Sm-Nd age of the Stillwater complex and the mantle evolution curve for neodymium

An internal isochron determined for a gabbro from the Stillwater complex by the Sm-Nd method yields a precise age of 2701 ± 8 Myr and initial ¹⁴³Nd/¹⁴⁴Nd = 0.508248 ± 12. The initial is close to the CHUR evolution curve but clearly displaced below it by ?_Nd = ?2.8 ± 0.2. A spectrum of total rocks in the Stillwater complex ranging from anorthosite to pyroxenite were found to lie on the same isochron to within experimental error indicating the same age and initial. These data demonstrate that some ancient mantle-derived rocks have initial ¹⁴³Nd/¹⁴⁴Nd which deviate substantially from the CHUR evolution curve at the time of their formation. This implies that there was early layering in the mantle with substantial REE fractionation (~6–12% Nd/Sm enrichment) or that the Stillwater complex was highly contaminated with REE from much older continental crust during emplacement. The results show the necessity of high-precision ages and initial ¹⁴³Nd/¹⁴⁴Nd values in order to properly describe REE fractionation in the mantle. While the Sm-Nd age results show no indication of any irregularities, we have confirmed that the Rb-Sr data for the Stillwater are highly disturbed. This comparison indicates that the Sm-Nd parent-daughter system may be much less susceptible to element redistribution during metamorphism, therefore permitting wide application of this technique to rocks of complex histories.     

Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite

⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase separates from three basaltic clasts (A, B, C), a pyroxene separate from clast B, and a total sample of a fourth basaltic clast (ρ) from the Kapoeta achondritic meteorite. The Ar data show that three of the four clasts crystallized ≥4.5 AE ago. Xe measurements indicate all four formed within a 0.1 AE period (Huneke, et al., 1977, Lunar Science VIII, pp. 484–486). Three clasts have suffered various degrees of ⁴⁰Ar loss since that time. The times of ⁴⁰Ar degassing do not cluster about a single time analogous to the lunar cataclysm. The survival of ≥4.5 AE ages contrasts with the general absence of ages ≥4.0 AE on the moon.The Ar retention age of clast B of ≥4.57 AE is atypically older than the Rb-Sr age of 3.6 AE (Papanastassiouet al., 1974, Lunar Science V, p. 583). The 3.5 AE Ar age of clast A is distinctly younger than the Rb-Sr age of 3.9 AE (Papanastassiou et al., 1974). The K-Ar and Rb-Sr systems are clearly not equivalent dating techniques in these instances.The combined evidence of Ar, Xe and Rb-Sr studies suggests the period of volcanism on the Kapoeta parent planet was restricted to the first ~0.2 AE of solar system history. The subsequent thermal metamorphic histories recorded in each of the four clasts after formation are distinctly different. The clasts must have existed as independent fragments at least as recently as 3.5 AE ago. The cosmic ray exposure ages of all the four clasts are similar (~ 3 Myr), and are not significantly different from that of the bulk meteorite. The clasts spent essentially all of the time prior to the formation of Kapoeta at depths greater than a few meters.     

Al‐Mg systematics of CAIs,POI, and ferromagnesian chondrules from Ningqiang

Abstract— We have made aluminum‐magnesium isotopic measurements on 4 melilite‐bearing calcium‐aluminum‐rich inclusions (CAIs), 1 plagioclase‐olivine inclusion (POI), and 2 ferromagnesian chondrules from the Ningqiang carbonaceous chondrite. All of the CAIs measured contain clear evidence for radiogenic ²⁶Mg* from the decay of ²⁶Al ( = 1.05 Ma). Although the low Al/Mg ratios of the melilites introduce large uncertainties, the inferred initial ²⁶Al/²⁷Al ratios for the CAIs are generally consistent with the value of 5 times 10^?5. There is clear evidence of ²⁶Al* in one POI and two chondrules, but with considerable uncertainties in the value of (²⁶Al/²⁷Al)₀. The (²⁶Al/²⁷Al)₀ ratios for the POI and the chondrules are 0.3–0.6 times 10^?5, roughly an order of magnitude lower than the canonical value. Ningqiang shows very little evidence of metamorphism as a bulk object and the (²⁶Al/²⁷Al)₀ ratios in its refractory inclusions and chondrules are consistent with those found in other unmetamorphosed chondrites of several different classes. Our observations and those of other workers support the view that ²⁶Al was widely and approximately homogeneously distributed throughout the condensed matter of the solar system. The difference in (²⁶Al/²⁷Al)₀ between CAIs and less refractory materials seems reasonably interpreted in terms of a ～2 million year delay between the formation of CAIs and the onset of formation of less refractory objects. The POI shows clear differences in ²⁵Mg/²⁴Mg between its constituent spinels and olivine, which confirms that they are partially reprocessed material from different sources that were rapidly quenched.     

