Kr and Xe isotopes from spontaneous fission of 248Cm and 250Cf: Comparison with noble gases in carbonaceous chondrites

Relative yields of Kr and Xe isotopes from the spontaneous fission of ²⁴⁸Cm and ²⁵⁰Cf have been determined mass spectrometrically. The yields are as follows: ⁸³Kr/⁸⁴Kr/⁸⁵Kr/⁸⁶Kr = 0.223/0.458/0.596/ ≡ 1.00 and 0.306/0.582/0.793/ ≡ 1.00;¹³¹Xe/¹³²Xe/¹³⁴Xe/¹³⁶Xe = 0.486/0.819/1.075/ ≡ 1.00 and 0.343/0.506/0.851/ ≡ 1.00 from ²⁴⁸Cm and ²⁵⁰Cf, respectively. The Xe yields from ²⁴⁸Cm agree with an earlier determination by Leich et al. [24]. Neither of these yield patterns matches that of “fissiogenic” Kr and Xe in carbonaceous chondrites and hence ²⁴⁸Cm and ²⁵⁰Cf are ruled out as progenitors of the meteoritic Kr and Xe. In general, none of the spontaneously fissioning nuclides of actinide elements can be identified as a possible progenitor. Even the mixtures of actinides, including a combination of ²⁴⁸Cm and ²⁵⁰Cm, are unsuitable. The origin of “anomalous” Kr and Xe in carbonaceous chondrites must then be traced either to the spontaneous fission of a superheavy element or to peculiarities in specific nucleosynthetic reactions.     

Investigations on cosmic-ray-produced nuclides in iron meteorites, 2. New results on41K/40K-4He/21Ne exposure ages and the interpretation of age distributions

The cosmic ray exposure ages of 16 iron meteorites were determined by the⁴¹K/⁴⁰K-⁴He/²¹Ne method. The ages measured in the present and in previous experiments are summarized and presented in form of various histograms characterizing the age distributions of the different chemical groups separately. Age clustering at 650 Ma (mega years) is typical for the group IIIAB. Age clustering at 400 Ma is observed for the IVA irons. Quasi-continuous age distributions are found for the groups IA, IIA, IIB, IVB and for the anomalous irons. The following interpretation is offered. The IIIA and IIIB irons have initially been core material of the same parent asteroid and were ejected in consequence of a single impact event about 650 Ma ago. The IVA irons represent core material of another asteroid which was hit and partially disrupted in consequence of an impact event about 400 Ma ago. The group IA exhibits meteorites with ages between 200 and 1200 Ma. The quasi-continuous character of this age distribution and cosmochemical evidence indicate for these irons a raisin-bread-like character of their initial distribution within the silicate mantle of their parent asteroid. In consequence of several or, perhaps, of many crater-forming impact events the mantle material was gradually destructed and ejected. In the age distribution of the IIA hexahedrites, ages <300 Ma predominate and ages >600 Ma seem to be missing. In attempting to understand this, the possibility must be taken into consideration that the mean life-time of hexahedrites in the interplanetary space might be shorter than that of other irons. The cause might be that the hexahedrite single crystals are perhaps easier cleavable in the space environment. A similar kind of selective mass wastage appears also to be the cause for the absence of stone meteorites with high exposure ages.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

40Ar/39Ar step heating of thermally overprinted biotite,hornblende and potassium feldspar from Eldora,Colorado

