Do Melt Inclusion Compositions always reflect the Compositions of Large-Volume Melts in the Magmatic System?
Abstract
Melt inclusions (MI) hosted by high-Fo olivine phenocrysts from various tectonic settings commonly have unusual major and trace element compositions, unlike any erupted magmas, whereas MI in more evolved olivine phenocrysts are usually compositionally similar to the host lavas. 'Anomalous' MI often have high CaO contents and CaO/Al2O3 values, although low-CaO compositions are also present. SiO2 contents are either lower or higher that those in lavas. Other major elements do not show any systematic behaviour and can display variable types of anomalous concentrations (eg, higher and lower than in the lavas) within different subsets of 'anomalous' inclusions from a sample. In some cases, particularly in subduction-related magmatic suites erupted through a thick arc crust, all MI in high-Fo olivines have anomalous compositions, whereas in other suites anomalous and 'normal' MI can be found in a single sample. In samples with dominant anomalous MI, high-Fo olivines usually contain a large number of MI of variable sizes, often > 100 microns. In contrast, in samples with MI of dominant 'normal' compositions, many high-Fo olivines are devoid of inclusions, however when present, inclusions are small and few. The above differences in the size and abundance of 'normal' and anomalous MI imply differences in the rate of crystallisation, with anomalous MI corresponding to faster cooling rates. The fastest cooling rates of a primitive magma are expected at the margins of a magma body where it is in contact with the wall rocks and/or pre-existing mush zones. Localised rapid cooling in such environments facilitates crystallisation and trapping of MI by the growing phenocrysts. The compositions of these MI are likely to record the complex process of partial dissolution of the wall rock or mush zone crystals, and mixing of the resulting melt with the intercumulus liquids and with the primitive magma. These MI thus have no bearing on the compositions of melts in the magmatic system. On the other hand, inside the magma body, far away from the reaction zones, slow cooling rates do not favour trapping of MI, and 'normal' MI are much less common. The population of MI in high-Fo phenocrysts is thus naturally biased towards anomalous (ie, hybrid) compositions, whereas in the macro system these reaction processes are far less important. A plumbing system model developed by B. Marsh can explain all observations well. Large abundant 'normal' MI are found in high-Fo olivine from magmatic suites where crystallisation occurred at shallow depths during ascent and was accompanied by degassing of H2O-rich fluids, resulting in rapid crystallisation.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....3156D