Distribution of Major and Trace Elements Between the 'Vapour' Phase and Silicate Melt Phase in Igneous Rocks

• Main Author: Hards, N J
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1975
• Subjects: Equilibrium; Fluorides; Geochemistry; Geology; Igneous rocks; Magma; Petrology; Temperature; Trace elements
• ISBN: 9798209810285

Distribution coefficients have been determined for Cl, F, Na, K, Si, Al, Mn, Sr, Ba, Nb, Zr, Hf, Be, Ce, Yb, Zn, Sn, Cs and Au between melts of nepheline syenite and granite compositions, and an aqueous ‘vapour’ phase, in the presence of HCl and HF. It has been found that Cl concentrates strongly in the ’vapour’ phase, especially for the granite, while F remains in the melt, even as the solidus is approached. Zn, Kn and Au all concentrate strongly in the ’vapour’ phase in the presence of Cl, especially for the granite, and the Cl-bearing ’vapour’ phase also contains significant amounts of the alkalis and, for tine nepheline syenite, the rare earth elements. In the absence of Cl, the ’vapour’ phase is much poorer in dissolved components, although Zn may concentrate in the ’vapour’ phase for the granite, and it is possible that 3e enters the ’vapour’ phase as the solidus is approached.The behaviours of Cl and F have been critically considered, and some suggestions have been made about the reasons for the different behaviour of these elements. The role of a ’vapour’ phase derived from salic magmas during crystallisation has been discussed, but it seems that there is rarely any unequivocal evidence for the existence of such a phase in nature. It seems probable that a fluoride-rich melt may be derived from granitic magmas in the final stages of crystallisation, and some suggestions are made about the possible role of such a phase in nature. It is also suggested that a process known as gravity differentiation or volatile transfer may be important in the genesis of some peralkaline magmas, but the process does not require an excess ’vapour’ phase.