The chemistry of some ancient zircons shifted starting around 3.8 billion years ago. It may mark the onset of plate tectonics.
, more than 4 billion years ago . One reason the debate has remained unresolved is that opportunities to investigate that distant past are scarce, as few rocks have survived the eons of tectonic activity.. Geologists know this because they can reliably estimate zircon ages by measuring how much uranium and lead is in the crystal. Growing zircons readily capture uranium but tend to exclude lead, so any lead found in zircon is assumed to have decayed from uranium.
Most of the world’s Hadean zircons have been found in a just a dozen sites, and most of those are from a site in Australia. But since 2018, Drabon and her colleagues have reported on a new source: the Barberton Makhonjwa Mountains in South Africa. Dozens of zircons collected from that region crystallized between 4.15 to 4 billion years ago, while thousands of others are at least 3.3 billion years old.
Prior to 3.8 billion years ago, most of the zircons contained quantities of different forms of hafnium through time that supported the existence of an old, stable protocrust. What’s more, these older zircons contained trace elements at levels that were similar to those found in zircons from Hawaii’s volcanic hotspot. There, new rock is created as a plume of molten material rises from Earth’s mantle and crystallizes — a process that could have operated in the absence of moving plates.
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