U/Th dating of impure carbonates: 230Th/232Th activity ratios in detrital material

U/Th dating ofimpure carbonates: 230Th/232Th activity ratios indetrital material A. Martínez‑Aguirre1 · J.M.Alcaraz‑Pelegrina2· J.Rodríguez‑Vidal3 Abstract In environmental age dating the 230Th/232Th activity ratio is widely used as an indicator of detrital contamination. A clear relationship between this activity ratio and the carbonate sample’s age has previously been demonstrated, whereby a detrital correction must be applied when the 230Th/232Th activity ratio of the leach drops to 20. We demonstrate that in some cases carbonate samples with 230Th/232Th well above 20 must be corrected, otherwise an overestimation of the sample’s age is obtained. Evaluation of the relationship between 230Th/234U and 230Th/232Th in the carbonate’s aliquots will enable evalua-tion of the limit at which a carbonate can be considered pure or impure. Keywords U/Th dating· Carbonates· Detrital material· Age corrections· Radio chronometry Introduction Isotopic imbalance between 230Th and 234U or 238U has been widely applied to the dating of carbonate precipitates from aqueous solutions under a variety of environmental conditions [1]. The solubility of U in natural waters allows it to co-precipitate with precipitating solid, whereas Th, as an insoluble element, is unavailable for co-precipitation. Hence, the decay ingrowth, 230Th, once the carbonate has been precipitated, supplies a method for age determination of geological systems. Dating of such systems, provided that the system has been closed since its formation and that 230Th is not co-precipitated with the U, is straightforward, using the following Bateman Eq.(1)[2]. In the Eq.(1), the activity ratios are at time t of sample measurement since sample precipitation and λo and λ4 are the decay constants for 230Th and 234U, respectively. However, many of these carbonates are impure and can contain large amounts of detrital particles, inter-grown or cemented by the carbonates. This detrital material can contribute significant though undetermined amounts of Th and U, which during dissolution of the carbonate sample for U and Th analysis can be carried over into the leach. Thus, if significant 232Th is present in the material to be dated, a correction for the 230Th, 234U and 238U which must have accompanied the 232Th needs to be made. All the various correction methods use 232Th to indicate the degree of detrital contamination carried over into the leach. At the time of deposition, the pure carbonate has negligible amounts of 232Th and 230Th. Thus, any 232Th found in the leach must be contributed by the detrital material. This material can also contain various amounts of 230Th which must be corrected to obtain the amount in the pure carbonate. Additionally, some U from the detrital material can also carry over into the leach (1) 230 Th∕ 238 U= (( 1−e −λot) + (234 U∕ 238 U ) −1 ) (λo∕(λo−λ 4))(1−e−(λo−λ4)t) or 230 Th∕234U=(1∕(234U∕238 U))(1−e−λot) + ( 1− ( 238U∕234U ))( λo∕ ( λo−λ 4 ))( 1−e−(λo−λ4)t ) *A. Martínez-Aguirre [email protected] *J. M. Alcaraz-Pelegrina f[email protected] *J. Rodríguez-Vidal [email protected] 1 Department ofApplied Physics I, ETSIA, University ofSeville, Ctra. Utrera km.1, 41013Seville, Spain 2 Department ofPhysics, Edificio C-2, University ofCórdoba, Campus de Rabanales, Ctra Madrid-Cádiz, km 396-A, 14071Córdoba, Spain 3 Department ofEarth Sciences, University ofHuelva, Avd. Tres de Marzo s/n, 21071Huelva, Spain during dissolution of the sample, whereby a correction for this effect is also necessary. A common approach is to analyse three or more cogenetic samples with variable 230Th/232Th, 234U/232Th and 238U/232Th activity ratios. Rosholt [3] and Schwarcz and Latham [4] used an isochron technique for dating travertines and caliches in which 232Th was used as the normalizing variable. Samples were leached with diluted acid (L/L technique), 230Th/232Th was plotted against 234U/232Th or 238U/232Th for each leach and the slope of the line connecting the pairs was considered as the 