H2O behavior in an iron-rich melt: application to the martian basaltic melts

Martian basalts are different from Earth basalts by their high FeO_tot content of about 20 wt % in average, compared to only 10 wt % for typical Earth's basalts. The observations of Mars' surface suggests the presence of an ancient ocean maintained by a dense atmosphere rich in volatile species. Based on the analyses of martian meteorites, the martian mantle is thought to be strongly depleted with respect to H₂O (hundreds of ppm) and CO₂. However, the volatile budget of the martian mantle is still debated. The lack of data prevent us to establish degassing processes of basaltic magmas contributing to the formation of the primitive atmosphere of Mars. Currently, there is no H₂O solubility data for a martian basaltic melt equilibrated at high pressures and temperatures.

We have conducted high pressure (0.5 - 1.5 GPa) and high temperature (>1550 °C) experiments on a simulated martian basalt-like composition (15.5 wt % FeO_tot). These experiments were in equilibrium with an H₂O fluid phase. Only a maximum at 3 wt % H₂O initially added enable us to obtain a glass, above this content, a full crystallization of the samples is observed. H₂O content was measured by Raman spectroscopy and IR spectroscopy. The results reveal that H₂O dissolution is extremely low in a Fe-rich melt. At 1.5 GPa, the H₂O dissolution is barely reaching 1 wt %. H₂O dissolution in martian basalts is well below solubility models established on terrestrial basalts. FeO content seems to affect the silicate structure of the melt, implying a different Raman signature for martian basalts compared to Earth's basalts.

Our results suggest that the FeO content appears to have a strong control onto the H₂O behavior in a martian basaltic melt by decreasing drastically its dissolution. It implies that a Fe-rich melt is not a good carrier of H₂O from the mantle to the surface.