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

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Danyushevsky, LV and McNeill, AW and Sobolev, AV (2002) Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications. Chemical Geology, 183 (1-4). pp. 5-24. ISSN 0009-2541

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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, Al2O3, CaO, Na2O, and K2O 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 H2O content of
primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the first liquidus (most magnesian) olivine that crystallised from these magmas.

Item Type: Article
Keywords: Melt inclusion; Homogenisation experiments; H2O; Primary magmas; Phenocrysts; Plagioclase
Journal or Publication Title: Chemical Geology
Page Range: pp. 5-24
ISSN: 0009-2541
Identification Number - DOI: 10.1016/S0009-2541(01)00369-2
Additional Information:

Journal Home page for Chemical Geology: http://www.elsevier.com/locate/chemgeo

Date Deposited: 01 Feb 2007
Last Modified: 18 Nov 2014 03:13
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