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The effect of the crust on the differentiation of basaltic magma


Olivier Namur
Dept. of Earth and Environmental Sciences, KU Leuven

ABSTRACT: Basalts represent the most common magmas erupting at the surface of the Earth and are produced by low- to high-degrees of wet or dry partial melting in the upper mantle. Chemical variability of basalts is partly attributed to melting of different mantle sources under contrasted pressure-temperature conditions. Crustal processing also plays a significant role on the compositions of erupted basalts. This talk will address the effect of melt migration and emplacement through the crust on the composition of melts that ultimately erupt at the Earth’s surface. Two different examples will be used to illustrate that constraints on crustal processes can be gained from the study of natural rocks and using experiments in the laboratory. The first one is an experimental study of the crystallization products of two near-primary basalts (an enriched one and a depleted one) from Iceland under crustal pressure conditions. Phase equilibria show a drop in melt fraction by increment of temperature significantly higher for the depleted melt than for the enriched one. As a consequence, for mantle melts ascending through the crust, the likeliness of reaching the surface before being fully solid is much higher for the enriched melt. This has important consequences for the major and trace element contents of erupted basalts, for estimates of the relative proportions of enriched and depleted melts produced in the mantle and is also important to constrain the mantle sources of continental and oceanic basalts. The effect of crustal differentiation of basaltic magmas can also be observed in the crystal record of erupted lavas. Examples of the Shatsky Rise oceanic plateau and the Calbuco volcano (Andes) will be used to illustrate that basaltic melts progressively crystallize in various storage regions before being erupted. Mass transport across the different magma chambers has important effects on crystal compositions and textures but also on the compositions of the magma that finally erupt and which can be far from representing liquid compositions. 

BIO:  Olivier Namur is an assistant Professor of Petrology at the KU Leuven, Belgium. He received his Masters at the University of Liege (Belgium) for a work on the formation of Fe-Ti ore deposits associated with Proterozoic massif-type anorthosites. He conducted a PhD thesis at the University of Liege under the supervision of Jacqueline Vander Auwera and Michael Higgins (University of Chicoutimi, Canada), where he studied the differentiation of ferrobasaltic magma and magma chamber processes in the Sept Iles layered intrusion (Canada). During his PhD, he also collaborated actively with Mikael Toplis (University of Toulouse) and together they proposed a new predictive model of plagioclase-silicate melt equilibrium. During his first post-doctoral research, Olivier worked under the supervision of Marian Holness at the University of Cambridge (NERC and Junior research fellowship from Magdalene College). They focused on rocks from the Skaergaard intrusion and proposed models of formation of igneous layering on the vertical walls of the magma chamber. They also illustrated the effect of latent heat buffering in a crystal mush on the zoning pattern of cumulus crystals, particularly plagioclase. After two years in Cambridge, Olivier moved to the University of Hannover (Germany) where he worked under the supervision of Francois Holtz (Von Humboldt fellowship followed by an individual intra-European Marie Curie Fellowship). In Hannover, Olivier’s focus shifted to experimental investigations and particularly on the differentiation of planet Mercury. Using low- to high-pressure, high-temperature experiments together with X-ray fluorescence data from the MESSENGER spacecraft (NASA) they investigated the mechanisms of mantle melting on Mercury as well as the mineralogy at the surface of the planet. They also investigated the effect of oxygen fugacity conditions on sulfur solubility in silicate melts. They showed that Mercury’s magmas could contain up to 15 wt.% sulfur at sulfide saturation.  Olivier has also been briefly the leader of a DFG-funded Emmy Noether Research group at the University of Hannover. In 2017, Olivier moved as an assistant professor at the KU Leuven. He is the leader of an Odysseus research group (FWO funding) that investigates the effect of crustal thickness of the eruptive products of basaltic volcanoes. Throughout his career, Olivier has actively collaborated with Bernard Charlier (University of Liege). They share more than 20 publications, including a recent book on layered intrusions.

O. Namur