Jason Jiun-San Shen

Precise Re-Os determinations and systematics of iron meteorites

The Re-Os system for samples of FeNi, sulphide, and phosphide from iron meteorites was investigated. Techniques were developed which yield reproducible analyses for Re/Os at the 2‰ level and which permit complete isotopic exchange between sample and tracer, as is necessary for concentration measurements of Re and Os by isotope dilution. High precision osmium and rhenium isotope data have been obtained using negative ion thermal ionization, with ionization efficiencies of up to 10% for Os and 20% for Re, both for normals and for Re and Os extracted from the samples. Replicate analyses of Re/Os are in good agreement, within ±2.5‰. The results show a well defined correlation line on a ^(187)Re-^(187)Os evolution diagram for iron meteorites from groups IAB, IIAB, IIIAB, IVA, and IVB, all taken together. This correlation line yields a slope of 0.07863 ± 0.00031 (2σ) and initial ^(187)Os/^(188)Os = 0.09560 ∓ 0.00018 (2σ). If the individual groups of iron meteorites for which there is sufficient dispersion in Re/Os are considered, data on the IIAB and on the IVA irons appear to indicate a difference in age of 60 ± 45 Ma, with the IVA group being older-This age difference is qualitatively the same as obtained for Pd-Ag data but is larger. Sulphides from two IAB iron meteorites show extremely low concentrations of Re and Os and indicate that Re and Os are not partitioned into this phase during planetary differentiation. There is evidence for recent element remobilization or contamination, corresponding to relative enrichment of Re or loss of Os in the sulphides. Schreibersites contain small but significant amounts of Re and Os, with high Re/Os relative to the metal phases and with ^(187)Os/^(188)Os much more radiogenic than in the metal. Model ages for the schreibersites are relatively young (4.3-3.5 AE) and indicate that the schreibersites were open-systems for Re-Os at least 0.5-1 AE after the original formation of the iron meteorites, It now appears possible to use metal-schreibersite pairs to determine internal isochrons. Based on the schreibersite model ages, the cooling rates for the two IAB meteorites are estimated to be ~ 1°C/Ma, more than an order of magnitude lower than the most recently determined metallographic cooling rates for IAB irons (Herpfer et al., 1994).

Geochemical and Sr-Nd isotopic characteristics of granitic rocks from northern Vietnam

Abstract Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [eNd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [eNd(T)=−4.7 to −9.7; TDM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate eNd(T) values (+0.6 to −2.8) and younger TDM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.

10Be study of rapid erosion in Taiwan

Cosmogenic 10Be was measured using accelerator mass spectrometry in soils and sediments to study the erosion of Taiwan, which has the highest denudation rate in the world. The river sediments in Taiwan have very low 10Be concentrations, around 5 million atoms per gram, about 145 the world wide average. This is the direct consequence of its high sediment yield of more than 1000 mg/cm2/yr, 70 times the world average. Combining these values we found that, for Taiwan as a whole, the 10Be output to the sea only slightly exceeds the input from the rain, a situation typical of many areas around the world. Therefore, even in this example of extremely rapid erosion, 10Be seems to remain a useful indicator for erosion status. The total 10Be inventory found in a soil profile from a geologically stable area is at least 18% of the maximum possible inventory. The minimum age thus estimated for the soil is 0.11 Ma, in reasonable agreement with estimates from other means. The maximum erosion rate for this area thus estimated is at most 0.14 mg/cm2/yr, four orders of magnitude slower than the average value observed for Taiwan. We also demonstrate that the shelf sediments around Taiwan have received the admixing of a 10Be rich component from the ocean and would give a poor estimate for material eroded from Taiwan.

48Ca HETEROGENEITY IN DIFFERENTIATED METEORITES

Isotopic heterogeneities of 48Ca have been found in numerous bulk meteorites that are correlated with 50Ti and 54Cr anomalies among differentiated planetary bodies, and the results suggest that a rare subset of neutron-rich Type Ia supernova (nSN Ia) was responsible for contributing these neutron-rich iron-group isotopes into the solar system (SS). The heterogeneity of these isotopes found in differentiated meteorites indicates that the isotopic compositions of the bulk SS are not uniform, and there are significant amounts of nSNe Ia dust incompletely mixed with the rest of SS materials during planetary formation. Combined with the data of now-extinct short-lived nuclide 60Fe, which can be produced more efficiently from an nSN Ia than a Type II supernova ejecta, the observed planetary-scale isotopic heterogeneity probably reflects a late input of stellar dust grains with neutron-rich nuclear statistical equilibrium nuclides into the early SS.

