GREENLAND 5ER\E5
II. EXPERIMENTAL PROCEDURES
A. Rubidium and Strontium.
The rubidium and strontium analytical procedure is essentially the basic isotope dilution procedures currently used in several laboratories, and described by Lanphere (1963). A brief description will suffice here.
Dissolution of silicate samples with HF and HCI0
4 was followed by complete solution in HCI except for zircon residues in some biotites and in all total rocks. At an early stage in this work it was realized in collaboration with G. J. Wasserburg and T. Wen that use of low-strontium laboratory glassware con- tributed significant amounts of strontium contamination {Wasserburg, Wen I and Aronson, 1964). It is felt that this possible contamination did not alter significant-
ly the ages of the samples so processed (ASH, SITR, 22, 4, and parts of 18(+), 6, and 48), and in all subsequent work teflon vessels were used in the post-dissolution, pre-ion exchange column steps. Dissolved samples were thoroughly mixed and al iquoted by weighing into a rubidium concentration aliquot, a strontium concen- tration aliquot and, for most lowly-radiogenic strontium samples, a strontium isotop-
ic composition aliquot. Weighed amounts of calibrated Rb87
enriched tracer and Sr86
~
Sr84 enriched tracer were added to their respective aliquots. The mixtures were then thoroughly mixed by vigorous stirring.Samples were evaporated dry under heat lamps. The rubidium concentration was washed with two drops of water and the remaining undissolved perchlorate
crystals were ready for the mass spectrometer run. A few rubidium samples were completely dissolved in 2 ml of 1% hydrochloric acid and Kand Rb were precipi- tated with sodium tetraphenyl borate. This precipitate was converted to perchlor- ates and then to sulfates by addition and evaporation of perchloric and sulfuric
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acid, respectively. The sulfates were run in the mass spectrometer.
The strontium aliquots were picked up in 2 .5N HCI and loaded on conditioned Dowex 50 columns 15-20 cm in length and 1 cm in diameter. The strontium aliquots were collected in glass beakers, evaporated dry, and loaded on a filament for the mass spectrometer.
The Rb87
tracer contained rubidium that was 99.2% Rb87
. The Sr86
tracer, used in the early stages of this work (see samples listed with glassware, above), was about 84% pure. The Sr84
tracer was used for all other samples. This "single"
isotope tracer ·of about 83% purity enables the determination of discrimination- corrected isotopic compositions during the spiked run. Detai Is of the discrimin- ation calculation are contained in a later section of this appendix. The tracers were calibrated regularly against normals carefully prepared by the author from
"spec-pure" SrC0
3 and Rb Cl supplied by Johnson and Matthey, Ltd. of England.
A slow drift toward an 0.6% increase in concentration in the Sr84
tracer was noted over a 2 .5 year period of constant use. No drift toward increased concentration was observable in the Rb 87
tracer over a 3 year period of constant use. This is probably only due to a lack of pr€cision in the measurement of rubidium concen- tration as explained below.
All mass spectrometer runs were made on the same 12-inch radius of curva- ture, 60° sector, single focusing, sol id source mass spectrometer described by Chow and McKinney (1956). The ion beam was accelerated by a 5 kilovolt potential. A ten stage electron multiplier received the ion current and the
resulting current was passed through a 2·1
o
9 resistor and a vibrating reed electrom- eter. All samples were run on single outgassed tantalum filaments.Blanks were processed regularly during the course of this study. These
indicate a strontium contamination which averages 0.012 /-gm per gram of sample processed and ranged from 0.007 ~gm/gm to 0.021/gm/gm. The rubidium blank ranged from 0. 0005.)A gm/ gm to 0. 006 )A gm/ gm.
B. Lead, Uranium and Thorium.
Zircon separates were carefully purified by several passes through 3.30 S .G. methylene iodide, discarding the floats. Visual inspection of grain mounts
and X-ray diffraction analysis showed no impurity, other than pyrite. The zircons were washed in 1-1 hot nitric acid until all pyrite was dissolved, and then rinsed several times with pure water. About 0.5-0.6 grams were fused in ten times their weight of purified borax. The entire fusion was generally complete within one hour at temperatures monitored by an optical pyrometer as between 930° C -1000°C.
Continuous agitation apparently eased the fusion.
The fused zircon was transferred completely with hydrochloric acid and dissolved in 250-500 ml of hydrochloric acid. Small amounts of flaky insoluble residue were centrifuged down, transferred to a teflon vessel and treated with HF and HCI0
4, dissolved in hydrochloric acid, and returned to the main solution.
The solution was partitioned by either weighing or volume into two parts, one con- taining about 90% for the lead composition run and the other 10% for the lead, uranium, and thorium concentration run. The lead, uranium, and thorium were coprecipitated with Zr(OH)
4 by passing NH
3 gas through the solution. The precipitate was dissolved in hydrochloric acid and the final normality was 1.2N-
1.5N. This solution was loaded on a 17 x 1 cm Dowex 100-200 mesh anion ex- change column. Lead was retained on the column during the loading and subsequent washing of the column by 1.2N hydrochloric acid, while zirconium, uranium, and
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thorium and most other elements were eluted. This eluate was collected from the spiked concentration aliquot for subsequent treatment for uranium and thorium.
The lead was eluted with water and collected. The lead was further purified by standard dithizone procedures extracting in chloroform from a pH 9, cyonide- citrote solution, and back-extracting into dilute nitric acid. The latter, contain- ing ionic lead, was evaporated dry, picked up in 2% NH
4 N0
3 and lead precipi- tated with H
2S gas. The sulfide was loaded on a rhenium single out gassed filament and run in the U.S.G.S. solid source moss spectrometers with simple collectors
in Denver or in the C.l.T. machine on a tantalum filament with electron multi- plier.
Lead from feldspars was extracted similarly, only the feldspar was more easily dissolved using standard treatment by HF and HCI 0
4.
The eluate containing the spiked uranium and thorium was again coprecipi- tated with Zr(OH)
4. This was dissolved in 6N nitric acid and loaded on a new Dowex 17 x l cm Dowex anion exchange column, previously conditioned with nitric acid. The sample-loaded column was washed with 6N nitric acid.
Uranium was eluted with woter,ond thorium with 6N hydrochloric acid into the same catch beaker. The solution was evaporated to near dryness, picked up in 6N HN0
3, and the above column procedure was repeated using a smaller 17 x 0.6 cm column. The uranium and thorium in the final evaporation were picked up in nitric acid and loaded on two side filaments of a triple filament source.
At the U.S.G.S. the overage lead blank incurred in separating lead from feldspar is about 0.1 gm or less. A blank run by Bruce Doe on the borax used for the fusion of zircons from samples 22(+) and 6 produced 0.54 gm of lead per