Method development for 234U and 230Th determination and application to fossil deep-water coral and authigenic carbonate dating from the Campos Basin - Brazil

as idades 230 Th/ 234 U determinadas por FIAS-ICP-QMS. Uma das amostras de carbonato autigênico analisada apresentou uma idade de aproximadamente 80 kanos. A outra amostra apresentou uma razão 230 Th/ 234 U próximo do equilíbrio e fora da faixa de aplicação do método. A 234 U and 230 Th determination method based on an extraction chromatographic separation followed by ICP-MS with quadrupole (ICP-QMS) was developed. For authigenic carbonates, a second separation step with ion exchange chromatography in a HNO 3 solution was added. These methods were applied to seven fossil deep-water coral and two authigenic carbonate samples from the continental slope of the Campos Basin - Brazil. The ages determined for the fossil corals samples from the same sediment core ranged from 9 to 202 ky with a 1% uncertainty, consistent with the values determined by 14 C dating and with those determined by 230 Th/ 234 U using flow injection coupled to an ICP-QMS. One of the authigenic carbonates analyzed presented an age of approximately 80 ky. The other sample exhibited a 230 Th/ 234 U activity ratio close to equilibrium and out of the application range of the method.


Introduction
The Brazilian continental margin contains substantial hydrocarbon reserves and thus has been the subject of research related to biostratigraphy, paleoecology and paleoceanography.Sequences of appearing and disappearing fossil deep-water corals have been found in sediments sampled from the Campos Basin continental slope. 1 Deep-water fossil corals could serve as records with high temporal resolution for the reconstruction of rapid changes in past ocean circulation. 2The earth's climate changes rapidly on the geological time scale, and the bottom of the sea plays an important role as a paleoceanographic record. 3,4Some authors have suggested that the deepwater corals are well suited to the study of rapid climate change. 2,5,6Past deep circulation patterns have been reconstructed using high-resolution records of deep-water corals obtained by 14 C or U/Th dating.
According to Miller, 7 seabed hydrocarbon seeps have been widely used as an indicator of deep hydrocarbon accumulations.Detailed study of the geochemical, geological and geochronological aspects of seep-related features can provide important information on the origin, evolution and thermal processes of hydrocarbon migration, thus contributing to a better understanding of petroleum systems.
Over the past few years, several studies on the underwater environment and fossil records rocks have emphasized the importance of carbonate formation associated with the age of hydrocarbon seepage and microbial activity. 8Because authigenic carbonate precipitation most commonly results from anaerobic methane oxidation, its carbon source is petrogenic and thus conventional 14 C dating is not applicable.][11] Depending on the sample age, the 230 Th activity can be low, which causes a high uncertainty in age determination by radiochemical methods, even after long counting times.][14][15] In this work, the 230 Th/ 234 U determination method by ICP-MS was performed by the Institute for Radioprotection and Dosimetry/Brazilian Nuclear Commission (IRD/CNEN), using on-line chemical separation (FIAS) employing UTEVA cartridges (Eichrom Co.). 16The method had to be adapted for application to samples of authigenic carbonates, because low chemical yields were observed, probably due to large amounts of iron.
The aim of the present work is to develop an analytical method for 230 Th/ 234 U dating of authigenic carbonate and deep-water coral samples with improved precision and accuracy over the on-line separation (FIAS) method currently used by the IRD/CNEN.The calculated ages were compared with values obtained by other techniques, such as 14 C dating by mass spectrometry coupled to a particle accelerator (AMS) by the Laboratory of the University of California (USA) and 230 Th/ 234 U dating with a flow injection system coupled to an ICP-QMS method by the Institute for Radioprotection and Dosimetry (Brazil).

