Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.237i&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 237
Other
3
Scientific paper
Zircons are becoming increasingly apparent in meteorites of the howardite-eucrite-diogenite association. The zircons generally occur as small (<=30 micrometers) inclusions closely associated with ilmenite and chromite. We have previously analyzed zircons from eucritic enclaves from Vaca Muerta [1], zircons from the brecciated eucrite Padvarninkai [2], and from the Yamato 791438 eucrite [3]. Here we report on our continued studies of meteoritic zircons with ion probe U-Th-Pb isotopic systematics and REE abundances of zircons from the Stannern eucrite. Eighteen zircons were identified and analyzed by electron probe in a polished section of the Stannern eucrite. An average composition is SiO2 31.78, TiO2 0.73, FeO 1.45, ZrO2 64.26, HfO2 1.04, total 99.26. The mean Zr/Hf ratio of 63 (range 21-129) is similar to the range observed in lunar zircons 30-60. Most of these zircons were not suitable for analysis (too small, proximity to cracks) and only four were analyzed for U-Th-Pb isotopes and trace-element abundances with the SHRIMP ion microprobe. All of these zircons occur in close association with ilmenite. The ilmenite grains range in size up to several hundred micrometers, but the zircons are at most only 20 micrometers in diameter. The analytical techniques for ion microprobe analysis are described in [1]. The U concentrations ranged from 30-80 ppm and the mean ^207Pb/^206Pb age of the four grains is 4550 +- 10 Ma (2 sigma); these values are very similar to the range in U concentrations (30-60 ppm) and the age (4553 +- 13 Ma) of the zircons analyzed from Padvarninkai [2]. The zircon age is compatible with the Sm-Nd age of Stannern (4.48 +- 0.07 Ga [4]) and the zircons have not been affected by the event(s) that disturbed the Rb-Sr system (3.1 +- 0.5 Ga [5]). The REE patterns (Fig. 1) are also very similar to the range shown in Padvarninkai. The LREE patterns are flat but the absolute abundances are different in the four grains ranging from chondritic to over 100 x chondritic. Europium is depleted in all zircons relative to the trend of the other REE. The HREE abundances are smoothly fractionated with overall absolute abundances that vary sympathetically with the overall LREE abundances. The similarity of the REE in zircons from Padvarninkai is noteworthy since the absolute abundances of REE in Stannern are 20 x CI compared with 10 x CI in Padvarninka. Therefore the zircon compositions do not appear to be controlled solely by the total REE inventory. References: [1] Ireland and Wlotzka (1992) EPSL 109, 1. [2] Bukovanska et al. (1991) Meteoritics 26, 325. [3] Ireland et al. (1992) Lunar Planet. Sci. XXIII, 569. [5] Birck and Allegre (1978) EPSL 39, 37. Figure 1, which in the hard copy appears here, shows Stannern REE.
Bukovanska M.
Ireland Trevor R.
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