Source depth of basaltic andesite magma beneath La Soufrière, St Vincent.

Many volcanoes erupt a limited compositional range of magmas over their lifetime. The composition of these erupted magmas is thought to be buffered by the crystal-rich mush from which melts are sourced (Blundy, 2022). Identifying the origin of erupted magmas helps us to constrain possible locations of melt accumulation in the mush and improve our interpretation of geophysical signals at restless volcanoes.

La Soufrière volcano, St Vincent (Eastern Caribbean), has throughout its lifetime produced predominantly basaltic andesite magmas, including most recently in 2020-21. We explore the origin and phase relations of these erupted magmas by performing a series of high-pressure, high-temperature experiments. Melt extracted from a mush will be multiply-saturated on its liquidus with the mush mineral assemblage at the P-T-fO₂-XH₂O conditions at the time of segregation. In a system with relatively low thermodynamic variance, for example, five or six independent chemical components (as determined by principal component analysis), a large number of coexisting mineral phases (e.g. plag+cpx+amph+oxides) and a well-constrained fO₂, multiple saturation can be reduced to an invariant point on the liquidus of the melt in P-T-H₂O space. The approach of finding liquidus multiple saturation for igneous rocks offers a novel magma source thermobarometer and hygrometer.

Equilibrium high-pressure, high-temperature experiments were performed at 3-8 kbar and 980-1200ºC, fO₂ ≈ Ni-NiO buffer, with initial H₂O contents of 2-10 wt%. Plagioclase, clinopyroxene and magnetite are found to be ubiquitous in the melt source region. Amphibole is a peritectic phase and forms on the rim of clinopyroxene, with decreasing temperature, in experiments with high water contents. The peritectic reaction involving amphibole is also observed in St Vincent xenoliths (Brown, 2023). Orthopyroxene is stable at high pressures (8 kbar) and low water contents (≤ 6wt% H₂O), with its stability field decreasing with pressure. Five-phase multiple saturation at the liquidus (melt fraction ≥85%) is found for initial H₂O contents of 7-8 wt%, at 6 kbar pressure (~22 km depth) and temperatures of 1030-1050ºC. The saturating assemblage is a hornblende-gabbro (cpx+plag+amph+Fe-Ti oxides), consistent with the mineralogy of plutonic xenoliths from historic eruptions of St Vincent (Tollan et al., 2012; Fedele et al., 2021). Mineral compositions in these multiply-saturated runs (e.g. very calcic plagioclase An_75-85) are similar to those in the xenoliths. Temperatures agree with mineral geothermometry estimates of the 2020-21 eruption (Weber et al., 2023), suggesting little cooling of the magma during ascent from its source region. Seismicity prior to the 2020-21 eruption is also consistent with mid-crustal source depths (Joseph et al., 2022). Magmas sourced from similar depths can account for the limited compositional diversity of La Soufrière over its volcanic history (Fedele et al., 2021).

 

References

Brown, JR. (2023) Doctoral dissertation, Durham University. 

Blundy, J. (2022) Journal of Petrology, 63(7), egac054.

Fedele et al. (2021) Lithos, 392, p.106150

Joseph et al. (2022) Nature Communications, 13(1), p.4129.

Tollan et al. (2012) Contributions to Mineralogy and Petrology, 163, pp.189-208.

Weber et al. (2023) Geological Society, London, Special Publications, 539(1), pp.SP539-2022.