Between 1.7 and 1.1 billion years ago, cold, subduction-related eclogites disappeared from the rock record, but why? In a recent study published by Dr. Renee Tamblyn (now at University of Bern) and a team of researchers at the University of Adelaide, including myself, offer a solution: heating of the mantle beneath continents resulting from the formation of the first major supercontinent Nuna. Our evidence comes from an unlikely source, granites.
On Earth, cold eclogites are a high pressure rock that unambiguously records subduction. However, the record of eclogites does not start when we think plate tectonics first began operating some ca. 3.6 billion years ago. Instead, the first known cold eclogite appears at ca. 2.5 billion years ago, found in an approx. 10 m x 15 m outcrop on Stolbikha Island, Russia—the so-called Belmorian eclogite. Subduction related eclogites are formed in high pressure settings (depths in excess of 60 km) where low temperatures are dragged into the mantle along with the subducting plate. The thermal gradients under which these rocks from is typically between 4 and 18 ºC/km (the average continental crust is between 20 to 30 ºC/km).
The billion year gap between the initiation of plate tectonics and this first eclogite occurs because the Earth's young mantle was hot. When the mantle is hot, subducting slabs will break off relatively easily, carrying these high pressure rocks with them deep into Earth's interior. As a result, the will not return to the surface to be potentially preserved. As Earth's interior cooled, the threshold of slab stability was reached and these rocks could return to the surface in the subduction channel, or by some other means of exhumation.
By 2.2 billion years, Earth's mantle broadly reached this threshold and eclogites were returned to the crust and preserved in several orogens that record the assembly of Nuna. Between 2.2 and 1.8 billion years ago, these cold eclogites appear in the Trans-Hudson Orogen and Snowbird Tectonic Zone in Canada, the Usagaran-Ubendian Belt in Tanzania, Eburnian-Transamazonian Orogen, both in Congo and south Cameroon, the Kola-Lapland Orogen in Russia, The Nagssustoqidian Orogen in Greenland, and the Trans-North China Orogen in the Hengshan Complex.
At 1.8 billion years ago, eclogites mysteriously disappear from the rock record until they reappear some 700 million years later during the formation of Rodinia. Some researchers have suggested that this disappearance is related to the paucity of orogens during this time, or that ultra-high pressure terranes including these eclogites are only recorded during supercontinent assembly events. Both explanations are lacking. There are subduction related orogens during this period, including some smaller continent–continent collisions and the eclogite record is nearly continuous from 700 Ma to the present across the construction and separation of Gondwana and Pangea.
So what happened? To answer this question, we examined the chemistry of granites, specifically, the Sr/Y ratio which is sensitive to the thermal gradients under which granites are formed. Because garnet stability is sensitive to the thermal gradient, and a reservoir of Y, but not Sr in the crust, when garnet is dissolved into a melt, the melt (and eventual granite) is high in Y relative to Sr (low Sr/Y ratio). When the thermal gradient is cool, garnet is stable and the resulting melts will exhibit a high Sr/Y ratio.
At ca. 2.0 billion years ago, the Sr/Y ratio of the continental crust suddenly decreases suggesting a rise in the thermal gradient. If Earth's mantle is cooling, why then would the Sr/Y ratio fall? We suggest the temperature beneath the continents increased as Nuna was assembled due to an increase in insulation of the mantle. Continental crust is a relatively poor conductor of heat, certainly much less efficient than mantle convection. When the supercontinent was created, it was like putting a large down blanket over the mantle. As a result heat built up beneath the continental crust. The higher temperatures fundamentally changed the chemistry of granites and crossed the stability threshold for the exhumation and preservation of eclogites. As a result eclogites disappeared from the rock record.
The phenomena may have happened again after the assembly of Rodinia, but at that point the increase in temperatures was either less than that of Nuna or Earth's mantle cooled sufficiently that the effect was only temporary (approx. 300 million years). There was no break in eclogite formation as a result of Gondwana or Pangea because mantle temperatures were now cool enough on average that the insulation effect did not raise temperatures above the eclogite stability criteria.
The question now is what other elements are affected by such heating events and whether such knowledge can be used to improve the exploration for specific elements that are sensitive to crustal/mantle temperatures when melts are produced.
Tamblyn, R., Hasterok, D., Hand, M., Gard, M., 2021. Mantle heating at ca. 2 Ga by continental insulation: Evidence from granites and eclogites, Geology, https://doi.org/10.1130/g49288.1.