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To constrain the crystallization of alkaline and volatile-rich lavas present in intraplate settings, we studied the petrological features and the geochemical composition of major, trace, and volatile elements of mineral and bulk-rock of two sodalite-bearing phonolites: (1) haüyne-plagioclase-bearing Si-K-rich phonolite from Laacher See (Germany) and (2) nosean-nepheline-bearing Si-poor phonolite from Saghro (Morocco). In haüyne-bearing phonolites (55-59 wt% SiO 2 , K &gt; Na, Na+K/Al = 0.96-1.08), we found that the low silica and low sodium activity promoted the early crystallization of S-rich haüyne (13.7-13.9 wt% SO 3 , 0.4 wt% Cl) + S-rich apatite (0.7-0.9 wt% SO 3 ), titanite, and rare pyrrhotite followed by clinopyroxene-plagioclase-sanidine at relatively low pressure and temperature (P = 250 MPa and T = 850 °C) and oxidized condition (ΔNNO-NNO+1, where NNO is nickel-nickel oxide buffer). The crystallization of haüyne occurred at fluid-undersaturated conditions from a silicate melt with 6 wt% H 2 O, 0.17-0.23 wt% Cl, 0.11-0.4 wt% S, and 0.07-0.14 wt% F. Nosean-bearing phonolites from Saghro are silica-poor and peralkaline (52-54 wt% SiO 2 , Na &gt; K, Na+K/Al = 1.2) and crystallized at higher P and T (300 MPa and 950 °C) and more reduced conditions (NNO) compared to haüyne-bearing phonolites. The incongruent reaction to form nosean requires high silica and Na 2 O activity. The mineral assemblage and composition suggest early crystallization of nepheline followed by nosean (7.8-8.8 wt% SO 3 ; 1-1.1 wt% Cl). The sequence of crystallization is: clinopyroxene + nepheline + S-poor apatite (&lt;0.04 wt% SO 3 ) + pyrrhotite followed by nosean and titanite. Noseanbearing magmas are fluid-undersaturated with relatively low volatile content (4 wt% H 2 O, &lt;0.25 wt% Cl, &lt;0.056 wt% S, 0.08-0.1 wt% F), although Cl may have exsolved during ascent and formed a fluid phase (NaCl-bearing).

american mineralogist

Role of volatiles (S, Cl, H 2 O) and silica activity on the crystallization of haüyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)