Microgravity and diamonds

Prospecting for diamonds relies either on lucky finds in sediments or locating the odd kimberlite pipes that brought diamonds from depths greater than 100 km in the mantle, where they form.  Such has been the centuries-old frenzy for diamonds that most deposits of the trip-over kind have been found.  One of the last major diamond fields turned up in Arctic Canada, after prospectors panned their way upstream of glaciers that had dropped the odd diamond in Canadian Shield tills.  It is simply too costly to keep repeating this painstaking exercise to satisfy the enduring demand for diamonds of all qualities.  New sources probably exist in huge, unexplored regions of Canada, Australia, Africa and north Asia, yet kimberlites, often having broken down to clays and forming little by way of topographic features, are not easy to find.  Great efforts have been made to harness conventional remote sensing that uses reflected and emitted electromagnetic radiation, but with little success.  Aside from the innocuous nature of kimberlites, most prime ground is either flat, vegetated steppe in areas once affected by glacial conditions, the featureless soil covered tracts of interior Australia or tropical rain forest, where remote sensing simply does not work well enough.

Kimberlite pipes have round traces at the surface and the rock has a different density from common rocks of the upper crust, so one means of locating them is by looking for circular patterns on gravity maps.  But they are small relative to the resolution of regional gravity maps, which are generally constructed by careful measurement of gravitational field potential at points on the surface.  It is not that gravimeters are incapable of detecting differences due to rocks with anomalous density, but that sample spacing is too coarse (>1km) because of the high cost of field surveys.  Maps of the Earth’s magnetic field and emissions of gamma-rays by radioactive isotopes are routinely created at suitable resolution by aerial surveys, but kimberlites show only subtle features on them.  Airborne gravity surveys have been a grail for explorationists for many physical resources, but insufficient economic interest has blunted the search for a way of overcoming the effects of turbulent accelerations during flight, which spoil measurements of the actual gravity force field.  Mining company Broken Hill Proprietary – Billiton’s venture into diamonds after their acquisition of the Ekati deposit in northern Canada has encouraged them to seek a cunning approach to the problem.  Whereas measuring gravitational potential from the air is a tough nut to crack, the US navy had developed an instrument to measure changes in the gradient of the gravitational field that can overcome varying accelerations, to help nuclear submarines navigate without recourse to giveaway sonar “pings”.  BHP-Billiton is into this technology in a big way, now that it has been declassified.  While gravity gradiometry offers one way of revolutionizing the precision of gravity surveys, other methods are possible, and it is rumoured that geophysicists who try to measure even tinier shifts in the gravitational field to monitor the rise and fall of magma in volcanoes are onto a cheaper and less convoluted method………

Source:  Nowack, R. 2002.  Pulling power.  New Scientist, 21 September 2002,p. 42-45.

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