230Th/234U or 230Th/238U activity ratios in the pure carbonate end-member. A similar isochron with 234U/232Th versus 238U/232Th is used to obtain the 234U/238U activity ratio in the pure carbonate. Once obtained, the activity ratios in the pure carbonate component of the calcite sample can be used in the above equation to obtain the sample’s age. High values of this activity ratio indicate high detrital contamination, so a detrital correction is necessary. Otherwise an overestimation of the sample’s age is obtained. Bischoff and Fitzpatrick [5] suggested that detrital contamination is significant when the 230Th/232Th activity ratio drops to 20; those with values above 20 may be considered pure carbonate samples and correction is not necessary. However, as we will show in some cases, even impure carbonate samples with 230Th/232Th well above 20 needs to be corrected for detrital contamination to obtain the real age of the samples, otherwise an overestimation of the respective sample age is obtained. Moreover, because the CaCO3 initially lacks 230Th and all 232Th has come from the detritus, all leaches must have the same initial 230Th/232Th as the detritus at the time of the carbonate’s crystallization. Allegre and Condomines [6] showed that for the simple case in which 234U and 238U are in equilibrium, the thorium ratio of the various cogenetic samples is described by the Eq.(2) In the above Eq.(2), the (230Th/232Th)o activity ratio is at the time of precipitation of the carbonate and the others are at the time of measurement of the sample. Decay of 238U and 232Th is negligible for the time under consideration, so their activities and ratios are constant over time. At the time of crystallization, t = 0, some cogenetic samples can have lower 230Th activity than 238U activity and 230Th activity will consequently increase with time. Others may have 230Th activity in excess of 238U activity, and 230Th will decrease with time. Both cases are described with the above Eq.(2). At t = 0, a plot of 230Th/232Th versus 238U/232Th for all cogenetic samples will show a horizontal line, with the y-intercept being the initial thorium ratio (230Th/232Th)o. After the time t, the same plot will show (2) 230 Th∕ 232 Th = (230 Th∕ 232 Th )o e −λot + (238 U∕ 232 Th )( 1−e −λot) data points on a line with a slope (1 − e-λot). At t >>1/λo, 230Th/232Th = 238U/232Th, representing secular equilibrium; the points will lie along a line with unit slope and passing through the origin. This means that the line rotates over time, starting from an initial slope of 0 and ending with a slope of unity. This rotation occurs around a point called the equipoint defining the initial 230Th/232Th, which should be the initial value in the detritus [5]. In the event of initial imbalance between 234U and 238U, an equation similar to the one above, with 234U instead of 238U, will hold. Twelve carbonate samples from the Rock of Gibraltar were analysed to conduct a study of its geological evolution. Samples GB01 to GB09 were collected in the same cave, St. Michael’s Cave entrance, samples GB010 and GB011 at Forbes’ Quarry and sample GB101 at Beefsteak Cave. The geological implications of dating such samples have already been published in Rodriguez-Vidal etal. [7]. Here we will present the study concerning the data obtained in the U/Th dating of the samples. Our paper only focuses on problems associated to dating, particularly two major problems. On the one hand, we will discuss the need for detrital correction in several carbonate samples in which the 230Th/232Th activity ratio of the leachate stays above 20 [5], to thereby avoid overestimation of the age sample. On the other, we will study the possibility of obtaining some interesting activity ratios in the detrital material included with the pure carbonate. This may enable us to evaluate the limit at which a carbonate could be considered a pure or impure carbonate. Experimental During development of the PaleoMed Project (see acknowledgements) focusing on the Rock of Gibraltar, numerous calcite samples were dated by the U/Th