Water in the Earth's mantle

Abstract

Three approaches has been used to estimate the amount of water in the Earth's mantle. I. The inhomogeneous two component accretion model suggests that most of the current water inventory of the Earth was derived from the very last portion of the volatile-rich component B added at the very end of accretion. The quantity of this late veneer was estimated from concentration of 3.2 ppb Ir in the Earth's mantle which amounts to 0.44 % CI carbonaceous chondrite material. With a new recently-determined Cl chondrite H2O-content of 12.8 % the estimated amount of water for the bulk Earth is 840 ppm of which 420 ppm represent crustal water. II. The water content measured in mantle-derived volcanic rocks reflects the water content in the source. In oceanic basalts F correlates well with H2O. Assuming the measured 0.05 F/H2O ratio in the oceanic basalts holds also for the Earth's mantle and with a mantle F value of 16.3 ppm a water content of 330 ppm is obtained. III. Direct measurements of water in upper mantle samples are impeded by late contamination and serpentinisation. Therefore the water contents of clean major anhydrous mantle minerals in a spinel lherzolite from San Carlos (Arizona) were measured. A considerable water content of 548 ppm could only be found in clinopyroxene. Considering the modal composition of the spinel lherzolite and the measured low water contents in olivine and orthopyroxene a water content for the upper mantle of about 100 ppm was obtained.