Isotopic abundances: inferences on solar system and planetary evolution

Members of the Royal Swedish Academy, ladies and gentlemen.In thinking over why I am here today, I recalled the story of Tom Sawyer, a character of Mark Twain's. On a beautiful, bright Saturday morning, when all of his friends were playing, he appeared by the wall in front of his aunt's house with a bucket of whitewash paint and a long-handled brush. Before him stood about thirty meters of continuous wooden fence, three meters tall. He dipped his brush and passed it over a plank and then repeated it. Then he stopped and compared the insignificant whitewashed streak with the far-reaching continent of unpainted fence and became melancholy. Then he took up the brush again and went to work. A friend came by and teased him for working on Saturday when he could be swimming or playing. Tom said it wasn't so bad, because a boy didn't get the chance to whitewash a fence every day. Tom took a new brush stroke at the fence and stepped back to note the beauty of the effect. His friend got interested and then asked permission to make a stroke or two. He then offered Tom the core of an apple he had just started to nibble on. Tom told him that it was doubtful that his friend could help because this was the front fence and that a very good and sophisticated job had to be done. Possibly only but one child in a thousand, possibly two-thousand, could do it. Well, after this denial, his friend just had to try and offered Tom the whole apple for a chance to paint. Then more young friends came around to attempt this challenging task. By the end of the afternoon, Tom was the wealthiest boy in town. He had twelve marbles, a key that didn't unlock anything, a piece of blue glass to look through, a tin soldier, a dog collar (but no dog) and lots of other valuable things. The fence had been coated three times and if he had not run out of whitewash, Tom would have bankrupted every kid in the village. My presence here is due to an ability to attract young talent to help whitewash the fence. The only difference is that we have just started on the job.     

The isotopic composition of silver and lead in two iron meteorites: Cape York and Grant

Silver in the metal phases of Cape York (IIIA) and Grant (IIIB) has been determined after an extensive surface cleaning process. The ${}^{107}\text{Ag}{}^{109}\text{Ag}$ was found to be enriched over that found in terrestrial Ag by ~7%. to 19%., demonstrating the presence of excess ¹⁰⁷Ag (¹⁰⁷Ag¹) in this class of meteorites. An effort was made to find schreibersite with a distinctive ¹⁰⁸Pd/¹⁰⁹Ag ratio in order to establish a three-point isochron, but the results are not markedly different from those obtained for the bulk metal. The Ag isotopic ratio of sulfides from the same meteorites were nearly normal in composition. These results demonstrate correlations of ${}^{107}\text{Ag}{}^{109}\text{Ag}$ with ${}^{108}\text{Pd}{}^{109}\text{Ag}$ between coexisting phases of two iron meteorites that are associated with planetary differentiation processes. The ratios ${}^{107}\text{Ag}{}^1{}^{108}\text{Pd}$ were found to be 1.7 × 10^?5 and 1.2 × 10^?5 for Cape York and Grant, respectively. These observations are in support of the widespread presence of ¹⁰⁷Pd in the early solar system. The difference in isotopic composition between metal and sulfide phases demonstrates that silver diffusion was small (over 6.5 × 10⁶ y) indicating a cooling rate much greater than 150°C/my for meteorites which have been attributed to small planetary cores. Uranium determinations were carried out on the metal phases and concentrations of ~ $\text{1 × 10}{}^{12}$ g U/g and 2 × 10^?10g U/g were found for Cape York and Grant, respectively. The Pb in these meteorites was determined using the improved cleaning procedures and chemical separations with low blank levels. The results confirm the presence of variable proportions of radiogenic Pb in both the metal and sulfide phases of iron meteorites. No simple explanation for the presence of radiogenic lead is apparent; while terrestrial contamination may appear to be the obvious explanation, it is possible that this effect could result from relatively recent metamorphism in the meteorite parent body.     

Willy: A prize noble Ur-Fremdling—Its history and implications for the formation of Fremdlinge and CAI

