Differentiation of tholeiites - the role of primitive composition and small amounts of H2O
Final Report Abstract
Tholeiitic basalt is the predominant type of basaltic magma on Earth and geochemical studies demonstrate the existence of a large variety of Mg-rich parental compositions. As a result of this diversity in primitive magmas, different possible crystallization trends can be expected in the range of thermodynamic conditions (P-T, fO2, volatile activities) which still need to be addressed by crystallization experiments. The problem is that most of such experiments in literature have been carried out at 1 atm, conditions which are not consistent with a presence of magma chambers at depths. High pressure experiments at 50-700 MPa are scarce and the residual liquid compositions do not match with natural evolved basaltic glasses (especially for FeO, SiO2). In this project, new crystallization experiments have been conducted using a spectrum of possible end-member mid-ocean ridge, ocean island and fore-arc tholeiitic basalt compositions to constrain the role of compositional variations of primitive magmas on differentiation paths and liquid lines of descent. Crystallization experiments have been performed in an internally heated pressure vessel at 50, 100, 200 and 300 MPa under dry and hydrous (<1 wt.% H2O) conditions as well as at 1 atm. in a gas-mixture furnace. For most of our high-pressure experiments the residual experimental liquid compositions can reproduce the compositions of evolved basalts observed in nature. Our experiments allow us to quantify the role of pressure and small water content of the melt on the geochemical evolution of basaltic melts with ongoing crystallization (liquid lines of descent). However, two studied mid-ocean ridge basalt compositions produced evolved liquids which deviate significantly from natural compositions, indicating that such basalts are unusual and not typical of mantle melts. Depending on the tholeiitic compositions used as starting material for the experiments, the crystal/melt ratio at given pressure, temperature, water activity may differ significantly, even if the compositions have the same liquidus phase. One important implication is that the compositional characteristics of depleted basalts inherited from the mantle may be overprinted by the reaction with other magmas en route to the surface. Enriched melts are more likely to survive the process of moving through the Earth's crust than depleted melts. As a result, the basalt chemistry is more influenced by melts from these enriched sources than from the lherzolitic upper mantle.
Publications
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Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources. Earth and Planetary Science Letters, 520, 199-211.
Neave, David A.; Namur, Olivier; Shorttle, Oliver & Holtz, François