⁴⁰Ar/³⁹Ar incremental heating experiments were applied to hornblendes, coarse-grained biotites and K-feldspars from 1400 m.y. old rocks near the contact with the ～60 m.y. old Eldora stock in the Front Range of Colorado. The aim was to distinguish, on the basis of argon isotopic data alone, a partially re-set K-Ar date from an undisturbed or a completely overprinted K-Ar date. In the laboratory heating of biotites the radiogenic argon (⁴⁰Ar^*) and potassium-derived³⁹Ar (³⁹Ar^*) were released in two stages — in the range ～600–900°C and above ～900°C. The two biotites furthest from the contact and the one adjacent to the contact give well-defined apparent-age plateaus at ～1230 m.y. and 63 m.y. respectively for all argon released above ～600°C. The 1230-m.y. date may represent a thermal event or the end of a long cooling while the 63-m.y. date essentially represents the time of reheating. Partially overprinted biotites at intermediate distances have significantly anomalous plunges in apparent ages for argon released above ～900°C, thus distinguishing them from undisturbed and completely outgassed biotites.The bulk of the⁴⁰Ar^* and³⁹Ar^* in the hornblendes was released in the range ～950–1100°C. The hornblende furthest from the contact gives a well-defined plateau at 1400 m.y. for the 98% of the argon that was released above ～950°C. A partially overprinted hornblende from near the contact gives an apparent plateau at ～1050 m.y. The existence of such a false plateau precludes the distinction of partially overprinted K-Ar hornblende dates from undisturbed K-Ar hornblende dates without independent evidence. Reasonable estimates of the time of reheating are not recovered in the age spectra for partially overprinted hornblende and biotites.For the feldspars the bulk of the⁴⁰Ar^* and³⁹Ar^* was released in the laboratory heating between about 900°C and 1200°C, probably reflecting phase changes near these temperatures. The argon released below about 900°C records reasonable maximum dates for the time of the thermal overprinting. For the microcline 22500 (the sample number specifies the distance, in feet, from the contact) this effect is slight — a minimum date of 147 m.y. occurs in 2.3% of the total³⁹Ar^*. For samples 2400, 1070, and 85 the respective minimum dates are similar at 72, 81, and 68 m.y. and dramatically improve on the total or integrated dates of 238, 358 and 211 m.y. The high-temperature (>900°C) apparent ages for these three feldspars do not define plateaus and are geologically meaningless. The high-temperature apparent ages for the last 50% of the³⁹Ar^* released from 22500 do define a plateau, but the 1060-m.y. date is also probably geologically meaningless.     

210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter

The distribution of²¹⁰Po and²¹⁰Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported²¹⁰Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved²¹⁰Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in²¹⁰Po, with²¹⁰Po/²¹⁰Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for²¹⁰Pb and a 0.6-year residence time for²¹⁰Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,²¹⁰Po maxima occur and unsupported²¹⁰Po is frequently present. Calculations indicate that at least 50% of the²¹⁰Po removed from the mixed layer is recycled within the thermocline. Similar calculations for²¹⁰Pb suggest much lower recycling efficiencies.Comparison of the²¹⁰Pb distribution with the reported distribution of²²⁶Ra at nearby GEOSECS stations has confirmed the widespread existence of a²¹⁰Pb/²²⁶Ra disequilibrium in the deep sea. Vertical profiles of particulate²¹⁰Pb were used to test the hypothesis that²¹⁰Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate²¹⁰Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred²¹⁰Pb flux. It is proposed instead that an additional sink for²¹⁰Pb in the deep sea must be sought. Estimates of the dissolved²¹⁰Pb/²²⁶Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of²¹⁰Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 10⁷ cm²/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,²¹⁰Po is deficient, on the average, by ca. 10% relative to²¹⁰Pb; a corresponding enrichment is found in the particulate phase. Total inventories of²¹⁰Pb and²¹⁰Po over the entire water column, however, show no significant departure from secular equilibrium.     

