کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6305834 | 1307326 | 2014 | 19 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Brachinite meteorites: Partial melt residues from an FeO-rich asteroid
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
علوم زمین و سیارات
ژئوشیمی و پترولوژی
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چکیده انگلیسی
Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (â¼36-39 wt.%), high in MgO (â¼27-30 wt.%) and notably high in FeO (â¼26-37 wt.%), and low in Al2O3 (â¼0.2-2.5 wt.%) and combined alkalis Na2O and K2O (â¼0-0.7 wt.%). They consist mostly of olivine (â¼71-96 vol.%; â¼Fo64-73), major clinopyroxene (minor to â¼15 vol.%; â¼En40-63Wo36-48), with variable small amounts of plagioclase (0 to â¼10 vol.%; â¼An15-33), and minor to trace amounts of orthopyroxene (none to â¼20 vol.%; En69-73Wo2-4), Fe-sulfides (trace to â¼7 vol.%), chromite (none to â¼5 vol.%), phosphates (none to â¼3 vol.%) and metallic Fe,Ni (trace to â¼2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (â¼0.1-1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from â¼0.01 to â¼0.9 CI. Oxygen isotope composition (Î17O) ranges from â¼â0.09 to â0.39â°, with the mean = â0.23 ± 0.14â°. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess 129Xe from the decay of short-lived 129I (half-life 17 Ma) and of fission tracks from the decay of 244Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using 53Mn-53Cr systematics, relative to the Pb-Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most â¼4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <â¼100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Chemie der Erde - Geochemistry - Volume 74, Issue 3, October 2014, Pages 311-329
Journal: Chemie der Erde - Geochemistry - Volume 74, Issue 3, October 2014, Pages 311-329
نویسندگان
Klaus Keil,