A record of metamorphic fluids in diopside

Geologic systems where oxygen isotope ratios have been used to study water-rock interactions can be used as testing grounds for evaluating the significance of hydrogen concentration measurements in nominally anhydrous minerals. Diopside grains from granulite-facies siliceous marbles from the Adirondack Mountains, New York were previously characterized by Edwards and Valley (1998) as having undergone faster (wet), slower (dry), or intermediate rates of oxygen isotope exchange during cooling. I found that OH content of the diopsides increases with increasing degree of oxygen isotope exchange, in this case a proxy for water fugacity. This is the first study to show that a relationship exists between OH concentration and oxygen isotope systematics, and therefore fluid history, in natural occurrences of nominally anhydrous minerals.