226Ra,210Pb and210Po in the Red Sea

Profiles of²²⁶Ra and dissolved²¹⁰Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including²²⁶Ra and dissolved and particulate²¹⁰Pb and²¹⁰Po. These profiles show several distinct features: (1)²²⁶Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved²¹⁰Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water²¹⁰Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)²¹⁰Pb and²¹⁰Po activities are essentially identical to each other in both particulate and dissolved phases although²¹⁰Po activities appear somewhat lower; (5) about 20% of the²¹⁰Pb and²¹⁰Po in the water column residues on particulate matter.Assuming the atmospheric²¹⁰Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm² yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

226Ra, 210Pb and 210Po disequilibria in the Western North Pacific

²²⁶Ra,²¹⁰Pb and²¹⁰Po were measured in oceanic profiles at two stations near the Bonin and Kurile trenches.²¹⁰Po is depleted by 50% on average relative to²¹⁰Pb in the surface water. In the deep water,²¹⁰Pb is about 25% deficient relative to²²⁶Ra. Based on the deficiency,²¹⁰Pb residence time with respect to removal by particulate matter was estimated to be less than 96 years in the deep water.²¹⁰Pb deficiency in the bottom water was significantly greater than that of the adjacent deep water, indicating more effective removal near or at the bottom interface.²¹⁰Pb,²¹⁰Po and Th appear to have similar overall rate constants of particulate removal throughout the water column.     

Depth versus (age)1/2: A perspective on mid-ocean rises

The straight-line relationship between depth and the square root of age predicted by recent variations of the thermal contraction model for ocean rise elevation is confirmed to an age of 80 m.y.We then examine this relationship in the immediate vicinity of the rise crest in an attempt to determine the sensitivity of the slopes thus obtained. Depth versus t^1/2 profiles from a variety of rise types ranging from the topographically smooth, fast-spreading Pacific-Antarctic rise to the rough, slow-spreading Mid-Atlantic rise are discussed, ages having been assigned using a finite rotation pole. Because of the variety of superimposed anomalous features concentrated within a limited and well-surveyed region, the Galapagos Spreading Center has provided a suitable arena for determining the precision with which the method can decompose such an agglomeration into distinctly recognizable components. Although topographic “noise” precludes precise quantification of the slopes, it is concluded that, by removing the first-order effect of thermal contraction, the method can be quite revealing when topography is examined in relation to other data. Slopes for several profiles across the Pacific-Antarctic and Pacific-Nazca rises reveal the pattern expected in the case of asymmetric spreading, a conclusion which has independently been derived from the magnetic anomalies. In the Galapagos region “jumps” of the spreading center, a basic compositional difference, and uplift from below are exposed by their predictable effect on the slopes obtained from the depth versus t^1/2 plots.     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

210Pb/226Ra disequilibrium in the Santa Barbara Basin

The vertical distributions of²¹⁰Pb and²²⁶Ra in the Santa Barbara Basin have been measured. The²¹⁰Pb/²²⁶Ra activity ratio is close to unity in surface water, but ranges from 0.2 to 0.6 in deep water with a mean value of 0.3 (d > 250m), suggesting rapid removal of²¹⁰Pb from the water column. The²¹⁰Pb concentrations in the particulate phase at different water depths indicate that the removal of²¹⁰Pb is due to adsorption on settling particles.It is estimated that the particulate²¹⁰Pb contributes about 50–70% of the total²¹⁰Pb measured on unfiltered water samples of the Santa Barbara Basin. The fate of²¹⁰Pb (and Pb) in the water column is thus strongly controlled by the settling particles, which have a mean residence time of one year or less in the basin. Material balance calculation for²¹⁰Pb in the basin suggests that there is an external source supplying about 70–80% of the²¹⁰Pb observed in particulate material or sediments. This excess²¹⁰Pb is most likely provided by particles entering the basin loaded already with²¹⁰Pb.     

Cosmic-ray-produced36Cl and53Mn in Allan Hills-77 meteorites

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been determined by neutron activation in nine Allan Hills-77 meteorites. Additionally,³⁶Cl (_1/2 = 3.0 × 10⁵years) has been measured in seven of these objects using tandem accelerator mass spectrometry. These results, along with¹⁴C (t_1/2 = 5740years) and²⁶Al (7.2 × 10⁵ years) concentrations determined elsewhere, yield terrestrial ages ranging from 0.1 × 10⁵ to 7 × 10⁵ years. Weathering was not found to result in⁵³Mn loss.     

