Composition and Evolution of Fluid-Saturated Calcium-Rich Melts:Inclusions in the Minerals of Olivine Melilitite from Mahlberg,Rhine Graben

摘要

Inclusions were studied in the minerals of olivine melilitite from the Mahlberg massif (Germany).The rock contains olivine and clinopyroxene phenocrysts of several generations,some of which are probably xenocrysts.Olivine,clinopyroxene,melilite,nepheline,phlogopite,perovskite,and Ti-magnetite were identified in the groundmass.The olivine phenocrysts contain crystalline inclusions of clinopyroxene and spinel,fluid inclusions,and silicate melt inclusions.Only sporadic melt inclusions were found in the clinopyroxene.The fluid inclusions always show evidence for partial decrepitation and contain CO_2 with densities in the range 0.01-0.73 g/cm~3.The melt inclusions show varying morphology and relationships with the host mineral,which are probably related to differences in the formation mechanism (primary,late primary,and pseudosecondary melt inclusions were distinguished).The melt inclusions are fully crystallized,and their mineral associations are in general identical to that of the groundmass.The inclusions were not completely homogenized during thermometric experiments,and a gas phase was retained at high temperatures.Glasses from the inclusions that were quenched from the temperature of complete melting of daughter minerals (1130-1220 deg C) were analyzed on an electron microprobe.The compositions of such glasses show low SiO_ 2 and high Ca,Na,K,Ti,P,Sr,Ba,and F concentrations.Recalculation of melt compositions to equilibrium with olivine Fo_(90) suggested that the primary mantle melts of the complex contained about 14 wt % MgO and 15 wt % CaO and were enriched in incompatible elements.The coexistence of melt inclusions with high-density fluid inclusions indicates that the melts were CO_2 saturated and degassed at a pressure of more than 5 kbar.Despite the high CO_2 pressure and the Ca- and alkali-rich melt composition,neither carbonate phases nor evidence for carbonate-silicate liquid immiscibility were found in the inclusions.Based on a comparison with the compositions of inclusions from the melilitolites of the Gardiner carbonatite-bearing massif (Greenland),we supposed that the absence of car-bonatite melts at Mahlberg was related to the high concentration of aluminum in the melt.