The protocol describes a method to purify and separate the U and Th nuclide in submarine hydrothermal sulfide sample with Fe co-precipitation and extraction chromatography for 230Th-U disequilibrium dating.
The age of a submarine hydrothermal sulfide is a significant index for estimating the size of hydrothermal ore deposits. Uranium and thorium isotopes in the samples can be separated for 230Th-U dating. This article presents a method to purify and separate U and Th isotopes in submarine hydrothermal sulfide samples. Following this technique, the separated U and Th fractions can meet measuring requirements by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). The age of the hydrothermal sulfide sample can be calculated by measuring the present-day activity ratios of 230Th/238U and 234U/238U. A super clean room is necessary for this experiment. Cleaned regents and supplies are used to reduce the contamination during the sample processes. Balance, hotplate, and centrifuge are also used. The sulfide sample is powdered for analysis and less than 0.2 g sample is used. Briefly, the sample is weighed, dissolved, added to 229Th-233U-236U double spike solution, Fe co-precipitated, and separated on an anion-exchange resin extraction column. Approximately 50 ng U is consumed for 230Th-U dating of sulfides sample by MC-ICPMS.
Submarine hydrothermal sulfides have been a steady source of metals like iron, copper, zinc and lead. They are also seen as economically viable resources of silver and gold. The location and size of the deposits are a record of the history of hydrothermal venting on the seafloor. Dating of a hydrothermal sulfide can provide important information regarding the formation and alteration mechanism of the sulfide ore deposit, seafloor hydrothermal activity history, and growth rate of large sulfide deposits1,2,3. 238U-234U-230Th disequilibrium dating is an effective isotopic method of age estimation for hydrothermal sulfides4,5,6,7,8,9,10,11,12, where the purification and separation of U and Th isotopes is necessary. This text describes a protocol for U and Th isotopes separation and 230Th-U dating of sulfides sample by MC-ICPMS.
Geological materials which contain U and Th remain undisturbed for several million years, and a state of secular equilibrium between all the nuclides in the radioactive series is established. However, a combination of chemical solubility and nuclear recoil factors often create disequilibrium, in which the members of the decay series are separated from each other through processes such as deposition, transport and weathering. For example, when a sulfide deposit is formed, the state of 238U, 234U and 230Th is of disequilibrium, and the long-lived 238U can decay gradually towards short-lived 234U and 230Th subsequently. Assuming (i) the system remains closed with respect to U and Th isotopes, and (ii) initial amount of 230Th and 232Th incorporated into sulfide samples is zero, it is possible to determine the time of deposition by measuring the present-day activity ratios of 230Th/238U and 234U/238U. However, the initial amount of Th is not zero in the sample, and we assume the initial 230Th/232Th atomic ratio is 4.4 ± 2.2 x 10-6. The applicable dating range of this method is approximately ~10-6 x 105 years13,14. However, the large difference between the abundance of uranium and thorium makes measurement challenging. Hence, it is very important to establish a chemical procedure for U-Th dating by MC-ICPMS.
In the past 30 years, most studies focused more measurements of carbonate materials14,15,16,17 and less on sulfide deposits11,12,18,19. Alpha-particle counting methods have traditionally been used for the study of 230Th/238U disequilibrium of submarine hydrothermal sulfides1. However, analytical uncertainty of 5-17% is a limiting factor that affects the precision of age determination of sulfides1,8,9. These techniques generally suffer from the use of relatively large columns and reagent volumes and the need for multiple column passes for purification and separation U-Th from a sample. Recent developments in MC-ICPMS have greatly improved the precision of U-Th isotopic measurements (<5‰ for ages)14 and have significantly reduced the sample size (<0.2 g) required for analysis. In these works, many chemical separation procedures have been developed, and have achieved excellent chemical yields with low chemical background12,13.
Here we present a chemical-based protocol to obtain samples that are sufficiently clean for MC-ICPMS analysis. It is suitable for the dating of hydrothermal sulfide samples of age <6 x 105 years14. With this technique, the separated U and Th isotopic fractions can meet measuring requirements by MC-ICPMS. The age of the hydrothermal sulfide sample can be calculated from the extent of disequilibria between 230Th and 234U and between 234U and 238U by using the described activity decay equation.