Identification of excess 40Ar by the 40Ar/39Ar age spectrum technique

⁴⁰Ar/³⁹Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess⁴⁰Ar indicate that saddle-shaped age spectra are diagnostic of excess⁴⁰Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar isochron diagram reliably reveal the crystallization age in such samples.     

The effects of assimilation of country rocks by magmas on 18O/16O and 87Sr/86Sr systematics in igneous rocks

Examples of positive correlations between initial ⁸⁷Sr/⁸⁶Sr and δ¹⁸O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-¹⁸O, high-⁸⁷Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ¹⁸O-⁸⁷Sr/⁸⁶Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ¹⁸O-⁸⁷Sr/⁸⁶Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.     

Land-air boundary environment as recorded by the18O/16O and13C/12C isotope ratios in the shells of land snails

A yearly cycle of carbon and oxygen isotope composition of shells of the Israeli land snailXeropicta vestalis is presented. The¹⁸O/¹⁶O values indicate that the snails use water from the land-air boundary zone. The¹⁸O/¹⁶O ratio of the shells is in isotopic equilibrium with the water condensate from the vapour during the winter months. During the summer months a contribution to the above water from soil water migrating upwards due to evaporation is noticeable. The δ¹³C values indicate that as in marine molluscs, the carbon isotopic composition in land snails is controlled mainly by the aqueous carbonate compound which is in equilibrium with the land-air boundary CO₂.     

Oxygen isotopes and the origin of high-87Sr/86Sr andesites

Andesites from the Peruvian Andes and the Banda arc of Indonesia are characterized by unusually high and variable ⁸⁷Sr/⁸⁶Sr ratios. The Banda arc samples, including two cordierite-bearing lavas from Ambon, show a clear positive correlation between ⁸⁷Sr/⁸⁶Sr and δ¹⁸O. The andesitic rocks have δ¹⁸O values that range from 5.6 to 9.2‰. Over that range in δ¹⁸O, ⁸⁷Sr/⁸⁶Sr increases from 0.7044 to 0.7095. The cordierite-bearing lavas have δ¹⁸O values of approximately 15‰ and ⁸⁷Sr/⁸⁶Sr ratios of approximately 0.717. The similarity between δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios in total rocks and separated plagioclase phenocrysts of the Banda arc samples indicates that the measured isotope ratios are primary and have not been affected by secondary, low-temperature post-eruptive alteration. The observed variation between O and Sr isotopic ratios can be modeled by two-component mixing in which one component is of mantle isotopic composition. As the crust beneath the Banda arc is probably oceanic, contamination of the manle component may have resulted from the subduction of either continentally-derived sediments or continental crust. Mixing calculations indicate that the contaminant could have an isotopic composition similar to that observed in the cordierite-bearing lavas.The Andean samples, despite petrographic evidence of freshness, exhibit whole-rock δ¹⁸O values significantly higher than those of corresponding plagioclase phenocryst separates, indicating extensive low-temperature post-eruptive alteration. The plagioclase mineral separates show a range of δ¹⁸O values between 6.9 and 7.9‰. The ⁸⁷Sr/⁸⁶Sr ratios of these same samples are, in most instances, not significantly different from those measured for the whole rock, thus signifying that the phenocrysts and groundmass were in isotopic equilibrium at the time of eruption. Unlike the lavas of the Banda arc, the Andean lavas show no strong positive correlation between ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values, but instead lower ⁸⁷Sr/⁸⁶Sr ratios appear to be associated with higher δ¹⁸O values. The δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of the Peruvian samples are both slightly higher than those of “normal” island arc volcanics.The small proportions of contaminant implied by the O isotope results seem to preclude continental crustal contamination as a primary cause of high ⁸⁷Sr/⁸⁶Sr ratios. The most plausible process that can explain both O and Sr isotope results is one in which sediments of continental origin are partially melted in the subduction zone. These melts rise into overlying mantle material and subsequently participate in the formation of calc-alkaline magmas.If the involvement of a sialic component in the genesis of andesitic magma occurs in the subduction zone, melting of that sialic material signifies temperatures of at least 750–800°C at the top of the subducted lithospheric slab at depths of approximately 150 km. The fact that contamination has apparently occurred in the Banda arc samples without producing any simple widespread correlations between Sr and O isotopic compositions on the one hand and major or trace element abundances on the other, shows that isotopic correlations, possibly including pseudo-isochrons, can be produced by mixing without producing trace element mixing correlations. Because O versus Sr isotope correlations are little affected by processes of partial melting of differentiation, they provide a direct means of testing whether Sr isotopic variations in volcanic rocks are of mantle origin or are due instead to mixing with sialic material.     

