Experimental constraints on evolution of leucite-basanite magma at 1 and 10-4GPa: implications for parental compositions of Roman high-potassium magmas

Abstract

The separate effects of pressure (10 −4 and 1.0 GPa), water, CO 2 , oxygen fugacity and calcium doping on the liquid line of descent of a primitive leucite-basanite magma (SiO 2 = 47.06 wt%, MgO = 12.76 wt% and Mg# = 75.1) from the Montefiascone Volcanic Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350–1160 °C temperature range. Results indicate that low-pressure liquidus temperatures are ≤1280 °C and that the high-pressure T liquidus is 1350 °C under anhydrous conditions; the latter is lowered to ~ 1275 °C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable f O 2 values, high and low pressure runs produced the same phase assemblage (spinel + olivine + clinopyroxene) up to 50 % crystallization, although olivine was partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the composition of residual liquids: the former increases SiO 2 content, whereas the latter induces the desilication of melts. The replacement of olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents. Comparison of the natural and experimental melts shows that many of major and trace element variations exhibited by high-K primitive ( i.e. , high Mg/Mg + Fe) magmas at Montefiascone, are consistent with their derivation from a single parental leucite-basanite melt by fractional crystallization of different proportions of mineral phases, plus carbonate assimilation. The changes in phases stability and melt composition caused by carbonate assimilation may also have fundamental implications for the origin of the calcic high-magnesium leucitites and melilitites. In particular, the complex metasomatic interactions that can develop at the interface between potassic magmas and carbonate wall rocks, may lead to melting of calcite. This low-viscosity melt readily mixes with the surrounding magma inducing the crystallization of Ca-Tschermak-rich pyroxene and hercynitic spinel, affecting significantly the SiO 2 , CaO and alumina composition of the resulting hybrid melt. A key finding of our study is that magmas such as the studied leucite-basanite may be considered parental to the wide spectrum of mafic high-K compositions in the Roman Province, which have been traditionally considered as representing near primary magmas reflecting distinct mantle source compositions and/or processes.