Sinking tectonic plates can transport vastly more water into Earth’s deep mantle than earlier models predicted. Common geological models assume that rocks steadily lose their trapped moisture as rising heat and pressure bake them during descent. Instead, cold serpentine rocks actively bind excess fluid into dense mineral lattices at extreme depths.
When a cold sinking slab carries more than six point nine weight percent excess water, extreme pressure drives fluid directly into the mineral structure. The rock behaves like a compressed sponge that absorbs surrounding moisture rather than squeezing it out. Above nine point three gigapascals of pressure and four hundred twenty degrees Celsius, the serpentine reorganizes into two dense hydrated phases known as phase E and the three point six five angstrom phase. If surrounding moisture drops below six point nine weight percent, the mineral still transforms into phase E without shedding any of its existing water.
Researchers tracked pure serpentinite samples across different temperature pathways using synchrotron X-ray diffraction and ultrasonic sound measurements. They varied the amount of excess water from zero point nine to thirty-one point five weight percent under laboratory pressures simulating deep slab descent. In the coldest conditions at roughly two hundred eighty kilometers underground, the rock expanded its water capacity from thirteen weight percent to about nineteen point three weight percent.
These findings show that subducting slabs possess a much higher capacity to haul surface water deep into the planetary interior. Under slightly warmer slab pathways at two hundred twenty kilometers depth, the minerals instead break down into phase E and clinoenstatite while expelling five point three weight percent water, creating sharp regional differences in rock density and seismic speeds.