et al., 1986;
Ludwig et al., 1992; Richards et al., 1994) are applicable to deposits less than 0.6 Ma; " -spectrometric 230 Th234 U238 U
(Gascoyne et al., 1978) have an age range up to % 0.4 Ma; and
" -spectrometric 227 Th/ 230 Th (Gascoyne, 1985) and TIMS
231 Pa235 U techniques (Edwards et al., 1997) can only be applied to younger deposits less than % 0.25 Ma. Electron-spin resonance (Gru¨n, 1989) and 234 U/ 238 U disequilibrium (Ludwig et al., 1992) have proved to be useful in extending the datable range beyond 0.5 Ma, but suffer from lack of precision. Uranium-lead dating uses the ingrowth of the stable Pb isotopes
206 Pb and 207 Pb from the decay of their respective parent
*Author to whom correspondence should be addressed (David.
Richards@bristol.ac.uk).
† Present address: School of Geographical Sciences, University of
Bristol, Bristol, BS8 1SS, UK.
isotopes 238 U and 235 U. Since the work of Moorbath et al.
(1987), U-Pb and Pb-Pb methods have been applied to various ancient carbonates (Paleozoic and Mesozoic), such as limestones (Smith and Farquhar, 1989), metamorphosed marbles
(Jahn and Cuvellier, 1994), calcite concretions (Israelson et al.,
1996) and paleosol calcite (Rasbury et al., 1997). For deposits less than a few million years, the amount of radiogenic Pb present is very small because of the long half-lives of the parent isotopes ( 238 U- 4.47
& 10 9 a; 235 U- 7.04
& 10 8 a) and, until recently, it has been considered impracticable to obtain ages with reasonable precision. Here, we show that some carbonate materials have high U concentrations, very low Pb concentrations, and a range in U-Pb that allows precise dating.
To demonstrate the feasibility of U-Pb dating of young secondary carbonates, we analysed a suite of subsamples from an independently-dated stalactite of late Quaternary age and high U and low Pb concentration using mass-spectrometric U and Pb isotopic measurements. Secondary carbonates are precipitated from fluids containing variable concentrations of U and Pb, negligible Th and Pa and, generally, 234 U and 238 U out of secular equilibrium. This being the case, initial U-series disequilibrium conditions are important considerations when dating young precipitates. We investigated this effect and subsequent closed system decay using " -spectrometric methods.
3683
2. U-PB ISOCHRON METHODOLOGY
Accurate dating using the U-Pb decay schemes requires appropriate corrections for Pb present initially; this is particularly important for the very young materials considered here. Where a range of U/Pb ratios exists in coeval samples, the isochron method allows simultaneous determination of the age and initial ratio. Closely spaced subsamples of a single stalactite, composed of pristine calcite and exhibiting a considerable range of U/Pb ratios, can thus be evaluated using the isochron treatment. In principle, two independent isochrons, based on
238 U and 207 Pb*/ 235 U, can be plotted, where 206 Pb* and 207
206 Pb*/
Pb* are the radiogenic Pb components. Alternatively, three-dimensional isotope plots which contain complete information about concordance between

3684
Fig. 1. Modified Tera-Wasserburg ( 207 Pb/ 206 Pb238 U/ 206 Pb) plot with disequilibrium concordia (after Wendt and Carl, 1985). The curvilinear trajectories (dotted lines), or concordia, show the change in
(
238
234
U/
U/
206
238
Pb and
U)
0 and
207 Pb/
231 Pa
0
206 Pb as age increases for different values of
( 230 Th
0
( 226 Ra
0
( 0. The near vertical lines
(solid) are isochrons. Ages reported in Ma. The secular equilibrium case, where the activity ratios of all parent-daughter pairs in the
U-series decay chains are unity is shown by a dashed line. Ages (in Ma) are represented by dots. Shaded region corresponds to region illustrated in Fig 3b.
the two decay schemes and common Pb (Wendt, 1984) can be used; we have used the variant developed by Wendt and Carl (1985) from Tera and Wasserburg (1972), as implemented in ISOPLOT 2.92 (Ludwig,
1994 , and pers. comm.).
