Saturation des mineraux phosphates dans les magmas silicates: Implications sur la genese des roches riches en fer-titane-phosphore

Understanding the genesis of layered intrusions and their associated ore-deposits is crucial due to economic concentrations of Cr, V, Ti, P and PGE, and for the developments exploration tools for these elements. To provide constraints on how Fe-Ti-P rich rods form, it would be helpful to understand the processes by which the various lithologies of layered intrusions form. Among the various types of rocks found in layered intrusions, nelsonites are very unusual, as they are mainly made up of apatite and Fe-Ti oxides. Nelsonites are also encountered in association with anorthositic complexes. Two models for the formation of nelsonites have been proposed: (i) two-liquid immiscibility and (ii) fractional crystallisation followed by apatite and oxide accumulation. This issue is highly debated and no clear argument has been found to favour either of these two hypotheses. In order to provide further constraints on the formation processes of the nelsonites, we have considered the problem in tree parts: (1) Firstly, we performed experiments controlling fO2, without volatiles. Our results allowed us to predict the magmatic composition at the phosphate saturation point of a silicate magma according to the following equation: Mliq-satP2O5= expT-0.8579 139.00-MliqSiO2 +0.0165-3.3333lnM liqCaO Where M is the molar concentration and T the temperature in Kelvin. This equation can be used for magma with a large spectrum of compositions (10-18% mol. SiO2, including per-aluminous liquids) over a large range of temperature (≈800-1400°C). In these experiments without volatiles we crystallized whitlockite and no apatite. (2) A second set of buffered experiments with H2O and F added were performed. The results were compared to fractional crystallisation modeling and allow us to clearly constrain the two-liquid immiscibility fields in a SiO 2-TiO2+FeO+P2O5+MgO+CaO-Al 2O3+Na2O+K2O system. We concluded that mafic magma would become saturated in apatite before reaching the two-liquid immiscibility fields. (3) Finally, we determined the trace elements in the apatite of nelsonites and associated apatite-rich rocks from the Sept-Iles intrusive complex (Quebec, Canada) and from the Rustenberg Layered Suite of the Bushveld Complex (South Africa). The trace element data in the apatites and whole-rock indicate that apatite is controlling the whole-rock budget of the rare earth elements; U, Th, Cl, Ca and Sr. We calculated the composition of the parent magma of the nelsonites by inversion and compared our modeling to the natural rock data. We also modeled fractional crystallisation in magmas assumed to be similar in composition to the marginal rocks of the layered intrusion in order to consider whether nelsonites and apatite and oxide-rich rocks could have formed through fractional crystallisation. Our calculations demonstrate that rocks similar to the Sept-Iles intrusive suite as well as the Rustenberg Layered Suite could form by fractional crystallisation.; Our experiments and modeling, suggest that fractional crystallisation and crystal accumulation of apatite and Fe-Ti oxides is the most likely process to generate nelsonites and apatite-rich rocks associated with the nelsonites.