method. Most samples resulted as being pure carbonates (230Th/232Th activity ratios well above 20) and the obtained ages were the ages expected by the researchers [7]. However, some of those sample results showed an age well above what was expected. Hence, even with the 230Th/232Th activity ratio above 20, those samples were treated as impure carbonates. Indeed, different aliquots of those samples result in different nominal ages (Eq.1), clearly confirming their impurity. Once corrected for their detrital material, the resulting ages were as expected [7]. In this paper we present this dating work. Twelve calcite samples, collected at the Rock of Gibraltar, were used to study the effect of detrital material on determining the age of the respective calcite sample. The Rock of Gibraltar is a north-south peninsula; the eastern side is very steep while the western side has a gentler slope. The results of geomorphological studies of this rock can be seen in [7]. Calcite samples were powdered and homogenized using a hammer and mortar prior to dissolution in a solution 2M of nitric acid (HNO3). For the solution HNO3, 69% purity and distilled water was used. After all the calcite dissolved, the residue was separated as soon as possible by centrifuging for 5min at 4000rpm in a Selecta Mixtasel centrifuge with 0.45μm pore size Millipore filter 47mm diameter filtration to eliminate all solid particles and thus minimize Th re-adsorption. The residue was discarded and the leach kept for further U and Th isotopes analysis. 1ml of FeCl3·6H2O (10mg/ml) as iron-carrier and well-known activities of 232U and 229Th (as U and Th tracers, calibrated and certified by CIEMAT, Spain), for yield determinations (both isotopes also alpha-emitters), were added before precipitation of iron hydroxides. The U and Th isotope precipitate with Fe hydroxides and the solution is discarded. The precipitate was dissolved in 20ml 8M HNO3 and a solvent extraction technique in a decantation funnel (100ml volume) with 5ml of tri-butyl-phosphate (TBP MERCK pro analysis) as the organic compound was used to separate U and Th from iron and other actinides. Both phases are mixed by shaking for 5min, followed by a 5-min rest until both phases are again separated. U and Th remain in the organic phase and the inorganic phase is discarded. This process is repeated twice with 10ml of 8M HNO3, to eliminate all Fe and other actinides that may have remained in the organic phase. The next step was to extract Th from this organic phase. For that purpose, 20ml of Xylene (Panreac pro analysis) is added to the TBP and 15ml 1.5M HCl is used to separate Th from the organic phase. Both phases are mixed by shaking for 5min followed by a 15-min rest until both phases are again separated. U remains in the organic phase whereas the Th is extracted in the inorganic phase. This is repeated twice to extract all Th from the organic phase. The Th fraction requires further purification and 15ml of distilled water is finally used as the inorganic phase to back-extract U from the same organic phase, TBP + Xylene [8], shaking for 5min followed by a 5-min rest. The process is repeated twice to extract as much U as possible. The U solution is then ready for electrodeposition onto a stainless-steel disk (see below). 0.5ml of FeCl3·6H2O (10mg/ml) is added to the Th fraction and a precipitation of iron hydroxides with ammonia solution 25% is carried out. This precipitate is dissolved in 5ml of 8M HCl and 5ml of HCl. As some U traces are also back-extracted with Th, an anion exchange resin (BIO-RAD AG 1 × 8, 100–200 mesh, hydrochloric form) is used for Th purification. A 4cm long resin column (100ml volume glass column 12mm diameter with a 4mm hole at the bottom) is conditioned twice with 20ml of 8M HCl and the solution is added to the column. U traces and iron stay in the column whereas Th passes through the resin and is collected in a beaker. Another 20ml of 8M HCl is added to the column to recover traces of Th. 0.5ml of FeCl3·6H2O (10mg/ml) is added to the solution and precipitation of iron hydroxides along with the Th with ammonia solution 25% is again carried out. Further Th purification is necessary, using the same resin in a narrower column (25ml volume glass column 6mm diameter with a 2mm hole at the bottom). 