PARGASITES IN LHERZOLITE AND WEBSTERITE INCLUSIONS FROM ITINOME-GATA, JAPAN

Abstract

Pargasite occupies up to 4 volume per cent of lherzolite inclusions and up to 8 per cent of websterite inclusions. It occurs as discrete primary crystals and as interstitial grains among other minerals averaging less than 2 volume per cent. New analyses are presented for six representative pargasites. They are characterized by high Cr2O3 and wide variation of K2O and Mg/(Mg+Fe) ratio. The following origin of the pargasites is hypothsized from the mineralogy and chemistry of the host rocks. Original garnet lherzolite and websterite were located at a depth of up to 50 to 75km. During upward transport, to 25km deep in the uppermost part of the mantle, the garnet was finally transformed to olivine+plagioclase+spinel and both orthopyroxene and clinopyroxene were recrystallized toward a relatively rich Tschermak's molecule. Jadeite and Tschermak's components separated from pyroxenes and silica liberated from the breakdown of garnet produced plagioclase, pargasite and orthopyroxene. However, minor newly-formed minerals were not always in equilibrium with major constituents. It is demonstrated that the lherzolite mantle just below the M-discontinuity contains only up to 0.04 per cent water.