For most applications of U-Pb methodology, departures from initial secular equilibrium of the U-series decay chain at the time of precipitation are insignificant relative to subsequent radiogenic ingrowth. For young samples, however, this becomes a significant consideration
(Ludwig, 1977; Wendt and Carl, 1985). It is well known that secondary carbonates such as speleothems are precipitated out of secular equilibrium; indeed, the initial radioactive daughter disequilibrium in such materials is utilised as a dating technique. Intermediate daughters such as 230 Th, 231 Pa are insoluble and strongly adsorbed on suspended sediments such that their incorporation in calcite is negligible (
( 231 Pa
0
( 0). In nearly all cases, expected for secular equilibrium with
230 Th
0
234 U is precipitated in excess of that
238 U. This must be taken into
( account by using an independent measurement of initial conditions,
234 U/ 238 U)
0
, if possible. The effects of 234 U/ 238 U disequilibrium on calculated age can be seen in Fig. 1, which illustrates the situation for a range of ( 234 U/ 238 a given 238 U/ 206
U)
0
Pb and typical of calcite deposited from freshwater: for
207 Pb/ 206 Pb the corresponding concordant age can vary significantly depending on initial conditions. It is likely that there will be some ported by its parent
226 Ra initially coprecipitated with U, but unsup-
230 Th; however, the effect of any initial abundance is minor because of the short half-life of 1600a and is irrelevant for the age range considered here.
In addition to the assumptions of initial conditions, the validity of
U-Pb methodology relies on the closed-system behaviour of U, Pb and intermediate nuclides in the decay chain. Concordance between the two
U-series decay chains is most likely to be compromised by Rn loss because Rn is the only gas in the decay chains and has a high diffusivity. Radon-222 in the 238 U decay chain has a half life of 3.8
days, much longer than the half-life of 219 Rn (3.96 s) in the 235 U decay chain. Therefore, partial loss of Rn will give rise to an apparent
206 Pb*/ 238 U age younger than the true age, whereas the 207 Pb*/ 235 U will remain unaffected. There is evidence that Rn loss is negligible from speleothems composed of dense calcite with columnar crystal fabric (Lyons et al., 1989). We investigated the possibility of Rn loss by measuring the activity of
238 U decay chain.
230 Th and 210 Po either side of 222 Rn in the
D. A. Richards et al.
3. SAMPLE AND LOCATION
A survey of published U concentrations and ages for UK speleothems dated by " -spectrometric 230 Th238 U techniques highlighted numerous samples from the caves of the Peak District in northern
England (Ford et al., 1983) as possible candidates for successful U-Pb dating. Winnats Head Cave (404 m above mean sea level, National
Grid Reference SK 1314 8282) is an abandoned swallet cave located at the northern edge of the Carboniferous Limestone outcrop of the Peak
District. Speleothem samples from phreatic chambers in the upper series were dated by Ford et al. (1983) to estimate the maximum age for the beginning of vadose conditions in the Winnats Head Cave. They obtained " -spectrometric 230 Th238 U ages (1 # ) of 176 $ ka for samples with U concentrations of 11.6 to 20.8
!
g g
8
7
" 1 to 191 $ 15
13
. T. D. Ford kindly sent us further samples from the same cave passage which were believed to be of a similar age. Sample WHC1 was found as a foundered sample and consists of two broken stalactites cemented by later flowstone deposition. Early growth in the larger stalactite is represented by an off-centre inner straw which is surrounded by asymmetric concentric growth up to 45 mm. Apart from the inner 5 mm, where a minor black detrital component is present, the stalactite is composed of pristine calcite. The microstructure of the speleothem shows no evidence for post-depositional dissolution or reprecipitation; primary morphology and crystal boundaries are preserved and secondary microcrystalline calcite is absent.
4. ANALYTICAL TECHNIQUES
Submillimetre wafers were cut from a slab of the larger stalactite using a diamond-wire saw dedicated to carbonate materials. Each wafer of 0.5–1.0 g was cut parallel to the growth bands from the inner straw to the outer edge. Cutting debris was removed by ultrasonification in acetone. Four of the subsamples analysed appeared to be composed of entirely pristine calcite, whereas two subsamples (WHC1-BA3 and 4) contained a small component of detrital material. A mixed 202 Pb233 U-
236 U tracer was added to the four pristine calcite samples prior to dissolution in a slight stoichiometric excess of 6N HCl. Subsamples
8B, 13 and 14A and 14B were divided at this stage into aliquots
(designated by bracketed letter) and run through chemistry separately.
