A I de Ao 151 d v tv 6 Lead 206 dating of rocks Uranium 238 decays to lead 206

A i de ao 151 d v tv 6 lead 206 dating of rocks

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A _(I,,, de" Ao = 15·1 d v :) tv, = 6
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Lead-206 dating of rocks Uranium-238 decays to lead-206 by a series of steps (see fig. 20.1). The half-life for 23&U is 4.5 x 10 9 years. Thus, the ratio oflead-206 to uranium-238 in a uranium-containing mineral is a measure ofthe time since the mineral was formed. A very sensitive and precise method (mass spectrometry) exist for measuring the mass of these two isotopes in a rock sample. 1. This method assumes that all of the lead-206 in a rock came from decay ofuranium-238. Is this assumption always valid? How could you tell if some of the lead-206 came from a different source? (No other radioactive decay. process produces lead-206.) lid I_~ {Q L-t."'c\ -10(. <:outd lot. t"'(.a"'ro~+~d ~J cl.el"\tc.~' -pr<><Je.S~S. r f '30 J fL.ct~ UJO ~ ~ rcz,~ a~\I~ o~ \i!.QO-20~ LU~\('" ~(d S\..c,La,) Uf ~S Of {o~~ r~q~ <l'" 1'4 MQSS Sf~C' 2. What are the youngest rocks that could be accurately dated by this technique? Al~\vt ct~~U~ t~+ {~<I cl.o~~ ~ t::: - 'i5 )C.lC~ (V\ Ca.qq): (,.5'" >< tC1 i <:i.Sl 0.(0. 3 - 3. A rock contains 0.257 mg oflead-206 for every milligram ofuranium-238. How old is the rock? To solve this problem, we need to know the amount ofuranium-238 that was present when the rock was formed (No). This is the amount ofuranium-238 present now plus the amount that decayed. Calculate the age of the rock. -, N = t{. '2.0,)<,\.0 lI"'ot No S".'is"Xta(~\ tv, = ~. ~'l \0 ~ 1 r 7
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