Only El Reg could use "golden shower" in a science story headline
Keep up the good work.
Rocks from Isua in south-west Greenland have been hailed as providing evidence for what geologists believe is the source of complex and heavy elements on Earth: an asteroid shower that endowed our young planet with gold (as well as platinum, iridium, nickel and tungsten). The research to be published in Nature suggests that …
I was thinking how I would explain this to my wife realising I would say 'well, they date the rocks....', which then did not make sense.
Surely, dating is the way to resolve this.
If 'gold' is dated to 3.9byo, then it is not a likely option, though not impossible.
If gold, and nickel and cadmium etc turns out to be , say 7.5byo,, then the asteroid answer sounds fine......
Or am I talking tosh as usual ?
to date things which contain a certain predictably-decaying isotope of carbon.
You can't just 'date' a lump of gold or other metal.
There is also a discussion about the accuracy of this method as some evidence suggests we may need to adjust for changes in 'predictable' patterns over geological timescales.
And this is where I get more confused.....
Gold [au] has atoms. Obviously.
It has protons, electrons and neutrons. But it also has isotopes. I know it is not carbon, so I was not suggesting "carbon dating" but, was thinking of "other dating", in the same way that other non carbon "things" are dated.
Or, is this where I am going wrong : one can only date using carbon ??????
Still confused.
You're right about using radioactive decay of isotopes to perform dating, but where it falls down is that there aren't any isotopes of gold with half lives long enough to perform geological dating. So you have to make an assumption that the gold was present when the rock was formed (in the case of Isua when sediments were metamorphosed into gneiss) and use the radiodate established from other elements in the rock - IIRC the Isua was dated using rubidium strontium dating.
If memory serves me right C dating is limited to a few hundred thousand years tops. The C14 halflife is not long enough to date prior to that.
Prior to that for the few M range you have to use K/Ar (pray for nicely solid rocks which do not release Ar) and for anything beyond a few M your primary choice are methods based on Pb isotope ratios (Pb mainline vs impurities from U and Tho alpha-decay products).
And if you want to date multi-billion-year-old isotopes, then you need multi-billion-year half-lives.
They mostly use Uranium/Lead or Thorium/Lead dating for things this old.
Radiodating measures the ratio of undecayed radioisotope to decay products, compares it to the half-life and then works out how long it's been. But this only works if the decay products are still stuck in the rock, ie there hasn't been a chemical process to separate them out.
Uranium that decayed before the rock was formed (ie in the asteroid) will have been separated out when it all melted when the asteroid hit earth. What you're measuring with radiodating is the length of time since the rock was formed.
[Radiocarbon is different because there is new C14 formed in the upper atmosphere - what you're measuring then is how long it's been since the carbon was attached using photosynthesis in a plant; in practise, that's effectively the length of time since the animal you're looking at died]
Doesn't sound very plausible to me. When's the last time y'all picked up a lump of gold that fell from the skies? Surely this should be happening all the time still?
Anyway, I'm definitely no scientist so maybe there are good examples around of golden meteorites. The ones I've seen were either made up of rock or iron. Must admit though that I probably wouldn't be in a hurry to report a meteor strike to the authorities if I woke up one morning and found my backyard covered in gold.
"When's the last time y'all picked up a lump of gold that fell from the skies? Surely this should be happening all the time still?"
It almost certainly is -- but since most of what comes through the atmosphere on a daily basis (more than 10,000,000 tons per year I think) is vaporised on entry, you're probably breathing it.