Tungsten Isotopes and the Origins of Enstatite Chondrites and the Earth

Details Tungsten Isotopes and the Origins of Enstatite Chondrites and the Earth Tungsten Isotopes and the Origins of Enstatite Chondrites and the Earth

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...22l&link_type=abstract

Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N

Other

Chondrites, Composition (Property), Earth (Planet), Enstatite, Planetary Evolution, Silicates, Tungsten Isotopes, Magnetic Materials, Hafnium

Scientific paper

Enstatite chondrites are highly reduced and this has led scientists to believe that they might have formed from an accretionary feeding zone located relatively close to the Sun. Enstatite chondrites have also been linked with the origin of the Earth because of their similarity in O-isotopic compositions. It is, therefore, of interest to study enstatite chondrites with the newly developed Hf-W-isotopic system to test if such an inferred genetic relationship is warranted. Despite a large Hf-W, the silicate Earth yields a W-isotopic composition identical to that of carbonaceous chondrites, implying that the Earth's core did not form until 250 m.y. after the formation of the solar system, or that the Earth's accretion was slower than previously thought. The fact that the W-isotopic compositions of the silicate Earth and carbonaceous chondrites are identical provides evidence that the bulk Earth must have accreted from materials with, on average, chondritic Hf-W. However, it is important to know if other classes of chondrites, in particular the enstatite chondrites, have the correct W-isotopic compositions to provide an appropriate source for the materials in the Earth. Four enstatite chondrites, Abee (EH4) Indarch (EH4) Hvittis (EL6), and ALHA81021 (EL6), have been analyzed in this study. In an attempt to test if we can resolve internal isochrons, we have analyzed magnetic (largely metal and nonmagnetic (largely silicate) fractions that were physically separated with a hand magnet in addition to whole rocks. In addition to bulk metal measurements , a portion of the metal separated from each sample was treated with hot 6NHCl until most of the metal was dissolved. This solution was then decanted from the residue (containing minor undissolved silicates) which then underwent additional treatment to achieve complete dissolution. These fractions are referred to as metal leachates and metal residues. It is found that, in general terms, the phases with higher Hf-W exhibit more radiogenic W. Furthermore on a plot of W-182/W-182 vs Hf-180/W-184, the metals, metal leachates and metal residues for all four enstatite chondrites are collinear with the metals, metal leachates, metal residues, mechanically separated silicates and whole rock Hf-W data for ordinary and carbonaceous chondrites.

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