Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications

Abstract

Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters ( P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in highmagnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to reequilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2 O 3 , CaO, Na 2 O, and K 2 O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2 O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the 0009-2541/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 5 4 1 ( 0 1 ) 0 0 3 6 9 -2

Volatile (S, Cl and F) and fluid mobile trace element compositions in melt inclusions: implications for variable fluid sources across the Kamchatka arc

“…To avoid the effect of crystallization within melt inclusions and to assess the major and trace element composition of the initially trapped melts, 133 inclusions were reheated, melted and partially homogenized (Danyushevsky et al 2002) and then analyzed by electron microprobe for major elements, sulfur and chlorine. A subset of 60 reheated inclusions was analyzed by ion probe for trace elements and fluorine.…”

“…To account for chemical changes of the included melts, all data were recalculated to melts in equilibrium with their host olivines. The recalculation was based on modeling fractional melting of olivine from the inclusion walls after Danyushevsky et al (2002). Because of the high Mg# in Ol from the Kluchevskoy samples the data for this volcano were recalculated accounting for FeMg exchange between glasses and host olivine (Danyushevsky et al 2000) using the model of Ford et al (1983) for olivine-melt equilibria.…”

“…Rather, they formed due to changes in P, FeO, H 2 O or ƒO 2 after entrapment (e.g. Metrich et al 1999;Danyushevsky et al 2002).…”

“…Because the volatile elements in whole rocks are strongly affected by degassing before and during eruption (Hansteen and Gurenko 1998;Metrich et al 1999;Edmonds et al 2001;de Moor et al 2005;Wallace 2005), melt inclusions trapped in pristine igneous minerals are potentially the best data source to study initial volatile contents in primary and parental melts (Danyushevsky et al 2002). Melt inclusions also provide an opportunity to extend the data on melt compositions beyond the range covered by whole rocks towards more primitive compositions .…”

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

“…They can record information on volatile contents before eruption and petrogenetic processes such as crystallization, magma mixing and crustal contamination and allow reconstruction of the primary melt compositions (Sobolev and Kostyuk 1975;Sobolev 1996;Fig. 1 continued Contrib Mineral Petrol (2013) 165:305-326 307 Lowenstern 1995Lowenstern , 2003Frezzotti 2001;Danyushevsky et al 2002;Schiano 2003;Ren et al 2005;Kamenetsky 2006;Kent 2008;Métrich and Wallace 2008;Moore 2008;Blundy and Cashman 2008; and references in these papers). Comparing with whole rocks, melt inclusions have advantages as below (Kent 2008): (1) Because of being trapped in different minerals growing at various stages of magma evolution, melt inclusions often bear wider compositions than bulk rocks (e.g., Sobolev and Shimizu 1993;Nielsen et al 1995;Gurenko and Chaussidon 1995;Saal et al 1998Saal et al , 2005Kent et al 1999Kent et al , 2002Norman et al 2002;Sours-Page et al 2002;Ren et al 2004Ren et al , 2005Jackson and Hart 2006;Kamenetsky and Gurenko 2007).…”