210Pb and226Ra distributions in the Circumpolar waters

²¹⁰Pb and²²⁶Ra profiles have been measured at five GEOSECS stations in the Circumpolar region. These profiles show that²²⁶Ra is quite uniformly distributed throughout the Circumpolar region, with slightly lower activities in surface waters, while²¹⁰Pb varies with depth as well as location or area. There is a subsurface²¹⁰Pb maximum which matches the oxygen minimum in depth and roughly correlates with the temperature and salinity maxima. This²¹⁰Pb maximum has its highest concentrations in the Atlantic sector and appears to originate near the South Sandwich Islands northeast of the Weddell Sea. Concentrations in this maximum decrease toward the Indian Ocean sector and then become fairly constant along the easterly Circumpolar Current.Relative to²²⁶Ra, the activity of²¹⁰Pb is deficient in the entire water column of the Circumpolar waters. The deficiency increases from the depth of the²¹⁰Pb maximum toward the bottom, and the²¹⁰Pb/²²⁶Ra activity ratio is lowest in the Antarctic Bottom Water, indicating a rapid removal of Pb by particulate scavenging in the bottom layer and/or a short mean residence time of the Antarctic Bottom Water in the Circumpolar region.²²⁶Ra is essentially linearly correlated with silica and barium in the Circumpolar waters. However, close examination of the vertical profiles reveals that Ba and Si are more variable than²²⁶Ra in this region.     

Fallout240Pu and239Pu in Atlantic marine samples

Mass spectrometric analyses of low levels of global fallout plutonium separated from Atlantic marine samples have differentiated fallout²³⁹Pu and²⁴⁰Pu in aquatic samples for the first time. The results show no single characteristic²⁴⁰Pu/²³⁹Pu ratio in marine samples; the observed range is from 0.11 to 0.24 on an atom basis. There are indications that differences exist in the chemical or physical form of plutonium from atmospheric fallout in Atlantic surface water and that selective concentration in surface organisms is occurring. No single²⁴⁰Pu/²³⁹Pu value is found in pelagic sediments collected from different depths and locations. Discounting sources other than fallout, our results show that the plutonium deposited at any given time since atmospheric testing began may have carried a unique²⁴⁰Pu/²³⁹Pu tag. This label may be extremely useful to trace fallout plutonium through biogeochemical cycles.     

87Rb-87Sr chronology of enstatite meteorites

A⁸⁷Rb-⁸⁷Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ⁸⁷Rb= 1.42 × 10^?11yr^?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the⁸⁷Sr/⁸⁶Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the⁸⁷Rb-⁸⁷Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the⁸⁷Rb-⁸⁷Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.     

The location of Fe3+ ions in forsterite (Mg2SiO4)

The electron spin resonance spectrum of Fe³⁺ in a single crystal of forsterite was studied. Two distinct patterns of about equal intensities were observed which are due to Fe³⁺ at two distinct positions with 4a (M1)and4c (M2or Si) symmetry of Pbnm. The assignment of the 4c pattern to Fe³⁺ ions at the Si position cannot be excluded by symmetry but it is unlikely. The Hamiltonian parameters A and E/D are consistent with the conclusion that Fe³⁺ in this crystal is disordered over two distinct octahedral positions.