Howardites

The answer sheet for the worksheet Oxygen isotopes: the fingerprint of origin. Each answer comes with the reason, so it can be used to mark or to go over the questions with the class.

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Answers: Oxygen isotopes: the fingerprint of origin

  1. C: Laser fluorination of about a milligramLaser fluorination releases oxygen gas from a sample of a milligram or so.
  2. B: AubritesEnstatite chondrites and aubrites lie on or very close to the TFL, like Earth and the Moon.
  3. A: Self-shielding of CO by ultraviolet lightUltraviolet self-shielding of carbon monoxide can separate oxygen isotopes independently of mass.
  4. D: The photosphere cannot be measured precisely enoughSpectroscopy of the solar photosphere cannot reach the precision that isotope comparisons need.
  5. A: Their parent body was never fully homogenisedA well-mixed body has one Δ17O; scattered values mean the ureilite parent body kept its varied ingredients.
  6. True (By exchange with water ice rich in 17O and 18O)Ice enriched in 17O and 18O is thought to have exchanged oxygen with dust early and widely.
  7. True (17O differs from 16O by one mass unit, 18O by two)Fractionation scales with mass difference, and 17O is about half as far from 16O in mass as 18O is.
  8. Solar wind, on ultrapure targets (accept any answer with the same meaning)Genesis exposed ultrapure targets to the solar wind and returned them to Earth.
  9. It shows the Moon formed from oxygen like Earth's (accept any answer with the same meaning)Lunar rocks share Earth's Δ17O, so any model must make the Moon from isotopically Earth-like material.
  10. +0.4 per mil (accept any answer with the same meaning)Δ17O ≈ δ17O − 0.52 × δ18O = 3.0 − 2.6 = +0.4 per mil.

Task

Answers will vary. The task comes from the lesson: Sketch a three-isotope plot with δ18O on the horizontal axis and δ17O on the vertical. Draw the TFL with slope 0.52 and the CCAM line with slope near 1. Mark approximate positions for H, L, LL, HED, Martian and enstatite meteorites.

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