Estudo experimental de magmas alcalinos subsaturados em s?-lica por meio do sistema Kalsilita-Nefelina-Diops?-dio-S?-lica a 4.0 GPa e condi?§?μes anidras

摘要

This study experimentally investigates the Kalsilite-Nepheline-Diopside-Silica system at high pressure and temperature,??with emphasis on silica-undersaturated volume (leucite-nepheline-diopside a?? Lct-Nph-Di; and kalsilite-nepheline-diopside??a?? Kls + Nph + Di a?? planes), at 4.0 GPa (~120 km deep), temperatures up to 1,400?oC and dry conditions, to better??understand the influence of K 2 O, Na 2 O, and CaO in alkali-rich silica-undersaturated magma genesis. In the Lct-Nph-Di??plane, we determined the stability fields for kalsilite (Kls ss ), nepheline (Nph ss ) and clinopyroxene (Cpx ss ) solid solutions,??wollastonite (Wo) and sanidine (Sa); and three piercing points (i) pseudo-eutectic Kls + Nph + Di + liquid (Lct 62 Nph 29 Di 9 )??at 1,000?oC; (ii) Kls + Sa + (Di + Wo) + liquid (Lct 75 Nph 22 Di 2 ) at 1,200?oC; and (iii) pseudo-eutectic Kls + Di + Wo + liquid??(Lct 74 Nph 17 Di 9 ) at 1,000?oC. Kalsilite stability field represents a thermal barrier between ultrapotassic/potassic vs. sodic compositions.??In the plane Kls-Nph-Di, we determined the stability fields for Kls ss , Nph ss and Cpx ss and two aluminous phases??in smaller proportions spinel (Spl) and corundum (Crn). This plane has a piercing point in Kls + Nph + Di(?± Spl) + liquid??(Kls 47 Nph 43 Di 10 ) at 1,100?oC. Our data showed that pressure extends K dissolution in Nph (up to 39 mol%) and Na in Kls (up??to 27 mol%), and that these solid solutions, if present, determinate how much enriched in K and Na an alkaline magma will??be in an alkaline-enriched metasomatic mantle. Additionally, we noted positive correlation between K 2 O and SiO 2 concentration??in experimental melts, negative correlation between CaO and SiO 2 , and no evident correlation between Na 2 O and SiO 2 .??