Experimental constraints on phlogopite chemistry in lamproites: 2. Effect of pressure-temperature variations
Foley, Stephen F.
European Journal of Mineralogy Volume 2 Number 3 (1990), p. 327 - 342
published: Jun 21, 1990
manuscript accepted: Jan 16, 1990
manuscript received: Jul 5, 1989
ArtNo. ESP147050203013, Price: 29.00 €
Abstract The chemistry of micas from high-pressure (5-40 kbar) experiments on leucite lamproite and olivine lamproite show much less variation as a function of pressure and temperature at fixed fluid composition (H2O > CH4 ; results reported here) in comparison to the variation due to fluid composition (H2O/CH4 and H2O/CO2 ; results given in Part 1). Micas at high pressures tend to have higher Al2O3, and lower TiO2, K/Al and Cr2O3. Decreasing temperature produces decreasing Mg/(Mg+Fe) and Al2O3, and increasing SiO2, K2O and F in micas, but these trends may be a function of the degree of crystallization and may differ in natural lamproites where fractional crystallization occurs. Mica has higher Ti02 where it coexists with titanate minerals. Since lamproites are thought to oxidize considerably during emplacement, the difference in composition between micas crystallized in these experiments and those crystallized under lower-pressure, more oxidizing conditions shows that most lamproite mica phenocrysts are not typical of high-pressure crystallization and can therefore not be used to model high-pressure fractionation. The strong decrease in Al2O3 contents during differentiation of natural lamproites appears to be due to fractionation of mica in water-rich conditions, in which micas have very high Al contents. High-TiO2 mica phenocrysts must have been crystallized under low-pressure conditions at high temperatures. Natural phlogopites may contain appreciable Ti3+ if crystallized at fO2 ≈ iron-wiistite, but not at 2 log units fO2 higher. Strongly olivine-normative lamproitic melts are likely agents of mantle enrichment in the sub-continental mantle at depths of 100-200 km. The sub-liquidus experimental mica compositions can be used to constrain models of vein chemistry in inhomogeneous mantle compositions.