Isotopic composition of silver in iron meteorites

Abstract

With a view to developing a new dating method based on extinct 6.8 m.y. Pd107, we isolated silver from 18 samples of the iron meteorites Bristol, Canyon Diablo, Grant, Odessa, Piñon, Sandia Mountains, and Toluca. The silver concentration (11 to 197 ppb) was determined on an aliquot by isotope dilution, and also by neutron activation analysis of separate samples. The isotopic composition was measured by surface ionization on the Argonne 100‐inch‐radius mass spectrometer. No enrichment of Ag107 was detected in any of these samples: our δAg107 values for Canyon Diablo and Toluca were 1.2±2.3 and 3.5±3.7 per mil, compared with Murthy's values of 40±4 and 22±4 per mil. The grand average of all our analyses was 0±1.5 per mil. That this discrepancy was not due to contamination is shown by our low blanks (<5 per cent), by the agreement of the neutron activation and isotope dilution results, and by other evidence. Sampling error is also not a likely explanation, since its probability is <1.6 × 10−3 under the most favorable assumptions. Possibly the apparent anomalies found by Murthy are in some way related to the anomalous value he obtained for terrestrial silver (Ag107/Ag109 = 1.064±0.002, compared with 1.082±0.002, the average ratio found by four other investigators).

Isotopic analyses of barium in meteorites and in terrestrial samples

“…These authors seem to be unaware of the other work in this field and the instrumental problems that invariably exist. Chakraburtty et al [1964] reported that no enrichment of Ag •ø• was found in seven iron meteorites, including Canyon Diablo for which an anomaly was reported by Murthy [1960], which was most likely the effect of instrumental fractionation.…”

Isotopic determination of silver in picomole quantities by surface ionization mass spectrometry

“…In relative determinations, one is not concerned with absolute values; it is only necessary to compare values of samples to measured values of an accepted or arbitrary standard. This latter method has been used in past geochemical studies (1)(2)(3)(4) and was used in this study. In either approach it is imperative that samples and standards be measured under identical conditions to avoid systematic errors.…”

“…The backgrounds are typically taken 8-10 gauss on both sides of the peak. In the case of Ag, which has two isotopes, 3 min of data acquisition are required for 1 set of 10 ratios. Data reduction is done both online and offline by a PDP-11 computer.…”

Anomalous Ar40contents of iron meteorites

“…Iron meteorites have not been so. extensively investigated, but in all cases where results have been reported the same situation holds true: T1 landers and Stevens, 1960], Mo [Murthy, 1962], S [Thode et al, 1961], Ge [Shima, 1963], and Ag [Chakraburtty et al, 1964;Murthy, 1962]. There is no obvious reason why potassium should behave differently from these other elements, and in order to account for the Ar 4ø anomaly with this explanation a difference in the ratio K•ø/K • of greater than an order of magnitude, not just a few per cent, would be necessary.…”