4cm of the resin is conditioned twice with 10ml of 7M HNO3. The Th precipitation is dissolved with 2ml of concentrated HCl plus 3ml of 7M HNO3 and added to the column along with 2 more ml of 7M HNO3. Iron and other impurities pass through the resin and Th remains. The resin in cleaned with another 20ml of 7M HNO3. Finally, Th is extracted from the resin with 20ml of 2M HCl [8]. This solution is ready for electrodeposition onto a stainless-steel disk. 1ml of 0.3M Na2SO4 is added to the U and Th final solutions to avoid deposition of U and/or Th on the beaker walls, evaporated until dry and dissolved with distilled 0.3ml H2SO4 and 4ml of distilled water. The pH is adjusted to 2.3–2.5 with ammonia solution 25%. The solution is added to the polyethylene electrolytic cell with 5ml of H2SO4 1%. At the bottom of the electrolytic cell a 2cm stainless-steel disk acts as the cathode and a platinum wire acts as the anode. The system is connected to a power supply. Electrodeposition [8, 9] of U or Th is carried out for 1h at 1.2 amperes. One min before the hour, 1ml of ammonia solution 25% is added to the solution. The disks are disconnected from the power supply and cleaned with acetone and distilled water. The U/ Th disk is finally measured by alpha spectrometry with PIPS detectors in a CAMBERRA Alpha Spectrometer previously calibrated for energy and efficiency with a mixed alpha source of known activity. Results anddiscussion U and Th isotope concentrations and some activity ratios were obtained in twelve samples collected at the Rock of Gibraltar. The data were used on the one hand to evaluate the extent of Th contamination in impure carbonates with 230Th/232Th activity ratios above 20 [5], and on the other to evaluate the 230Th/232Th activity ratio in the detrital material of the impure carbonates. Aliquots with230Th/232Th above20 The activity concentration of U and Th isotopes in mBq/g and corresponding 234U/238U, 230Th/234U and 230Th/232Th activity ratios are presented in Table1 for twelve carbonate samples, collected at the Rock of Gibraltar. The results in Table1 show that it is clear that except for sample GB06, which must be corrected for detritus (230Th/232Th below 20), all the other samples seem to be pure carbonates, as their 230Th/232Th is above 20. Except for that sample, Bateman equations were used to obtain the age of the carbonate samples. The results, as well as the 234U/238U activity ratio in the pure carbonate at the time of deposition, are also presented in Table1. Except for three samples (GB01, GB04 and GB101) with higher activity concentrations, probably precipitated from waters with higher U content, U concentration range from 2 to 4mBq/g, no significant differences were observed between the three locations. However, the samples at Forbes’s Quarry have U concentrations slightly lower than the samples from St. Michael’s cave and a sample collected at Beefsteak Cave (GB101) is one of the samples with the highest activity concentrations, 6.3–6.9mBq/g for each isotope, respectively. The similarities and differences are more evident in the 234U/238U activity ratios at the time of deposition (column 7). All samples collected at St Michael’s Cave clearly contain both U isotopes in secular equilibrium, whereas in the others there is a slight excess of 234U. The similarities and differences must be related to the activity ratio in the waters from which the carbonate was precipitated. As the 230Th/232Th activity ratio in sample GB06 was below 20, it was considered an impure carbonate and correction for its detrital material was thus necessary. Several aliquots (from the same original homogenized sample) were prepared by dissolving with different HNO3 concentrations (from 2 to 8M), from 100 to 250ml depending on the sample’s mass; the results are shown in Table2. As expected, different acid concentrations