| 其他摘要 | Phosphorus-rich peraluminous magmatic system is one of the major types of peraluminous magmatic systems. It’s always characterized by high in P, and ASI [alumina saturation index: mol. Al2O3/(Na2O+K2O+CaO)], low in Ca, Mg and Fe, strong depletions of REE, Th and Y, and mineralizations of W, Sn, Nb and Ta. Studies that deal with the geochemical characteristics in the petrogenesis and evolution of phosphorous-rich peraluminous magmatic system, liquid immiscibility, the phosphate-silicate equilibria, and behaviors of trace elements(including REE)in magmatic-hydrothermal transition stage in phosphorous-rich peraluminous magmatic system, are remaining to be determined and lack of the direct supports in experimental geochemistry at present. It will be helpful for understanding of the geochemical characteristics of phosphorous-rich peraluminous magmatic system and the impact of phosphorus on the behaviors of trace element(including imcompatible element, REE)to study the geochemistry of phosphorous-rich peraluminous magmatic system. Moreover, the studies of the subject metioned above are of much importance to understand the theory of the petrogenesis and metallogenesis of peraluminous magmatic systems.
In order to resolve the questions mentioned above, experiments have been conducted at different PTX conditions with albite granite. The main results and conclusions have been achieved as follows:
1) The effect of phosphorus on liquidus temperature of peraluminous granite has been determined at 100 MPa. With increasing phosphorus content in melt, the liquidus temperature of the granite decreases from 780 C at 1.91 wt%P2O5, 760C at 4.83 wt%P2O5, to 740 C at 7.71 wt%P2O5. Namely, as the addition of per 1 wt% P2O5 to the granitic melt, the liquidus temperature drops to 7~10 C.
2) At different PTX conditions, the immiscibility structures, such as immiscibility droplets or flow structure, are not recognized in all run products. So we infer that phosphorus is not the key fact that is account for the liquid segregation in peraluminous magmatic system.
3) The spessartine-apatite equilibrium buffers the phosphorus content of peraluminous melt between 0.47 and 0.80 wt% at 750 C, 0.35 and 2.26 wt% at 830 C. The P content of melt is function of ASI(P2O5 wt%=3.5×ASI2-11.3×ASI+9.5). The reasons have two aspects: one is the dissolution of spessartine that enhances the activity of Al2O3 in melt; the other is the equilibrium between Mn-rich apatite and end-number apatite that lead to reduce the P content of melt.
4) Partition coefficients of W, Sn, Be, Nb, and Ta between P-rich peraluminous granitic melt and coexisting aqueous fluid are less than 0.1. The results show that W, Sn, Be, Nb, and Ta tend to distribute in the melt, and then exist in the form of independent minerals(such as beryl, cassiterite, zircon, tantalite, columbite, and so on)in the late period of crystallization of magma and maybe form the granite-type or pegmatite-type rare metal deposits.
5) Partition coefficients of REEs between melt and fluid decrease with the atomic number and show right-dipping pattern at different PTX conditions. The REE patterns have no discontinuity at Nd-Pm, Gd, and Ho-Er. Partition coefficients ratio of Y/Ho equals about 1 and is insensitive to T, P and the phosphorus content of melt. The results show that at the magmatic-hydrothermal transition stage, the interaction between P-rich peraluminous melt and aqueous fluid doesn’t cause to the decoupling of Y-Ho and REEs. So REE tetrad effect cannot be ascribed to the fluid/melt interaction in the late period of evolution of P-rich peraluminous magmatic system. |
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