Olivine crystals from Mauna Loa volcano, Hawaii, with a width of less than one millimeter. The brown ovals are solidified, glassy inclusions trapped as droplets of melt by the growing olivine crystal. They contain strontium isotope ratios which are inherited from 500 million year old seawater. Photo: Sobolev, Max Planck Institute for Chemistry
recycling of the Earth’s crust in volcanoes happens much faster than
scientists have previously assumed. Rock of the oceanic crust, which
sinks deep into the earth due to the movement of tectonic plates,
reemerges through volcanic eruptions after around 500 million years.
Researchers from the Max Planck Institute for Chemistry in Mainz
obtained this result using volcanic rock samples. Previously, geologists
thought this process would take about two billion years.
all of the ocean islands are volcanoes. Several of them, such as
Hawaii, originate from the lowest part of the mantle. This geological
process is similar to the movement of coloured liquids in a lava lamp:
hot rock rises in cylindrical columns, the so-called mantle plumes, from
a depth of nearly 3,000 km. Near the surface, it melts, because
the pressure is reduced, and forms volcanoes. The plume originates from
former ocean crust which early in the Earth’s history sank to the
bottom of the mantle. Previously, scientists had assumed that this
recycling took about two billion years.
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.
microscopically 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 isotopic ratio of strontium which changes
with time. Strontium is a chemical element, which occurs in trace
amounts in sea water. The isotopes of chemical elements have the same
number of protons but different numbers of neutrons. Mainz scientists
developed a special laser mass spectrometry method which allowed the
detection of isotopes of strontium in extremely small quantities.
their surprise, the Max Planck researchers found residues of sea water
with an unexpected strontium isotope 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
strontium from sea water has reached deep in the Earth’s mantle, and
reemerged after only half a billion years, in Hawaiian volcano lavas,”
says Klaus Peter Jochum, co-author of the publication. “This discovery
was a huge surprise for us.”
surprise for the scientists was the tremendous variation of strontium
isotope ratios found in the melt inclusions in olivine from the single
lava sample. “This variation is much larger than the known range for all
Hawaiian lavas,” says Alexander Sobolev, who participated in the study.
“This finding suggests that the mantle is far more chemically
heterogeneous on a small spatial scale than we thought before.”
heterogeneity is preserved only by melt inclusions but is completely
obliterated in the lavas because of their complete mixing.
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 precisely.