1. Preparing the sample, reagents, and containers
2. Weigh the samples
3. Dissolve and spike the sample
4. Ferric hydroxide co-precipitation for U-Th
5. Preparation of anion exchange column
Figure 1: Ion-exchange column filling with anionic exchange resin. Please click here to view a larger version of this figure.
6. Purification and separation of U and Th fractions
Figure 2: Uranium and thorium fractions of the submarine hydrothermal sulfides. Please click here to view a larger version of this figure.
7. MC-ICPMS measurement
Instrument | Parameter | Value |
MC-ICPMS | RF power | 1325 W |
Cool gas | 16.00 L min-1 | |
Auxiliary gas | 1.78 L min-1 | |
Sample gas | 1.00 L min-1 | |
Low resolution | 300~400 | |
CETAC Aridus II | Sample injection rate | 50~60 μL min-1 |
Ar Sweep Gas | 2~5 L min-1 | |
Nitrogen Gas | 2~10 mL min-1 | |
Spray Chamber Temperature | 110 °C | |
Membrane Oven Temperature | 160 °C |
Table 1: Instrument parameters for measuring U-Th isotopes by MC-ICPMS (using the instrument listed in the Table of Materials).
Using this procure, a submarine hydrothermal sulfide sample can be completely dissolved. Following this protocol, the Th fraction was eluted from the hydrothermal sulfide sample using 8 M HCl. Meanwhile, the U fraction of the hydrothermal sulfide sample was eluted with 0.1 M HNO3. U and Th fractions were dissolved in the 2% HNO3 (+0.1% HF) solution (see Figure 2) and stored in 2 mL capacity vials. The mixture was then analyzed by MC-ICPMS.
With the MC-ICPMS instrument, U and Th isotopes ratio and the age of submarine hydrothermal sulfide is determined precisely. The ages were calculated by an iterative method13. The test results are listed in Table 2. U content ranged from 178.0 to 5,118.2 ng·g-1, and Th content ranged from 603 to 7,212 pg·g-1. Five samples had ages of 567 ± 52, 1,585 ± 27, 3,345 ± 132, 14,211 ± 727 and 21,936 ± 91 years B.P. (B.P. stands for “before year 2000 A.D.”). Sample consumption was about 60 mg except S32 wherein only 17 mg sample was consumed.
Sample | Sample Mass | 238U | 232Th | 230Th/232Thb | 234U/238Ub | 230Th/238Ub | 230Th Age(yr)c | 230Th Age (yr BP)d, e | (234U/238U)initialf | ||||||||
No. | (mg)a | (ng g-1) | (pg g-1) | (uncorrected) | (corrected) | ||||||||||||
S12 | 58 | 182.8 | ±0.2 | 7212 | ±144 | 11.7 | ±0.3 | 1.156 | ±0.002 | 0.1511 | ±0.0018 | 15221 | ±193 | 14211 | ±727 | 1.163 | ±0.002 |
S15 | 57 | 569.3 | ±0.7 | 1200 | ±24 | 310.3 | ±6.3 | 1.166 | ±0.002 | 0.2140 | ±0.0007 | 22006 | ±84 | 21936 | ±91 | 1.177 | ±0.002 |
S32 | 17 | 5118.2 | ±10.4 | 5173 | ±104 | 51.9 | ±1.2 | 1.157 | ±0.003 | 0.0172 | ±0.0002 | 1628 | ±20 | 1585 | ±27 | 1.158 | ±0.002 |
Y3 | 55 | 178.0 | ±0.2 | 865 | ±17 | 23.0 | ±0.8 | 1.162 | ±0.002 | 0.0366 | ±0.0010 | 3484 | ±100 | 3345 | ±132 | 1.164 | ±0.002 |
Y4 | 59 | 347.1 | ±0.4 | 603 | ±12 | 11.7 | ±0.8 | 1.159 | ±0.002 | 0.0067 | ±0.0004 | 629 | ±42 | 567 | ±52 | 1.159 | ±0.002 |
a Sample mass for separation of uranium and thorium nuclide and U and Th analysis. | |||||||||||||||||
b All ratios are radioactivity ratio, which calculated based on the decay constants λ238=1.55125 ×10-10 a-1 as described by Jaffey et al.(1971)20, λ234=2.82206 (±0.00302)×10-6 a-1 as described by Cheng et al.(2013)15, and 9.1705(±0.0138)×10-6 a-1 as described by Cheng et al.(2013)15. | |||||||||||||||||
c Calculated 230Th age following the equation | ![]() |
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d Corrected 230Th ages assume the initial 230Th/232Th atomic ratio of 4.4 ±2.2 x10-6. Those are the values for a material at secular equilibrium, with the bulk earth 232Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%15. | |||||||||||||||||
e B.P. stands for “Before year 2000 A.D.”. | |||||||||||||||||
f | ![]() |
Table 2. 230Th dating results for submarine hydrothermal sulfides. The error shown is 2s error.
