Estimation of the Content and Composition of Trapped Melt in Dunite

Аннотация

A method was developed for the quantitative estimation of the content of trapped melt in various dunite types and the composition of this melt on the basis of the major- and trace-element characteristics of the dunites and compositions of their chrome spinels. Our approach is advantageous over the method based on clinopyroxene geochemistry and clinopyroxene–melt partition coefficients for the contents of the light REE and more incompatible elements in melt, comparable with it for the middle REE, and possibly less accurate for the heavy REE and Sr. The estimated mean contents of trapped melt in dunites from ophiolite and concentrically zoned complexes are 1.0–1.5 wt %, which is probably typical of various dunite types, including cumulate dunites from layered complexes. These values are an order of magnitude higher than previous estimates. The correspondence between the compositions of calculated trapped melts in dunites and real natural melts indicates that the estimated contents of trapped melt in dunites are realistic, and the mineral–melt partition coefficients that were used in our calculations are valid for the complexes considered in this paper. In general, the proposed method is suitable for serpentinized dunites, including dunitic serpentinites.