Al dating system
Correspondingly, H chondrites exhibiting different metamorphic grades are assumed to come from different depths in this parent body.Another reason was that Christa Göpel had previously used the uranium clock on phosphate crystals from Ste.Marguerite and Forest Vale and had obtained absolute ages of 4562.7±0..9±0.7 million years.These ages are so-called metamorphic ages because phosphates are metamorphic minerals that formed during heating of the H4 region on the parent body.These nagging doubts were amplified by the recent discovery by Kevin Mc Keegan (University of California, Los Angeles) and colleagues that another short-lived isotope, beryllium-10 (half-life 1.5 million years) was also originally present in CAIs.This radioisotope is not produced by stellar nucleosynthesis but most likely formed as energetic particles from the early sun bombarded material in the accretion disk.The temperature at which the uranium-lead system stops being equilibrated during decreasing temperature is believed to be approximately 730 degrees Kelvin.This temperature is reached at different times for different H chondrites.
Because lead isotopes are the daughter products of uranium decay, uranium ages are usually called Pb-Pb ages.
This heating was the cause of metamorphic changes in the rocks making up this asteroid.
Rocks from different depths experienced different peak temperatures and duration of heating.
Peter Hoppe and I measured the magnesium isotopic ratios and the aluminum/magnesium ratios in many different spots on a single feldspar crystal with such an instrument at the Max-Planck-Institute for Chemistry in Mainz, Germany.
Mg excesses as a function of aluminum/magnesium ratios was made with a special type of mass spectrometer, the ion microprobe.
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On a large crystal from Forest Vale we could make these measurements in many different areas.