Phase Relations and Liquid Lines of Descent in Hydrous Ferrobasalt—Implications for the Skaergaard Intrusion and Columbia River Flood Basalts

Crystallization experiments using a hydrous ferrobasalt as starting material, conducted at 200 MPa, 940–1200°C, at a wide range of water activities (0·1–1) and redox conditions (QFM − 3 to QFM + 4, where QFM is the quartz–fayalite–magnetite oxygen buffer), show that H2O influences significantly the differentiation history of ferrobasaltic magmas. A combination of our data with published experiments on dry ferrobasalt at 1 atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical Fe-rich tholeiitic system under both dry and hydrous conditions. The addition of H2O decreases liquidus temperatures and changes significantly the proportions, temperature range and sequence of crystallizing mineral phases. The dry liquidus is at about 1170°C whereas the liquidus for H2O-saturated melts is at ∼1060°C. The main phases crystallizing from H2O-bearing ferrobasalt at the investigated conditions are olivine (OL), clinopyroxene (CPX), plagioclase (PL), magnetite (MT), hematite (HM), ilmenite (ILM) and amphibole (AM). The phase assemblage is similar to that of the dry system except for the presence of HM at extremely oxidizing conditions and AM at low temperatures (< 950°C) and H2O-saturated conditions. The important observation made in this study is that the stability of Fe–Ti-oxides, and in particular MT, as well as the simultaneous coprecipitation of MT and ILM, are almost independent of the activity of H2O (aH2O) in the system, whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing aH2O. The stabilities of oxides are controlled mainly by the redox conditions prevailing in the system. The most pronounced effect of aH2O on the liquidus temperatures of silicates is observed for PL, which shows a considerable delay in crystallization with progressive magma differentiation. Early crystallization of Fe–Ti-oxides in H2O-bearing ferrobasaltic compositions precludes any significant Fe enrichment during differentiation. As Fe enrichment is a characteristic feature of the Skaergaard intrusion, it implies that the Skaergaard parental magma did not contain considerable amounts of water. On the other hand, our experiments indicate that the differentiation of some ferrobasaltic series from the Columbia River flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles (∼0·5–3 wt % H2O).