HR: 09:15h
AN: V51E-06    [Abstracts]
TI: Phosphorus, a Minor Element to Track UHP conditions: an Experimental Insight
AU: * Brunet, F
EM: brunet@geologie.ens.fr
AF: CNRS-ENS Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005 France
AB: In the continental crust, where the P₂O₅ concentration amounts to around a weight percent, the phosphorus budget is mainly controlled by apatite, Ca₅(PO₄)₃(OH,F,Cl), and monazite REEPO4. Phosphorus is therefore mainly hosted in phosphate minerals rather than in silicates where it can potentially replace tetrahedral silicon. In phosphorus-poor environments, as in the Earth's mantle, this trend reverses and most of the phosphorus budget is believed to be controlled by silicates. Interestingly, ultra high-pressure seems to favor the Si-P replacement in minerals. For example, a complete solid-solution series between a phosphate and a silicate is achieved within the ellenbergerite structure under UHP conditions (Dora Maira, western Alps). Phosphorus incorporation in garnet is also favored by UHP conditions as evidenced in pyrope garnets from the same occurrence (Na + P = Mg + Si substitution). I show experimentally, here, that the Na₃Al₂(PO₄)₃ content in garnet increases with increasing synthesis P-T conditions. A Na₃Al₂(PO₄)₃ garnet is obtained at 1673 K and 17 GPa (multi-anvil experiments at GRC, Matsuyama, Japan). The increasing phosphorus solubility in garnet with P and T should be kept in mind when interpreting P-bearing exsolution in this mineral. According to recent experimental data presented at the last AGU Fall meeting (Brunet et al. 2004), the relevance of the Si-P substitution extends to octahedrally coordinated silicon (0.5 wt. % P₂O₅ in stishovite at 1873 K and 18 GPa) what opens perspectives for the study of lower mantle inclusions in diamonds. With respect to UHP phosphate mineralogy, the apatite breakdown above 12 GPa (in its own chemical system) into γ-Ca₃(PO₄)₂ (Murayama et al., 1986) deserves further consideration since it is believed to proceed at lower pressure in silicate-bearing parageneses. Finally, the preservation of phosphorus growth zones observed in garnet and ellenbergerite indicates a low diffusion kinetics for this element in silicates. This is an interesting characteristics to decipher crystallization stages in UHP minerals that incorporate phosphorus.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005