A detailed mineralogic and chemical study of Willy, a very large (150 μm diameter) Fremdling from the Allende CAI 5241, was performed and compared to other Fremdlinge from Allende CAI's 5241 and TS-34 in an attempt to understand the nature and mode of formation of these exotic and complex objects. Willy is composed primarily of V-rich magnetite, V-rich fassaite, and Ni-Fe metal containing Co and Pt. Minor phases include an Fe-Ni-sulfide, V-rich spinel, scheelite (the first reported occurrence in a meteorite), and Cl-apatite. Phases found in trace amounts include nuggets of Os-Ru-Re metal, molybdenite, an unidentified Fe-, Mg-molybdate, and diopside. The Fremdling is concentrically zoned and contains a complex porous core of magnetite, metal, sulfide, scheelite, and other minor phases surrounded by a compact mantle of magnetite with minor apatite. The mantle is surrounded by a dense rim composed of fassaite with minor spinel that appears to be a typical occurrence around oxide-containing Fremdlinge. At the boundary between the fassaite-rim and the magnetite-mantle of Willy is a thin zone (<20 μm wide) of an apparent reaction assemblage consisting of V-rich MgAl₂O₄, FeAl₂O₄, and a third V-rich spinel, possibly FeV₂O₄, in intimate intergrowth.From the observed chemistry and texture, a multistage sequence of formation of Willy, possibly occurring in the solar nebula and involving major changes in T and $\text{?O}{}_2$, can be deduced. The first phases that may have formed in the interior are magnetite and an Fe or Ca tungstate. Refractory metal nuggets and sulfide were introduced after this stage followed by two stages of Ni-Fe formation during which Pt was dissolved in the metal. This was followed by formation of the magnetite mantle, introduction of apatite and possible alteration of ferberite to scheelite. Finally, the V-rich fassaite rim formed, accompanied or followed by reaction forming the complex Fe-, V-rich spinels. All of these steps preceded introduction of the Fremdling into the CAI precursor which in turn appears to have occurred prior to formation of spinel and the major silicate phases in the CAI.The concentric mineral zoning and texture of Willy may indicate that it is one of the few Fremdlinge that was not substantially recrystallized after having been captured in the CAI. It thus may represent the precursor material for many of the other Fremdlinge, since numerous Fremdlinge studied exhibit many of the mineral and chemical features observed in Willy. The co-existence of magnetite, sulfide, and Ni-Fe metal in the interior of Willy and the co-existence of metal, hercynite spinel and magnetite at the rimmantle boundary suggests that the maximum temperature at which Willy could have existed for long periods of time in its present state was about 500–600°C. Similarly, the existence of highly heterogeneous V-, Ti-rich fassaite rims around Willy and other Fremdlinge implies rapid cooling rates for the CAI's.     

U,U andTh in seawater

We have developed techniques to determine²³⁸U,²³⁴U and²³²Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a²³³U²³⁶U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that²³⁴U/²³⁸U ratios can be measured accurately and reproducibly.²³⁴U/²³⁸U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 10⁹ atoms of²³⁴U (3 × 10⁻⁸ g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.²³⁸U is measured to ± 2‰ (2σ) for a sample of 8 × 10¹² atoms of²³⁸U ( 3 × 10⁻⁹ g of U, 1 ml of seawater).²³²Th is measured to ± 20‰ with 3 × 10¹¹²³²Th atoms (10⁻¹⁰ g²³²Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the²³²Th concentration in the oceans. Using these techniques, we have measured²³⁸U,²³⁴U and²³²Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and²³⁸U and²³⁴U in vertical profiles from the Pacific. Determinations of²³⁴U/²³⁸U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these²³⁴U/²³⁸U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10⁻⁵. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).²³²Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the²³⁴U/²³⁸U ratio is consistent with the predicted range of²³⁴U/²³⁸U using a simple two-☐ model and the residence time of deep water in the ocean determined from¹⁴C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.     

40Ar-39Ar and Rb-Sr age determinations on Quaternary volcanic rocks

⁴⁰Ar-³⁹Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides⁴⁰K and⁸⁷Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the⁴⁰Ar and the³⁹Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×10⁵ years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the⁴⁰Ar/³⁶Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic⁴⁰Ar from trapped⁴⁰Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in⁸⁷Sr/⁸⁶Sr as large as 16 parts in 10⁴ and⁸⁷Rb/⁸⁶Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×10⁵to3.3±0.2×10⁵ years and are in good agreement with the⁴⁰Ar-³⁹Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial⁸⁷Sr/⁸⁶Sr. However, different tuffs show small differences in initial⁸⁷Sr/⁸⁶Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the⁴⁰Ar-³⁹Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.     

Neodymium isotopic study of Baffin Bay water: sources of REE from very old terranes

Water samples were collected from Baffin Bay and surrounding areas in order to evaluate this region as a potential source of Nd from old continental material to Atlantic water. The isotopic data ranged from ε_Nd(0) = −9.0 to −26 with most of the data around ε_Nd(0) = −20 compared with values of North Atlantic Deep Water (NADW) with ε_Nd(0) = −13.5. The concentration of Nd in Baffin Bay waters was as high as 6 × 10⁻¹² g/g compared with 2.5 × 10⁻¹² g/g for NADW. The combination of low ε_Nd and high Nd concentration indicates that Baffin Bay may be a significant source of Nd from very old crustal material. A simple box model was used to evaluate the contribution to the Nd budget of NADW and it was concluded that a substantial fraction of the Nd from ancient crustal sources that is required to maintain the isotopic composition of NADW could be supplied by Baffin Bay outflow.