Other
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....3818j&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #3818
Other
Scientific paper
Angrites, a very rare meteoritic class, formed very early in the solar system history. The U-Pb crystallization ages of angrites studied so far are 10 to 11 Ma younger than the "Allende Inclusions" and about 8 Ma younger than chondrules. Recently, we have determined the U/Pb cystallization age of the new D'Orbigny angrite. We obtained three concordant Pb206/U238, Pb207/U235, and Pb207/Pb206 ages of 4,557±1.5 Ma for the pyroxene confirming the above mentioned time period of angrite crystallization. Surprisingly, this meteorite contains abundant black glass grains which fill druses and hollow spheres. This phase has not been detected in any other angrite. The composition is ultrabasic, similar to the bulk composition. It decomposes in strong acids leaving a white silica residue. However, the origin of this material is still enegmatic. Based on a trace element study it was concluded that the glass could have formed either by melting of D'Orbigny bulk rock or by condensing at high temperature from a vapor which had chondritic relative abundances of refractory lithophile elements. To get a better understanding of the mechanism of glass formation we measured the Pb/U isotopic data. In the Pb-Pb diagram the data of three glass samples plot close or near the geochron. However, its U/Pb isotopic system was strongly disturbed mainly by our leaching procedure. Therefore, a precise determination of the age was not yet possible. We obtained three strong differing Pb206/U238, Pb207/U235, and Pb207/Pb206 ages. However, the Pb207/Pb206 ages of two different samples gave agreeing but unrealistic values of ˜4,700 Ma. Based on these results we believe that the glass is very old comparable to the pyroxenes. New Mn-Cr isotopic data support our suggestion.
Jagoutz Emil
Jotter R.
Kurat Gero
Varela Maria Eugenia
Zartman R.
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