Over What Period of Time Did Chondrule Formation Occur

Introduction: There have been many suggestions for the nature of the chondrule forming event(s) including lightning, impacts, X-winds and shocks e.g. [1]. Models of chondrule formation which invoke shock-heating agree well with constraints inferred from petrographic and chemical observation of chondrules [2], however, the location of chondrule formation and the origin of the shocks remain uncertain. suggestions for the origin of the shocks include planetary bow-shocks...Whatever the origin of the heating event that produced chondrules, its seems likely that information about the frequency of occurence, and about the period of time over which chondrules were formed, is likely to provide constraints on proposed mechanisms. Chondrule ages: Individual chondrule ages have been determined by the I-Xe, Pb-Pb and Al-Mg radiometric dating techniques. The majority of data comes from the I-Xe system which is known to readily respond to parent-body processes such as aqueous alteration, or thermal events. The interpretation of radiometric ages from chondrules relies on a petrographic assessment of the sample for textures or minerals that may be attributable to secondary events. Where textures and compositions (e.g. the continuing presence of glassy phases) are consistent with rapid cooling from a melt, it is assumed that a measured radiometric age reflects that event. In the case of the IXe system, this assumption can be further checked by the use of step-heating when performing the analysis – the presence of iodine bearing phases which degas at low temperatures suggests chemical reactions in the parent body, especially if the inferred I/I ratio varies with temperature. Radiometric age determinations of individual chondrules remain technically challenging due to the small amounts of material involved, and scientifically challenging due to the difficulties in proving that no disturbance of the radiometric isotope system has occurred subsequent to the chondrule forming event. The available dataset is thus rather small. I-Xe ages. The high sensitivity and low background of noble-gas mass spectrometers, together with the simple sample preperation mean not just that many more I-Xe chondrules ages are available than for PbPb or Al-Mg, but also, as noted above, isochrons for individual chondrules with many datapoints are the norm. Fig. 1 is a summary of most available chondrule data, after (Whitby and Gilmour, 2001); only chondrules with simple isochrons have been selected. (In some cases, a chondrule may appear to have recorded two events which can be thermally separated). I-Xe ages for chondrules generally cluster close to the reference values of Bjurbole and Shallowater (which are only ~ 0.6Myr different). Ages of chondrules from Chainpur scatter over a much larger extent, attributed to shock processing [3]. For the remaining data, well-determined I-Xe ages believed to reflect closure to diffusion after chondrule formation, cover at least 2.5 Myr from any single meteorite. This is a significantly larger range than the uncertainties due to the experimental technique. The poster presentation will compare the spread in I-Xe ages to the smaller range of values observed using other isotopic systems, and discuss the significance of an extended period of chondrule formation.