Sr-Nd-Pb-U-Th Isotope Procedures

 

Leaching Procedures

Where required, samples may be washed or leached prior to dissolution, if they are being analysed for isotopic composition only.  Please contact us if you are uncertain as to what procedures may be required for / were carried out on your samples.

Silicate Sample Dissolution

Between 100 and 250 milligrams of powder (occasionally more for low-concentration samples) are dissolved in 50% HF-12N HNO3, then attacked with 8N HNO3 and finally 6N HCl.  The residue is taken up in 1N HBr for Pb/Sr/Nd, or 2.5N HCl for Sr/Nd only.

Carbonate Sample Dissolution

Calcites are washed in nanopure water prior to being leached in 5% acetic acid in an ultrasonic bath. Dolomites are washed in nanopure water and dissolved in 6N HCl. The residues are taken up in 7N HNO3 for Sr-spec columns.

Sulphide Sample Dissolution

Sulphides are dissolved in 7N HNO3 on a hotplate overnight. Galenas are normally dissolved in 7N HNO3 at room temperature.

U/Th/Pb

Pb is separated in Bio-Rad 10-ml polyethylene columns and Dowex AG1-8X anion resin, using 1N HBr to elute other elements and 6N HCl to elute Pb.  The collected HBr solution is dried, the residue attacked with 8N HNO3 to eliminate Br, dried again, and the residue is taken up in 8N HNO3 for U/Th.  The collected Pb solution is dried, redissolved in 1N HBr, and the above procedure is repeated with a small volume resin bed.  U and Th are separated together using the same columns and resin:  other elements are eluted with 7N HNO3, and U and Th are eluted with ultrapure H2O.  The HNO3 solution is dried down and the residue taken up in 2.5N HCl for Sr/Nd.  The U/Th solution is dried, redissolved in HNO3, and the above procedure is repeated with a small resin bed.

Total procedural blanks for Pb are < 100 picograms (25 - 65 in 2012).  We use a 208Pb spike (J. Blenkinsop, Carleton University) for Pb concentrations only, and a 233U spike (from GEOTOP, Montreal).  Concentrations are precise to + 0.5%.  Samples are loaded onto single Re filaments with H3PO4 and silica gel, and are run at filament temperatures of 1250-1350oC, depending on the gel-type used. 

Analysis of USGS Standard BCR-1 yields Pb = 13.56 ppm, U = 1.70 ppm, Th = 5.86 ppm, 206Pb/204Pb = 18.818, 207Pb/204Pb = 15.633, and 208Pb/204Pb = 38.633 (average of 2 runs, see Tatsumoto et al.)  All mass spectrometer runs are corrected for fractionation using NIST SRM981. 

The average ratios measured for SRM981 are:

TRITON: 206Pb/204Pb = 16.889 +/- .007, 207Pb/204Pb = 15.426 +/- .0092, and 208Pb/204Pb = 36.494 +/- .030 (2 s.d.), based on 43 runs between October 2010 – June 2012. Our subsequent fractionation correction is based on the values of Todt et al. (1984).

Rb/Sr

Each silicate sample, dissolved in 2.5N HCl, is pipetted into a 14-ml Bio-Rad borosilicate glass chromatography column containing 3.0 ml of Dowex AG50W-X8 cation resin.  Rb and Sr are eluted in succession using 2.5 N HCl.  If required, the rare earth elements are then eluted using 6N HCl.  The REE solution is dried and the residue dissolved in 0.26N HCl.

Carbonate and water samples are loaded in 7N HNO3 onto small (5cm x 5mm i.d.) polypropylene columns containing Eichrom Sr-spec resin, and Sr is eluted in water.

Total procedural blanks for Sr are < 250 picograms.  We have two mixed 87Rb/84Sr spikes, one for felsic rocks and another for mafic to intermediate rocks.  Sr is loaded onto a single Ta filament with H3PO4 and is run at filament temperatures of 1350-1500oC.  Isotope ratios are normalized to 86Sr/88Sr = 0.11940 to correct for fractionation.  Two standards are run at Carleton, NIST SRM987 (TRITON: 87Sr/86Sr = 0.710239 +/- 14, n=42, August 2010 – August July 2012) and the Eimer and Amend (E&A) SrCO3 (TRITON –  87Sr/86Sr = 0.708012 +/- 15, n=11, August 2010 – August July 2012).  Rb is loaded with H3PO4 onto one side of a double Re filament assembly and run at temperatures of 1250-1300 oC.  No correction for fractionation is made.

Sm/Nd

REE fractions are dissolved in 0.26N HCl and loaded onto Eichrom Ln Resin chromatographic columns containing Teflon powder coated with HDEHP [di(2-ethylhexyl) orthophosphoric acid, Richard et al., 1976]. Nd is eluted using 0.26N HCl, followed by Sm in 0.5N HCl. 

Total procedural blanks for Nd are < 50 picograms; < 6 picograms for Sm.  Samples > 250Ma are spiked with a mixed 148Nd-149Sm spike prior to dissolution.  Concentrations are precise to +/- 1%, while 147Sm/144Nd ratios are reproducible to 0.5%.  Samples are loaded with H3PO4 on one side of a Re double filament, and run at temperatures of 1700-1800 oC.  Isotope ratios are normalized to 146Nd/144Nd = 0.72190.  Analyses of the USGS standard BCR-1 yield Nd = 29.02 ppm, Sm = 6.68 ppm, and 143Nd/144Nd = 0.512668 +/- 20 (n=4).  The international La Jolla standard produced: TRITON: 143Nd/144Nd = 0.511847 +/- 7, n = 26 (Feb 2005 – June 2007).  Internal lab Nd standard = 0.511818 +/- 8, n = 28 (Feb 2005-June 2007); 0.511819 +/- 10 n = 94 (Feb 2005 – Aug 2009); 0.511823 +/- 12 n = 65 (Oct 10 – July 12).

References:

Richard, P., N. Shimizu, and C. J. Allègre, 143Nd/146Nd, a natural tracer: an application to oceanic basalts, Earth Planet. Sci. Lett., 31, 269-278, 1976.

Tatsumoto, M., R. J. Knight, and M. H. Delevaux, Uranium, thorium, and lead concentrations in three silicate standards and a method of lead isotopic analysis, U.S.G.S. Prof. Paper 800-D, , D111-D115, 1972.

Todt, W., R. A. Cliff, A. Hanser, and A. W. Hofmann, 202Pb -205Pb spike for Pb isotopic analysis, Terra Cognita, 4, 209, 1984.