High-Resolution 40Ar-39Ar Dating of K-rich Chondritic Inclusions
Abstract
The origin of K-rich chondritic inclusions of the Krahenberg type is still poorly understood. Within the dark lithology of the LL breccia Krahenberg, Zahringer [1] noted K-enrichment by an order of magnitude, which was confirmed by Kempe and Muller [2] and found to be even stronger for the heavier alkali elements Rb and Cs. Rb-Sr dating [2] yielded an isochrone with a slope corresponding to 4.60 +- 0.14 Ga (re-calculated to new constants [3]) indicating that this material equilibrated very early during solar system formation. Despite this result, a separate origin of the dark and light lithologies, respectively, was favored and compaction of the breccia was assumed to have occurred later, around 4.0 Ga ago inferred from K-Ar ages [1]. Wlotzka et al. [4] revealed the alkali fractionation among the K-enriched and Na-depleted dark lithology having Na/K ratios of 0.33, compared to 11 for the hosts. Realizing an opposite fractionation process in crystalline feldspar and noting that the Na + K atom sum and the Si, Al, Mg, Ca, and Fe concentrations are not different, the authors suggested a formation process by interaction via vapor phase, probably related to impact heating. K-rich inclusions having the same microporphyritic texture of euhedral to skeletal olivines in a partly devitrified K-rich glass were also noticed in other LL chondrites, e.g., Bhola, Vishnupur, and Kelly [4], and even in other meteorite classes as H chondrites. By high resolution ^40Ar-^39Ar stepheating extraction we dated Krahenberg (LL) dark and light lithologies, a K-rich fragment of Bhola (LL), and a whole rock sample of the H chondrite Tulia, in order to obtain chronological information related to their formation processes. The age spectra are severely affected by secondary ^40Ar loss as well as by ^39Ar recoil redistribution [5]. The magnitude of the latter within the dark fragments requires a micrometer-sized K-rich phase, which we identified as devitrified glass containing numerous dendritic pyroxene crystals. Nevertheless, the Bhola inclusion yielded a well defined age plateau (Fig. 1) at 4.231 +- 0.016 Ga (1 sigma), a major time mark also for other LL chondrites [6,7]. It probably indicates a large impact on the LL parent body with a complete K-Ar-clock reset. The age spectra of the Krahenberg lithologies are more complex: They also indicate a last complete reset around 4.2 Ga ago but, in addition, a later partial degassing, though with different responses of the two lithologies. Tulia shows a very late major reset given by a low temperature (partial) plateau age of 457 +- 24 Ma as is also evident for other L chondrites [e.g. 8-10]. Rb-Sr and ^40Ar-^39Ar chronometer obviously seem to date different events in the history of Krahenberg. It should be clarified if a nearly total degassing around 4.2 Ga or subsequent strong disturbances of the two lithologies could have occurred without affecting the Rb-Sr system of the whole meteorite. To this end we evaluate the intensity of the thermal event using measured diffusion parameters and shaping the age spectra taking into account ^39Ar recoil effects. This then also allows a comparison to the estimated thermal history required to form the dark lithology [4]. References: [1] Zahringer J. (1968) GCA, 32, 209. [2] Kempe W. and Muller O. (1969) Meteorite Research (P. M. Millman, ed.), 418-428. [3] Steiger R. H. and Jager E. (1977) EPSL, 36, 359-362. [4] Wlotzka F. et al. (1983) GCA, 47, 743-757. [5] Huneke J. C. and Smith S. P. (1976) LPSC 7th, 1987-2008. [6] Trieloff M. (1990) Dipl. Thesis, University Heidelberg, 87. [7] Bernatowicz T. J. et al. (1988) GCA, 52, 1113-1120. [8] Bogard D. D. et al. (1976) JGR, 81, 5664-5678. [9] McConville P. et al. (1988) GCA, 52, 2487-2499. [10] Kaneoka I. et al. (1988) Proc. NIPR Symp. Ant. Met., 1, 206-214.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29Q.541T
- Keywords:
-
- Argon Isotopes;
- Gabbro;
- Potassium;
- Pressure Effects;
- Shock Waves;
- Solar System Evolution;
- Electron Probes;
- Metamorphism (Geology);
- Phase Transformations;
- Republic Of South Africa;
- Temperature Effects;
- Lunar and Planetary Exploration;
- ARGON-ARGON AGES; BHOLA (LL); H CHONDRITES; K ENRICHMENT; KROHENBERG (LL); LL CHONDRITES; TULIA (H)