Stability of amphibole and carbonate in an oxidized upper mantle and the genesis of carbonatites

Amphibole, phlogopite, dolomite and magnesite are the volatile-bearing mineral phases in the upper mantle. As such, their stability effects the H\(_2\)O and CO\(_2\) content of the mantle and therefore solidus positions and phase relationships. The stability of pargasitic amphibole in the upper mantle is a function of water content and bulk rock composition. Under water-undersaturated conditions, the stability of amphibole controls the solidus position. Under these conditions, amphibole is stable to the highest pressures and temperatures in fertile peridotites and its pressure and temperature stability limits are decreased as bulk rock composition becomes more refractory. At high pressures (>21 kb), the temperature stability limit of dolomite and magnesite are controlled by the reaction carbonate+ Cpx -> sodic, dolomitic carbonatite melt + A1 phase (Gt/Sp). In the system (dolomite (5%)-bearing Pyrolite + 0.2% H\(_2\)O) this reaction, and therefore the high pressure solidus, occurs at 930°C. It is the lowest temperature peridotite solidus found to date. At low pressures ( Cpx + 01 + CO\(_2\). This produces an S-shaped solidus in Pyrolite (H\(_2\)O, CO\(_2\) undersaturated) similar to that found in the CMAS system. The sodic, dolomitic carbonatite melt field is bounded by the solidus and amphibole-out boundary. The melt becomes melilititic at temperatures above the stability limit of amphibole. The pressure and temperature stability limits of the carbonatite depend on the stability field of amphibole which is a function of water content and bulk rock composition. The carbonatite melt field is limited to between 21-31 kb and 930-1075°C suggesting that carbonatite melts are only formed in areas which are oxidized and have oceanic (or rift valley) type geotherms. Carbonatite melts may migrate rapidly to the surface; however during upward melt migration the melt would cross the reaction boundary carbonatitic melt + Opx -> Cpx + Ol + CO\(_2\) causing metasomatism of spinel lherzolite to olivine wehrlite. This process also causes Na, K, P, and LREE metasomatism of the lithosphere at ≤ 60 kms depth. This is additional to and differs mineralogically and chemically from Ti, Fe, Al metasomatism by crystallization of hydrous basaltic melts as they migrate across the amphibole-out boundary, i.e. metasomatism at up to 80-90 kms depth. These processes are illustrated in peridotite nodules from the Tumut-Eucumbene Tunnel, N.S.W. This suite is distinctive as it contains amphibole, dolomite and magnesite. Mineral textures suggest that this suite records amphibole related Fe-Ti-Al metasomatism and the trapping of a carbonate-rich mel