Sunday, April 27, 2008

92Nb in meteorite accessory phases

ResearchBlogging.orgI must admit to being a fan of the element niobium. It is quite possibly my favorite element on the periodic table. So imagine my delight when I came across this paper on short-lived Nb chronology of meteoritic zircon and rutile.

Today, Nb is monoisotopic. But although all isotopes with a mass other than 93 are radioactive, 92Nb has a half-life of several million years, and is stable enough to have still been present in the early history of the solar system. It decays into 92Zr.

So, the 92Zr we find today is a combination of the solar systems original 92Zr plus the solar system’s original 92Nb, all of which has now decayed. Before the 92Nb decay was complete, there would have been less 92Zr around relative to the other Zr isotopes. So a phase with high Zr and low Nb (e.g. zircon) should show a 92Zr deficit if it formed while 92Nb was still around.

Similarly, a phase with high Nb and low Zr that formed when 92Nb was still live should have a 92Zr excess. Rutile is a prime example of a high Nb, moderate Zr phase.

So, armed with a multicollecter ICPMS and a laser, our intrepid authors pick meteorites with zircon and rutile in them, fire away, and observe the expected isotopic anomalies.

They then combine their results with those of other short-lived P process isotopes and attempt to constrain the nature of the supernova that created the isotopes, and the time between this supernova and the formation of the solar system.

I honestly don’t know enough about astronomy to know what an SNII-sources neutrino-driven wind is, so I can’t really comment on that unless a friendly blogging astronomer can help me out.

As for the 10 MA time period, this really depends on the exact age of the analysed phases. Amelin et al. 2002 give the age of CAI as 4567 MA, with chondrules being 2-3 Ma younger. So unless we know the age of the Chaunskij zircon relative to CAI’s, it isn’t apparent what this 10 MA means. In theory either U-Pb or Hf-W should help answer this question, but the absolute age of this zircon isn’t given. I didn’t chase up the references for the reported values for other short-lived P-process elements.

Full paper:
Yin, Q.Z., Jacobsen, S.B., McDonough, W.F., Horn, I., Petaev, M.I., Zippel, J. (2000). SUPERNOVA SOURCES AND THE 92Nb-92Zr p-PROCESS CHRONOMETER. The Astrophysical Journal,, 537, L49-L53.

Amelin et al. 2002 Science Vol. 297. no. 5587, pp. 1678 - 1683

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