Potassium Amphibole Stability in the Upper Mantle: an Experimental Study in a Peralkaline KNCMASH System to 8.5 GPa

Abstract

Experiments were performed from 1·0 to 8·5 GPa in a peralkaline INTRODUCTION system K2O‐Na2O‐CaO‐MgO‐Al2O3‐SiO2‐H2O (KNCMASH) K-rich amphiboles are exotic members of the amphibole to investigate the stability and composition of richteritic amphiboles family in terms of their frequency of occurrence at in the MARID (mica‐amphibole‐rutile‐ilmenite‐diopside) as- the Earth’s surface. With few exceptions they are semblage amphibole + phlogopite + clinopyroxene. The results represented by K-rich varieties of richterite Nawere compared with phase relations and the composition of natural CaNaMg5Si8O22(OH)2, katophorite NaCaNaMg4‐ MARIDs to assess possible mechanisms of formation for MARID- AlSi7AlO22(OH)2 and, more rarely, arfvedsonite type rocks. K-richterite is stable in a wide range of bulk K/Na NaNa2Mg4Fe 3+ Si8O22(OH)2. The vast majority of known ratios in the MARID assemblage to 8·5 GPa and 1300∞C. In occurrences is confined to extremely alkaline, Kthis assemblage the amphibole can accommodate significant amounts dominated mainly magmatic environments which are of K on the M(4)-site and shows a systematic increase in the K/ poor in Al and partly Si undersaturated. Experimental Na ratio with increasing pressure. At P >7·0 GPa, K-richterite results on K-richterite stability show that this amphibole can coexist with garnet. Phase relations of K-richterite in a natural can act as a principal reservoir for alkalis and water MARID composition are consistent with those in the simplified as well as for a number of incompatible trace elements system and confirm the potential stability of K-richterite and K- in the Earth’s mantle down to transition zone depths, richterite + garnet within the diamond stability field. The as- and may be involved in deep-seated magma generation semblage K-richterite + phlogopite + clinopyroxene is incompletely processes: K-richterite (or Ti- and/or F-rich varieties)