Separation of Uranium and Thorium for ²³⁰Th-U Dating of Submarine Hydrothermal Sulfides

This article presents a method to purify and separate uranium and thorium isotopes in submarine hydrothermal sulfide samples. With this technique, uranium and thorium isotopic can be measured by MC-ICPMS, as suitable for dating samples with age less than 600, 000 years and provide important information of sulfide ore deposits, seafloor hydrothermal activity history and growth rates of large sulfide deposits. With this technique, less than 0.2 gram powdered sulfide sample is used and only 50 nanogram uranium is consumed for uranium series dating by MC-ICPMS.

Some critical steps must be followed to ensure success of this protocol. Purify all chemical reagents and clean the apparatus. Be sure all operations are carried out under the fume hood and clean air circulation.

Avoid the cross-contamination from the adjacent samples during the sample processing and reaching the condition of uranium and the thorium in accordance with your technique. The major imitation of this technique is related to the uranium 238 and the thorium 232. Concentration of the sample, it is best to choose samples where the uranium is more than and the thorium is less than 10 ppb.

First, use sprayed alcohol, or ultra pure water, to clean the fume hood, hot plate and the balance room bench for the chemical experiment. In the fume hood, prepare the sub-boiled hydrochloric acid and nitric acid and sulfide samples in vials. Label 30 milliliter PFA bottles twice, to prevent erasure and weigh the blank bottles on a balance accurate to plus or minus 0.0001 grams.

Pour the samples into the PFA bottles, cover with a lid and weigh the bottles. Add approximately one liter of ultra-pure water into the bottle, rotate the bottle to rinse the inner wall and shake the bottle carefully to have water cover all the samples. Place the sample containing bottles in the fume hood, open the lid and use a pipette to add three milliliters of 14 molar nitric acid, or aqua regia, into each bottle containing the samples.

Place the PFA bottles on the hot plate and set the hot plate temperature to 170 degrees Celsius to dissolve the samples completely. Leave the solution to cool for at least 30 minutes. Then add 100 to 300 milliliters of thorium-229, uranium-233 and uranium-236 spike solution of known activity into each sample solution.

Place the bottles on the hot plate at 170 degrees Celsius for five hours to dry. Dissolve each sample in two drops of 14 molar nitric acid and dry them on the hot plate at 170 degrees Celsius again. Prepare clean, 15 milliliter centrifuge tubes, label and place them in a tube stand, add approximately 0.1 milliliters of two molar hydrochloric acid into each bottle containing the dried samples and shake gently to completely dissolve the samples.

Transfer each sample into a centrifuge tube. Add several drops of ammonia solution into each tube until the acid is neutralized, showing a reddish-brown uranium and thorium isotope precipitate, biferric oxyhydroxide. Seal the centrifuge tubes, level them and centrifuge at 2340 times g for seven minutes.

Discard the supernatant. Add ultra-pure water to the tubes and shake to resuspend. Centrifuge again to wash and repeat the wash two more times.

Next, dissolve the precipitate with 1.5 milliliters of seven molar nitric acid. Transfer the solution into the corresponding bottle, add one drop of perchloric acid to remove organic matters and dry each sample on the hot plate at 170 degrees Celsius for about 30 minutes. To prepare an anion exchange column, insert frit into each PTFE column slowly at the bottom, with the help of a PTFE pipe.

Put the columns on the holder, pipette cleaned anion exchange resins into the columns. Fill the whole column with ultra-pure water and add one drop of 14 molar nitric acid. Then add two column volumes of seven molar nitric acid twice to remove the trace elements.

Dissolve each sample in 0.5 milliliters of 7 molar nitric acid. Load it onto the column carefully, let it drip across the column, into the waste beaker. Next, add two column volumes and one column volume of seven molar nitric acid successively into each column.

Iron and other metal elements in the samples are removed, while uranium and thorium are retained by the resin. After that, add two column volumes and one column volume of eight molar hydrochloric acid successively into each column, to elute thorium fraction. Collect the thorium fraction, using a seven milliliter PFA bottle for each column.

Add one drop of perchloric acid into the bottle and dry the fraction on a hot plate at 170 degrees Celsius for about 30 minutes. Next, elute uranium fraction from the resin with two column volumes of 0.1 molar nitric acid twice. Collect the eluate in new PFA bottles then add one drop of perchloric acid and dry it on the hot plate at 170 degrees Celsius for about 30 minutes.

Now prepare and label two milliliter capacity vials. Dissolve each sample in one drop of 14 molar nitric acid and dry it on the hot plate at 170 degrees Celsius for less than five minutes, until half a drop is left. Transfer them, along with 0.2 milliliters of 2%nitric acid and 0.1%hydrofluoric acid, into the corresponding vials for instrument measurement.

Using this procedure, submarine hydrothermal sulfide samples can be completely dissolved for uranium and thorium analysis. In this example, uranium content ranged from 178 to 5, 118.2 nanograms per gram and thorium content ranged from 603 to 7, 212 picograms per gram. Five samples had ages ranging from 567 to 21, 936 years before the year 2000 AD.Sample consumption was about 60 milligrams, except S32 wherein only 17 milligrams of sample was consumed.

In these steps, the concentration was acid reagents but we determined actually, to ensure success. Following this procedure, uranium and thorium isotopes reduce out and they still find it also determined the It can provide real chemical information for sulfide deposits. Our reagents are harmful to human body.

Please strictly comply with the relevant specifications to ensure personal safety.