Storage conditions and magma processes triggering the 1818Â CE Plinian eruption of Volcán de Colima

- The 1818 and 1913 andesitic Plinian pumice of Colima contain Pl > Opx > Cpx ≫ Amph + Fe-Ti oxides + Ap ± resorbed Ol.
- Pumice contains three groups of amphibole with different abundances, textures and compositions (variable Aliv contents).
- Two amphiboles groups (low and medium Aliv) were reproduced at 890 °C and 150-200 MPa.
- High Aliv amphiboles were not reproduced and are xenocrystic injected during mixing events at pressures > 200 MPa.

Volcán de Colima has an eruptive history punctuated with explosive Plinian eruptions followed by long periods of effusive events. Both of the best documented Plinian eruptions, which occurred in 1818 CE and 1913 CE, emitted andesitic magma (~ 58 wt% SiO2) that contains Pl > Opx > Cpx ≫ Amph + Fe-Ti oxides + Ap ± resorbed Ol, whereas effusive magmas from the 1819-1868 and 1962-2015 periods are particularly scarce in amphibole, and are slightly more silicic (59-61 wt% SiO2). Pumice from the 1818 and 1913 deposits contain three groups of amphibole with different abundance, texture and composition. Composition is the most distinctive between them; the first group consists of crystals with high AlIV (1.84-2.32), the second group of amphiboles is less abundant in AlIV (1.45-1.76) and the third has low AlIV (1.29-1.39). In contrast, amphibole from two Colima prehistoric lavas can only be grouped as medium AlIV (1.45-1.76).A set of hydrothermal experiments were carried out using a 1818 natural sample to investigate amphibole compositional variability with changes in pressure and temperature. Two groups of amphibole found in the Plinian deposits can be reproduced experimentally between 875 and 900 °C and 150-200 MPa. Another type of amphibole could not be equilibrated in the 1818 explosive Colima magma and their origin can be attributed to mixing with more mafic magmas. We found that amphibole abundance is greater in explosive events and that mafic inherited amphiboles can only be found in Colima Plinian deposits. When the influx of mafic magma is large or frequent enough, and they mix with more silicic Colima magma, then explosive events are more likely. If mafic melts cannot mix with the resident magmas or just pond and heat the reservoir, regardless of location in the plumbing system, then magmas can decompress more slowly and amphibole will have time to completely react out.