Forged deep in the Earth during the earliest days, diamonds lie dormant in thick, cold lithosphere awaiting a thermal event to transport them to the surface at supersonic speeds. Important for diamonds' stability is that after creation they stay at high pressures and low temperatures. Too low a pressure, or too high a temperature, and diamonds change to graphite - which is universally spurned as a token of everlasting love. Kimberlites, which are the express train bringing mantle material to the surface, are diamondiferous if they pass through thick lithosphere (>150 km), or non-diamondiferous if the lithosphere is too thin.  Accordingly, cost-effective diamond exploration requires not just discovering kimberlites, which are found in ever increasing numbers, but discovering those likely to have passed through thick lithosphere thereby possibly bringing diamonds to the surface.

Current thinking is that the lithosphere-asthenosphere boundary (LAB) is characterized by the onset of partial melt.  Even a very low melt fraction, (0.5%) will interconnect, thereby increasing the electrical conductivity of the medium by 2-3 orders of magnitude through ionic conduction in the fluid.  Accordingly, deep-probing electromagnetic surveying, using the natural-source magnetotelluric (MT) technique, is an inexpensive and efficient means of determining the depth to the top of the LAB, i.e., the thickness of the lithosphere.

This paper will demonstrate the resolving power of MT for lithospheric thickness determination, and present an overview of global results. Recent data, and their interpretation, from Canada's Slave Province, which hosts the oldest dated rocks on Earth (4.127 Ga) and is the location of the first diamond mine in North America, will be presented.