yield different activity ratios. The LL method and Rosholt diagrams (see Fig.1) were thus used for detrital correction and the activity ratios and age of the pure carbonate obtained using the ISOPLOT program (3D plot) [10]. These results are also presented in Table2. As expected, the age of the pure carbonate (70 ky), once corrected, clearly shows that when the 230Th/232Th activity ratio drops to 20, detrital material correction is necessary. As stated above, the 230Th/232Th activity ratios in Table1, above 20, indicate that those samples can be considered pure Table 1 Specific activities (mBq/g) and activity ratios in the analysis of 12 carbonate samples collected at the Rock of Gibraltar. It can be seen that, except in sample GB06, 230Th/232Th activity ratios are well above 20; they can thus be considered pure carbonate samples. In the last two columns the nominal age (obtained by Eq. 1) in ky and the 234U/238U activity ratio at the time of calcite precipitation are also given. For error determination, propagation of errors was used (for example for 238U activity concentration the equation used was 238U = (N238/N232) 232U, where N238 and N232 are the alpha counts for 238U and 232U, respectively and 232U the added activity of the U tracer) 238U230Th 234U/238U230Th/234U230Th/232Th T (ky) (234U/238U)o GB01 5.57 (0.14) 4.06 (0.14) 0.974 (0.014) 0.747 (0.032) 298 (80) 151 (14) 0.960 (0.021) GB02 2.90 (0.09) 1.15 (0.07) 1.047 (0.027) 0.379 (0.024) 29.9 (6.8) 51.5 (4.2) 1.054 (0.031) GB03 2.69 (0.06) 1.59 (0.05) 1.067 (0.020) 0.551 (0.021) 164 (37) 86.1 (4.9) 1.085 (0.025) GB04 7.02 (0.13) 5.06 (0.15) 0.997 (0.012) 0.723 (0.025) 699 (221) 140 (10) 0.995 (0.017) GB05 3.06 (0.06) 2.32 (0.06) 1.045 (0.018) 0.725 (0.025) 119 (18) 138 (9) 1.066 (0.027) GB06 3.93 (0.23) 3.00 (0.10) 1.031 (0.064) 0.741 (0.049) 10.5 (0.7) 145 (20) 1.046 (0.097) GB07 3.38 (0.08) 1.36 (0.04) 1.023 (0.022) 0.394 (0.015) 35.8 (4.1) 54.3 (2.7) 1.027 (0.026) GB08 3.05 (0.06) 1.02 (0.03) 1.052 (0.017) 0.319 (0.012) 154 (38) 41.5 (1.8) 1.058 (0.019) GB09 2.82 (0.07) 2.21 (0.06) 1.021 (0.023) 0.767 (0.028) 552 (13) 157 (13) 1.032 (0.036) GB010 1.82 (0.07) 1.29 (0.06) 1.239 (0.052) 0.571 (0.034) 28.8 (5.7) 88.7 (7.9) 1.306 (0.067) GB011 2.00 (0.04) 0.39 (0.02) 1.159 (0.024) 0.168 (0.008) 40.2 (9.3) 19.9 (1.1) 1.168 (0.025) GB101 6.28 (0.15) 4.96 (0.16) 1.104 (0.022) 0.715 (0.029) 37.1 (4.5) 132 (10) 1.151 (0.032) Table 2 Specific activities (mBq/g), activity ratios and nominal age in ky (using Eq.1) in the analysis of five aliquots of carbonate sample GB06. The data error was obtained by error propagation. Samples were dissolved with different nitric acid concentrations (2–8M). The last line shows the results of the corrected activity ratios and age of the pure carbonate sample obtained using the ISOPLOT program [10] GB06 238U230Th 234U/238U230Th/234U230Th/232Th T (ky) (234U/238U)o 1 3.93 (0.23) 3.00 (0.10) 1.031 (0.064) 0.741 (0.049) 10.5 (0.7) 145 (20) 1.046 (0.097) 2 3.65 (0.07) 3.44 (0.11) 1.082 (0.018) 0.871 (0.031) 6.52 (0.32) 210 (22) 1.147 (0.032) 3 4.22 (0.10) 4.56 (0.16) 1.119 (0.027) 0.966 (0.041) 6.34 (0.40) 298 (67) 1.275 (0.062) 4 3.63 (0.14) 3.16 (0.10) 1.115 (0.045) 0.781 (0.039) 8.29 (0.52) 158 (17) 1.178 (0.070) 5 4.83 (0.09) 3.62 (0.12) 1.073 (0.016) 0.699 (0.027) 8.64 (0.55) 127 (9) 1.105 (0.023) ISOP 1.029 (0.035) 0.476 (0.051) 70 (11) 1.035 (0.043) carbonate, so correction is not necessary. This would mean that all aliquots of a same sample, with activity ratio above 20, should result in the same age. However, when comparing the sample ages with their stratification profile [7], the ages of samples GB02, GB010 and GB101 do not fit their position and it seems they should be younger. To verify their ages, several respective aliquots (from the same original sample) were analysed by dissolving with different nitric acid concentration, as in sample GB06; the results are shown in Table3. As can be seen in the table, the activity concentrations are different in each aliquot; the exceptions are the U concentration in GB02 and GB010, quite similar