Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980-1986

We introduce a novel scheme that enables natural silicic glasses to be proj ected into the synthetic system Qz-Ab-Or-H2O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) co nditions in the sub-volcanic magma system. By this means an important disti nction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimat es of the conditions of last equilibrium between crystals and melt can be m ade. Formation of highly silicic melts (i.e. >77 wt% SiO2) is a simple cons equence of the contraction of the silica phase volume with decreasing press ure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO2 (68-80 wt%, anhydrous). The compositions of the least evolved (SiO2-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with less than or equal to 5.4 wt% water, corresponding to a water saturation pressure of similar to 200 MPa at 900 degreesC. The c ompositions of more evolved (higher SiO2) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and mic rolites to low pressures. During this polybaric crystallisation, plagioclas e phenocrysts trapped melts with a wide range of dissolved water contents ( 3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 er uption were derived from a large reservoir at depths of greater than or equ al to6 km. Subsequent magmas ascended to varying depths within the sub-volc anic system prior to extraction. From glass chemistry and groundmass textur e two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, trid ymite and quartz, testifying to temperatures of at least 885 degreesC at 11 MPa. These shallow storage conditions are comparable to those in the crypt odome formed during spring 1980. The corresponding thermal gradient, less t han or equal to 0.2 degreesC MPa-1, is consistent with near-adiabatic magma ascent from similar to8 km. We argue that the crystallisation history of M ount St Helens dacite magma was largely a consequence of decompression crys tallisation of hot magma beyond the point of water saturation. This challen ges the conventional view that phenocryst crystallisation occurred by cooli ng in a large magma chamber prior to the 1980-1986 eruption. Because the cr ystallisation process is both polybaric and fractional, it cannot be simula ted directly using isobaric equilibrium crystallisation experiments. Howeve r, calculation of the phase proportions in water-saturated 910+/-15 degrees C experiments by Rutherford et al. (1985) over the pressure range 220-125 M Pa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, an d small temperature differences between nature and experiment, phase compos itions can also be matched to the natural samples. We conclude that decompr ession of water-saturated magma may be the dominant driving force for cryst allisation at many other silicic volcanic centres.