Formation ages and evolution of Shergotty and its parent planet from U-Th-Pb systematics
- Creators
- Chen, J. H.
-
Wasserburg, G. J.
Abstract
The isotopic composition of lead was determined in different phases from Shergotty, Zagami and EETA 79001. Using phases with low ^(238)U/^(204)Pb, the initial leads of these meteorites could be well defined and are distinct from each other and have high ^(204)Pb/^(206)Pb (from 0.0652 to 0.0739). These leads evolved in different reservoirs over most of solar system history in a low ^(238)U/^(204)Pb ≈ 5 environment. It follows that the parent planet of the shergottites has a high ^(204)Pb concentration relative to U and must also be high in other volatiles. If the parent planet of the shergottites is Mars, then this planet must be volatile-rich compared to the Earth by a factor of ~ 1.8. This implies that Mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed Martian upper mantle. The U-Th-Pb isotopic relationships involving phases with high ^(238)U/^(204)Pb are quite regular and give ages for three shergottites which indicate an event of U-Th-Pb fractionation at ~200 m.y. The initial leads at that time were highly evolved relative to the primordial lead. The absence of substantial additions of radiogenic lead (above the initial lead) in high U phases precludes an older age for the U-Th-Pb fractionation. However, details of the data arrays are complex. The young age is in general agreement with some of the ages obtained by other methods, but precise concordance between the different methods is not established. It is suggested that the shock event that produced maskelynite may possibly be associated with the late-stage breakup at ~2.5 m.y. and not necessarily related to the ~ 180 m.y. record. The major event of chronometer resetting is taken to be ~ 180 m.y. This event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target. Problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes.
Additional Information
© 1986 Pergamon Journals Ltd. Received June 10, 1985; accepted in revised form December 5. 1985. Presented at the Shergotty-Nakhla-Chassigny Symposia, 16th Lunar and Planetary Science Conference, 1985. The authors wish to thank J. C. Laul for setting up the consortium and encouraging them to participate and also making many phone calls. We thank S. K. Mukerjee, Director General of the Geological Survey of India, for recognizing the possible scientific significance of this type of research and giving it his strong personal and professional support. L. Nyquist was most helpful and responsive in providing us with the end of Zagami. This Zagami sample was originally obtained through the courtesy of S. A. Oyesola of the Geological Survey of Nigeria. The Curator's office at the Johnson Space Center has continued to be helpful in providing well documented samples that were carefully processed in a clean environment. The reviewers were both numerous and very helpful in their constructive critical comments. It is doubtful that we have achieved the clarity and lucidity that they sought from us. This is mostly due to the fact that we really don't understand. We thank Daniel Unruh, Larry Nyquist and John Morgan for their most serious efforts in studying this paper. This work was supported by NASA Grant No. NAG 9-43. Editorial handling: R. A. SchmittAdditional details
- Eprint ID
- 42101
- Resolver ID
- CaltechAUTHORS:20131028-154907008
- NAG 9-43
- NASA
- Created
-
2013-10-29Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Other Numbering System Name
- Lunatic Asylum Lab
- Other Numbering System Identifier
- 511