Quartz conglomerates from Nunavut in northern Canada (near the Coronation Gulf) are one part of the puzzle to figure out when and how the world's oldest diamond-bearing mantle formed.
These conglomerates were recently found to contain both diamonds and gold - very similar to the quartz conglomerates from the Wits Basin in South Africa that has mined around 40 % of the world's gold!
These conglomerates were deposited around 2.85 billion years ago, meaning that the diamonds must be even older. Unfortunately there are no mineral inclusions in these diamonds that can be used for definitive age dating, so the best that can be done is some smart estimates, that suggest the diamonds are between 2.8 and 3.8 billion years old.
Was the Archaean (> 2.5 billion years ago) mantle a lot hotter than today?
Pressure and temperature estimates from the diamond and an olivine inclusion show that a thick cool diamond-bearing mantle was already established between 3.8 to 2.8 billion years ago. The cratonic mantle was already at least 180 km deep at this time. Since diamonds are only stable below 120 km, they may have already had opportunity to grow since 3.8 billion years ago.
Read an interview with Graham Pearson about this discovery here and here
Watch Suzette Timmerman's presentation about this discovery here
A Tiny cubo-octahedral diamond recovered from the conglomerate. This diamond allowed for some pressure and temperature estimates of the diamond-bearing mantle between 3.8 and 2.8 billion years ago. Image from Graham Pearson (University of Alberta).