Subsamples BA3 and BA4 were leached with a slight stoichiometric excess of 2N HCl and separate aliquots of tracer were added to the leachate and the insoluble residue fraction. The insoluble fractions of
BA3 and BA4 were centrifuged, rinsed, dried and weighed prior to addition of tracer. The major elements Ca and Mg were removed from solution in the leachate fractions by centrifugation after precipitation as fluorides on addition of a few drops of concentrated HF (after Smith et al., 1991). The supernatant liquids were adjusted to 1N HBr, and Pb was purified by standard HBr methods (AG1 x 8 in Br-form, 50 !
L x
2). The same separation technique was applied to the insoluble residue fraction after dissolution in HF and HNO
The U-bearing fraction was converted to NO drying in 8N HNO resin in NO "
3
3 three times prior to column chemistry (AG1 x 8 form; 500 !
L).
3 and conversion to 1N HBr.
"
3 form by dissolving and
Determinations of U and Pb concentrations and isotopic ratios by
TIMS were performed on a VG Micromass 30 in single Daly peak jumping mode. Pb was loaded onto single Re filaments with 2 !
L of Si gel and H
3
PO
4 and U was loaded onto a single Re filament with 0.5–1
!
L drop of graphite powder in suspension. Total procedural Pb blank was 200 –300 pg whose isotopic composition was carefully measured
( $ 2%): 207 Pb/ 204 Pb ( 15.34, 206 Pb/ 204 Pb ( 17.61, 208 Pb/ 204 Pb ( 36.74.
In most cases, the blank represented # 10% of total Pb; larger corrections of % 15% blank contribution were needed for subsamples BA8-B(B) and
BA14-B(B). Mean mass fractionation corrections, based on repeated analysis of NIST SRM 981 were 4.5‰ per a.m.u. for Daly collection. At the time of these analyses, 204 Pb abundances were consistently biased by 4‰ and an additional correction was made. All uncertainties due to the blank correction, 204 Pb bias and fractionation were fully propagated into the reported analytical errors.
Determinations of U, Th, and 210 Po isotopic ratios for pristine calcite from WHC1 were obtained by " -spectrometry. The radionuclides were separated from major elements and deposited on discs for " -counting using standard techniques (Gascoyne et al., 1978; Hamilton and Smith,
1986). Uranium and thorium were determined by isotope dilution using

U-Pb dating of a Quaternary speleothem 3685
Subsample a,b
U conc c
( !
g g " 1 )
Pb conc c
(ng g " 1 )
Table 1. U and Pb concentrations and Pb isotopic ratios
238 U/ 204 Pb
( & 1000)
238
(
U/
&
206 Pb
1000) 207 Pb/ 206 Pb 208 Pb/ 204 b 207 Pb/ 204 Pb 206 Pb/ 204 Pb
3(L)
3(R)
4(L)
4(R)
8B(B)
8B(C)
9
13(A)
13(B)
14A(B)
14B(B)
22.14
$ 2.5
— — — — — — —
0.155
$ 0.001
6.0
$ 0.2
1.58
$ 0.09
0.090
$ 0.005
0.868
$ 0.008
36.52
$ 0.43
15.25
$ 0.18
17.58
$ 0.15
19.78
$ 0.10
78.9
$ 0.6
15.4
$ 0.4
0.863
$ 0.008
0.864
$ 0.001
36.81
$ 0.92
15.43
$ 0.38
17.85
$ 0.44
1.04
$ 0.29
88.4
$ 0.7
0.70
$ 0.19
0.041
$ 0.011
0.887
$ 0.003
36.16
$ 0.25
15.15
$ 0.09
17.08
$ 0.09
29.11
29.40
$
$
0.24
0.14
37.06
$ 0.26
17.94
$ 0.14
17.80
26.78
20.58
$
$
$
0.50
0.26
0.72
2.3
$ 0.3
1141 $ 217 22.94
$ 1.75
0.346
$ 0.037
38.39
$ 1.00
17.22
$ 0.55
49.77
$ 6.10
8.8
$ 0.3
224 $ 9 9.37
$ 0.27
0.659
$ 0.006
37.45
$ 0.25
15.81
$ 0.10
23.98
$ 0.25
8.1
—
$ 0.3
143
—
$ 6
6.0
$ 0.3
198 $ 11
— —
2.8
$ 0.3
551 $ 71
6.74
8.49
16.91
—
$
$
—
$
0.24
0.36
1.19
0.731
0.679
0.499
—
$
$
—
$
0.006
0.011
0.027
36.62
37.40
37.38
—
$
$
—
$
0.45
0.56
0.67
15.55
15.79
16.24
—
$
$
—
$
0.17
0.25
0.31
21.25
23.26
32.58
—
$
$
—
$
0.25
0.43
1.95
c a
All errors are 2 #
(L) and (R) denote leachate and residuum b A, B denotes sample split prior to dissolution, (A), (B) denotes split after dissolution and addition of tracer.
