Zirconium isotope evidence for incomplete admixing of r-process components in the solar nebula     

Maria Schönbächler  Der-Chuen Lee  Alex N. Halliday  Bodo Hattendorf 《Earth and Planetary Science Letters》2003,216(4):467-481

Isotopic anomalies in Mo and Zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula. We report precise Zr isotopic measurements of carbonaceous, ordinary and enstatite chondrites, eucrites, mesosiderites and lunar rocks. All bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty. No anomalies in ⁹²Zr are found in any samples including high Nb/Zr eucrites and high and low Nb/Zr calcium-aluminum-rich inclusions (CAIs). These data are consistent with the most recent estimates of <10⁻⁴ for the initial ⁹²Nb/⁹³Nb of the solar system. There exists a trace of isotopic heterogeneity in the form of a small excess of r-process ⁹⁶Zr in some refractory CAIs and some metal-rich phases of Renazzo. A more striking enrichment in ⁹⁶Zr is found in acetic acid leachates of the Allende CV carbonaceous chondrite. These data indicate that the r- and s-process Zr components found in presolar grains were well mixed on a large scale prior to planetary accretion. However, some CAIs formed before mixing was complete, such that they were able to sample a population of r-process-enriched material. The maximum amount of additional r-process component that was added to the otherwise well-mixed Zr in the molecular cloud or disk corresponds to ∼0.01%.  相似文献   

Propagation of microwaves in pulsar magnetospheres     

Gianluigi Bodo  Attilio Ferrari  Silvano Massaglia 《Astrophysics and Space Science》1981,80(2):261-266

We discuss the dispersion relation of linearly-polarized waves, propagating along a strong background magnetic field embedded in an electron-positron plasma. The results are then applied to the study of the propagation conditions of coherent curvature radio radiation inside neutron stars magnetospheres, as produced by electric discharges following current pulsar models.  相似文献   

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)     

Bodo Weber  Margarita López Martínez 《Mineralium Deposita》2006,40(6-7):707-725

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.  相似文献