aSample mass for separation of uranium and thorium nuclide and U and Th analysis.
bAll ratios are radioactivity ratios, which are calculated based on the decay constants λ238 = 1.55125 x 10-10 a-1 as described by Jaffey et al.20, λ234 = 2.82206 (±0.00302) x 10-6 a-1 as described by Cheng et al.15, and 9.1705 (±0.0138) x 10-6 a-1 as described by Cheng et al.15.
cCalculated 230Th age following the equation in section 7.
dCorrected 230Th ages assuming the initial 230Th/232Th atomic ratio to be 4.4 ± 2.2 x 10-6. These are the values for a material at secular equilibrium, with the bulk earth 232Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%.
eB.P. stands for “Before year 2000 A.D.”.
fUsing the equation in section 7.
Some critical steps must be followed to ensure success of this protocol. Ensure that all operations are carried out in clean chemistry room under fume hood with clean air circulation. Purify all regents in this process in advance and clean the apparatus before use. Dissolve the samples completely in the process of making the 7 M HNO3 solution which is then loaded onto the 7 M HNO3-conditioned resins. If there is any insoluble substance in the sample, it will be redissolved after drying. Additional important steps are suggested: (i) avoid the cross contamination from the adjacent samples during the sample processing; (ii) for each elution step allow the liquid to drain completely before the next step; and (iii) complete the process from the conditioning of the columns to collecting Th and U fractions within 2 h, otherwise the strong acid tends to break down the resin.
The major limitation of this technique is related to the 238U and 232Th concentration of the sample. It is best to choose samples with U > 50 ppb and Th < 10 ppb. The AG 1-X8 resin used can be replaced by UTEVA resin in the process.
With this method, five submarine hydrothermal sulfides samples from the South Atlantic were measured. Ages were 567 ± 52 to 21,936 ± 91 year B.P., indicating that this region has been experiencing hydrothermal activity events from 21,936 ± 91 years B.P.
U-Th purification and separation refers to isotopic methods of age estimation based on the measurement of uranium (238U and 235U), thorium (232Th), and certain members of the intermediate daughter nuclides in the three naturally occurring radioactive decay series for hydrothermal sulfide sample. It is also useful to determine the U and Th concentration of deep-sea sediments19. The technique can be applied to the dating of carbonate and phosphate, and to environmental tracer studies, assisting in building the age framework for the formation of minerals.
The authors have nothing to disclose.
This study was financially supported by Experimental Technology Innovation Foundation of Institute of Geology and Geophysics, Chinese Academy of Sciences (No. 11890940), and China Ocean Mineral Resources R & D Association Project (No. DY135-S2-2-07).
AG 1-X8 anion-exchange resin | BIO-RAD | 140-1441 | Separating rare elements |
Ammonia solution | Kanto Chemical CO., INC. | 1336-21-6 | Reagent |
Glass vials | BOTEX | None | Sample collection |
Hydrochloric acid | Sinopharem chemical reagent Co. Ltd | 7647-01-0 | Reagent |
Hydrofluoric acid | EMD Millipore CO. | 7664-39-5 | Reagent |
Neptune Plus | Thermo Fisher Scientific CO. | None | Apparatus |
Nitric acid | Sinopharem chemical reagent Co. Ltd | 7697-37-2 | Reagent |
Perchloric acid | Kanto Chemical CO., INC. | 32059-1B | Reagent |
Ultrapure water | Merck Millipore | None | Producted by Mill-Q Advantage systerm |
Wipe paper | Kimberley-Clark | 0123-12 | Wipe and clean |
2 ml vial | Nelgene | 5000-0020 | Sample collection |
229Th-233U-236U spike | None | None | Reagent |
7 ml PFA beaker | Savillex | 200-007-20 | Sample treatment |
10 ml centrifuge | Nelgene | 3110-1000 | Sample treatment |
30 ml PFA beaker | Savillex | 200-007-20 | Sample treatment |