Re-Os systematics in pallasite and mesosiderite metal

Re-Os data on pallasite metal yield a sufficient range in Re/Os to permit the determination of a whole rock isochron, if pallasites from different groups are considered together. The results are indistinguishable from the data obtained on Groups IAB, IIAB, IID, IIIAB, IVA, and IVB iron meteorites. Pallasite data were obtained on samples of the Main Group, which have been considered related to Group IIIAB irons, and on samples of the distinctive pallasites Eagle Station, Finmarken, and Marjalahti. It appears that iron meteorites and pallasites crystallized within a time interval of 20 Ma. This result is consistent with the evidence from the short-lived chronometers ^(107)Pd-^(107)Ag and ^(53)Mn-^(53)Cr. It is also consistent with the evidence from ^(182)Hf-^(182)W that the time interval over which molten FeNi metal from different iron meteorites equilibrated with and became segregated from silicates was ∼10 Ma. The well-defined ^(187)Re-^(187)Os whole-rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated FeNi metal and FeS and then all crystallized within a 20 Ma time interval. This conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic. The short time-scale requires that the irons and pallasites were formed in small parent planets of ≲10 km radius or near the surface of larger bodies. In contrast to the pallasites, we find that FeNi samples from mesosiderites have a narrow range in Re/Os so that it is not possible to determine a whole-rock isochron for these samples. The mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi-stage evolution. Based on Sm-Nd data on mesosiderite silicate clasts, the Re-Os results on mesosiderite metal are compatible with the model of melted or partially molten FeNi cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 Ma after early formation and the FeNi was splashed onto the surfaces of other small, differentiated planetesimals.

Lanthanum isotopic composition of meteoritic and terrestrial matter

Abstract Because 138 La is rare, refractory, and of peculiar nucleosynthetic origin, we have attempted to search for La isotopic anomalies hoping to find evidence for an early solar irradiation and clues for the astrophysical source of this nuclide. Whole rock samples from six meteorites and two terrestrial standards were measured using our newly developed analytical method. No anomalies could be definitively established. Our 95% confidence interval of 0.8 permil limits any differential proton fluence exposure to 19 / cm 2 . Our limit implies either that the putative irradiation was uniform or that the mixing between irradiated and unirradiated matter was highly efficient on our sampling scale. This result contradicts the prediction of the model of Lee (1978) which was proposed to account for the production of both 26 Al and pure 16 O by proton irradiation. In general, our experiment does not support models for the production of 26 Al inside the solar system but a more direct test still awaits the analysis of La from Ca-Al-rich meteoritic inclusions containing different amounts of 26 Al. 138 La excesses of about 1.2 permil in carbonaceous chondrites Murchison and Allende were found but these marginal (three standard deviations) effects require further confirmation.

138La Anomaly in the Early Solar System

For every 1100 lanthanum atoms in the solar system, only one is 138La. Relative to this low abundance, even a tiny additional 138La made by irradiating its more abundant neighboring nuclides with energetic protosolar flare particles would cause a large, hence detectable, percentage increase in 138La. Such early solar irradiation can produce many now-extinct short-lived radio nuclides (e.g., 41Ca, 53Mn, and 26Al) and is the only way to make the newly discovered 10Be and the possibly detected 7Be, because stars destroy rather than produce Be. The alternative hypothesis to produce extinct nuclides is the injection of freshly synthesized radioactivity from a nearby asymptotic giant branch star or supernovae during solar system formation. Hoping to clarify the origin of extinct nuclides, we have been searching for 138La excess and its possible correlation with extinct nuclides. Here we report the detection of up to 0.6% (~7.5 σ) 138La excesses in five Allende calcium-aluminum-rich inclusions. Surprisingly, they do not correlate with 26Al, thus offering no support for making 26Al by early irradiation. Instead, 138La excess correlates with 50Ti excess. Current nucleosynthesis models produce 50Ti in a rare subset of Type I supernovae whose core underwent significant gravitational collapse before carbon deflagration. Our observed correlation thus suggests that 138La also came from these rare sources, perhaps in the mantle of the white dwarf, by reactions induced by the neutrino burst emitted during core neutronization. After the explosion, 138La was incorporated into Ti-rich dusts that later became the building material of our solar system.

Precise compositional and trace-elemental analysis by chemical methods in compound semiconductors

Abstract The elemental composition and trace element impurities are known to be two important factors which greatly influence the physical properties of compound semiconductor materials. For stoichiometric analysis of compound semiconductors, high precision and accuracy with an error of less than or equal to 0.5% are prerequisite. Chemical methods, especially classical wet chemical techniques based on gravimetry, titrimetry and electrochemistry, can meet this analytical requirement. In this study, constant potential and constant current coulometry was employed for stoichiometric analysis of binary compounds such as GaAs, CdTe and Hg x Cd 1− x Te, and ternary compounds such as CuInS 2 and CuInSe 2 . For analysis of the trace elements in the materials, high detection sensitivity capable of determining trace elements down to levels of manograms per gram is normally required. In this study, the concentrations of trace and dopant elements including silicon, boron, chromium, tellurium, zinc, calcium, magnesium, copper and iron in the bulk samples of various semiconductors were investigated. Instrumental techniques based on neutron activation analysis and atomic absorption spectrometry combined with prior chemical treatments were employed to determine the levels of trace element impurities.