(June 9, 2025 at 6:46 pm)SubtleVirtue Wrote:(June 9, 2025 at 6:43 pm)Paleophyte Wrote: That depends a little on the exact minerals. For a silica-rich rock I'd look for zircons, titanite, monazite, or other uranium-rich minerals to date by U-Pb and/or potassium-rich minerals (K-feldspar, biotite, hornblende) to date by Ar-Ar and/or Rb-Sr. For a silica-poor rock you have a more limited selection. There are still a few minerals (apatite, baddeleyite, titanite, rutile, and possibly even zircon) that you might be able to get an age from but the selection is more limited and you have to be a little more selective. If you have strong metamorphic overprint that may have reset the ages on your minerals you may need to pass on the trickier silica-poor volcanics in favour of the more robust minerals in the silica-rich rocks.
U-Pb for example; how do you know how much U is in the zircons by dating?
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That is one of the quantities that you'll need to measure, along with the amount of Pb decay product. You have a couple of options for that:
- Extract the zircons and dissolve them. Once dissolved, run them through an ion exchange column to separate the U and Pb from everything else (mostly Zr and Si). Spike the separated U and Pb, load it onto a filament, and analyse by TIMS. That'll get you the most precise result, but tends to be very labour-intensive.
- Put the zircons under a microscope and shoot them with a laser (LA-ICPMS). Feed the material that the laser ablates into a plasma mass-spectrometer and it'll tell you how much U and Pb you have. Much faster and cheaper but less precise.
- Instead of a laser, use an ion beam for in situ analysis. The ions can be produced inside of a mass-spectrometer that can tell you how much U and Pb you have (SIMS). A middle ground between LA-ICPMS and TIMS.
