Melt density and the efficiency of fractional crystallization for the generation of phonolite magmas

The natural Plateau phonolites from the Kenya rift exceed the total combined volume of phonolite lava found elsewhere in the world by several orders of magnitude [1]. These alkali-rich magmas are thought to form as fractionation products from low-silica, primary compositions such as nephelinite and melililite [2] or are generated directly within the mantle by melting or fractionation [3]. Although the density contrast between melt and crystal has an important effect on crystal fractionation and magma differentiation rates, density data for alkaline magmas typically found in continental rift zone setting is not available, precluding the assessment of the efficiency of crystal fractionation processes as a mechanism to generate phonolitic melts. We present in-situ density measurements of phonolitic melts at crustal and upper mantle conditions (1.0-3.1 GPa, 1585-1855 K) using synchrotron X-ray absorption in a Paris-Edinburgh press [4]. The starting material is a synthetic haplo-phonolite glass based on the Plateau flood phonolites from the Kenya rift [5]. Single-crystal diamond cylinders were used as sample containers and the density was determined as a function of pressure and temperature from the X-ray absorption contrast between the sample and the diamond capsule. The results were combined with available density data at room conditions to derive the first experimental equation of state (EoS) of phonolitic liquids at crustal and upper mantle conditions. The equation of state is calibrated up to 3.1 GPa, but can be reliably extrapolated to higher pressures; melt densities range from 2.45 g/cm3 up to 2.62 g/cm3 for a depth of 30 to 60 km, respectively - A comparison of our results with literature data shows that the compressibility of silicate melts decreases with increasing silica content in a similar manner observed for silicate glasses: phonolitic melts are more compressible than peridotitic and basaltic melts. The low compressibility of phonolites causes small density differences between crystals (e.g. nepheline) and melt, resulting in slow crystal settling velocities, leading to slower crystal fractionation processes. The data are used to discuss the efficiency of crystal fractionation as a mechanism to generate phonolite melts as those found in continental rift zones. [1] Lippard, S. J., 1973, Lithos, 6, 217-234. [2] Edgar, A. D., 1987, Geological society, Spec. Pub. No. 30, 29-52. [3] Price, R. S., Green D. H., 1972, Nature Physical Science, 235, 133-134. [4] van Kan Parker M. et al., 2010, High Pressure Research, 30: 2, 332 - 341. [5] Hay D.E., Wendlandt R.F., 1995, J.Geophys. Res. 100, 401-410.