Geochemistry: Noble gases in Earth's mantle come from oceans

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Geochemistry: Noble gases in Earth's mantle come from oceans
Geochemistry: Noble gases in Earth's mantle come from oceans

Noble gases in the mantle originate from oceans

The noble gases in the earth's mantle, such as xenon or argon, get there, at least for the most part, with circulating seawater. Previously accepted theories, on the other hand, denied that the gases pass into the surrounding rock during the subduction of tectonic plates and through water and instead reach the earth's surface immediately through volcanic activity.

When examining noble gases from the earth's mantle, the two geoscientists Greg Holland and Chris Ballentine from the University of Manchester discovered that the isotopic composition of the xenon released there was largely the same as that in seawater and the earth's atmosphere. The ratio of the three xenon isotopes 124, 126 and 128 to the isotope 130 indicates that about ninety percent of this noble gas originated in the atmosphere and reached the depths via the oceans; only ten percent come from other sources. For the researchers, this can only be explained by the subduction of tectonic plates and the resulting penetration of seawater into the crust and mantle.

Accordingly, two percent pore water in the subducted material was sufficient to explain the corresponding noble gas deposits and their isotope ratios. In this way, since the formation of the earth and the formation of its oceans, an estimated ten percent of the current ocean volume has entered the earth's mantle and core. According to Holland and Ballentine, this process also explains why, for example, volcanoes such as those on Hawaii or Iceland have a higher water content in their lava than others: they get their magma from deeper regions of the earth's mantle, where the transport of subducted seawater is more efficient over long periods of time In their opinion, they refute the thesis of a subduction barrier, according to which water from the oceans is not absorbed by the mantle material when the plates subside, but is immediately transported back again by volcanic activity. Consequently, the water and noble gases in the lava of many volcanoes are not from the early days of the earth, but are geologically younger and form part of a geohydrochemical cycle that is continually replacing geotectonically vented fluids.

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