DIAMONDS ARE FOREVER (WE HOPE!)

Alan G Jones, Geological Survey of Canada
Gary McNeice, Phoenix Geophsyics

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-efective 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
characterised 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.