Search

Numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods, including analyses of crystal size distributions in natural samples and decompression experiments on hydrous magmas. Experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation, which has typically been the focus of decompression-induced crystallization studies. For this study, samples of eruptive clasts were collected from a Vulcanian explosion that occurred following the July 12-13, 2003 dome collapse of Soufrière Hills volcano, Montserrat. Plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry (SIMS) depth profiling. Lithium inflection depths within the profiles, along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples, were used to calculate the growth rates as a result of magma devolatilization, with an average of 8.3 x 10-8 mm/s. Anorthite content of the plagioclase rim growth indicates an average decompression magnitude of 40 MPa, inducing an undercooling of ~45 °C that favors crystal growth over microlite nucleation. However, variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions. In such events occurring over short timescales (<10 h), lithium is a more reliable indicator of decompression-induced growth than changing anorthite content due to lithium's rapid diffusivity. [ABSTRACT FROM AUTHOR]

In the petrologic and geochemical study of volcanic rocks, microlites have become an important tool with which to determine magma decompression rate and ascent path prior to eruption. However, studies in experimental petrology and kinetic modeling indicate that growth of pre-existing phenocrysts will occur over a much wider range of decompression-induced undercoolings than microlite nucleation. Consequently, we have developed a method using secondary ion mass spectrometry (SIMS) depth profiling to measure geochemical trends recorded during the final stage of phenocryst growth. To test and demonstrate the new method, we examined explosive and effusive eruptive products collected from Soufrière Hills volcano, Montserrat. Plagioclase feldspar crystals were removed from clasts of pyroclastic deposits. Phenocrysts were individually selected on the basis of euhedral morphology and a relatively homogeneous distribution of surface contamination, such as volcanic glass. Crystals were cleaned, embedded in In, and analyzed by SIMS in depth-profiling mode. We used an O+2 primary ion beam, which provides a faster sputtering rate than the typically utilized O- primary beam. A normal-incidence electron gun is used for charge compensation. Ten isotopes were examined over 2-10 h periods and the resulting crater depth was determined. Data show variable trends in An, K, and Fe that we interpret to represent late-stage physical variations within the magma prior to eruption. The continuous chronology of geochemical data obtained from the SIMS analyses presented here provide information with unprecedented (~0.1 µm) resolution, allowing researchers to determine physical conduit conditions during magma ascent from the chamber to eruption at the surface. [ABSTRACT FROM AUTHOR]