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Nepheline syenites and related magmatic fluids in the Ditr u Alkaline Massif, Transylvania, Romania a, a b András Fall *, Robert J. Bodnar and Csaba Szabó a Fluids Research Laboratory, Dep. of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, VA 24061, USA b Lithosphere Fluid Research Laboratory, Dep. of Petrology and Geochemistry, Eötvös University, Pázmány sétány 1/C, H-1117, Budapest, HU ã 2. Geology and petrography of DAM The Ditrãu Alkaline Massif (DAM) is situated in the Eastern Carpathians, in Romania, and is a Mesozoic alkaline igneous complex formed during an extensional phase of the Alpine evolution 2 associated with a rifted continental margin. The DAM is an intermediate size massif (about 800 km ) with a quasi-circular shape with imperfect ring structure and irregular indentations. The lithology includes a series of ultramafic rocks in the northwest, silica-oversaturated alkaline rock mostly at the northern end, and undersaturated alkaline rocks (nepheline syenites) predominating in the central and eastern margin of the massif. The nepheline syenite is a coarse- to medium-grained rock that occurs in massive or foliated varieties, and consists of large (5-10 mm) euhedral grains of feldspar and nepheline. The mafic components are present in smaller amounts, and include biotite and clinopyroxene (mostly aegirine), and rarely, amphibole. Other important phases are calcite, and the secondary phases cancrinite (Fig. 1), sodalite (Fig. 2) and analcime (Fig. 3) formed mostly at the expense of nepheline. Apatite, titanite and zircon are present as accessory phases. 1.0 mm 1.0 mm Cc Cn Ne 0.5 mm 0.5 mm Cn So 2.0 cm 2.0 cm Ne Anc 1.Introduction As silicate magmas crystallize they may become saturated in volatiles and exsolve fluids that include aqueous-carbonic or hydrosaline chloride-rich liquids, among others (Bodnar, 1995; Webster, 2004). These volatiles may be trapped as fluid inclusions. Primary fluid inclusions in minerals representing different stages in the paragenesis document the presence and evolving composition of fluids in the nepheline syenites of the Ditrãu Alkaline Massif. Fig. 1. Cancrinite (Cn) formed at the boundary between nepheline (Ne) and calcite (Cc). Fig. 2. Cancrinite (Cn) and interstitial sodalite (So). Fig. 3. Analcime (Anc) reaction rim around nepheline (Ne). 30 μm A b H V a 3 μm B H V 15 μm C V 10 μm D b V H a 10 μm E V H b a 15 μm F V 3. Fluid inclusions Primary aqueous fluid inclusions were observed in nepheline, albite, aegirine and cancrinite. At room temperature the inclusions in nepheline range from polyphase high-salinity inclusions to 2-phase low- salinity inclusions. The large polyphase inclusions (Type I) consist of L+V and 3 (Fig. 3A) or more solid phases, and occur randomly, or in small clusters. The 3- (Type II) and 2-phase (Type III) inclusions (Fig. 3B,C) occur mostly in clusters, oriented parallel to a cleavage direction of the nepheline. In most cases all compositional varieties occur in the same crystal, however some of the crystals contain only Type II or Type III inclusions. The polyphase inclusions are considered to be primary based on their distribution within crystals (Bodnar, 2003a). Type II and III primary inclusions also occur in late stage nepheline. When all compositional varieties occur in the same crystal, the Type II and III inclusions are younger than Type I inclusions. The crystals are interpreted to have formed early and fluids representing Type II and III inclusions were trapped later along the cleavage planes of the crystals. Only Type I inclusions (L+V+3 solids) were observed in the aegirine (Fig. 3D) and albite (Fig. 3E). They occur randomly, as individual inclusions, and mostly have irregular shapes. The cancrinite contains 2-phase (L+V) inclusions (Fig. 3F) and most of the inclusions have a specific elongated triangular shape. The “tip” of the inclusion faces opposite to the growth direction of the host mineral. 4. Microthermometry The fluid inclusion compositions were determined by microthermometry and Raman spectroscopy. The initial ice-melting temperatures of Type II, III and cancrinite-hosted fluid inclusions occur between -23 and -21°C, therefore these inclusions were interpreted using the H O-NaCl system 2 (Bodnar, 1993). The salinity of Type II inclusions was determined from halite dissolution temperatures (Bodnar, 2003b) which ranged from 264 to 325°C. According to this the salinities range from 35.6 to 40.2 wt% NaCl eqv., based on the equations of Sterner et al. (1988). The salinity of Type III inclusions and inclusions in cancrinite was determined from the ice-melting temperature (or freezing-point depression) using the equation of Bodnar (1993). Ice-melting temperatures of Type III inclusions ranged from -17.3 to -20.7°C, corresponding to salinities between 20.3 and 22.8 wt% NaCl eqv. Total homogenization temperature ranged from 243 to 305°C. Ice-melting temperatures of inclusions in cancrinite ranged from -3.7 to -6.3°C, corresponding to salinity of 6.0 to 9.6 wt% NaCl eqv., and homogenize between 180 and 230°C. The relationship between salinity and homogenization temperature for the different inclusion types is shown on Fig. 4. 