Concentrations quoted relative to total sub-sample mass.
a 228 Th/ 232 U spike. This was calibrated in 1981 and now has a transient equilibrium activity ratio of 1.027 (Ivanovich et al., 1984). The specific activity of
(t
1/2 errors.
210 Po (t
1/2
( 138.3 d) was determined using a 208 Po spike
( 2.9 yr). All isotope ratios and concentrations are quoted with 2 #
5. RESULTS AND DISCUSSION
5.1 U and Pb Concentrations
Lead concentrations for subsamples of pristine calcite from
WHC1 were very low and ranged from 2.3 to 14.2 ng g " 1 . The detritally contaminated subsample BA4, in contrast, had a combined value for leachate and residue of 167.3 ng g " 1 . In spite of the pristine nature of the calcite in most of the subsamples, these are maximum estimates of calcite Pb concentration because it is difficult to isolate the pure calcite phase from minor impurities. Aliquots from the same dissolved subsample (BA8B and BA13) show different Pb concentrations
(Table 1). This is because of very small (negligible in comparison with total mass of subsample) but different amounts of the much less soluble detrital component in each subsample. Uranium concentrations for separate aliquots of BA8B and BA13 are in agreement and demonstrate that the U is predominantly from the calcite phase and Pb from the detrital phase. Published
Pb concentrations for carbonate materials exhibit a wide range of values, typically much higher than those obtained for pristine calcite in WHC1. The lowest reported values for aragonite are
23 ng g " 1 for an unaltered nautiloid of mid-Pennsylvanian age
(Smith et al., 1994), and 24 ng g " 1 for an unaltered Pliocene coral (Getty et al., 1996). Ancient limestones have Pb concentrations of 100 to 550 ng g " 1 (Smith et al., 1991; Jones et al.,
1995; Winter and Johnson, 1995), whereas Pb concentrations as low as 15 ng g " 1 have been reported for secondary calcite spar (Smith et al., 1991). Calcite concretions in Upper Cambrian black shales of southern Scandinavia (Israelson et al,
1996) contain much higher Pb concentrations (1.7 to 12 !
g g " 1 ).
g
Uranium concentrations in WHC1 are high (17.8 to 37.06
!
g
" 1 ), significantly greater than most published values for secondary carbonates, although calcite concretions in black shales also have high concentrations (23.4 to 167.7
!
g g " 1 ; Israelson et al., 1996). Ford et al. (1983) report U concentrations of up to
74.8
!
g g " 1 for speleothems from the lower series in Winnats
Head Cave. Because of the high U and very low Pb concentrations, an exceptionally large range in !
values ( 238 U/ 204 Pb) from 700 $ 190 to 1,141,000 $ 217,000 was obtained for
WHC1. The previously highest recorded !
value for carbonates was 288,000 for a 400 Ma calcite concretion in Upper Cambrian black shales (Israelson et al., 1996). It is clear that speleothems must have precipitated from waters with a very high dissolved U content, possibly derived from one or more of the following sources:
1) Former cover of Namurian Edale Shales and Millstone
Grit or locally derived loessic soil. Sediment derived from the westward erosion of overlying marine black shales and
Millstone Grit has been deposited in such karst features as fissures, sinkholes, collapsed depressions, and within caves. The entrance of the nearby cave of Treak Cliff
Cavern, 500 m from Winnats Head Cave, is in a remaining pocket of these U-rich shales and speleothems therein have similarly high U contents—7.0 to 9.8
!
g g " 1 (Ford et al.,
1983). Authigenic U concentrations of up to 24 !
g g " 1 have been measured by XRF on pressed powder pellets of cave sediments from the nearby cave passages of Speedwell Cavern (Bottrell, 1993). Most of the limestone plateau is covered by a thin ( # 0.3 m) locally-derived loessic or
‘silty drift’ layer at present and, although the age is indeterminant, it could have been deposited during one or more of the middle to late Pleistocene glaciations.