in each aliquot. Differences in U and Th concentrations are quite evident in sample GB101. These facts should indicate the extraction of different U and Th isotope activities from the detrital material contained in the carbonate samples. To verify this fact, in Figs.2, 3 and 4 Rosholt diagrams for each sample are presented. Rosholt diagrams show a linear relationship between activity ratios in the several aliquots of each sample; this means that samples GB02, GB010 and GB101 cannot be considered pure carbonate samples and their age must be corrected. The ISOPLOT program (3D plot) was also used to obtain the real age of these samples and the results are shown in Table4. As can be seen, in the case of sample GB010 the differences between aliquots are small considering the error in the data. However, it seems that only aliquot 3, with the highest 230Th/232Th ≅ 100, has the age of the sample (see Table4). Results from aliquots of samples GB02 and GB101, with aliquots with different nominal ages, show the clear need to correct the activity concentrations to obtain the real age of the pure carbonate. Indeed, the corrected ages (see Table4) are clearly lower than those obtained in the aliquots. Only for the high 230Th/232Th activity ratio in the aliquot (67 in 56789101112131415 6 7 8 9 10 11 12 13 14 15 16 Gibraltar - GB06 234U/232Th 238U/232Th 46810 12 14 16 5 6 7 8 9 10 11 12 Gibraltar - GB06 230Th/232Th 234 U/ 232 Th Fig. 1 Rosholt diagrams for sample GB06 Table 3 Specific activities (mBq/g), activity ratios and nominal age (Eq.1) in ky in different aliquots of samples GB02, GB010 and GB101. The different aliquots were dissolved with different concentrations of HNO3 (2–8M). The data errors were obtained by error propagation 238U230Th 234U/238U230Th/234U230Th/232Th T (ky) (234U/238U)o GB02 1 2.90 (0.09) 1.15 (0.07) 1.047 (0.027) 0.379 (0.024) 30 (7) 51.5 (4.2) 1.054 (0.031) 2 3.09 (0.10) 1.11 (0.06) 1.069 (0.028) 0.335 (0.021) 37 (9) 44.1 (3.4) 1.078 (0.032) 3 3.15 (0.10) 0.93 (0.04) 1.048 (0.026) 0.282 (0.015) 67 (14) 35.9 (2.3) 1.053 (0.029) 4 3.79 (0.08) 1.49 (0.07) 1.039 (0.019) 0.378 (0.019) 24 (4) 51.3 (3.2) 1.046 (0.021) GB010 1 1.82 (0.07) 1.29 (0.06) 1.239 (0.052) 0.571 (0.034) 28.8 (5.7) 88.7 (7.9) 1.306 (0.067) 2 1.80 (0.04) 1.21 (0.05) 1.182 (0.030) 0.570 (0.027) 10.6 (1.2) 89.2 (6.4) 1.234 (0.039) 3 1.75 (0.05) 1.12 (0.08) 1.247 (0.043) 0.512 (0.040) 108 (31) 75.6 (8.3) 1.305 (0.053) GB101 1 6.28 (0.15) 4.96 (0.16) 1.104 (0.022) 0.715 (0.029) 37.1 (4.5) 132 (10) 1.151 (0.032) 2 5.34 (0.11) 4.91 (0.14) 1.130 (0.028) 0.814 (0.028) 27 (2) 172 (14) 1.211 (0.030) 3 6.67 (0.14) 6.00 (0.19) 1.151 (0.020) 0.782 (0.030) 28.6 (2.7) 156 (13) 1.234 (0.031) 4 3.97 (0.08) 4.01 (0.15) 1.148 (0.021) 0.883 (0.038) 18.7 (2.0) 210 (26) 1.266 (0.038) sample GB02) does the uncorrected age agree with the corrected age (see Table4). In the case of sample GB101 all uncorrected ages are overestimated. In Fig.5, the 230Th/234U activity ratios versus the 230Th/232Th activity ratios in the aliquots for each sample (including sample GB06) are plotted. As can be seen, for sample GB06, a constant 230Th/234U activity ratio is obtained for 230Th/232Th at about 20. However, for the other three samples constancy is obtained for 230Th/232Th activity ratios well above 100. This means we need to be careful when considering a carbonate sample as pure or impure when dating is required. 230Th/232Th activity ratios inthedetritus It is interesting to obtain activity ratios in the detrital material, particularly the 230Th/232Th activity ratio in this material at the time of deposition. All aliquots of an impure carbonate sample share the same detrital material and consequently the same value of this activity ratio. We applied several methods to evaluate this activity ratio in the above impure carbonates. First as we already know the samples’ age, obtained using the L/L method and the activity ratios in leachates of each aliquot, it is possible by using the SL (single leach) method [1, 11] to determine the actual and initial 230Th/232Th activity ratios