We're in the Hall Planet Earth, and I'mstandingin front of one of my favorite samplesfrom theStillwater Complex in Montana, and it'sone of myfavorite samples because I've actuallydone research on this rock.What I want to do with it is toillustrate how we can date rocks byradiometric techniques.Now geologists for more than 200 yearshave known the earth is very old.They've made observationsof relatively simple things such asunconformities suchas folded rocks, such as eroding mountainsand realized theearth has to, has to be not thousands ofyears, but millions of years old, andperhaps greater.But they never knew how old the Earthreally was, or how long basically geologicprocesses require.But since the advent of radiometric datingwe have been able to answer thosequestions, and we've been able todetermineprecisely and quantitatively the ages ofrocks.So now in the case of the stillwater watercomplex, the stillwater water is afossil magma chamber, if you will, itsa solidified magma chamber, it solidifiedabout 2.7 Billion years ago, and it crops outrightnow in the bear tooth range insouthwestern Montana.So this rock is known as a Gabro, a Gabrois arock that contains the white mineralsplagioclase, green minerals which areclinopyroxene.And brown minerals which are,orthopyroxene.Now the question is how are we going to goabout dating this rock, and I'm going toexplain to you a method known assamarium-neodymium, dating.Well, first of all you have toprepare the rock.So we take a sample of this rock, and wecrush it up.and then we separate out the differentminerals,so, and we've got them in vials righthere.So, this vial of white minerals is a vialof the plagioclase.

The second vial is a vial of just thewhole rock, the bulk rock.In other words, not separating theminerals.This next vial is the vial of, full ofclinopyroxene.So again, these are very, very puremineral separates.And this fourth vial is a vial oforthopyroxene.So we're going to analyze these foursub-samples to determine age.So now I'd like to explain this method ofdating using samarium and neodymium.And the things that you have to know aboutsamarium neodymium are first of all thatthey're rare earths.they're one of agroup of elements known as rare earthelements that are trace elements.but exist in almost everything, so they'rerather common, and they'rein concentrations that are easily measuredby any number of techniques.the second thing you need to know, isthat samarium, the isotope samarium 147 isradioactive.A samarium 147decays to neodynium 143.With a half-life of 1.06 times 10 to the11 years.Which is a lot of years, and I'm sure youcan figure out how many billions of yearsthat is.The other thing to realize is that thereare a numberof isotopes of neodymium, the stableisotope of neodymium is neodymium 144.There's one more thing that's reallyimportant to understand.When plagioclase, clinopyroxeneand orthopyroxene all crystallize fromthis liquid.They all start with the same ratio ofneodymium 143 to neodymium 144.Okay.But they'll have different ratios ofsamarium 147.To neodymium 144, 143.So, now what we can do is plot thetwo ratios that I just mentioned againsteach other.So here's a plot, the ratio of neodymium143 divided by neodymium 144,against samarium 147 divided by neodymium144.Now initially, the three differentminerals, remember they all startedout life with the same neodymium 143 to144 ratio.

But they have different samarium toneodymium ratios.So they start out as a straight line here.So this is plagioclase, this isclinopyroxene, and this is orthopyroxene.So with time, the samarium decays away.This ratio, samarium 147 to neodymium 144,decreases,and the ratio of neodymium 143 toNeodymium 144 increases.And again, the slope of that line isproportional to theage, and that allows us to calculate theage very precisely.Again, the age of this rock turns out tobeapproximately 2.7 billion years old, andwe know it very precisely.