John H. Reynolds

High Sensitivity Mass Spectrometer for Noble Gas Analysis

A high‐sensitivity mass spectrometer for noble gas analysis is described in detail. Ultra‐high vacuum techniques have been used throughout so that small rare gas samples can be closed off within the spectrometer envelope and analyzed statically. This procedure is of considerable advantage in analyzing argon and lighter rare gases. The background spectrum when the spectrometer pressure is 5×1010 mm Hg is described. Sensitivity data are presented. The detectability of the instrument for xenon atoms (any isotope) is 0.5×106 atoms; for A36, the detectability is 14×106 atoms.

Plutonium-244: confirmation as an extinct radioactivity.

The mass spectrum of xenon from spontaneous fission in a laboratory sample of plutonium-244 is precisely what meteoriticists predicted it would be; this discovery completes a web of proof that this nuclide is a bona fide extinct radioactivity of galactic origin, that r-process nucleosynthesis was ongoing in the galaxy at the time of the birth of the sun, and that the early meteoritic abundances of plutonium-244, heretofore tentative, can be utilized with confidence in models for the chronology of galactic nucleosynthesis. The search for an explanation for anomalous fission-like xenon in carbonaceous chondrites can now be narrowed.

Intensive sampling of noble gases in fluids at Yellowstone: I. Early overview of the data; regional patterns

Abstract The Roving A utomated R are G as A nalysis (RARGA) lab of Berkeleys Physics Department was deployed in Yellowstone National Park for a 19 week period commencing in June, 1983. During this time 66 gas and water samples representing 19 different regions of hydrothermal activity within and around the Yellowstone caldera were analyzed on site. Routinely, the abundances of five stable noble gases and the isotopic compositions of He, Ne, and Ar were determined for each sample. In a few cases the isotopes of Kr and Xe were also determined and found to be of normal atmospheric constitution. Correlated variations in the isotopic compositions of He and Ar can be explained within the precision of the measurements by mixing of only three distinct components. The first component is of magmatic origin and is enriched in the primordial isotope 3 He with 3 He 4 He ≥ 16 times the air value. This component also contains radiogenic 40 Ar and possible 36 Ar with 40 Ar 36 Ar ≥ 500 , resulting in a 3 He 36 Ar ratio ≥ 41,000 times the air value. The second component is assumed to be purely radiogenic 4 He and 40 Ar ( 4∗ He 40∗ Ar = 4.08 ± .33 ). This component is the probable carrier of observed excesses of 21∗ Ne, attributed to the α,n reaction on 18 O. Its radiogenic character implies a crustal origin in U. Th, and Krich aquifer rocks. The third component, except for possible mass fractionation, is isotopically indistinguishable from the noble gases in the atmosphere. This component originates largely from infiltrating run-off water saturated with atmospheric gases. In addition to exhibiting nucleogenic 21∗ Ne, Ne data show anomalies in the ratio 20 Ne 20 Ne , which correlate roughly with the 21 Ne 22 Ne anomalies for the most part, but not as would occur from simple mass fractionation. Some exaggerated instances of the 20 Ne 22 Ne anomaly occur which could be explained by combined mass fractionation of Ne and Ar isotopes to a severe degree coupled with remixing with normally isotopic gases. Otherwise exotic processes have to be invoked to explain the 20 Ne data. Relative abundances of the non-radiogenic and non-nucleogenic noble gases ( 22 Ne, 36 Ar, 84 Kr, and 132 Xe) are highly variable but strongly correlated. High Xe/Ar ratios are always accompanied by low Ne/ Ar ratios and vice versa . Except for water from the few cold ( T C ) springs analyzed, none of the samples have relative abundances consistent with air saturated water and the observed variations are not readily explained by the distillation of air saturated water. In characterizing each area of hydrothermal activity by the highest 3 He 4 He ratio found for that area, we find that within the caldera this parameter is somewhat uniform at ~7 ± 1 times the air value. There are exceptions, most notably at Mud Volcano, an area located along a crest of recent and rapid uplift. Here the maximum 3 He 4 He ratio is ~ 16 times the air value. Also noteworthy is Gibbon Basin which is in the vicinity of the most recent rhyolitic volcanism and exhibits a 3 He 4 He ratio ~ 13 times the air value. Immediately outside the caldera the maximum sol 3 He 4 He ratio decreases rapidly to values