in the detrital material for each aliquot. This method considers that the detrital material contributes negligible amounts of both uranium isotopes to the leach of the sample and that the 230Th in it comes from that in the pure carbonate along with part of the 230Th of detrital material which is also extracted together with 232Th. The method presumes that the 230Th/232Th activity ratio in extracted fractions is the same as in the actual detrital material. The actual activity ratio is the activity ratio at the time of deposition, corrected by the 230Th decay. Thus, 50 100 150 200 250 300 50 100 150 200 250 300 Gibraltar - GB02 234U/232Th 238 U/ 232 Th 050100 150 200250 300 350 20 30 40 50 60 70 80 90 Gibraltar - GB02 230Th/232Th 234 U/ 232 Th Fig. 2 Rosholt diagrams for sample GB02 020406080 100 120 140 160 180 200220 240 0 50 100 150 200 250 300 Gibraltar - GB010 234 U/ 232 Th 238 U/ 232 Th 050100 150200 25 03 00 0 20 40 60 80 100 120 140 Gibraltar - GB010 230Th/232Th 234 U/ 232 Th Fig. 3 Rosholt diagrams for sample GB010 The subscript C means the concentration in the pure carbonate, L in the leachate of the impure carbonate and D in the detrital material of the impure carbonate. (3) 238 UC= 238 UL 234UC=234UL 230 ThC=230ThL−232ThL ( 230Th∕232Th )D e−λ ot By introducing these equations in the Bateman dating equation it is possible to obtain a general equation between activity ratios in the leachates, the 230Th/232Th in the detrital material and t, the time of deposition. Hence, for each aliquot, given that its age is already known, it is possible to calculate the initial 230Th/232Th activity ratio in the detrital material. These results are shown in Table5. As the detrital material is the same in all aliquots of the same impure carbonate, the activity ratio in the detrital material can be considered as the mean value of all aliquots. In such a Table, it can be seen that generally (except sample GB010) the actual and initial 230Th/232Th activity ratios in aliquots of the same impure carbonate are practically constant and a mean value can be used. This activity ratio is clearly lower in the case of sample GB06. A large discrepancy between aliquots of sample GB010 was obtained, and thus a mean value cannot be considered. Second as described by Allegre and Condomines [6], at t = 0 (time of deposition), a plot of 230Th/232Th versus 238U/232Th for all cogenetic samples would show a horizontal line, with the y-intercept being the initial thorium ratio (230Th/232Th)o. After time t, the same plot will show data points on a line with a slope (1 − e-λot). At t >>1/λo, 230Th/232Th = 238U/232Th representing secular equilibrium; the points will lie along a line with unit slope and passing through the origin. This means that the line rotates over time, starting from an initial slope of 0 and ending with a slope of unity. This rotation occurs around a point called the equipoint, defining the initial 230Th/232Th, which should be the initial value in the detritus [5]. In the case of initial disequilibrium between 234U and 238U, an equation similar to the one above, with 234U instead of 238U, will hold. Thus, in the event of secular equilibrium between uranium isotopes, a plot of 230Th/232Th versus 238U/232Th activity ratios of the aliquots will yield a line (isochron) that will cut the equiline line at a point (equipoint); the horizontal line cutting the same point will provide the 230Th/232Th activity ratio at the time of deposition in the detrital material. Furthermore, the y-intercept of the isochron line will show the 230Th/232Th at the current time; thus, from both data the time of deposition can also be obtained. If there is no equilibrium, the same can be obtained by switching 238U/232Th for 234U/232Th. These plots are presented in Figs.6, 7, 8 and 9 for samples GB06, GB02, GB010 y GB101, and the results of the y-intercepts of the isochron and horizontal lines are respectively presented in Table6. Rather high 230Th/232Th activity ratios in detrital material from samples GB02 and GB101, well above 10 for the initial value, were obtained