10 20 30 40 100 200 300 400 L+V+Halite (Type II) L+V (Type III) L+V (in cancrinite) Homogenization temperature (°C) Wt% NaCl eqv. 5. Raman spectroscopy Daughter minerals in the Type I inclusions were identified by Raman spectroscopic analyses. This information was used to estimate the chemical composition of fluids in the inclusions. Most of the Type I inclusions contain 3 solid phases, and are described here in the order in which they dissolve during heating. The first solid is a tabular, elongated, anisotropic phase (a on Fig. 3A) that dissolved at about -1 138-148°C. Raman spectra display two fundamental vibrations at 152 and 1063 cm (Fig. 5A) that -1 -1 correlate most closely with thermonatrite (156 and 1062 cm , Na CO ·H O), trona (164 and 1060 cm , 2 3 2 -1 Na (CO )(SO ) ) or gaylussite (164 and 1071 cm , Na Ca(CO ) ·5H O). This phase is probably 3 3 42 2 32 2 thermonatrite mixed with the other above mentioned phases, and can be considered to be a complex hydrated carbonate. The second solid is a tabular, anisotropic phase (b on Fig. 3A) that dissolved around 173-187°C. The Raman spectrum displays fundamental vibrations at 145, 685, 1045, 1266 -1 and 1434 cm (Fig. 5B). This phase was identified as the sodium bicarbonate nahcolite (NaHCO ). All 3 Type I inclusions contained a cubic, isotropic phase ( H on Fig. 3A) that showed a very weak Raman signal or no signal at all, and was identified as halite after the optical and dissolution properties (dissolved around 272-325°C). 90 80 70 60 90 80 775°C 700°C 500°C NaAlSiO 4 KAlSiO 4 SiO 2 Qz Ne Ks Nepheline compositions of DAM nepheline syenites Morozewicz and Buerger ideal nepheline compositions (Tilley, 1954) M B 1 200 400 600 800 2 3 4 5 6 0 245 305 Pressure (kbars) 180 230 L+V in cancrinite Type III in nepheline A Temperature (°C) Polyphase Type I fluid inclusion in nepheline. Three-phase Type II fluid inclusion in nepheline. Two-phase Type III fluid inclusion in nepheline. Polyphase Type I fluid inclusion in aegirine. Polyphase Type I fluid inclusion in albite. Triangular two-phase inclusion in cancrinite. Fig. 3. Photomicrographs of different fluid inclusion types from minerals of the nepheline syenites. 6. Discussion The nepheline-kalsilite-silica (Ne-Ks-Qz) diagram is a geothermometer because the amounts of kalsilite and silica that can be incorporated into nepheline vary with temperature (Fig. 6). Nepheline compositions of the DAM (Constantinescu and Anastasiu, 1979; Morogan et al., 2000) plotted on the Ne-Ks-Qz diagram indicate crystallization temperatures in the range 500-700°C. 99.1273 304.46 505.423 702.151 894.772 1083.42 1268.21 1449.25 -1 Wavenumber (cm ) A 152 1063 Intensity (a.u.) 100.163 305.474 506.416 703.123 895.727 1084.35 1269.12 1450.15 B 145 685 1045 1266 1434 -1 Wavenumber (cm ) Intensity (a.u.) Fig. 5. Raman spectra of (A) a complex hydrated carbonate daughter mineral and (B) a nahcolite daughter mineral in Type I fluid inclusions in nepheline, albite and aegirine. Isochores representing the minimum and maximum homogenization temperatures of the Type III inclusions and inclusions in cancrinite are plotted on Fig. 7 along with the calculated temperature of crystallization of nepheline. The intersection of the isochore field for Type III inclusions with the formation temperature range of nepheline defines a lower pressure of about 2.5 kbars. The upper pressure limit of 5.0 kbars is estimated from the position of DAM relative to the units of the Bucovina nappe system penetrated by the massif, which suggest that the melt crystallized not deeper than about 15 km. The deep crustal level is supported by the long isobaric cooling history, which lasted about 20-25 Ma (Kräutner and Bindea, 1998). According to this, crystallization of secondary phases occurred approximately in the same pressure range as the nepheline. Hence, the P-T trapping conditions of inclusions in cancrinite can be estimated from the intersection of the isochore field with the 2.5-5.0 kbars pressure range. The paragenesis (Fig. 8A) shows early phases containing high-salinity carbonate rich fluid inclusions and the late phases containing low- salinity fluid inclusions. The partitioning of chlorine between the silicic melt and the exsolving aqueous