Experimental
The seven fossil coral and two authigenic carbonate samples, which originated from the continental slope of the Campos Basin -Brazil, were obtained from Petrobras.
To remove the U and Th fractions on the coral samples carried by the Fe-Mn coatings and clay particles, mechanical cleaning was first performed, followed by ultrasonic washing with 1 mol L -1 HNO 3 for 20 min.A second chemical cleaning procedure adapted from Lomitschka and Mangini 12 was then performed.One gram of cleaned coral sample was ground in an agate mortar and dissolved in 30 mL of 3 mol L -1 HNO 3. When some residue remained, it was dissolved completely with an acid mixture composed by HNO 3 plus HF, evaporated to eliminate HF, and the obtained residue re-dissolved with 3 mol L -1 HNO 3 and reserved for its U and Th content determination by ICP-QMS.These results were used to correct the 234 U and 230 Th values on the carbonate phase for the detritic contribution by applying the 232 Th concentration on the carbonate phase and the U/Th ratio on the residual phase.Thorium-230 and 234 U concentrations on the leaching solution were determined according to the method illustrated in Figure 1.The 230 Th, 234 U and 238 U values, corrected for the detritic contribution, were used to calculate the coral ages according to equation 3.
The external layer of the authigenic carbonate samples was removed, and the remaining part was ground in an agate mortar and sieved into four fractions < 170, 170-80, 80-42 and 42-24 mesh.One gram of each fraction was leached with 7 mol L -1 HNO 3 in order to yield the carbonate phase.The remaining residual fraction was totally dissolved with HNO 3 plus HF mixture, and then 238 U and 232 Th were determined by ICP-QMS.The 230 Th, 232 Th, 234 U and 235 U results on the carbonate phase were used to construct both Rosholt and Osmond isochrones 17 and to obtain the 230 Th/ 234 U and 234 U/ 238 U activity ratios.The results obtained using these two different approaches were applied to equation 3.
The uranium and thorium elemental and isotope determinations were performed using a Perkin-Elmer ELAN 6000 ICP-QMS instrument using the conditions described in Table 1.The 233 U and 229 Th spikes used are traceable to BIPM and were diluted according to the Radionuclides Metrology Section of the Institute for Radioprotection and Dosimetry (SEMRA/IRD).
Mass bias was corrected by applying equation 1, where β was determined from NIST CRM U020A (1 ng mL -1 ) analysis and equation 2: 18 (1) (2)   To reduce the mass discrimination effect, the 234 U/ 235 U isotope ratio was measured instead of the 234 U/ 238 U isotope ratio.To calculate the 234 U/ 238 U isotope ratio, the measured 234 U/ 235 U isotope ratio was multiplied by the 235 U/ 238 U isotope ratio (0.0072526).To calculate the 230 Th/ 234 U mass concentration ratio, the obtained 230 Th concentration was divided by the product of the 234 U/ 238 U isotope ratio multiplied by the 238 U mass concentration.To obtain the 230 Th/ 234 U activity ratio, the calculated mass concentration ratio was multiplied by the ratio of the specific activities of the isotopes in question.
The 234 U/ 238 U and 230 Th/ 234 U activity ratios were calculated.Based on the obtained values, the ages were determined according to equation 3, 19 (3) where λ 230 is the 230 Th decay constant and, λ 234 is the 234 U decay constant.
The developed methods were tested for 234 U and 230 Th determinations of the NIST SRM 4357 standard reference ocean sediment material.Three 2 g aliquot samples were used for each method.

