DEPTHS OF BASALTIC MAGMA CRYSTALLIZATION

The ratios Ca/Al in clinopyroxenes and rocks (melts) are indicators of crystallization depth and evolution of tholeiitic basalt melt. Analysis of experimental data for clinopyroxenes shows regular decrease of Ca content and increase of Al content with increasing pressure. This causes systematic decrease of the ratio Ca'= 100CaO/Al2O3 from 400 - 500 or more under pressure 0 - 5 kbar to 100 or less within the interval 8 - 30 kbar. It makes it possible to estimate the depth of early stages of magma crystallization beginning if the mineral phenocrysts are present in the rocks (melts). For example, Horhona kamatiites crystallization started at the depth of 45 km and was in progress at the different levels of Earth crust. The main crystallization of basaltic melts corresponds to Ca' in clinopyroxenes > 400 - 500 and pressures no more than 5 kbar. As compared to Ca' in clonopyroxenes and analogous parameter for basalts (for glasses and rocks as a whole) describes pressure dependence of magma evolution. Natural melts form trends in terms of different compositions and proportions of olivine, plagioclase, and pyroxene. Comparison of these trends with the barometric diagram CaO/Al2O3 - FeO/MgO shows their evolution took place under pressures less-than-or-equal-to 8 kbar in the presence of intermediate chambers forming close but different in composition melt series.