and about three times lower for the other two samples (GB06 and GB010). Considering that the correction for sample GB010 was quite small, it seems that the limiting ratio at which contaminations affect a sample’s age must be related to the 230Th/232Th activity ratio 10 15 20 25 30 35 40 45 50 55 15 20 25 30 35 40 45 50 55 60 Gibraltar - GB101 234 U/ 232 Th 238U/232Th 10 15 20 25 30 35 40 45 50 55 60 15 20 25 30 35 40 45 Gibraltar - GB101 230 Th/ 232 Th 234 U/ 232 Th Fig. 4 Rosholt diagrams for sample GB101 Table 4 Results obtained in samples GB02, GB010 and GB101, considered impure carbonates using the L/L method and the ISOPLOT program (3D plot) [10] Sample 234U/238U230Th/234U T (ky) (234U/238U)o GB02 1.060 (0.035) 0.245 (0.026) 30.5 (3.5) 1.066 (0.038) GB010 1.258 (0.043) 0.532 (0.040) 79.8 (8.4) 1.323 (0.054) GB101 1.094 (0.034) 0.604 (0.058) 98.8 (15.5) 1.124 (0.045) 50 100150 20 02 50 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40 Gibraltar - GB02 230Th/234U 230Th/232Th 050100 150200 250 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 Gibraltar - GB010 230Th/234U 230 Th/ 232 Th 20 40 60 80 100 120 140 160 180 200 220 240 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 Gibraltar - GB101 230 Th/ 234 U 230Th/232Th 5101520253035404550 0.5 0.6 0.7 0.8 0.9 1.0 Gibraltar - GB06 230 Th/ 234 U 230Th/232Th Fig. 5 230Th/234U versus 230Th/232Th activity ratios in samples GB06, GB02, GB101 and GB010 Table 5 Activity ratios in the detrital material for aliquots of impure carbonates GB06, GB02, GB010 and GB101. Results are obtained by using SL method Eq.(3) and the ages previously calculated with the L/L method and the ISOPLOT program [10] Sample 230Th/232Th)D 230Th/232Th)Do 230Th/232Th)D 230Th/232Th)Do GB06-1 2.93 5.59 GB02-1 10.5 14.0 2 3.73 7.12 2 9.90 13.1 3 3.19 6.08 3 8.76 11.6 4 3.25 6.19 4 8.53 11.3 5 2.72 5.19 Mean 3.16 (0.38) 6.03 (0.76) Mean 9.42 (0.94) 12.5 (1.3) GB010-1 2.11 4.39 GB101-1 6.86 17.1 2 0.758 1.58 2 6.37 15.8 3− 4.4 − 9.25 3 5.73 14.3 4 5.83 14.5 Mean Mean 6.20 (0.52) 15.4 (1.3) in the detrital material. High activity ratios would produce different ages in leachates with 230Th/232Th activity ratios about 20. Comparing the data in Tables5 and 6, the 230Th/232Th activity ratios obtained by both methods agree quite well and the age of the carbonates obtained through the equipoint is the same as that obtained by applying the L/L method. Furthermore, it has now been possible to determine the activity ratios in the case of sample GB010 and the age obtained agrees with that obtained by applying the L/L method. The study has shown the possibility of evaluating the 230Th/232Th activity ratio in detrital material of impure carbonates at the present time, at the time of precipitation with the carbonate. These activity ratios furthermore provide a method to obtain the age of the sample by using Eqs.(2) and (3) with a similar result. Conclusions Analyses of U and Th isotopes in carbonate samples from the Rock of Gibraltar were carried out to obtain their age using the U/Th method. The data on 230Th/232Th activity ratios showed, barring one sample, values well above 20, 6810 12 14 16 5 6 7 8 9 10 11 12 Gibraltar - GB06 equiline (t>350 ky) t=0 230Th/232Th 238U/232Th 6810 12 14 16 5 6 7 8 9 10 11 12 Gibraltar - GB06 equiline (t>350 ky) t = 0 230Th/232Th 234 U/ 232 Th Fig. 6 230Th/232Th versus 238U/232Th (a) and 234U/232Th (b) activity ratios in sample GB06. At t = 0, cogenetic samples will fall along a horizontal line. This line would rotate over time and at secular equilibrium all points will lie along a line with unity slope and passing through the origin (equiline). The y-intercept of the horizontal line provides the 230Th/232Th activity ratio in the detrital material originally included in the carbonate sample 020406080 100 120140 160180 200220 240260 280300 0 20 40 60 80 100 equiline (t>350 ky) t=0 Gibraltar - GB02 230 Th/ 232 Th 238 U/ 232 Th 050 100 150200 25 03 00 0 20 40 60 80 equiline (t > 350 ka) t = 0 Gibraltar - GB02 230 Th/ 232 Th 234 U/ 232 Th Fig. 7 Same as Fig.6 but for sample GB02