2) The Asbian age shelf-margin reef complex. Much greater
U concentrations have been observed by field $ -spectrometry for the limestones of the reef complex (6 to 24 !
g g " 1 ) compared with the surrounding basinal and lagoonal limestones (2.5 to 12 !
g g
" 1 ) (Hyland, 1995).
3) A high density of major and minor mineral veins cross the region, associated with extensive lead-zinc-baryte-fluorite mineralization. These play an important part in the karst hydrology of the area and if there are minor uranium minerals associated with these deposits they might be selectively mobilised by vadose waters.

3686 D. A. Richards et al.
204
Fig. 2. Uranium-lead isochrons for the WHC1 stalactite. (a)
Pb206 Pb/ 204 lower !
238 U/
Pb isochron plot. Inset shows data for subsamples with values. (b) 235 U/ 204 Pb207 Pb/ 204 Pb isochron plot.
5.2 U, Th and Pb Isotopes and Age Calculation
Figure 2a shows a U-Pb isochron plot where measured values of 206 Pb and 238 U are normalised to 204 Pb. Initial 206 Pb/
204 Pb from Fig. 2 is 17.28
$ 0.33, which is within error of the values obtained for the insoluble residue fractions BA3(R) and
BA4(R) (Table 1). There is a large range of !
(defined as
238 U/ 204 Pb) which leads to a well-defined isochron slope
( 206 Pb*/ 238 U) value of (2.864
$ 0.14) x 10 " 5 (95% confidence limits) with a mean square weighted deviates (MSWD) value of
15.4. Alpha-spectrometric U and Th measurements yielded an age of 255
$
$ 53
40 ka and an initial
0.08
0.07
. By modifying the standard
234 U/ 238 U activity of 1.32
206 Pb/ 238 U equations assuming an initial state of U-series disequilibrium calculated from the measured 234 U/ 238 U and negligible 230 Th and 226 Ra (Ludwig, 1977; Wendt and Carl, 1985), we obtained an age of
248 $ 10 ka that compares very well with the " -spectrometric
230 Th age. The effect of radioactive disequilibrium was significant; ignoring secular disequilibrium conditions at the time of formation would yield an age estimate of 187 $ 9 ka.
207
Fig. 3. Tera-Wasserburg style three-dimensional concordia plot for the WHC1 stalactite. Shown is the projection onto the
Pb/ 206
238 U/ 206 Pb-
Pb plane (see Fig. 1 caption). The intercept of the threedimensional fit with this plane is shown by the shaded error ellipse. (a)
Three-dimensional regression, disequilibrium concordia [( 234 U/
238 U) with
0
(
238 U/
1.32; dashed line], and plane intercept. (b) Detail of intercept
206 Pb207 Pb/ 206 Pb plane, disequilibrium concordia ( $ 2 # error), regression line, and the secular equilibrium concordia.
Figure 2b illustrates a 235 U/ 204 Pb207 Pb/ 204 Pb isochron plot for subsamples of WHC1. It is clear that the relative errors are much larger in this case, mainly because of the much smaller abundance of the parent 235 U than 238 U and, therefore, smaller effective range in suming no initial
235 U/ 204 Pb. The resultant apparent age, as-
231 Pa, is 333 $ 79 ka based on a regression with MSWD of 2.5. The initial 207 Pb/ 204 Pb ratio is 15.24
$ 0.13.
