Humphreys, Madeleine C. S

Magma reservoirs are thought to grow relatively slowly, assembling incrementally under volatile-saturated conditions. Eruptions may be triggered by injections of volatile-rich melt, or generation of over-pressure due to protracted crystallization. Here, we analyse fluorine, chlorine and water in apatite crystals trapped at di erent stages of magma evolution, and in melt inclusions from clinopyroxene and biotite crystals expelled during an explosive eruption of the Campi Flegrei caldera, Italy, about 4,000 years ago. We combine our geochemical analyses with thermodynamic modelling to reconstruct the evolution of magmatic volatile contents leading up to the explosive eruption. We find that the magma reservoir remained persistently water-undersaturated throughout most of its lifetime. Even crystals in contact with the melt shortly before eruption show that the magma was volatile-undersaturated. Our models suggest that the melt reached volatile saturation at low temperatures, just before eruption.We suggest that late-stage volatile saturation probably triggered the eruption, and conclude that ‘priming’ of the magma system for eruption may occur on timescales much shorter than the decadal to centennial timescales thought typical for magma reservoir assembly. Thus, surface deformation pulses that record magma assembly at depth beneath Campi Flegrei and other similar magmatic systems may not be immediately followed by an eruption; and explosive eruptions may begin with little warning.

Volatile elements play an important role in many aspects of the physicochemical architecture of sub-volcanic plumbing systems, from the liquid line of descent to the dynamics of magma storage and eruption. However, it remains difficult to constrain the behaviour of magmatic volatiles on short timescales before eruption using established petrological techniques (e.g. melt inclusions); specifically, in the final days to months of magma storage. This study presents a detailed model of pre-eruptive volatile behaviour in the Campi Flegrei system (Italy), through combined analyses of apatite crystals and glass. The deposits of eight eruptions were examined, covering the full spectrum of melt compositions, eruptive styles and periods of activity at Campi Flegrei in the past 15 kyr. Measured apatite compositions are compared with thermodynamic models that predict the evolution of the crystal compositions during different fractional crystallization scenarios, including (1) volatile-undersaturated conditions, (2) H2O-saturated conditions and (3) varying P–T conditions. The compositions of clinopyroxene-hosted and biotite-hosted apatite inclusions are consistent with crystallization under volatile-undersaturated conditions that persisted until late in magmatic evolution. Apatite microphenocrysts show significantly more compositional diversity, interpreted to reflect a mixed cargo of crystals derived from volatile-undersaturated melts at depth and melts that have undergone cooling and degassing in discrete shallow-crustal magma bodies. Apatite microphenocrysts from lavas show some re-equilibration during cooling at the surface. Clinopyroxene-hosted melt inclusions within the samples typically contain 2–4 wt % H2O, indicating that they have been reset during temporary magma storage at 1–3km depth, similar to the depth of sill emplacement during recent seismic crises at Campi Flegrei. Comparable apatite compositional trends are identified in each explosive eruption analysed, regardless of volume, composition or eruption timing. However, apatites from the different epochs of activity appear to indicate subtle changes in the H2O content of the parental melt feeding the Campi Flegrei system over time. This study demonstrates the potential utility of integrated apatite and glass analysis for investigating pre-eruptive volatile behaviour in apatite-bearing magmas

We use volatiles in melt inclusions and nominally anhydrous phenocrysts, with volcanic gas flux and composition, and textural analysis of mafic inclusions to estimate the mass of exsolved vapour prior to eruption at Soufrière Hills Volcano (SHV). Pre-eruptive andesite coexists with exsolved vapour comprising 1.6–2.4 wt% of the bulk magma. The water content of orthopyroxenes indicates a zone of magma storage at pressures of approximately 200–300 MPa, whereas melt inclusions have equilibrated at shallower pressures. Inclusions containing >3 wt% H2O are enriched in CO2, suggesting flushing with CO2-rich gases. Intruding mafic magma contains >8 wt% H2O at 200–300 MPa. Rapid quenching is accompanied by crystallization and vesiculation. Upon entrainment into the andesite, mafic inclusions may undergo disaggregation, where expansion of volatiles in the interior overcomes the strength of the crystal frameworks, thereby recharging the vapour content of the andesite. Exsolved vapour may amount to 4.3–8.2 vol% at 300 MPa, with implications for eruption longevity and volume; we estimate the magma reservoir volume to be 60–200 km3. Exsolved vapour may account for the small volume change at depth during eruptions from geodetic models, and has implications for magma flow: exsolution is likely to be in equilibrium during rapid magma ascent, with little nucleation of new bubbles.