[Magazine] Scientific American. Vol. 293. No 4

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E. Brady and L. E. Limbird in Cellular Signalling, Vol. 14, No. 4, pages 297–309; April 2002. A Pharmacology Primer: Theory, Application, and Methods. Terry Kenakin. Academic Press (Elsevier), 2003. COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. 4 billion years ago contrasts sharply with the hot, hostile world typically depicted in textbooks. 58 SCIENTIFIC A MERIC A N COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. A Cool Early Earth? The textbook view that the earth spent its first half a billion years drenched in magma could be wrong.

In the past five years, however, geologists — including my group at the University of Wisconsin–Madison — have discovered dozens of ancient crystals of the mineral zircon with chemical compositions that are changing our thinking about the earth’s beginnings. The unusual properties of these durable minerals — each the size of the period in this sentence — enable the crystals to preserve surprisingly robust clues about what the environment was like when they formed. These tiny time capsules bear evidence that oceans habitable to primitive life and perhaps continents could have appeared 400 million years earlier than generally thought.

To lead. When a zircon forms from a solidifying magma, atoms of zirconium, silicon and oxygen combine in exact proportions (ZrSiO4) to create a crystal structure unique to zircon; uranium occasionally substitutes as a trace impurity. Atoms of lead, on the other hand, are too large to comfortably replace any of the elements in the lattice, so zircons start out virtually lead-free. The uranium-lead clock starts ticking as soon as the zircon crystallizes. Thus, the ratio of lead to uranium increases with the age of the crystal.

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