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Deep fluids, evolved magmatic fluids, and non-silicate magmas

Deep fluids and evolved magmatic fluids

Water has played and still plays a major role in the evolution of the planet Earth. In particular, the behaviour and role of water in the differentiation of the continental crust into a lower, anhydrous, granulite zone and an upper, granitic, hydrated zone is still poorly understood. Similarly, the interactions between the mantle and lower crust are poorly constrained and are particularly difficult to measure. However, the distribution and movement of water in the crust and lithosphere exert significant control over the transfer of heat and matter across the lithosphere and on its rheology. To address all of these questions, we propose to carry out a systematic study of the distribution and migration of water in the lower crust and upper mantle, at selected sites that represent a large variety of geodynamic settings spanning a wide age range. Similarly, the flow of aqueous fluids in the upper crust plays a critical role in the chemical evolution and geodynamics of the crust. This role is particularly marked in the development of late granitoids, which are generally associated with remobilisation of elements by hydrothermal fluids and the development of veins and mineral deposits. To advance the understanding of the processes involved, to quantify the elemental fluxes, and to determine the ultimate sources of metals, we will develop new tools for tracing hydrothermal circulation. Particular attention will be given to strategic metals, addressing the conditions of their solubility and their sources. The sphalerite – bearing vein deposits (Zn-Ag-Ge [Pb, Cd]) of Noilhac St. Salvy (Massif Central) will be studied in order to understand the temporal variations of thermodynamic conditions of mineralising fluids, combining an isotopic approach (MC-ICPMS) and in situ measurements of the concentrations of trace elements (ICP-MS laser) (in collaboration with M.C. Boiron, Georessources, Nancy). Another key point that we will develop is the dating of metal-bearing phases. To this end, the Re-Os isotopic system is a valuable tool because it allows the mineralisation itself to be dated as opposed to the associated silicate phases. This method will be used to date hydrothermal mineralisation in the Massif Central and gold deposits in West Africa, in collaboration with the Georessources Laboratory, in the context of the WAXI project, and will be the subject of a Ph.D. thesis.

Sphalerite from St Salvy Mine (Tarn, France) (A. Filia, 2010, Master 2)

Non-silicate magmas

On Earth, magmas are not limited to liquid silicate. They also appear in less common forms, such as carbonatite magmas, liquid salts, or molten sulphates. The objective of this section is to understand in detail the formative and evolutionary processes of these atypical magmas. Carbonatite magmatism will be studied by examining the products of the only active carbonatite volcano, Ol Doinyo Lengaï, in the East African Rift. The petro-geochemical study of the different series of lava and the cumulates carried as xenoliths in the recent 2007-2008 eruptions will help to provide new information on the creation and evolution of carbonatite magmas. This approach will be combined with the analysis of noble gases trapped as fluid inclusions in minerals and also with those of volcanic gases sampled directly on the volcano. Along with this field-based study, the distribution of noble gases between carbonatite and silicate liquids under mantle conditions will be determined experimentally at the LMV, Clermont-Ferrand (in collaboration with K. Koga). This information will allow us to understand the role of these carbonate phases in the distribution of noble gases in the mantle.

Oldoinyo Lengai viewed from the Natron Lake south margin (2010, August)

Similarly, the fusion of salts and sulphates and their interactions with calc-silicate formations can produce alkaline, hyper-aluminous magmas that are rich in B, Li, and F and depleted in SiO2. This type of magma, observed in ancient evaporite sequences, is responsible for the creation of Cenozoic rubies associated with marbles and also for the formation of vanadium-bearing grossular garnet (tsavorite) from the Neoproterozoic metamorphic belt of Mozambique. We propose to characterise the geochemistry and thermobarometry of fluid inclusions and molten salts trapped in these garnets and rubies, to perform thermodynamic calculations on the equilibrium of molten salts in the C-H-S and C-H-S-O systems, and to perform dissolution and synthesis experiments on these two gems (in collaboration with J. Dubessy, Lab. Georesources). The goals are to understand the mechanisms which mobilise the metals Al, Cr, and V in the continental crust by ionic liquids derived from molten evaporites, to improve our knowledge of phase equilibria in the C-H-S-O system, and to approach P-T-X-fO2 formation conditions for per aluminous magmas.

Tsavorite formed by the interaction between molten salts from evaporitic levels and graphitic gneiss rich in Ca.