Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.528k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 528
Mathematics
Logic
Ages, Contamination, Terrestrial, Isotopes, Meteorites, Allan Hills 77257, Goalpara, Kenna, Novo Urei, Pecora Escarpment 82506, Rare Earth Element, Ureilites
Scientific paper
The origin of acid-leachable light (L) REE-rich components in ureilites is now a controversial issue between two groups who favor two different ideas: terrestrial contamination [1, 2] or metasomatism on the ureilite parent body [3, 4, 5]. Although Goodrich and her co-workers and we have obtained the same Sm-Nd trend from Kenna, Novo Urei, ALH 77257 [3, 5], and Goalpara [1, 2], Goodrich and others interpreted the trend as an isochron with an age of 3.79 Ga [5] whereas we interpreted the data as terrestrial contamination yielding a mixing line [2]. In this paper, we further discuss the Sm-Nd data of possible contaminants based on continental crustal data of from various parts of the world with various ages. Recently, Goodrich and Lugmair [5] pointed out that four different ureilites (Kenna, Novo Urei, Goalpara, ALH 77257) from the different recovery locations define a single Sm-Nd line, which can not be explained by terrestrial contamination because of the wide range of Nd compositions. However, by comparing recovery locations to their geological characters, it will be noted that all of these four ureilites were recovered from areas with Phanerozoic ages: Kenna which is Neogene, Novo Urei which is Paleogene to Jurassic, Goalpara which is Quaternary, and ALH 77257 which is Jurassic to Paleozoic. Phanerozoic sediments show similar Sm-Nd compositions all over the world [6] and plot very close to LREE-rich samples of Kenna. For this reason, it is not surprising that these four ureilites define a single contamination mixing trend. On the other hand, PCA 82506, which defines a "4.23 Ga isochron" using its acid-leachate and residue [4], was recovered from an area with Permian and Precambrian ages. Data from East Antarctica Precambrian rocks [7] plot along this "isochron", indicating that this ureilite was contaminated with terrestrial REE typical of old crustal material in Antarctica Similarly, the Rb-Sr systematics of ureilite are also explained as contamination by multiple terrestnal sources. Otherwise calculated Rb-Sr isochron ages of 0 Ga for Kenna, 1 Ga for Novo Urei, 2 Ga for PCA 82506 [3], are significantly inconsistent with their "Sm-Nd isochron ages" and cannot be understood. Sm-Nd, Rb-Sr and U-Th-Pb [1, 2] systematics of ureilites all refute the conclusion by Goodrich and her co-workers that LREE-rich components are indigenous in ureilites. References: [1] Torigoye N. et al. (1993) Meteoritics, 28, 450-451. [2] Tongoye-Kita N. et al. (1995) GCA, 59, 381-390. [3] Goodrich C. A. et al. (1991) GCA, 55, 829-848. [4] Goodrich C. A. and Lugmair G. W. (1992) LPS XXIII, 429-430. [5] Goodrich C. A. and Lugmair G. W. (1995) GCA, in press. [6] Goldstein S. L. et al. (1984) EPSL, 70, 221-236. [7] DePaolo D. J. et al. (1982) Nature, 298, 614-618.
Kita Noriko T.
Misawa Kazuhiko
Tatsumoto Mitsonobu
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