Gneiss age dating

Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 12 Ma and document granitoid activity over a period of about 23 Ma.A migmatitic granitic gneiss from Wadi Bitan, south-west of Ras Banas, has a zircon age of 704 ± 8 Ma, and its protolith was apparently generated during the same intrusive event as the granitoids at Hafafit.With rocks in hand, it becomes possible to analyse every aspect of them and retrace their history. However, the neodymium-142 levels may not be an indicator of the rock’s age.O’Neil himself admits his team may instead be measuring the age of the magma from which the rocks formed.Jeff is extremely knowledgeable and enthusiastic about geology, and in the course of cramming a semester's worth of geology into the two hour lab, he mentioned that he had in his office one of the oldest rocks in the world: a nice chunk of Acasta gneiss.

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Any rocks that formed while samarium-146 was still around would today contain larger than usual quantities of neodymium-142.The dating method relies on the amount of the common isotope neodymium-142 in the rock.All rocks contain some neodymium-142, but rocks older than 4.2 billion years should contain more of it.Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (≈900 to 600 Ma) supracrustal and intrusive assemblages.A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called ‘Older Granites’ of Egypt.The energy of the blow was such that the Earth’s upper layers melted into an ocean of magma.The next period for which we have evidence in the evolution of Earth is around 3.8 billion years ago, by which time most geologists agree plate tectonics were in place.It is clear that O’Neil and his colleagues have discovered one of the oldest signals from the very early stages in our planet’s development.But how the signal should be interpreted “is going to be very controversial”, says Whitehouse.“What we really want to get to is what Earth looked like before 4 billion years ago,” says Martin Whitehouse of the Swedish Museum of Natural History, who was not involved in the study.“We want to understand how the early Earth transitioned from a magma ocean through to the Earth at 3.8 billion years.


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