The Time Lord

Old_Joe

OK, here's excerpts from a really interesting article from NPR describing the time-clock provided by Zirconium, of all things. Turns out that it can tell us a lot about things that happened a really, really long time ago.

To peer into Earth's deep time, meet a hardy mineral known as the Time Lord

The oldest known Earth stuff that remains on the surface of our planet is a mineral that's been called the "Time Lord" because it's so incredibly good at keeping geologic time. The mineral is zircon, and scientists have found bits of it that formed 4.37 billion years ago, not too long after the proto-Earth's epic collision with a Mars-sized object that spawned our moon.

Tiny crystals of zircon can look like sand, or useless crud. But with a radioactive tick-tock that marks the passing of billions of years, these small but mighty minerals offer us a peek into the Earth's early development.

"They are really the best markers of Earth's time, or the history of the Earth," says Michael Ackerson, a geologist with the Smithsonian's National Museum of Natural History. Zircon crystals originate in cooling magmas in continental crusts, along with other minerals. But those other minerals tend to disappear over time- "Most of the minerals don't survive, so things like quartz, things like feldspar – they are chemically or physically weathered and eroded to a point where they are no longer quartz and feldspar." By contrast, zircon is very resilient – which is one of the main reasons that this mineral is so useful.

In the face of harsh winds, crushing pressures, or high heat, these hardy crystals persist. And eventually, they can end up getting incorporated into other rocks that are still forming. That means scientists can crush up the Earth's oldest rocks, pick through the debris, and find little grains of zircon that are even older.

In the Jack Hills region of western Australia, for example, there's rock that formed from a beach 3 billion years ago. The oldest zircons ever discovered came from this rock. Ackerson once found a zircon that's 4.32 billion years old. Zircons that old "are extremely, extremely, extremely rare, and they're the only windows we have into the earliest Earth," he says.

These days, to know a zircon's exact age, scientists can zap it with a laser that blasts a little hole into it and knocks off miniscule pieces. Then, an argon plasma [is used] to break them down to their smallest constituents, and a detector counts atoms of different chemical elements.

The important ones are uranium and lead. Zircon loves uranium and will take it in as it grows, but zircon hates lead. That means if you find lead inside, it pretty surely came from the decay of uranium, which happens at a steady rate, like the ticking of a clock.

But looking at the chemical makeup of zircon can do more than just reveal its age or the age of its associated rock. It can also give scientists clues about the conditions that existed when that zircon originally got created. For example, Ackerson recently looked at aluminum concentrations inside ancient zircons to infer that plate tectonics may have begun 3.6 billion years ago.

The above excerpts from the NPR article were edited for brevity.