METEORITIC KRYPTON AND BARIUM VERSUS THE GENERAL ISOTOPIC ANOMALIES IN METEORITIC XENON

Abstract The general isotopic anomalies in meteoritic xenon are described in detail. Where superior isotopic analyses exist, the xenon anomalies appear to be the same for all meteorites. In other cases there is fair evidence that the xenon examined is a mixture of “meteoritic” and contaminating atmospheric xenon. Two superior krypton analyses for carbonaceous chondrites show no anomalies which are significant in comparison with those for xenon. Barium from the Richardton chondrite is of normal isotopic composition. Cyclotron deuterons produce no Xe 124 in a tellurium target, although the other xenon isotopes, which are in excess in meteorites, are produced. A number of possible mechanisms for producing the general anomalies are discussed and found wanting. One of them, due to Kuroda and Cameron , calls for excess terrestrial fission xenon and for transfer of solar xenon to the atmosphere. It thus involves reasonable processes, but, as we show, requires unreasonable yields for spontaneous fission. A mechanism we propose calls for excess meteoritic fission xenon and for gross mass fractionation of terrestrial xenon. It thus produces the observed anomalies accurately, but by somewhat unlikely processes.

Potassium-argon dates of basaltic rocks from Southern Brazil

Abstract Potassium-argon ages are reported for 20 basalts (including a few core samples) and 15 diabases from the Parana basin of Southern Brazil. Histograms of both basalt and diabase ages exhibit strong peaks close to 120 m.y. Thus the principal Brazilian volcanism was Mid-Lower Cretaceous and not Jurassic as earlier supposed. While the few dates younger than 120 m.y. are likely due to argon loss, there is a real “tail” of higher ages which shows that there was precursory volcanic activity 147 m.y. ago, in the Upper Jurassic, and perhaps earlier. The span of at least 28 m.y. for the volcanism is similar to that which has been found for South Africa. The basaltic rocks of Brazil differ, however, both in age and potassium content from the older diabase rocks of Southeast Africa, Antarctica and Tasmania which have been dated thus far. Auxiliary studies included a careful evaluation of the analytical precision in our techniques and control experiments in dating series of spaced samples from narrow diabase dikes which represent examples of “instantaneous” cooling. The latter experiments show that whole-rock dating of diabase from Brazil is occasionally susceptible to discrepancies of up to 8 per cent even when fairly rigid thin-section criteria have been exercised in the selection of the samples for dating. An argon diffusion experiment on diabase material gave results characteristic of feldspar. Acid etching was found not to be effective in “rectifying” discordant feldspar separates. Auxiliary studies by electron microprobe showed that a diabase sample, in which only plagioclase feldspar would be noted in cursory visual examination, actually contains feldspars ranging from plagioclase through potash oligoclase and anorthoclase to sodic sanidine, where inconspicuous weathering of the last might be important in creating discrepancies of the type noted.

Rare-gas-rich separates from carbonaceous chondrites

Samples studied were residual, carbonaceous /Alates—a coined word to designate colloids prepared sometimes before and sometimes after acid demineralization—from Murray, Murchison, Cold Bokkeveld (type C2s) and Allende (type CV3) meteorites. Characterization: C2 /Alates, comprising 0.5% of the bulk meteorite are fine-grained (< 100 A), amorphous, sulfide-free, oxidizable, 95% carbonaceous materials which pyrolyze bimodally at 200–700 and 800–1200°C. Allende /Alates are similar but with traces of inferred spinel and chromite and of sulfur, Rare gas results: Elemental: Release from stepwise heated Murray is bimodal with maximum release and upper temperature peak at 1000°C, probably accompanying chemical reaction. All /Alates studied had very nearly the same elemental concentrations, distinctly planetary in pattern. Isotopic: Trapped neon compositions are unprecedentedly close to Pepins neon-A corner but nevertheless show signs of complexity, as if accompanied by neon-E. The trapped 3He/4He ratio is essentially constant at (1.42 ± 0.2 × 10−4. The isotopically anomalous heavy noble gases, easily detected in the residues of oxidized /Alates, were not conspicuous in this particular study. Comparison and Chicago results: Concentrations of heavy rare gases in our /Alates agree with concentrations measured directly (as opposed to inferred by difference) in acid resistant residues at Chicago. Alone, our results support the idea of a carbonaceous gas-carrier uniformly present in meteorites of various types, but Chicago characterizations of the samples can apply to both their samples and ours provided that the right amount of gas was lost in the Berkeley procedures to make the uniform gas contents in various samples a coincidence.

Xenon-iodine dating: sharp isochronism in chondrites.

