A new theory on the thermal evolution of Earth explains why the planet’s upper mantle was cool enough to produce diamonds in the Archean Eon between 4 billion and 2.5 billion years ago, rather than turning into lumps of common graphite.
After studying thousands of rocks samples dating at least 2.5 billion years old, Kamber & Tomlinson determined the Earth’s upper mantle was much cooler than previously thought. Image credit: NASA / JPL-Caltech.
Queensland University of Technology’s Professor Balz Kamber and Trinity College Dublin’s Professor Emma Tomlinson looked at the magnesium oxide levels in thousands of volcanic rocks, dating at least 2.5 billion years old, that had been collected from around the world.
“The analysis of the magnesium oxide levels in rock samples from the Archean era contradicts the conventional belief that the Earth’s mantle was a lot hotter than it is in the current day,” Professor Kamber said.
“We know for a fact that the Earth produced a lot more heat back then — about 2.5-3 times,” Professor Kamber said.
The prevailing theory is that the Earth’s whole mantle was significantly hotter until 2.5 billion years ago.
“But our analysis is that the prevailing theory is only half right. While the lower mantle was significantly hotter, the upper mantle which is the area down to 670 km was no hotter than it is in the present day,” Professor Kamber said.
“It’s the upper mantle that matters because the volcanic rocks that we observe, they come from the upper mantle.”
To explain their theory, the researchers use the analogy of someone trying to warm their bedroom in winter by turning up the heater but failing to close the windows.
“You can produce as much heat as you like but it doesn’t get any warmer,” Professor Kamber said.
“So what we’re actually interested in is not how much heat we’re producing, but how warm it was in the interior of the Earth.”
“The assumption has been: more heat, therefore it was hotter. But what we show is: more heat but not hotter. The Earth was producing more heat but was also getting rid of it, opening more windows so to speak.”
The cool upper mantle theory helps to explain the formation of diamonds, most of which were formed during this time period and would have turned into lumps of graphite if the upper mantle was too hot.
The understanding that the upper mantle 2.5 billion years ago was a lot cooler than previously thought also answers another long-standing area of dispute that has split geologists concerning the movement of tectonic plates. If the upper mantle had been much hotter 2.5 million years ago, then the oceanic plates would have been thicker and difficult to move under each other.
The new evidence of a cooler upper mantle, which would have been churning hot rocks from the lower mantle upwards towards the surface to release the heat, explains how the plates riding on top of this would have moved fast and collided with each other.
“The understanding the thermal evolution of the Earth was critical to understanding the many aspects of our planet, such as the evolution of the atmosphere, the emergence of land, and the evolution of life,” Professor Kamber said.
“A geologist views the present state as the accumulated history of more than 4 billion years. We can’t understand the present fully if we don’t understand this journey.”
The research is published in the journal Chemical Geology.
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Balz S. Kamber & Emma L. Tomlinson. 2019. Petrological, mineralogical and geochemical peculiarities of Archaean cratons. Chemical Geology 511: 123-151; doi: 10.1016/j.chemgeo.2019.02.011
