Taking Water to Extremes

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Science  14 Apr 2000:
Vol. 288, Issue 5464, pp. 229
DOI: 10.1126/science.288.5464.229c

Water is perhaps the most thoroughly studied substance at ambient conditions; however, in most of the Earth, it is at conditions well beyond its critical point (373°C and 2.21 megapascals), and its properties are less well established. Water is of crucial importance in volcanism and in many reactions in the mantle and lower crust, where temperatures are 1000°C or more and pressures lie in the gigapascal range. Because it has been difficult to measure even the most basic properties of water under these conditions, thermodynamic relations have been used to extrapolate from data derived at much lower pressures.

Withers et al. have developed a method to measure the density of water at extreme conditions and have obtained results up to 1100°C and 4 gigapascals (this pressure and temperature correspond to a depth of about 120 kilometers in the Earth). They synthesized water inclusions in corundum (aluminum oxide, a very hard mineral that is insoluble in water) at these conditions, so that the density was preserved in the inclusion when the sample was returned to ambient conditions (because the inclusion size and mass of water are constant). They then show that the hydrogen nuclear magnetic resonance shift can be related to the fluid density. Their data confirm some of the extrapolations (equations of state), but not others.—BH

Geochim. Cosmochim. Acta,64, 1051 (2000).

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