Basaltic andesite is a common magma type in a wide variety of tectonic settings. Basaltic andesite is conventionally described as a magma with 52 to 57 wt% SiO2 and ≤5 wt% total alkalis. However, this definition masks significant variation in other components, notably MgO, FeOt, Al2O3 and CaO. This variability likely reflects different source conditions of in terms of pressure (P), temperature (T), redox (fO2), and magmatic volatile (H2O, CO2) content. These different source conditions can be evaluated using experimental petrology.
The experimental constraints on source conditions involve determination of multiple saturation points (MSP) at the liquidus of a given melt composition (Asimow and Longhi, 2004; Blundy, 2022). MSP constrains the point of last equilibrium of an erupted magma with its crystal-rich source region. Once a melt detaches from its source and ascends to the surface, changes in P-T conditions lead to crystallisation and degassing that modify the composition of the liquid phase but not the magma's bulk composition.
Equilibrium piston-cylinder experiments were carried out on three basaltic andesites: high-MgO basaltic andesite from Kluchevskoy, Kamchatka; low-MgO, subalkaline basaltic andesite from Hekla, Iceland; and low-MgO basaltic andesite from La Soufriere St Vincent, Lesser Antilles.
Experiments show that high-MgO basaltic andesite is a mantle lherzolite-derived magma (~30km, 1220 ºC, DNNO+1) with H2Oinitial of ~3.5 wt%. Subalkaline basaltic andesite with H2Oinitial of ~ 4 wt% last equilibrated with apatite-bearing gabbro at ~15 km, 1000ºC and DNNO-1.5; and low-MgO basaltic andesite with H2Oinitial of ~ 8 wt% was last equilibrated with amphibole gabbro at ~18 km, 1020 °C, DNNO+2. In all three cases, source temperatures agree with eruption temperatures.
The obtained P-T-H2O conditions agree well with geophysical constraints, and chemistry and mineralogy of erupted magmatic products and illustrated different source conditions for these magmas.
| Subject : | : | Oral |
| Topics | : | Open session |
| Topics | : | Magmatic Petrology |
| Keywords | : | basaltic andesite ; multiple saturation point ; mantle derived magma ; crustal derived magma ; lherzolite ; gabbro |
| PDF version | : |  PDF version |