Results and Discussion
For accurate 234 U and 230 Th determination in the fossil coral and authigenic carbonate samples, a separation procedure was needed to eliminate the main matrix elements and to obtain a sample solution with a chemical composition similar to that of standards. 20An off-line separation procedure was applied based on extraction chromatography with 2 mL UTEVA (Eichrom Co.) columns [21][22][23][24][25][26] (Figure 1).The elution curves of U and Th with 0.1 mol L -1 HCl, 0.1 mol L -1 HCl + 0.1 mol L -1 HF, and 0.1 mol L -1 HCl + 0.01 mol L -1 HF, were tested.The feed solution (40 mL of 3 mol L -1 HNO 3 + 0.025 mol L -1 Al(NO 3 ) 3 ) was added to 1000 ng of U nat and 1000 ng of Th nat .Based on the obtained results (Figure 2), 10 mL of 0.1 mol L -1 HCl + 0.01 mol L -1 HF was used as the U and Th eluent to avoid higher HF concentrations and to obtain 100% recovery for both elements.This condition ensures that Th and U are quantitatively retained and eluted during separation from the matrix.The U and Th decontamination factors (DF) from Ca, Fe, Al, Mg and Sr, which are representative of the major matrix elements, are shown in Table 2.The obtained decontamination factors are considered satisfactory, because they are equal to or greater than 10 3 . 27e to the low thorium chemical yield obtained, the initial tests with authigenic carbonate samples revealed the need for a matrix separation step before chromatographic extraction with the UTEVA column (Figure 3).Based on previous work (Godoy et al. 28 ), ion exchange chromatography with DOWEX 1X8 was tested.Using 40 mL of 7 mol L -1 HNO 3 + 0.025 mol L -1 Al(NO 3 ) 3 as feed solution containing 1000 ng of U and 1000 ng of thorium, the elution curves of Th with 6 mol L -1 HCl + 0.26 mol L -1 HF, 1 mol L -1 HCl + 0.26 mol L -1 HF and 0.1 mol L -1 HCl + 0.26 mol L -1 HF were determined.Based on the obtained results (Figure 4), 25 mL of 6 mol L -1 HCL + 0.26 mol L -1 HF was chosen as the Th eluent.Thorium was strongly adsorbed, whereas uranium was not retained by the DOWEX 1X8 resin and instead was distributed between the feed and washing solutions.Both solutions were collected together for U determination, followed by a second separation step with UTEVA columns similar to that applied for the coral samples.
The obtained 234 U and 230 Th concentrations for the NIST SRM 4357 certified reference marine sediment sample using both methods are shown in Tables 3 and 4. The results are within the confidence interval for both isotopes.For these samples, the 230 Th achieved accuracy (2.8 and 2.2%), by applying this off-line separation method are better than those obtained by Godoy et al., 16 whose values were above 10%, but within the 95% confidence interval of the certified value.The 230 Th precisions (1.9 and 3.1%) are similar to that (1.5%) reported by Godoy et al. 16 Although 234 U does not have a certified reference value, the precision and accuracy values for this isotope using the proposed method were similar to those using on-line methods. 16r the fossil deep-water corals samples from Campos Basin, the 230 Th/ 234 U and 234 U/ 238 U activity ratios as well as the calculated ages are shown in Table 5. Taking into account the peak areas, the residual fraction contribution and the uncertainty related to the spikes, the uncertainties  associated with coral were about 1%.Table 6 shows the fossil coral ages determined using the proposed method, with the previous method based on flow injection, and by 14 C AMS dating at the University of California, Irvine, United States.To verify if there were statistically significant differences between the obtained results by the three methods, ANOVA tests were applied, as shown in Table 7.
Because the calculated F (0.50) value was lower than the critical F (3.89) value, no significant differences were found among the three methods.The authigenic carbonate samples contained detritic contamination not only at the surface, but throughout the entire sample.Therefore, usual cleaning procedures, such as mechanical and chemical removal of the surface layers, did not generate a final sample with a low 232 Th content, as observed for the coral samples (range of ng g -1 ).The procedure described by Ku 29 was applied with sample fractionation by size to generate independent sub-samples.Based on the obtained results, two different isochrons were constructed: 230 Th/ 232 Th vs. 234 U/ 232 Th and 234 U/ 232 Th vs. 238 U/ 232 Th for the Rosholt plots (Figure 5) and 230 Th/ 238 U vs. 232 Th/ 238 U and 234 U/ 238 U vs. 232 Th/ 238 U for the Osmond plots (Figure 6).The 230 Th/ 234 U and 234 U/ 238 U activity ratios were obtained using the slopes of the Rosholt plots and the intercepts of the Osmond plots.The obtained values were statistically equivalent: (0.542 ± 0.014) 230 Th/ 234 U and (1.1004 ± 0.0028) 234 U/ 238 U and (0.564 ± 0.046) 230 Th/ 234 U and (1.0976 ± 0.0036) 234 U/ 238 U for the Osmond and Rosholt plots, respectively.Applying these results to equation 3, the age of this authigenic carbonate sample was (83.6 ± 0.1) ky according to the Osmond plot and (88.8 ± 0.1) ky based on the Rosholt plot.According to Ludwig, 17 the Osmond plot provides a more realistic value for samples with a high 232 Th content.Furthermore, because 232 Th and 234 U each appear in both of the isochrons, the results of the regressions are correlated   in a complex way.However, these correlations are never taken into account when calculating the ages or age-errors.Therefore, a simultaneous solution using all three of the ratios for the modified Osmond ratios is needed to generate a regression for an isochron line in three dimensions.The calculated age, determined using Isoplot 3.7 software, 17 used to construct the 3D 232 Th/ 238 U-230 Th/ 238 U-234 U/ 238 U isochron was (78.66 ± 0.17) ky.

Figure 1 .
Figure 1.Method for 234 U and 230 Th determination -fossil coral samples.

Table 1 .
Instrument and data acquisition settings for Perkin-Elmer ELAN 6000 ICP-MS

Table 3 .
230Th and 234 U activity values for NIST SRM 4357 certified reference marine sediment sample using the method for coral samples Data represent the average ± 1.96 SD of three replicates.c Value not certified.
a Confidence interval.b

Table 4 .
Authigenic carbonate samples Data represent the average ± 1.96 SD of three replicates.c Value not certified.
a Confidence interval.b

Table 5 .
230Th/ 234 U and 234 U/ 238 U activity rations and calculated ages of the fossil coral samples Data represent the average ± 4.30 SD of three ICP-MS readings.b Data represent the average ± 4.30 SD of three ICP-MS readings. a

Table 6 .
Ages of fossil corals based on different methods

Table 7 .
Test and least significant difference between the methods at 95% confidence level a LSD = Least Significant Difference.