Volume 38: January - December 1997

Issue 5: May 1997

Abstract

Potassium amphibole stability in the upper mantle: an experimental study in a peralkaline KNCMASH system to 8.5 GPa

J. Konzett, RJ. Sweeney, AB. Thompson and P. Ulmer Institut fur Mineralogie und Petrographie, Eth-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland and Corresponding author Email: juergen@erdw.ehz.ch
ABSTRACT
Experiments were performed from 1.0 to 8.5 GPa in a peralkaline system K2O, Na2O-CaO-MgO-Al2O3-SiO2-H 2O (KNCMASH) to investigate the stability and composition of richteritic amphiboles in the MARID (mica-amphibole-rutile-ilmenite-diopside) assemblage amphibole + phlogopite + clinopyroxene. The results were compared with phase relations and the composition of natural MARIDs to assess possible mechanisms of formation for MARID-type rocks. K-richterite is stable in a wide range of bulk K/Na ratios in the MARID assemblage to 8.5 GPa and 1300^oC. In this assemblage the amphibole can accommodate significant amounts of K on the M(4) site and shows a systematic increase in the K/Na ratio with increasing pressure. At P >7.0 GPa, K-richterite can coexist with garnet. Phase relations of K-richterite in a natural MARID composition are consistent with those in the simplified system and confirm the potential stability of K-richterite and K-richterite + garnet within the diamond stability field. The assemblage K-richterite + phlogopite + clinopyroxene is incompletely buffered in the KNCMASH system, resulting in a systematic relation between bulk- and mineral compositions observed in the experiments. Such a correlation, however, cannot be observed in natural MARIDs. Therefore, MARID-type rocks do not represent the bulk composition from which they formed and, hence, must be products of an open-system crystallization.

Keywords: K-richterite; high-P experiments; MARID xenoliths; mantle metasomatism

Pages: 537 - 568

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