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Time scales of crystal residence and magma chamber volume from modelling of diffusion profiles in phenocrysts: Vesuvius 1944

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Abstract

Diffusional smoothing of Fe–Mg compositional gradients in clinopyroxene crystals from the 1944 eruption of Vesuvius is used to determine the preeruption residence times of crystals at magmatic temperatures. The result is a distribution of crystal residence times from one sample that can be used to constrain magma chamber volumes. Diffusional zones between compositionally distinct cores and rims were observed by backscattered electron (BSE) imaging to vary in width from 1.6 to 11.9 μm and represent storage of crystals at magmatic temperatures for periods ranging from 4.5 months up to 9 years prior to eruption. The distribution of residence times is skewed to young ages and is best explained by the open-system behaviour of an 8.0×107 m3 chamber that received an input of between 3.5×107 and 7.0×107 m3 in the final 6 months prior to eruption. The calculated inputs, compared with contemporary observations of the emitted magma volumes, implies that some 3.0×107–6.5×107 m3 of magma input did not erupt but was accommodated by chamber expansion before the 1944 eruption. This would place the preeruptive chamber volume between 1.15×108 and 1.5×108 m3. The eruption was then triggered by a further input of ∼1.9×107 m3 which forced the extrusion of a similar volume of relatively degassed magma as lava flows prior to the fire-fountaining episode. The backscattered electron imaging technique for residence time determination has great potential in unravelling the histories of populations of phenocrysts in many volcanic systems.

Introduction

Volcanic eruptions represent one of the more dramatic of geological hazards, and the possible threat posed is nowhere better typified than in the location of Vesuvius, adjacent to the densely populated area around the Bay of Naples [1]. The dynamics of magma storage and replenishment beneath volcanoes are fundamental to understanding the behaviour of the volcanic systems and have direct implications for the forecasting of the severity of future volcanic hazards. The time scales and volumes of magmatic storage prior to eruption are a key part of understanding these dynamics but are difficult to measure with certainty. In this paper, we exploit the diffusional broadening of originally sharp compositional discontinuities in clinopyroxene phenocrysts to calculate the magmatic residence times of a number of crystals erupted during the 1944 eruption. In addition, the profiles are used to determine the volume of the preeruptive chamber.

Our method is a general one and can be applied widely as it relies on analysing phenocrysts that display compositionally distinct cores and rims, a common phenomenon in volcanic rocks [2], [3], [4], [5]. The formation of such zonation is commonly attributed either to changes in physical conditions during crystallisation, e.g., pressure reduction and/or magma degassing during ascent [6], or to changes in magma composition such as those that occur during magma mixing and contamination [7]. Once an interface between two zones has been established, any compositional contrasts tend to smooth out by diffusion, turning an originally sharp boundary into a broader band. This process is strongly temperature-dependent and is effectively quenched on eruption. If the rate of atomic diffusivity across the boundary is known then the time that the core–rim boundary spent at magmatic temperatures can be calculated from its width. Previous workers have applied this approach to single crystals to infer residence times in magmatic systems [7], [8], [9], [10], [11], but here we apply the technique to a number of crystals in one sample. The results give a distribution of residence times, the characteristics of which are directly related to the volume of the preeruptive magma chamber.

Section snippets

Background, samples and methods

The 1944 eruption started on the 18th of March 1944 following the collapse of the spatter cone in the centre of the crater platform. Initial activity from the 18th to the 21st of March consisted of the effusion of ∼2×107 m3 of lava [27], mainly to the E and NE. This was followed on the 21st–22nd of March by eight episodes of fire-fountaining which dispersed lapilli and scoriae downwind to the SE. From late on the 22nd until the 29th of March, activity underwent a transition to a buoyant column

Calculation of diffusion time scales

The width of the boundary is related to the interdiffusivity of iron and magnesium (D). The residence time at high temperature, t, can be obtained from Eq. (1), modified from [24], to take account of the pixellated nature of the image data, as explained in Appendix A.C(x,t)=n2(n+1)=1−0.5erfcx2DtC represents the compositional contrast on a scale of 0–0.5 between the junction and the observed maximum extent of diffusion at the half-width x, and t is the time elapsed at magmatic temperature since

Results

The widths of the diffusional boundaries measured in 16 crystals are listed in Table 4 together with the residence times calculated from Eq. (1) using the average temperature recorded by the crystals. The ages range from a maximum of 9.0 years, equivalent to a diffusional width of 11.9 μm, to 0.4 years from the narrowest resolvable boundary of 1.6 μm. In addition to these results, seven other crystals with core compositions but without rims, or with narrow rims and diffusional boundaries of

Discussion

The age distribution shows a distinct bias towards shorter residence times, emphasised by the strongly concave-upwards cumulative distribution profile in Fig. 3b. The relative abundance of young crystals could be the result of two processes. In the first instance, they could reflect an increased arrival rate of magma and crystals immediately prior to the 1944 eruption, with the older crystals being diluted by the large volume of new magma. Alternatively, the upper magma chamber can be regarded

Conclusions

The method of using BSE images to determine residence time scales in clinopyroxene phenocrysts at Vesuvius produces a consistent residence time distribution for the crystal populations given what we know about the state of the volcano during open vent activity. Such crystal population data can be used to infer directly the volume of the preeruptive magma chamber because admission into this chamber is the datum to which individual crystal residence time scales refer. Very simplistic estimates of

Acknowledgements

DJM would like to acknowledge the support of a NERC studentship during the work that this paper represents, and would also like to thank Dr. A.Tindle for assistance with the Open University Cameca SX100 microprobe, and Professor Steve Sparks and Dr. Boyan Bonev for constructive comments that improved this manuscript during the early stages. The authors would like to thank Dr. J. Van Orman and Professor J. Davidson for challenging and constructive reviews. [KF]

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