Diamond study holds clue to continental drift

Scientists have studied diamonds in bid to know about the Earth`s distant past, as impurities are unaltered in these materials.

Washington: Scientists have studied diamonds in bid to know about the Earth`s distant past, as impurities are unaltered in these materials.

Researchers analyzed data from the literature of over 4,000 of these mineral inclusions to find that continents started the cycle of breaking apart, drifting, and colliding about 3 billion years ago. The research pinpoints when this so-called Wilson cycle began.

Lead author Steven Shirey at the Carnegie Institution`s Department of Terrestrial Magnetism explained: "The Wilson cycle is responsible for the growth of the Earth`s continental crust, the continental structures we see today, the opening and closing of ocean basins through time, mountain building, and the distribution of ores and other materials in the crust. But when it all began has remained elusive until now. We used the impurities, or inclusions, contained in diamonds, because they are perfect time capsules from great depth beneath the continents. They provide age and chemical information for a span of more than 3.5 billion years that includes the evolution of the atmosphere, the growth of the continental crust, and the beginning of plate tectonics."

The largest diamonds come from cratons, the most ancient formations within continental interiors that have deep mantle roots or keels around which younger continental material gathered. The inclusions in diamonds come in two major varieties: peridotitic and eclogitic. Peridotite is the most abundant rock type in the upper mantle, whereas eclogite is generally thought to be the remnant of oceanic crust recycled into the mantle by the subduction or sinking of tectonic plates.

Shirey and co-author Stephen Richardson of the University of Cape Town, using their own work with other co investigators published in more than 20 papers over a 25-year period, found that before 3.2 billion years ago, only diamonds with peridotitic compositions formed—whereas subsequent to 3 billion years ago, eclogitic diamonds dominated.

"The simplest explanation is that this change came from the initial subduction of one tectonic plate under the deep mantle keel of another as continents began to collide on a scale similar to that of the super continent cycle today. The sequence of under thrusting and collision led to the capture of eclogite in the sub continental mantle keel along with the fluids that are needed to make diamonds," added Shirey.

The research has been published in Science.


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