Toralf Kirsten

129I/127I: a Puzzling Early Solar System Chronometer

Abstract We report I-Xe ages and other relevant xenon data for seven ordinary chondrites from H and L-groups of petrologic types 4-6, which were selected on the basis of minimum weathering and shock effects. Nevertheless, no chronological order with respect to the I-Xe ages exists among the different petrologic types. We demonstrate, however, that the degree to which the 1-Xe record is preserved in these chondrites, but not necessarily the age, is dependent on the thermal metamorphic history. In order to explain the lack of chronological order among the chondrites, spatiotemporal variations in the condensation-accretion process or inhomogeneities in the isotopic composition of iodine in the solar nebula is required.

Limited response of the K–Ar system to the Nordlinger Ries giant meteorite impact

FOR the proper interpretation of radiometric rock ages it is important to know which processes are able to reset radioactive clocks. In particular, the resetting by intense shock effects is significant in lunar chronology because most lunar highland rocks are impact breccias produced in the course of multiple large meteorite impacts. For the important K–Ar scheme, we have dated variously shocked samples from ejecta and from a drill core through one of the best documented terrestrial meteorite craters the Nördlinger Ries (Southern Germany)1. From K–Ar and fission track dating of thoroughly melted impact glasses the 24 km diameter Ries crater was found to have formed 14.7±0.7 Myrago by meteorite impact into the variscian bedrock (for summary of ages see ref. 2). The modern version of K–Ar dating (39Ar–40Ar technique) can reveal quantitative information about partial losses of radiogenic argon (fractional resetting) in addition to yielding ages eventually not affected by these gas losses3,4. We have applied this dating technique to mineral separates (hornblende, biotite, chlorite) from ejecta, fall-back breccias and underlying bedrock of the Ries Crater, including drill core samples taken at depths up to 1,201 m(ref. l).The samples exhibit various degrees of mechanical strain (from ∼ 10 kbar up to >400 kbar shock pressure). Apart from the thoroughly molten glasses which reproduced the impact age of ∼ 14.7 Myr, we found that all other rocks involved in the impact irrespective of the degree of shock metamorphism and within the limits of error, yield the same 39Ar–4oAr age of 313±3 Myr, probably the age of the bedrock. Apparently, rocks exposed to the very intense shock effects frequent on the Moon give reliable K–Ar ages when measured as K–Ar plateau ages. Age resetting, if present, probably results from heating associated with such impacts. In the case of the Ries, we found from the diffusion characteristics of argon that the post-shock temperature of the crystalline fragments within the suevite layer has not exceeded 600 °C.

Isotopenanalyse der Edelgase in einem Tellurerz von Boliden (Schweden)

The stable He-, Ne-, Ar-, Kr- and Xe-isotopes and the Te-, K-, U-, Th- and Se-concentrations in a Precambrian bismuth-tellurium ore have been measured. We have found an age of 0.53 x 109 y with K-Ar, 0.55 x 109 y with U/Th-He4 and 1.56 x 109 y by the U-Xe136 method. The Xe131-excess and part of the Xe129-excess result from n-capture and subsequent decay of Te128and Te130. The remaining part of the Xe129-excess and the high U-Xe136 age give evidence for a higher neutron flux before the last cooling of the ore. The tellurium was probably associated with much more uranium than now. Before 0.54 x 109 y, differentiation of this uranium occured. Moreover, He and Ar but not all of the Xe were removed at this time. After cooling, the Xe129excess was formed by the decay of the long-lived I129 which had been formed from (n,γ) reactions on Te128 by the higher neutron flux. From the excess in Xe130, a half life for the double β-decay of Te130 of 6.2 x 1020 y is calculated. Besides fission-Kr, there is an excess of Kr with atmospheric composition and in addition an excess of Kr82 and Kr83 also. From the Kr82-excess and the selenium content a lower limit for the Se82-double β-decay half life of T1/2≥ 1018 y is calculated. No evidence is given for Ar38 from U-fission.