Elsevier

Lithos

Volume 65, Issues 3–4, December 2002, Pages 299-311
Lithos

Fractal analysis of mingled/mixed magmas: an example from the Upper Pollara eruption (Salina Island, southern Tyrrhenian Sea, Italy)

https://doi.org/10.1016/S0024-4937(02)00197-4Get rights and content

Abstract

Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO2 rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO2 rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO2 rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10−4 and 10−2 m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.

Introduction

It is widely accepted that many geological structures (e.g. faults, minerals) show scale invariance, i.e. they are self-similar within a large range of scales (Turcotte, 1992). This property allows geologists to use fractal geometry to understand geological processes (e.g. earthquakes, mineral growth and fluid flow in fractures; Barton and La Pointe, 1995). In more recent times, petrologists and volcanologists used fractal analysis to study mixing processes in magmas. However, these studies focus on morphological mixing structures (e.g. banded pumices; Wada, 1995) or chemical maps from heterogeneous intrusive rocks (Perugini and Poli, 2000). Both studies suggest that the fractal dimension of poorly mixed magmas is higher than that of mixed liquid. On the contrary, numerical models on the convective mixing of fluids (Ten et al., 1997) evidence that homogeneous, well-mixed liquids show a higher fractal dimension with respect to the mingled liquids. These observations indicate that little is known about the dynamics of the fractal mixing in magmas.

In this study, we investigate the mixing/mingling structures of the pumices of the Upper Pollara eruption (Salina Island, southern Tyrrhenian Sea) by using a novel approach. We merge data from image and geochemical analyses and find a correlation between the degree of geochemical interaction (mixing) of the two end-member magmas and the fractal dimension of the mingling/mixing morphologies. The data are discussed in light of the available experimental and numerical models. The results allow us to constrain (a) the fluid-dynamical regime (i.e. laminar vs. turbulent flow) responsible for the interaction between magmas and (b) models on fractal mixing in magmatic flows.

Section snippets

Geological setting

The Pollara morphological depression occupies the northwestern edge of Salina Island (Aeolian Archipelago, southern Tyrrhenian Sea), which consists of five main eruptive centers (Corvo, Rivi-Capo, Fossa delle Felci, Porri and Pollara) that developed between 430 and 13 ka (Fig. 1a; Gillot, 1987, Barca and Ventura, 1991). Volcanic activity within the Pollara depression began with the emission of the P.ta Perciato lava flow (30 ka; Keller, 1980, Gillot, 1987). Two main explosive eruptions named

Evidence of mingling/mixing processes in the UPP pumices

Homogeneous and heterogeneous juvenile fragments occur within the UPP sequence. Homogeneous fragments are dark to white in color. The homogeneous dark fragments are aphyric to low-porphyric (10–15 vol.%) pumices. The light pumices are aphyric to subaphyric. Intermediate, homogeneous grey fragments show a crystal content not exceeding 10 vol.%. Plane-parallel bands to vortex-like and wave-like mingling structures characterize heterogeneous clasts (Fig. 2). In some cases, sub-circular to

Analytical approach

Poorly vesiculated (vesicles<5 vol.%) and subaphyric (crystals<5 vol.%) UPP mingled clasts have been selected for the determination of the fractal dimension of the morphological mingling structures. These criteria have been adopted in order to select glasses whose color reflects the chemical composition and not differences in microcrystal or vesicle content. Selection has been done after observation under the microscope and SEM of a larger number of clasts.

A water-polished thick slice, which

Discussion

Data presented in this study may be summarized as follows: (a) UPP heterogeneous pumices display mingling structures (bands, vortices, blobs and folds; Fig. 2) constituted by magmas ranging in composition from HK andesites to high-SiO2 rhyolites. (b) These structures are fractal objects within two orders of scale (i.e. 10−2–1 cm) and their fractal dimension is between 1.1 and 1.84. (c) Coupled geochemical and image analysis data reveal that a positive correlation exists between the fractal

Conclusions

The results of the present study may be summarized in five points:

  • (a)

    Coupled image and chemical analysis of UPP heterogeneous pumices indicates that a correlation between magma composition and macro-microscopic morphological features of mingling/mixing structures exists.

  • (b)

    A relationship exists between the fractal dimension and the degree of homogenization: well-mixed magmas show higher D-values with respect to the end-members. Therefore, higher D-values reflect larger degrees of interaction between

Acknowledgements

GV thank J. Ottino for introducing us in the dynamics of chaotic flows. This research has been supported by GNV grants. We are grateful to Dr. M. Davoli for his technical assistance in the SEM-EDAX analyses. GV thanks the IPF-OV staff for the technical assistance in the image analysis procedures. The comments of G. Jeffrey Taylor and anonymous are greatly appreciated. Also, thanks to G. Poli and D. Perugini for the editorial handling.

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