Understanding Earth's Early Mantle

Understanding the geologic history of Earth will no doubt be a decades, if not centuries long process, as we gather more and more data and expand our knowledge. New research published in the April 25 issue of the journal Nature has contributed an interesting new data point, however, shining a light on the processes involved in tectonic subduction.

An international team of researchers led by scientists from Boston University’s Department of Earth and Environment have found evidence that materials recently found in young oceanic lava flows originated more than 2.45 billion years ago in the Archean.

The researchers found mass-independently fractionated (MIF) sulphur isotope signatures in olivine-hosted sulphides in geological-young ocean island basalts from Mangaia, Cook Islands (Polynesia). Current understanding states that MIF sulphur isotope signatures were generated exclusively through atmospheric photochemical reactions until approximately 2.5 billion years ago, their appearance in samples today must have originated at the Earth’s surface during the Archean.

“The discovery of MIF-S isotope in these young oceanic lavas suggests that sulfur—likely derived from the hydrothermally-altered oceanic crust—was subducted into the mantle more than 2.5 billion years ago and recycled into the mantle source of the Mangaia lavas,” says Rita Cabral, a graduate student in BU’s Department of Earth and Environment, who is the study’s primary author.

Tectonic plates wander across the face of the planet, moving at excruciatingly slow geologic pace. When one encounters another, they collide at areas that are known as subduction zones, where one plate is eventually forced under another and then transported down into the Earth’s mantle.

Scientists have believed for some time that these subducted tectonic plates must be reerupted at some later time, but how long between subduction and reeruption, indeed, whether it does at all, has lacked sufficient evidence.

This newest discovery may help scientists “constrain the timing of the initiation of plate tectonics, [understand] the origin of some of the chemical heterogeneity in the Earth’s mantle, and may shed light on how the chaotically convecting mantle could preserve such material for so long.”

Source: Boston University

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