The Earth’s mantle is an important reservoir of H2O, and even a small amount of H2O has a significant influence on the physical properties of mantle rocks. Estimating the amount of H2O in rocks from the Earth’s mantle would, therefore, provide some insights into the physical properties of this volumetrically dominant portion of the Earth. The goal of this study is to use mineral equilibria to determine the activities of H2O (aH2O) in orogenic mantle peridotites from the Western Gneiss Region of Norway. An amphibole dehydration reaction yielded values of aH2O ranging from 0.1 to 0.4 for these samples. Values of fO2 of approximately 1 to 2 log units below the FMQ oxygen buffer were estimated from a fO2-buffering reaction between olivine, orthopyroxene, and spinel for these same samples. These results demonstrate that the presence of amphibole in the mantle does not require elevated values of aH2O (i.e., aH2O1) nor relatively oxidizing values of fO2 (i.e., >FMQ).

It is possible to estimate a minimum value of aH2O by characterizing fluid speciation in C-O-H system for a given value of oxygen fugacity (fO2). Our results show that the estimates of aH2O obtained from the amphibole dehydration equilibrium are significantly lower than values of aH2O estimated from this combination of fO2 and C-O-H calculations. This suggests that fluid pressure (Pfluid) is less than lithostatic pressure (Plith) and, for metamorphic rocks, implies the absence of a free fluid phase.

Fluid absent condition could be generated by amphibole growth during exhumation. If small amounts of H2O were added to these rocks, the formation of amphibole could yield low values of aH2O by consuming all available H2O. On the other hand, if the nominally anhydrous minerals (NAMs) contained significant H2O at conditions outside of the stability field of amphibole they might have served as a reservoir of H2O. In this case, NAMs could supply the OH necessary for amphibole growth once retrograde P-T conditions were consistent with amphibole stability. Thus, amphibole growth may effectively dehydrate coexisting NAMs and enhance the strength of rocks as long as the NAMs controlled the rheology of the rock.

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