Differentiation of tholeiites - the role of primitive composition and small amounts of H2O

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Toleiitic basalt is the predominant type of basaltic magma in mid-ocean ridges (MORB - mid-ocean ridge basalt), in plume-related oceanic plateaus (Large Igneous Provinces, LIPs) and in a number of ocean islands (e.g. Hawaii) and seamounts. The geochemical studies of oceanic magmatic rocks demonstrate the existence of large variety of Mg-rich glass compositions, representative of a variety of mantle-derived primitive magmas. As a result of the diversity of primitive magmas, due to different mantle mineralogy and degree of mantle melting as well as different possible crystallization conditions (P, fO2, volatile activities), and mechanisms of differentiation (crystallization, magma mixing, assimilation etc.), there is a wide compositional range of evolved basalts which have been collected in the oceanic crust. In contrast to the large experimental dataset at very high pressure (> 1 GPa) and at 1 atmosphere and dry conditions, the geologically relevant conditions (pressures from 50 to700 MPa, fluid-undersaturated systems) prevailing during the magma differentiation and the transport from their source to ocean floor have been still poorly investigated. The pressure range of 50-700 MPa is particularly important since it covers the range between shallow axial magma chambers in fast-spreading ridges and deep reservoirs in the mantle below slow-spreading ridges or at the mantle-crust transition for LIPs. As a consequence, the available thermodynamic models predicting differentiation paths are calibrated for a restricted range of conditions and cannot account for the whole variety of primitive melt compositions. Most of them also fail in predicting accurately the role of water on fractionation processes. In this project, crystallization experiments are planned to constrain liquid lines of descent for six selected compositions, which cover the large compositional spectrum of primitive end-member tholeiites (e.g. MORBs derived from refractory and fertile mantle, N-MORB, E-MORB, high vs. low SiO2). The focus will be given to the role of pressure (50, 200, 500 and 700 MPa) and of melt water content (0.04 to 1 wt% H2O) on the composition of cotectic melts saturated with Olivine+Clinopyroxene+Plagioclase at different conditions (T,P,H2O). The experimental database will be used to understand the role of compositional differences in primitive tholeiites (along with thermodynamic variables) on the position of the Ol+Cpx+Pl cotectics projected on pseudoternary diagrams, a method which is widely used in geobarometric simulations. Apart from general implications, each investigated natural composition can be used to interpret the differentiation conditions at the corresponding MORB settings (e.g. Indian Ridge, Costa Rica Rift, East Pacific Rise) using available geochemical data already published.