Effect Of Oxygen Fugacity On OH Solubility In Pyroxenes

Trace amounts of water present as H-related point defects in the structure of nominally anhydrous mantle minerals affect significantly many physicochemical properties of the host minerals and even dynamics of the mantle.Clinopyroxene and orthopyroxene are two important minerals in the upper mantle,with their abundance only less than olivine.Usually,pyroxenes are more H-enriched than olivine,and therefore,they contribute greatly to H budget in the upper mantle.The storage of OH groups in pyroxenes has received the interest of some scholars,and the available studies were usually on the potential effect of pressure,temperature and chemical composition on the OH incorporation.Oxygen fugacity(fO2)is an important thermodynamic parameter controlling many physicochemical properties and geological processes of the Earth's different reservoirs,and affects the partitioning of elements between coexisting phases and the speciation of degassed volatiles in melts.fO2 influences greatly the defect population of many minerals and thus the incorporation of OH in pyroxenes.However,this has so far received no attention in the community.In this work,we have conducted a series of annealing experiments OH on the effect of fO2 on OH storage in clinopyroxene and orthopyroxenes.The runs were carried out by equilibrating single crystal minerals at 1-3 GPa and 800-1200 ?,and Fe2O3-Fe3O4(HM),Ni-NiO(NNO)and Fe-FeO(IW)were used as the fO2 buffers.Water contents of the pyroxene samples were determined by polarized Fourier-Transform infrared(FTIR)spectroscopy.The H-annealed diopside samples show typical OH related absorption bands at?3355,3450,3535 and 3646 cm-1,and the annealed enstatite samples show typical OH related absorption bands at?3075,3370,3424,3513,3544,3600 and 3685 cm-1 The peak position and spectral shape of the OH bands are similar for each mineral,regardless of the experimental conditions.The solubility of OH groups in each of the minerals is a strong function of fO2,temperature and pressure.In general.the OH solubility of either clinopyroxene or orthopyroxene increases with increasing temperature or pressure,but is strongly reduced by increasing fO2 from IW to NNO to HM.Over the same range of JO2 ranging from IW to NNO(by?4 log units),the OH solubility is reduced by a factor of?2-3 for clinopyroxene and of?1-2 for orthopyroxene.The effect of fO2 on the OH solubility of pyroxenes is different from that of olivine,and as such,the partitioning of OH groups between pyroxenes and olivine is strongly affected by fO2.The results provide new constraints on water storage in the upper mantle.