Petrogenesis of basanite-phonolite series of an oceanic intraplate volcano: combining experimental data and field observations
Zusammenfassung der Projektergebnisse
We carried out an experimental investigation of the basanite-phonolite magma series of Cumbre Vieja volcano on La Palma, Canary Islands. The main objective of the study was to constrain the conditions under which phonolitic melts can form from more primitive basanite and tephrite compositions. This goal was addressed by two experimental series. First, a set of experiments was conducted to determine the H2O+CO2 solubilities in basanite and tephrite melts, which is needed to determine the redox conditions in experimental charges with variable H2O activities. Second, a number of classical phase equilibrium crystallization experiments was conducted to constrain the effect of H2O on stability fields of the crystallizing minerals at fixed pressures. These experiments involved four different starting compositions (primitive basanite, evolved basanite, tephrite, tephriphonolite) investigated at pressures of 700, 400 and 200 MPa. In total, nearly 370 successful individual experiments were carried out. Most experimental melts produced in the course of the project have compositions overlapping with those of natural rocks from Cumbre Vieja volcano. One major result of our study is that phonolitic melts cannot form by closed-system (equilibrium) crystallization from a basanitic parental melt within the uppermost mantle beneath La Palma (>450 MPa), which is the depth where Cumbre Vieja basanites and tephrites are thought to be stored before eruption. Our data suggest that phonolites can form by closed-system crystal fractionation at the following conditions: in the uppermost mantle (around 700 MPa) from a tephriphonolitic parent; in the lowermost crust (around 400 MPa) from a tephritic to tephriphonolitic parent; and in the crust (around 200 MPa) from an evolved basanitic to tephriphonolitic parent. The experimental results are in good agreement with our barometric data on natural Cumbre Vieja phonolites and tephriphonolites indicating pressures of 260-350 MPa. In any case, differentiation of basanite to phonolite requires effective separation of a large amount of crystals from the evolving melt (open-system crystallization), or closed-system crystallization of evolved basanite within the shallower crust. Another major result of our study is that residual melts produced by equilibrium crystallization extend to more evolved compositions as the pressure decreases. Our data strongly support a phase equilibria control of the natural petrochemical trend at Cumbre Vieja and other localities. They suggest a genetic link between mantle-derived primitive basanite, evolved basanite, tephrite and phonolite via crystallization under hydrous conditions in the interval of pressures from 700 to 400 MPa and probably to 200 MPa. We conclude that the main reason for phonolites to form at a particular volcano is the development of a magma plumbing system in which crystals can effectively be separated from evolving melts, and melts can be stored at crustal levels (200-400 MPa). This depends on the longevity of volcanism, on the magma flux rate, and on the thermal condition of the lithosphere. If the magmatic plumbing system beneath a particular volcano does not permit these conditions, formation and eruption of phonolites are not likely. Factors such as major element composition, H2O contents, or oxygen fugacity seem to have subordinate influence.
