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Title: Melt inclusions from the Southwest Indian Ridge and Reykjanes Ridge : insights into melt extraction and magma chamber processes
Author: Font Morales, Laura
ISNI:       0000 0001 3472 7767
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2003
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Naturally glassy melt inclusions trapped in olivine, plagioclase and pyroxene crystals in basalts from the Southwest Indian Ridge (SWIR) (49°E-69°E) and the Reykjanes Ridge (RR) (57°N63°N) are used to study the diversity in composition of primary melts, the conditions of mantle melting, source composition and magma chamber processes occurring beneath theses slow spreading ridges. The SWIR is characterised by cold mantle and low degrees of melting and the RR is characterised by high degrees of melting in the northern part of the ridge related to the anomalously high mantle temperature associated to the Iceland mantle plume. The different geological mantle conditions of the SWIR and RR influence the composition of the melt inclusions. Along the SWIR, there is an increasing diversity in Mg# from east to west (69°E-49°E), between the glasses and olivine hosted melt inclusions with the plagioclase hosted melt inclusions. This suggests an entrapment of primitive melts in plagioclase phenocrysts and more evolved melts within late olivine crystals, which are close in composition to the matrix glasses. The eastern-most melt inclusions show high concentrations in incompatible elements (P, K, Ti, Na, (La/Sm), Zr/Y, Ba/Yb), which decrease towards the western part of the ridge studied area. This suggests low degrees of melting in the eastern part, increasing towards the west. The variable fractionation histories between the melt inclusions and the matrix glass along the SWIR, and the slight increase in the degree of melting from east to west suggest that the melts in the eastern side of the ridge migrated more slowly upwards from the mantle while fractionating en route compared to the melts in the western area. Once in the magma chamber, residence times were short prior to eruption, allowing little opportunity from different melt batches to mix and differentiate. The primitive composition and the low concentration of incompatible elements of the plagioclase hosted melt inclusions indicate rapid migration of melt increments into the magma chambers and entrapment shortly after arrival. However, the melts had sufficient time to fractionate and mix, producing low Mg # basalts represented by the matrix glass and olivine hosted melt inclusions. The glasses and melt inclusions of the RR have lower MgO wt.% and Mg# towards the north of the ridge, which suggest that they become more fractionated. This suggests that these melts have longer residence time in the magma chamber in the north or the magmas fractionate during the ascent to the magma chamber. The Nag and Sig of the matrix glasses and melt inclusions decrease towards the north of the ridge suggesting an increase in the degree and depth of melting northwards. This correlates with the temperature anomaly related to the mantle plume and with the garnet signatures of some olivine and pyroxene hosted melt inclusions from the north of the RR. The matrix glasses and melt inclusions show similar composition along the ridge. They also show an increasing enrichment in trace elements towards the north related to the enrichment of the source composition. This indicates that the southern melts are generated by depleted mantle components from the depleted upper mantle, whereas the northern melts are generated by enriched mantle components segregated at greater depths in the mantle beneath the RR. The similarity between the composition of the melt inclusions and host glasses along the SWIR and RR suggests that the melt inclusions represent late melts, which are the result of the mixing of primary melts segregated during the early stages of melting beneath the ridge. This correlates with the lack of large primitive olivine crystals in the SWIR and RR rock samples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Olivine crystals