RICERCA ITALIANA     

Crystallization kinetics and time scales of magmatic proceses as recorded in volcanic rock textures: experimental, theoretical and observational studies

Università degli Studi di Camerino

Abstract

The crystals present in an igneous rock and the observed variations in their dimensions and compositions reflect the integrated pressure (P) -temperature (T) - composition (X) - time (t) history of the sample. Because crystals take a finite time to nucleate and grow (rates depending on undercooling, T, composition, etc), variations in their sizes, size distributions, and compositions can provide insights into magmatic processes and their time-scales. In order to use observed variations in the textures of igneous rocks to gain quantitative insights into the timescales of magmatic processes we require quantitative information on the rates of nucleation (J) and growth (G) as a function of undercooling (T) for the crystals and melts of interest. Unfortunately, experimental data on nucleation and growth kinetics are not abundant (none for the shoshonitic or trachytic compositions of interest in this proposal) and measured values in other magma types vary by many orders of magnitude, depending on type of crystal, melt composition and degree of undercooling. Furthermore, there are no suitable theoretical models capable of calculating nucleation and growth rates without some experimental constraints. In this proposal we seek funding to allow experimental measurements of crystal nucleation and growth rates in melts of shoshonitic, trachybasaltic and trachytic composition, for application to natural samples from Etna, Stromboli and Campi Flegrei. We will study >>>

Principal Investigator

Michael Robert CARROLL Università degli Studi di CAMERINO

Research Objectives

The objective of the research described in this proposal is to obtain experimental, observational and theoretical data necessary for quantitative evaluation of timescales of magmatic processes (storage, ascent, eruption) for alkaline magmas produced at Etna, Stromboli and the Campi Flegrei. In order to used observed textural variations to obtain insights into the dynamics of volcanic systems it is necessary to have quantitative information on the rates of nucleation (J) and growth (G) of crystals in the magma compositions of interest, under P-T-X conditions of interest. Experimental data for the magma compositions of interest (shoshonite, trachybasalt, trachyte) are not available, the few available studies of other magma compositions (mainly basalts and calc-alkaline rhyolites) show that J and G may vary by many orders of magnitude, and there are no suitable theoretical models capable of calculating these parameters. Of particular interest for this study is the kinetics of crystallization of plagioclase and clinopyroxene in shoshonitic and trachybasaltic (hawaiitic) melts, the kinetics of crystallization of alkali feldspar and plagioclase in trachytic melts, and the relations between magma ascent dynamics, crystallization kinetics, and textural evolution of phenocryst and groundmass crystal populations. The major questions to be answered include the following :

1. how does growth rate (G) vary with undercooling [T=T-T(liquidus)] and melt >>>

First Results

The research described in this proposal concerns the acquisition of new experimental data on crystallization kinetics of plagioclase, pyroxene and alkali feldspar in shoshonitic to trachytic melt compositions, and application of these data to observations concerning CSDs and groundmass crystallinity in eruptive products of Stromboli, Etna and the Campi Flegrei. The integration of experimental kinetic data with observational data on natural samples and appropriate models will allow calculation of timescales ranging from magma storage (phenocryst dimensions) to ascent and eruption timescales (microphenocryst and groundmass crystallinity). Specific results from different activities within the overall research program include the following:

Measurements of plagioclase and clinopyroxene growth rates in shoshonitic and trachybasaltic melts subjected to drops in temperature at atmospheric pressure (growth rate variations with T, with undercooling due to cooling at constant pressure).

Measurements of plagioclase growth and nucleation rates in shoshonitic melt for different values of T, with undercooling imposed by a drop in P(H2O).

Measurement of plagioclase equilibrium liquidus in shoshonitic melt for P(H2O) from 100 to 0.1 Mpa (necessary to define effective T in decompression experiments)

Measurements of plagioclase and alkali feldspar growth and nucleation rates in trachytic melts >>>

Timescale

24 months

National and international background

Studies of the equilibrium properties of magmatic systems have a long history and today the basic phase relations are known for a wide variety of magma types under a broad range of P-T-X conditions. There also exist quite advanced thermodynamic models for calculating phase relations in complex natural systems (e.g., Ghiorso et al., 1983, and numerous subsequent revisions) The same cannot be said for kinetic aspects of magmas, such as the rates of crystal nucleation and growth, yet it is these parameters that ultimately control the textures that are observed. The crystals present in an igneous rock and the observed variations in their dimensions and compositions reflect the integrated pressure (P) -temperature (T) - composition (X) - time (t) history of the sample. Because crystals take a finite time to nucleate and grow, variations in their sizes, size distributions, and compositional variations can provide insights into magmatic processes and their time-scales. Such information is complementary to time scale information provided by studies of short-lived radiogenic isotopes (e.g., U-series isotopes), which typically provide information on timescales on the order of 10^3 to 10^5 years (e.g., Hawkesworth et al., 2000; Turner et al., 2000). Recent improvements in analytical methods for U-series isotopes should soon allow more reliable extension of these isotopic methods to overlap the timescales (hours to days to centuries) sampled by studies of textural variations and >>>