Tribology in the Earth’s mantle
R. David Whitby | TLT Worldwide May 2018
Water winding its way underground reveals how the planet’s plates are lubricated.
Water enters the Earth’s mantle through subduction zones and reduces the melting temperature of rocks to generate magmas.
© Can Stock Photo / jasony00
In the July 2017 TLT I wrote
a column titled Tribology, Water and Plate Tectonics
in which I described research that indicated the presence of water deep in the Earth’s crust and the effects it might have on the ways in which the plates that make up the crust might be lubricated. At the time of writing, it was not clear how the water got so deep into the mantle.
Water is the most abundant volatile compound on Earth. It continuously enters the mantle through subduction zones where it reduces the melting temperature of rocks to generate magmas. A global compilation of the thermal structure of subduction zones was used by researchers at the Universities of Michigan, California (Santa Barbara), Wisconsin-Madison and Columbia (New York) (1
) in 2011 to predict the metamorphic facies and water content of slabs of rock that are being subducted.
Their calculations indicated that mineralogically bound water can pass efficiently through old and fast subduction zones such as those in the Western Pacific Ocean, whereas hot subduction zones such as the Cascadia subduction zone in the Eastern Pacific Ocean, see nearly complete dehydration of the subducting slab. The researchers believe the top of the slab is sufficiently hot in all subduction zones so that the upper crust, including sediments and volcanic rocks, was predicted to dehydrate significantly. The degree and depth of dehydration in the deeper crust and uppermost mantle were considered to be highly diverse and depend strongly on rock composition and local pressure and temperature conditions.
They concluded that the upper mantle dehydrates at intermediate depths in all but the coldest subduction zones and, on average, about a third of the bound water subducted globally in slabs reaches 240 km in depth, carried principally and roughly equally in the gabbro and peridotite sections. They predicted global flux of water to the deep mantle is smaller than previous estimates but still amounts to about one ocean mass over the age of the Earth.
At this rate, the overall mantle water content was calculated to increase 370 ppm over the age of the Earth. This was reported to be qualitatively consistent with inferred water concentrations in the Earth’s mantle, assuming that secular cooling of the Earth has increased the efficiency of volatile recycling over time.
However, toward the end of 2017 research work at Yonsei University in Seoul, South Korea (2
), looked at the behavior of kaolinite clay while it is subducted. Reseachers simulated the heat the clay encounters during its descent into the mantle. The dehydration process in subduction zones, which determines whether water is released from the slab or transported into the deeper mantle, is an essential component of the deep-water cycle. The researchers used in situ
and time-resolved high-pressure/high-temperature synchrotron X-ray diffraction and infrared spectra to characterize the structural and chemical changes of the kaolinite.
At conditions corresponding to a depth of about 75 km in a cold subducting slab (2.7 GPa and 200 C) and in the presence of water, researchers observed the pressure-induced insertion of water into kaolinite. This super-hydrated phase has a unit-cell volume about 31% larger, a density that is about 8.4% lower than the original kaolinite and, with 29 wt% of water, the highest water content of any known aluminosilicate mineral in the Earth. As pressures and temperatures approached 19 GPa and about 800 C, they observed the sequential breakdown of the super-hydrated kaolinite.
The formation and subsequent breakdown of super-hydrated kaolinite in cold slabs subducted below 200 km leads to the release of water deep in the Earth’s mantle. The loss of water continues until the kaolinite reaches the bottom of the upper mantle. The high water content is roughly twice the 14 wt% found in serpentines, previously the most water-rich minerals widely subducted into the mantle. Kaolinite accounts for between 5% and 60% of ocean sediments depending on location.
This appears to be a major route by which water gets deep underground.
1. J. Geophys. Res.; Solid Earth
, 116, B10401 (2011). DOI: 10.1029/2010JB007922.
2. Nature Geoscience
, 10, 947-953 (2017). DOI: 10.1038/s41561-017-0008-1.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at firstname.lastname@example.org