* current address: Department of Terrestrial Magnetism, Carnegie Institute of Washington, 5241 Broad Branch Road N.W., Washington D.C. 20008
We present a method for on-line dehydration of small quantities of hydrous and nominally anhydrous minerals followed by measurement of the absolute abundance of hydrogen released from the sample by continuous-flow mass spectrometry. This method is appropriate for measuring water content between 18 ppm and 10 wt % H2O and requires a minimum of ~ 2 × 10-8 moles of hydrogen per analysis. The hydrogen needed for an analysis corresponds to 10-200 µg of hydrous minerals or 5-40 mg of nominally anhydrous minerals. We develop measurement protocols for garnet and pyroxene, two nominally anhydrous minerals that are potentially major reservoirs of hydrogen in the mantle.