That the Moon formed and Earth’s geochemistry was reset by our planet’s collision with another, now vanished world, has become pretty much part of the geoscientific canon. It was but one of some unimaginably catastrophic events that possibly characterised the early Solar System and those around other stars. Since the mantle geochemistry of the Earth’s precursor was fundamentally transformed to that which underpinned all later geological events, notwithstanding the formation of the protoEarth about 4.57 Ga ago, I now think of the Moon-forming event as our homeworld’s ‘Year Zero’. It was the ‘beginning’ of which James Hutton reckoned there was ‘no vestige’. Any modern geochemist might comment, ‘Well, there must be some kind of signature!’, but what that might be and when it happened are elusive, to say the least. One way of looking for answers is, as with so many thorny issues these days, to make a mathematical model. James Connelly and Martin Bizzarro of the University of Copenhagen, Denmark, have designed one based on the fact that one of the volatile elements that must have been partially ‘blown off’ by such a collision is lead and, of course, that is an element with several isotopes that are daughters of long-term decay of radioactive uranium and thorium (Connelly, J.N. & Bizzarro, M. 2016. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth and Planetary Science Letters, v. 452, p. 36-43. doi:10.1016/j.epsl.2016.07.010).
Loss of volatile daughter isotopes of Pb produced by the decay schemes of highly refractory isotopes of U and Th would have reset the U-Pb and Th-Pb isotopic systems and therefore the radiogenic ‘clocks’ that depend on them in the same way as melting or high-temperature metamorphism resets the simpler 87Rb-87Sr decay scheme. Each radioactive U isotope has a different decay rate that produces a different Pb isotope daughter (235U Þ 207Pb; 238U Þ 206Pb, so it is possible to devise means of using present-day values of ratios between Pb isotopes, such as 207Pb/206Pb, 206Pb/204Pb and 207Pb/204Pb, to work back to such a ‘closure’ time. In short, that is the approach used by Connelly and Bizzarro. The most complicated bit of that geochemical ruse is estimating values of the ratios for the Earth’s modern mantle and for the Solar system in general – a procedure based on what we can actually measure: lots of mantle-derived basalts and lots of meteorites. Cutting out some important caveats, the result of their model is quite a surprise: ‘Year Zero’ on their account was between 4426 and 4417 Ma years ago, which is astonishingly precise. And it is pretty close to the measured age of the of lunar Highland anorthosites – products of fractional crystallisation of the Moon’s early magma ocean – and also to that of the oldest zircons on Earth. But is also about 60 Ma later than previous estimates
The Connelly and Bizzarro paper follows hard on the heels of another with much the same objective (Snape, J.F. and 8 others 2016. Lunar basalt chronology, mantle differentiation and implications for determining the age of the Moon. Earth and Planetary Science Letters, v. 451, p. 149-158. doi.org/10.1016/j.epsl.2016.07.026). Once again omitting a great deal of argument, Snape and colleagues end up with an age for the isotopic resetting of the lunar mantle of 4376 Ma to the nearest 18 Ma; i.e. an age significantly different from that arrived at by Connelly and Bizzarro. So the answer to the question, ‘When was there a vestige of a beginning?’ is, ‘It depends on the model’… Thankfully, neither estimate for ‘Year Zero’ has much bearing on the big, practical questions, such as, ‘When did life form?’, ‘Has there always been plate tectonics?’