Measurements of the accumulation of Xel29 from radioactive decay of extinct 1129 in meteorites show that the 1129/ 1127 ratio in high-temperature minerals in diverse chondrites was 10-4 at the time of cooling. The uniformity in the ratio indicates that the minerals cooled simultaneously within 1 or 2 million years.

Potassium-argon ages of alkaline rocks from southern Brazil*

Abstract We present potassium-argon ages for 66 samples (micas, feldspars, fine-grained whole rocks and amphiboles in descending frequency) from 21 alkaline rock localities in southern Brazil. A general geological description is given for each locality. Analytical precision is about one per cent for potassium and two per cent or better for radiogenic argon determinations. It is possible to classify many samples as concordant (within 4 m.y.) or discordant with a preferred age for their localities. On this basis, 41 samples are found concordant and 8 samples, discordant. Four examples of anomalously “old” alkali feldspar were encountered, exhibiting a hitherto unreported reversal of the orthodox mica-feldspar discrepancy in K Ar dating. Pyroxene ages were erratic and have been discarded. Studies with the electron microprobe indicate that the potassium content of pyroxene is much lower than that often reported, the difference being contamination. With some exceptions, the alkaline occurrences fall into two age groups, one Early Cretaceous (122–133 m.y.) and the other Late Cretaceous to Early Tertiary (51–82 m.y.). The older group coincides, in age, with the basaltic vulcanism of the Parana Basin, but the younger group, to which belongs the major part of Brazilian alkaline occurrences, cannot be associated in age with these basalts. The intrusions with older ages are clustered in southern Sao Paulo State, and the younger group, with one exception, lies to the north and east of the older group. At some localities, like Pocos de Caldas, the alkaline magmatism apparently took place at several different stages. There is no close correlation in time between Brazilian and African alkaline intrusions the latter being generally older, but there seem to exist similar age patterns (including coincidence between the vulcanism and the oldest alkaline rocks) in both continents.

Rare gases in tektites

Abstract Tektites from a number of localities have been examined for their contents of potassium, atmospheric argon, radiogenic argon, atmospheric neon, and possible cosmogenic neon. At 300°K, the coefficient of diffusion for helium in tektite glass from Kalgoorlie, Australia is 4 × 10 −10 cm 2 /sec. The activation energy for helium diffusion is 0.27 electron volts. The helium solubility constant is 0.005. Corresponding quantities for neon diffusion are 2 × 10 −15 cm 2 /sec, 0.61 electron volts and 0.005. The helium diffusion rate is so high that tektites retain no radiogenic or cosmogenic helium. The recent group of tektites, however, should retain a substantial amount of any cosmogenic neon contained at time of fall. In no case was any excess Ne 21 detected. From the upper limits of original Ne 21 content, a maximum “flight time” since last melting has been computed for each tektite. For a tektite from Kalgoorlie, this is 28,000 years. This result is in apparent disagreement with the finding by Ehmann and Kohman of radioactive Al 26 in australites. Potassium-argon ages have been computed for each tektite. These range from 0 to 32 m.y. and are usually in agreement with the stratigraphie age of the formations on which the tektites lie, according to presently accepted time scales. Data in this paper do not support a recent suggestion that the moldavites of Czechoslovakia and the recently discovered tektites from Georgia U.S.A. are part of the same fall.

Comparative study of argon content and argon diffusion in mica and feldspar

Abstract The A 40 /K 40 ratios for coarse and fine-mesh samples of 100-m.y.-old feldspar (perthitic orthoclase-microline) were compared with those for coarse- and fine-mesh samples of lepidolite from the same pegmatite and found to be in agreement, although there was some evidence for migration of argon from the potash feldspar phase to the soda feldspar phase of the perthite. Fine samples of both minerals were then heated to various temperatures for 48 hours and the radiogenic and atmospheric argon released was measured. At all temperatures below that at which mica is disrupted, leakage from the feldspar was greater. The temperature-dependence of the leakage from the two minerals was found to be quite different. That for the mica suggests that there are two components of argon in the mineral: a minor component which is loosely bound, and a major component tightly bound. The leakage curve for feldspar is quite regular, suggesting that there is only a single argon component. If this is assumed to be so, an extrapolation of the curve provides a lower limit of 10 −19 cm 2 /sec for D , the diffusion coefficient. This value of D is high enough to account for about 35 per cent argon loss in 1-cm grains of 2000-m.y.-old potash feldspar. Atmospheric argon in the minerals appears to be more loosely bound than the radiogenic argon. The A 40 /K 40 ratio for samples from the Pala District, California, pegmatite is 0.00526 ±0.00016, corresponding to an age of 92.0 ± 2.5 m.y..