Alteration of Feldspathoids Changes pH of Late-Magmatic Fluids: A Case Study from the Lovozero Peralkaline Massif, Russia

The 360-370-Ma-old Lovozero peralkaline massif (NW Russia) is a layered nepheline syenitic–foidolitic pluton. In the rocks of massif, late-stage (auto)metasomatic alterations rock-forming minerals are quite intense. We studied products alteration and sodalite via microtextural, microprobe, spectroscopic methods. found that these extensively replaced by association between natrolite + nordstrandite ± böhmite paranatrolite in accordance with following reactions: 3Nph 4H2O → Ntr Nsd NaOH; 6Nph 9H2O Pntr 2Nsd 2NaOH; Sdl NaOH NaCl, where Nph nepheline, natrolite, nordstrandite, paranatrolite, sodalite. As result, about one-third sodium from (and sodalite) set free passes into fluid. This leads to an increase Na/Cl ratio and, hence, pH An stabilizes hyperagpaitic (e.g., ussingite, villiaumite, thermonatrite, trona), which can crystallize close proximity pseudomorphized Thus, feldspathoids increases late-magmatic fluids, turn lead crystallization minerals.