PHASE-RELATIONS AND CHEMISTRY OF TI-RICH K-RICHTERITE-BEARING MANTLE ASSEMBLAGES - AN EXPERIMENTAL-STUDY TO 8.0 GPA IN A TI-KNCMASH SYSTEM

Experiments have been conducted in a peralkaline Ti-KNCMASH system rep resentative of MARID-type bulk compositions to delimit the stability f ield of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a f unction of P and T, and to characterise the composition of partial mel ts derived from the hydrous assemblage. K-richterite is stable in expe riments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200-1250 degrees C at 4.0 GPa and 1300-1400 degrees C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a syste matic increase in K with increasing P to 1.03 pfu (per formula unit) a t 8.0 GPa/1100 degrees C. In the absence of phlogopite, K-richterite a ttains a maximum of 1.14 K pfu at 8.0 GPa/1200 degrees C. Titanium in both amphibole and mica decreases continuously towards high P with a n early constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa Sigma Si + Al in K-ric hterite is less than 8.0 when normalised to 23 oxygens + stoichiometri c OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al a nd Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-K NCMASH system are extremely peralkaline [(K2O+Na2O)/Al2O3 = 1.7-3.7], Si-poor (40-45 wt% SiO2), and Ti-rich (5.6-9.2 wt% TiO2) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first m elt is saturated in a phlogopite-rutile-lherzolite assemblage. Both ph logopite and rutile disappear ca. 150 degrees C above the solidus. At 8.0 GPa, the solidus must be located at T less than or equal to 1400 d egrees C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does n ot buffer the melt composition at 8.0 GPa. The experimental results su ggest that partial melting of MARID-type assemblages at pressures grea ter than or equal to 4.0 GPa can generate Si-poor and partly ultrapota ssic melts similar in composition to that of olivine lamproites.