Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..333b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 333
Other
2
Ages, Eucrites, Ion Probe, Trace Elements, Zircon
Scientific paper
In our previous studies of HED zircons we have found that the zircon U-Pb systems have been undisturbed and consistent with a formation age of 4.56 Ga. Zi from the eucrites Padvarninkai [1], Stannern [2], and Yamato 791438 [3] all show concordant compositions and even the eucritic enclaves in the Vaca Muerta mesosiderite appear to be undisturbed [4]. We have analyzed zircons found in pol sections from another six eucrites, Bereba, Cachari, Jonzac, Juvinas, Millbillil and Pasamonte. In addition, a relatively large (~10 micrometers) baddeleyite was discovered in Pasamonte and gives us a chance to examine the systematics of this phase in comparison to zircon. The zircons in these meteorites are typically less than 10 micrometer and occur in close association with ilmenite and chromite. Zircons and baddeleyite were analyzed for U-Th-Pb isotopes and trace-element abundances with the SHRIMP ion microprobe. The analytical techniques for ion microprobe analysis are described [4]. Owing to the small size of the zircons, most of them were entirely consumed during the U-Pb analyses and REE patterns were not measured. In addition, the sm size of the grains meant that there was significant beam overlap onto the surrou chromite and ilmenite with the possibility that the U-Pb normalization could be affected. The zircons are fairly typical of meteoritic zircons with U concentrat around 50-100 ppm with a high value of 300 ppm in a Pasamonte zircon. REE are al the range previously observed for meteoritic zircon. The baddeleyite has ~60 ppm and, in contrast to the extreme heavy REE enriched zircon patterns, has a slight light REE ennched pattern (La 1800 x CI; Lu 800 x CI) with a large Eu depletion. All U-Pb analyses are either concordant or reverse discordant (i.e. they have excess radiogenic Pb). Three of the sections (Jonzac, Juvinas, Millbillillie) ha large surface Pb contributions, but their compositions were consistent with concordant 4.56 Ga U-Pb. Pasamonte zircon was also concordant at 4.56 Ga and Ber was concordant to slightly reverse discordant at 4.534 +- 0.016 Ga. Cachari, how is distinct in that it shows a mixing line between concordant 4.56 Ga and a radiogenic common Pb with a ^207Pb/^206Pb of ~0.4. The lowest apparent ^207Pb/^2 age is 4.20 Ga. Cachan also has a number of petrographic features associated wit deformation; in particular one of the zircons has been sheared with an offset th the center of the grain. It is possible that the U-Pb systematics are reflecting event, which has caused isotopic disturbance in the mesosiderites. The Pasamonte baddeleyite may also be slightly disturbed with a young ^207Pb/^206Pb age of ~4. Ga; as yet we have no standard to calibrate U-Pb ratios. References: [1] Bukovanska M. et al. (1991) Meteoritics, 26, 325. [2] Ireland T. R. and Bukovanska M. (1992) Meteoritics 27, 237. [3] Ireland T. R. et al. (1992) XXIII, 569-570. [4] Ireland T. R. and Wlotzka F. (1992) EPSL, 109, 1-10.
Bukovanska M.
Ireland Trevor R.
No associations
LandOfFree
Zircons in Eucrites: Pristine and Disturbed U-Pb Systematics does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Zircons in Eucrites: Pristine and Disturbed U-Pb Systematics, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Zircons in Eucrites: Pristine and Disturbed U-Pb Systematics will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1072321