Melt Inclusions in Minerals as a Source of Principle Petrological Information 论文

摘要

Secondary ion mass spectrometry and electron microprobe studies demonstrated that melt inclu- sions in magnesian olivine bear evidence of the composition and formation conditions of the primary melts and their mixing products, including the initial water contents of the mantle melts. This information is partially or completely obliterated in the compositions of rocks and glasses by later processes of mixing, contamination, degassing, and crystal fractionation. The typical oceanic basalt of the Mid-Atlantic Ridge is shown to have formed through the mixing of primary melts that were derived by the critical melting of a mantle column, more than 50 km thick. The melting began in the presence of garnet and proceeded in an open system which could retain no more than 2-3 wt % melt. The mixing of the primary melts took place both in magma chambers, simultaneously with crystallization and, probably, during melt transport through the mantle. The initial concen- trations of H 2 O in the mantle magmas of suprasubduction zones were as high as 2-3 wt %, which is significantly higher than was estimated previously. No correlation was found between the concentrations of H 2 O in the pri- mary MORB magmas and those of elements with a similar incompatibility degree (La, Ce, K). This fact may indicate the continuous interaction of the melts with H 2 O-bearing CO 2 -rich fluid.