3 days ago
Rocks Date Onset Of Modern Plate Tectonics To 2 Billion Years
When plate tectonics first emerged on Earth is still debated. Some models suggest that Earth at first possessed a single protoplate covering the entire planet. This protoplate eventually broke open, the fragments forming tectonic plates. Other models suggest plate tectonics may have evolved gradually over billions of years.
A new study, Xiaoli Li and colleagues from the Earth and Space Sciences Department, Peking University, published evidence from the petrologic record suggesting that the modern style plate tectonics begun in the Paleoproterozoic — over 1.6 billion years ago.
The team analyzed eclogites from the Belomorian Province in Baltica. Eclogite is a metamorphic rock containing red almandine-pyrope garnet hosted in a matrix of green omphacite (a sodium-rich pyroxene). Eclogites typically results from high to ultrahigh pressure metamorphism of basaltic lava at low thermal gradients as it is subducted to the lower crust to upper mantle depths in a subduction zone. Subduction zones, where one tectonic plate slides beneath another plate, are a key element of modern plate tectonics.
Radiometric dating of the rocks revealed an age around 2 and 1.8 billion years ago, making them the oldest known subduction eclogites. The protolith, the original lava rock, is even older with an age ranging between 2.7 and 2.5 billion years.
The eclogite from Baltica shows some chemical similarities to 'modern' eclogite, formed just 65 million years ago, from the Himalayas. The Himalaya orogen formed when the former ocean between Asia and India was subducted, and remains of oceanic crust and continental fragments were uplifted by the collision. The authors suggest that in a similar way the eclogite from Baltica demonstrates the existence of a mountain range, comparable to the Himalaya range, on the supercontinent Columbia over two billion years ago.
Plate tectonics has, so far, only been observed on Earth, and may be essential to making a world hospitable for life by constantly remixing and renewing the outer layers of the planet. The new results, together with previous research, suggests that something happened around 3.8 to 2 billion years ago, with Earth switching from a single plate towards plate tectonics and changing its geological evolution forever.
The study,"Orosirian cold eclogite from Baltica marks the onset of modern plate tectonics," was published in the journal Earth and Planetary Science Letters.
