Over geological time plate tectonics has played a key role in making Earth’s environment unique within our solar system. Central to stabilizing these special near-surface conditions is the process of subduction (where one tectonic plate sinks beneath another).
At subduction zones, chemical elements from Earth’s surface are taken down into the mantle by the downgoing tectonic plate, both as hydrothermally altered ocean crust and as marine sediment. As this plate sinks, the temperature and pressure increases, which cause the rock to release water and other chemicals into the mantle. This dehydration leads to mantle melting and ultimately to volcanoes at the planet’s surface, including many of the world’s most hazardous volcanoes, such as most of those in the ‘Pacific ring of fire’. This volcanism also transports some downgoing chemicals back to Earth’s surface thus “recycling” them.
Not all chemicals from Earth’s surface that are taken into the mantle during subduction get directly recycled in this way. Some get taken deeper into Earth’s interior where their ultimate fate is poorly known. How long are they stored in the mantle, and what geological circumstances must conspire to result in their eventual release? Understanding the balance between the inputs and outputs of elements like carbon to the deep Earth at subduction zones has profound implications for the evolution of Earth’s atmosphere and surface environment . Investigating processes that occur 100 or more kilometers below Earth’s surface is extremely challenging, and requires combining clues from seismology, high temperature and pressure experiments, computer modeling, and analysis of erupted volcanic rock samples found at the surface.
In a recent paper in Geophysical Research Letters DCO scientists from the UK, along with colleagues in the Chilean Geological Survey (SERNAGEOMIN), have found new clues about subduction zone processes in a suite of volcanic rocks from the volcanoes of southern Chile . They found subtle but significant and systematic differences in the chemistry of the rocks depending on the depth of the downgoing tectonic plate beneath the volcanoes. These observations suggest that, over distances of just a few kilometers, changes in temperature and pressure conditions within Earth cause a shift from the release of more water-rich fluids from the downgoing slab, to those dominated by surface-melting of the downgoing tectonic plate. This melting includes sediments transported on the upper surface of the plate, and has the potential to release more carbon, potentially by breakdown of sediment-bound carbonates. The similar patterns seen in rocks from subduction zones in Chile, Kamchatka  and Izu-Bonin  suggest that we may be getting a rare glimpse into the inner workings of our planet.
"It's remarkable to observe such sharp variations in melt chemistry over length scales of a few kilometers. We think that this means that temperatures in the subducting plate are key to understanding the nature and quantity of chemical elements transported via melts in subduction zones. Surprisingly, the chemical pattern we observe in southern Chile looks to be replicated in other subduction zones with different characteristics, suggesting that these types of studies have the potential to tell us something fundamental and ubiquitous about how Earth works." says Sebastian Watt, lead author of the study.
The team plans to continue their work in Chile, using this area as a natural laboratory to understand more about how carbon cycles between Earth’s surface and its deep interior, and the role of these processes in shaping our habitable planet.
Photo credit: S. Watt. Olivine crystals within a fine groundmass in a clast erupted explosively at Apagado. Such crystals commonly trap melt inclusions, recording the melt composition at the time of their growth. In several of the region's explosive deposits, the rapid ascent of magma transported olivine crystals from depth and deposited them as free grains in scoria deposits. This rapid transport and cooling resulted in limited alteration of melt-inclusion compositions, making them ideal for the investigation of deep melting processes.