Water has sat in the crust for a billion years

Brine from a mine in Timmins, Ontario, has a minimum residence time of about 1.5 billion years. The dating is from xenon isotopes that match an ancient atmosphere.

Fluid inclusions in minerals can be old. Flowing fracture water is a harder claim, because the crust cracks and water moves. In a mine near Timmins, more than two kilometres down in the Canadian Shield, the fracture brine carries excesses of the light xenon isotopes that record the atmosphere as it was billions of years ago. Holland and colleagues used those excesses to set a minimum mean residence time of about 1.5 billion years. Other noble gases in the same fluids point the same way: this water has been isolated from the surface since the Precambrian.

It is brine, ancient and salty and full of dissolved hydrogen and methane from reactions between water and rock. It is not a sample of the ocean that stood above that ground, bottled unchanged. ‘Residence time’ means how long this fluid system has been shut. Molecules exchange. The pocket does not. A diamond, in another entry, brought up a wet mineral from the mantle. This is the opposite delivery: the water stayed, and we dug down to it.