Density contrast of fluid inclusions associated with melt (glass) from two distinct suites of mantle peridotites from the West Eifel, Germany; implications for melt origin

CO (sub 2) -rich fluid inclusions associated with silicate melts (glass) covering a wide variety of densities from 0.13 to 1.17 g/cm (super 3) are present in two suites of genetically different mantle peridotites from the West Eifel, Germany. The two suites consist of low temperature hydrous peridotites (1a), which contain amphibole, and high temperature dry peridotites (lb), respectively. None of the inclusions is primary. Highest fluid densities of 1.17 g/cm (super 3) from the high temperature dry suite corresponding to a maximum pressure of close to 1.5 GPa and a depth of about 47 km compared to 1.05 g/cm (super 3) and a maximum pressure of 0.99 GPa, equivalent to a depth of 31 km, for the amphibole-bearing hydrous suite. Except for similar silica contents (49 to 59 wt. % SiO (sub 2) ), melts from both suites are of different major element composition and origin. Small proportions of melt (1 vol.%) along grain boundaries in the lb suite were derived from the Quaternary alkali basalt lavas from the West Eifel which infiltrated the peridotites at mantle depths considerably greater than 47 km and were chemically modified by reaction with orthopyroxene. Melts in the la suite result from the breakdown of amphibole. Blebs of melt (glass) with microcrysts of clinopyroxene, olivine and spinel replace former amphibole. The wide variety of homogenization temperatures of fluid inclusions associated with the melts in the la suite is believed to be due to re-equilibration at crustal levels shallower than 31 km. This is consistent with the view that the hydrous peridotites were sampled from immediately below the Moho.