کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
4703665 1352874 2010 18 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Chronology and shock history of the Bencubbin meteorite: A nitrogen, noble gas, and Ar–Ar investigation of silicates, metal and fluid inclusions
موضوعات مرتبط
مهندسی و علوم پایه علوم زمین و سیارات ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
Chronology and shock history of the Bencubbin meteorite: A nitrogen, noble gas, and Ar–Ar investigation of silicates, metal and fluid inclusions
چکیده انگلیسی

We have investigated the distribution and isotopic composition of nitrogen and noble gases, and the Ar–Ar chronology of the Bencubbin meteorite. Gases were extracted from different lithologies by both stepwise heating and vacuum crushing. Significant amounts of gases were found to be trapped within vesicles present in silicate clasts. Results indicate a global redistribution of volatile elements during a shock event caused by an impactor that collided with a planetary regolith. A transient atmosphere was created that interacted with partially or totally melted silicates and metal clasts. This atmosphere contained 15N-rich nitrogen with a pressure ⩾3 × 105 hPa, noble gases, and probably, although not analyzed here, other volatile species. Nitrogen and noble gases were re-distributed among bubbles, metal, and partly or totally melted silicates, according to their partition coefficients among these different phases. The occurrence of N2 trapped in vesicles and dissolved in silicates indicates that the oxygen fugacity (fO2) was greater than the iron–wüstite buffer during the shock event. Ar–Ar dating of Bencubbin glass gives an age of 4.20 ± 0.05 Ga, which probably dates this impact event. The cosmic-ray exposure age is estimated at ∼40 Ma with two different methods. Noble gases present isotopic signatures similar to those of “phase Q” (the major host of noble gases trapped in chondrites) but elemental patterns enriched in light noble gases (He, Ne and Ar) relative to Kr and Xe, normalized to the phase Q composition. Nitrogen isotopic data together with 40Ar/36Ar ratios indicate mixing between a 15N-rich component (δ15N = +1000‰), terrestrial N, and an isotopically normal, chondritic N.Bencubbin and related 15N-rich meteorites of the CR clan do not show stable isotope (H and C) anomalies, precluding contribution of a nucleosynthetic component as the source of 15N enrichments. This leaves two possibilities, trapping of an ancient, highly fractionated atmosphere, or degassing of a primitive, isotopically unequilibrated, nitrogen component. Although the first possibility cannot be excluded, we favor the contribution of primitive material in the light of the recent finding of extremely 15N-rich anhydrous clasts in the CB/CH Isheyevo meteorite. This unequilibrated material, probably carried by the impactor, could have been insoluble organic matter extremely rich in 15N and hosting isotopically Q-like noble gases, possibly from the outer solar system.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Geochimica et Cosmochimica Acta - Volume 74, Issue 22, 15 November 2010, Pages 6636–6653
نویسندگان
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