The two standard isochron methods give similar ages within error, demonstrating a reasonable degree of concordance. The degree of concordance can also be visualized by plotting the data on a projection from the common Pb plane of a threedimensional Tera-Wasserburg plot to the concordia plane 238 U/

U conc ( !
g g " 1 )
24.4
All errors are 2 #
U-Pb dating of a Quaternary speleothem
( 234 U/ 238 U)
1.155
$ 0.029
Table 2. Alpha-spectrometric U-series results for WHC1
( 234 U/ 238
1.321
$
U) init
0.080
0.069
( 230 Th/ 234 U)
0.939
$ 0.034
( 230
900
Th/
$
232 Th)
200
3687
( 210 Po/ 230 Th)
0.942
$ 0.064
Age (ka)
256 $ 53
40
206 Pb207 Pb/ 206 Pb (Fig. 3). The radiogenic component of the sample is represented by the error ellipse of the concordiaplane intercept of the regression line (Ludwig and Titterington,
1994). The error ellipse intersects the disequilibrium concordia line at % 245 ka, in agreement with the two-dimensional U-Pb isochron and U-Th ages. The regression line has a MSWD of
8.04, indicating that the scatter in the data is greater than that expected from analytical error alone. The spread in the data is better explained by a plane ( Kent et al., 1990) with a MSWD of 1.36, which is likely to have been caused by a mixture of more than one component of common Pb. A statistically significant correlation between 208 Pb/ 204 Pb and 206 Pb/ 204 Pb suggests that there are two components; non-radiogenic detrital Pb and calcite lattice bound radiogenic Pb.
A test of the possibility of recent Rn loss was performed by measuring the activities of 210 Po and 230 Th. Because WHC1 is known to be much greater than 20 ka, 210 Po and 230 Th should be in isotopic equilibrium. The measured ( 210 Po/ 230 Th) activity ratio is 0.942
$ 0.064, which is within error of unity (Table 2).
The lack of significant loss of 222 Rn from the sample is consistent with the findings of Lyons et al. (1989), who only found significant 222 Rn loss from porous speleothem materials.
6. IMPLICATIONS
This study demonstrates that some speleothems have high enough U concentrations and low enough Pb concentrations for substantial changes in Pb isotopic composition to be generated in periods as short as 100 ka. For such samples, precise ages of geologically young carbonate material can be determined by
U-Pb dating isochron methodology and accounting for initial state of U-series disequilibrium. For the particularly favourable sample analysed from the Winnats Head cave, the 206 Pb*/ 238 U age is insensitive to uncertainties in the initial Pb composition.
For example, substituting a higher value of 18.45 for initial
206 Pb/ 204 Pb, based on galenas from the South Pennine orefield
(Coomer and Ford, 1975), does not reduce the age outside error. We have screened numerous speleothems from around the world for U and Pb concentrations by ICPMS and find that, while the range of U/Pb ratios is greater than eight orders of magnitude, the vast majority of samples have lower U/Pb ratios than WHC1. This being the case, the isochron approach evaluated in this study will be essential for most of the favourable samples and, for many, alternative techniques with higher precision will be required. Where the major limitation on precision is instrument isotope-fractionation effects, uncertainties can be reduced by double spiking (Todt et al., 1996) or using the new generation of plasma-source magnetic-sector multi-collector mass spectrometers with an internal thallium standard (Freedman et al., 1997).
Consideration of the initial state of U-series disequilibrium is important for accurate U-Pb dating of young samples. For samples less than 500 ka old, typically, estimation of initial
234 U/
234 U/
238
238
U is possible by back-calculation from the measured
U ratio. For older samples, where secular equilibrium is re-established, the effect of initial disequilibrium on U-Pb ages becomes increasingly less important, and regional estimates of initial 234 U/ 238 U may be used. Closed-system behaviour after deposition is also essential and the possibility of Rn loss is a concern, particularly for porous samples. The result of our 210 Po and 230 Th measurements is an encouraging indication that some speleothem calcite is closed with respect to Rn loss.
This technique will prove valuable to those studying landscape development, paleoclimate, hominid evolution or hydrogeochemistry in carbonate terrains prior to 0.5 Ma. Having demonstrated consistent age determination in a young sample that has been independently dated by 230 Th234 U238 U methodology, we envisage the principal application of U-Pb dating in extending the record of speleothem deposition back beyond the limits of the more usual U-series disequilibrium series methods. For these older samples, the minimum U/Pb ratio required for precise age determination diminishes with increasing age and a wide range of older speleothems may be datable.
Acknowledgements —We are grateful to Dr. T. D. Ford for providing the speleothem samples from the Peak District. We thank Dr. K.
Ludwig for insightful comments and provision of the latest version of his invaluable program ISOPLOT, and an anonymous reviewer for a thorough and helpful review. This work was supported by a NERC grant GR3/9358.
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