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Geochemistry and neodymium‐strontium isotope signature of tektite‐like objects from Siberia (urengoites, South‐Ural glass)
Author(s) -
DEUTSCH ALEXANDER,
OSTERMANN MARKUS,
MASAITIS VICTOR L.
Publication year - 1997
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1997.tb01552.x
Subject(s) - geology , isotopes of strontium , geochemistry , strontium , isotope , mineralogy , crust , trace element , rare earth element , isochron dating , continental crust , isochron , rare earth , chemistry , physics , organic chemistry , quantum mechanics
— We report Sr‐Nd isotope parameters, rare earth element (REE), and major element data for isolated findings of tektite‐like objects from western Siberia (urengoites, South‐Ural glass), as well as for two indochinites. The latter were recovered in Vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the Australasian strewn field. The three urengoites (∼24 Ma) are extremely silica‐rich (89 to 96 wt% SiC 2 ), and their REE abundances vary between 45 and 76 ppm. With La N /Yb N ranging from 7.6 to 10.4 and Eu N /EU* between 0.69 and 0.75, their REE distribution patterns match that of average upper crust. The urengoites have present‐day ɛ Sr of +155 to +174 and ɛ Nd ranging from −18 to −23. Their model ages in million years are: T Sr uR = 1200 up to 4060 and T Nd cHUR = 1570 up to 2070. Data points for the urengoites plot colinearly in the Rb‐Sr evolution diagram. The age corresponding to the slope is 183 ± 30 Ma (2s̀), which is indistinguishable from the intercept age of 211 Ma in the T Sr UR vs. l/f Rb diagram. Rubidium‐strontium and Sm‐Nd systematics of the urengoites indicate a heterogeneous precursor material, derived from Paleoproterozoic continental crust, which underwent Rb/Sr fractionation and partial Sr isotope homogenization in Jurassic times. Any relation between the urengoites and the Haughton impact crater, having within 2s̀ errors an identical age, can be excluded on the basis of isotope relationships and geochemical data. The only known South‐Ural glass (∼6.2 Ma) is characterized by intermediate SiO 2 (65 wt%), high Al 2 O 3 (14 wt%) and CaO (12 wt%), and low FeO TOT (0.4 wt%) contents. This unique tektite‐like object contains 110 ppm REE displaying a steeply negative C1 normalized distribution with La N /Yb N of 17, and Eu N /Eu 1 of 0.71. The Rb abundance (10 ppm) and Rb/Sr ratio are low, and combined with a “crustal” 87 Sr/ 86 Sr ratio of 0.722, yielding an unrealistic T Sr uR age of 2.5 Ga. The Rb‐Sr systematics imply a rather recent parent/daughter element decoupling. The T Nd CHUR age of the South‐Ural glass is ∼1690 Ma. Geochemical data suggest that urengoites and the South‐Ural glass belong to two discrete groups of tektites, whose source craters remain to be discovered.