地球外壳循环比预想要快

The recycling of Earth's crust（外壳） in volcanoes happens much faster than scientists have previously¹ assumed. Rock of the oceanic crust, which sinks deep into the earth due to the movement of tectonic plates（构造板块） , reemerges through volcanic² eruptions³ after around 500 million years. Researchers from the Max Planck Institute for Chemistry in Mainz obtained this result using volcanic rock samples. Previously, geologists⁴ thought this process would take about two billion years. Virtually all of the ocean islands are volcanoes. Several of them, such as Hawaii, originate from the lowest part of the mantle⁵. This geological process is similar to the movement of coloured liquids in a lava⁶ lamp: hot rock rises in cylindrical⁷ columns, the so-called mantle plumes⁹, from a depth of nearly 3000 kilometres. Near the surface, it melts, because the pressure is reduced, and forms volcanoes. The plume⁸ originates from former ocean crust which early in Earth's history sank to the bottom of the mantle. Previously, scientists had assumed that this recycling took about two billion years.

The chemical analysis of tiny glassy inclusions in olivine crystals from basaltic lava on Mauna Loa volcano in Hawaii has now surprised geologists: the entire recycling process requires at most half a billion years, four times faster than previously thought.

The microscopically¹⁰ small inclusions in the volcanic rock contain trace elements originally dissolved in seawater, and this allows the recycling process to be dated. Before the old ocean crust sinks into the mantle, it soaks up seawater, which leaves tell-tale trace elements in the rock. The age is revealed by the isotopic¹¹ ratio of strontium which changes with time. Strontium is a chemical element, which occurs in trace amounts in sea water. The isotopes¹³ of chemical elements have the same number of protons but different numbers of neutrons¹⁴. Mainz scientists developed a special laser mass spectrometry method which allowed the detection of isotopes of strontium（锶） in extremely small quantities.

To their surprise, the Max Planck researchers found residues¹⁵ of sea water with an unexpected strontium isotope¹² ratio in the samples, which suggested an age of less than 500 million years for the inclusions. Therefore the rock material forming the Hawaiian basalts must be younger.

"Apparently¹⁶ strontium from sea water has reached deep in the Earth's mantle, and reemerged after only half a billion years, in Hawaiian volcano lavas¹⁷," says Klaus Peter Jochum, co-author of the publication. "This discovery was a huge surprise for us."

Another surprise for the scientists was the tremendous variation of strontium isotope ratios found in the melt inclusions in olivine（橄榄石） from the single lava sample. "This variation is much larger than the known range for all Hawaiian lavas," says Alexander Sobolev. "This finding suggests that the mantle is far more chemically heterogeneous¹⁸ on a small spatial¹⁹ scale than we thought before." This heterogeneity²⁰ is preserved only by melt inclusions but is completely obliterated²¹ in the lavas because of their complete mixing.

Sobolev, Jochum and their colleagues expect to obtain similar results for other volcanoes and therefore be able to determine the recycling age the ocean crust more precisely²².