Stability and chemical composition of pargasitic amphibole in MORB pyrolite under upper mantle conditions

The stability field of pargasitic amphibole in a model mantle composition ( MORB pyrolite) has been experimentally determined for a fixed water content . A solidus for a pargasite-bearing Iherzolite has been defined at pressure s below the limit of amphibole stability of 30 kbar at T = 925 degrees C. T he maximum temperature for pargasitic amphibole in MORE pyrolite occurs at 1075 degrees C between P = 18 and 25 kbar. This maximum lies between that d etermined for a fertile peridotite composition (Hawaiian pyrolite) and a de pleted peridotite composition (Tinaquillo Iherzolite). A comparison of the new results with those from earlier studies suggests that the stability for a particular bulk H2O content is mostly controlled by alkali content of th e Iherzolite composition. The systematic compositional variation of pargasi tic amphibole as a function of pressure and temperature can be represented as an increase of the richterite component with increase in both pressure a nd temperature. For a given pressure the tschermakite component increases w ith increasing temperature. The compositions of coexisting clinopyroxenes a lso show a systematic variation with pressure and temperature. The phase re lationships in MORE pyrolite combined with the modal abundance of coexistin g phases show that the breakdown reactions of pargasitic amphibole occur co ntinuously throughout the subsolidus region studied. The temperature stabil ity limit of pargasitic amphibole coincides with the water-undersaturated s olidus (amphibole-dehydration solidus) at pressures below 30 kbar. The expe rimental results are applicable to pargasitic amphibole-bearing natural per idotites. Cooling and decompression paths and heating events observed in na tural peridotites can be interpreted from changes in the composition of par gasitic amphibole. The data are also applicable to a model for peridotite m elting and hydration process in the subduction environment.