Yves Thibault

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Experiments at 3.0 GPa were done to define the nature of near-solidus melts formed from phlogopite lherzolite (PLZ) and carbonated phlogopite lherzolite (CPL) model mantle sources. The reactivity of these melts towards harzburgite and wehrlite were then investigated at 2.0 GPa and 1000{dollar}\sp\circ{dollar}C.;At 3.0 GPa and 1225{dollar}\sp\circ{dollar}C, PLZ yields 7 wt% of melt. The partial breakdown of phlogopite to olivine and liquid results in a silicate melt enriched in K, OH, Al and Fe and a residual phlogopite enriched in Ti. This alkaline silicate melt is only slightly reactive towards harzburgite and wehrlite at 2.0 GPa and 1000{dollar}\sp\circ{dollar}C.;In CPL, at 3.0 GPa and 1100{dollar}\sp\circ{dollar}C, 4 wt% of alkali-bearing dolomitic melt coexists with residual garnet phlogopite lherzolite. The alkalinity is related to the partial breakdown of phlogopite to olivine and pyrope yielding K and OH to the melt. At 2.0 GPa and 1000{dollar}\sp\circ{dollar}C, this dolomitic melt metasomatizes a harzburgite to a phlogopite wehrlite, whereas infiltration into wehrlite could result in the formation of calcite- and phlogopite-bearing dunite.;A process active below continental rifts is proposed in which a carbonated phlogopite lherzolite horizon, formed at the base of the lithosphere by the release of dense alkaline fluids, migrates via melting-migrating-solidifying/reacting cycles. The results of the CPL and PLZ experiments suggest that fractional melting of the carbonated phlogopite lherzolite horizon at 100 km depth could yield a dolomitic melt followed by an alkali-rich silicate melt. The consecutive infiltration of these distinct agents into lithospheric mantle at 65km depth could result in a decoupled metasomatic/enrichment event. The dissociation of the carbonate components of the dolomitic melt will create an harzburgite {dollar}\to{dollar} wehrlite metasomatic trend. The infiltration of the silicate melt will enrich the metasomatized rocks in clinopyroxene and phlogopite. The variety of rocks that result bear similarities with a suite of mantle xenoliths from the West Eifel, Germany.



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