fluid is strongly pressure dependent (Fig. 8B, Cline and Bodnar, 1991). In higher pressure systems (~2.0 kbars) the first fluid exsolved from the melt has high salinity and salinity decreases as crystallization proceeds. Phase equilibria confirm a minimum pressure of formation for the DAM nepheline syenites of about 2.5 kbars. As a result, the first exsolved fluids had the highest salinity, and fluid salinity decreased as crystallization progressed. Fig. 7. P-T diagram showing the isochores for the Type III fluid inclusions in nepheline with an average salinity of 21 wt% NaCl eqv., and for inclusions in cancrinite with an average salinity of 7 wt% NaCl eqv. The dashed lines show the estimated P-T conditions of crystallization of nepheline in the DAM. Crystallization progress 0.83 0.85 0.87 0.89 0.91 0.93 0.95 0.97 0.99 0.980 0.985 0.995 0.990 1.000 a (aq) HO 2 nepheline amphibole, biotite early (40.2-35 wt%NaCl) middle (22.8-20.3 wt%NaCl) late (9.6-6.0 wt%NaCl) cancrinite, analcime a (m) HO 2 aegirine, albite The decreasing salinity observed in the DAM is consistent with formation of late primary and secondary phases mostly by alteration of nepheline (and albite) according to the following scenario. Calcite crystallizes as a late primary phase, removing most of the carbonate from the fluid. In the sub- solidus region, nepheline is altered to cancrinite (1), removing additional carbonate and chlorine and sulfate from the fluid. In situations where cancrinite is formed at the grain boundary between nepheline and calcite (Fig. 1), calcite serves as the source of carbonate and calcium to form cancrinite. Nepheline and, to a lesser extent, albite are altered to sodalite and remove NaCl from the fluid (2,3). The albite alteration generates silica that reacts with nepheline to form analcime (4). 6 NaAlSiO + 2 Ca(Cl ,CO ,SO ) + H O = Na Ca (AlSiO ) (Cl ,CO ,SO ) ·(H O) 4 2 3 4 2 6 2 46 2 3 42 2 6 NaAlSiO + 2 NaCl = Na (AlSiO ) Cl 4 8 46 2 6 NaAlSi O + 2 NaCl = Na (AlSiO ) Cl + 12 SiO 3 8 8 46 2 2 (1) nepheline complex carbonate cancrinite nepheline from fluid from fluid albite sodalite sodalite NaAlSiO + H SiO = NaAlSi O ·H O + H O 4 4 4 2 6 2 2 silica in fluid nepheline analcime from fluid (2) (3) (4) Biotite and amphibole occur as late phases in the nepheline syenites. The solubility of water in a nepheline syenite melt at 3.5 kbars is estimated to be approximately 7 wt.%, and the fluid inclusion evidence suggests that the melt was saturated in H O during much of its 2 crystallization history. The activity of water in the melt and in the coexisting aqueous phase both increase during crystallization, consistent with the appearance of the hydrous phases biotite and amphibole late in the paragenesis (Fig. 9). 7. Conclusions During crystallization, the melts exsolved a high salinity, carbonate-rich magmatic fluid that evolved to lower salinity as crystallization progressed. Phases that occur early in the paragenesis contain high- salinity inclusions while late phases contain low-salinity inclusions. The decrease in salinity is consistent with formation of mineral phases that remove Cl, CO and SO from the aqueous solution. 3 4 The activity of water (a ) in the melt increases during crystallization, resulting in the formation of H2O hydrous phases during late-stage crystallization of the nepheline syenites. Salinity ~2.0 kbar melt solid aegirine nepheline albite perthites calcite cancrinite sodalite analcime Paragenesis A B Crystallization progress Fig. 8. A. Paragenesis of nepheline syenites. B. Evolution of salinity of a silicic melt at 2.0 kbars. Fig. 4. Relationship between salinity and homogenization temperature for the various H O- 2 NaCl fluid inclusion types studied. Type II inclusions homogenize by halite dissolution after the bubble disappearance. Fig. 9. Relationship between the activity of water in the melt (a (m)) H2O and the activity of water in the coexisting aqueous solution (a (aq)). H2O Fig. 6. Nephelines from the DAM fall mostly between the 500 and 700°C isotherms. References Bodnar, R.J., 1993. Geochim. Cosmochim. Acta 57, 683-684. Bodnar, R.J., 1995. Mineral. Assoc. of Can. Short Course 23, 139-152 Bodnar, R.J., 2003a. Mineral. Assoc. Can. Short Course 32, 1-8. Bodnar, R.J., 2003b. Mineral. Assoc. Can., Short Course 32, 213-231. Cline, J.S., Bodnar, R.J., 1991. J. Geophys. Res. 96, 8118-8126. Constantinescu, E., Anastasiu, N., 1979. Analele Univ. Buc., Geol., XVIII, 15-27. Kräutner, H.G., Bindea, G., 1998. Slovak Geol. Mag. 4, 213-221. Morogan, V., Upton, B.G.J., Fitton, J.G., 2000. Mineral. Petrol. 69, 227-265. Sterner, S.M., Bodnar, R.J., 1984. Geochim. Cosmochim. Acta 48, 2659-2668. Tilley, C.E., 1954. Am. J. Sci. 252, 65-75. Webster, J.D., 2004. T Chem. Geol. 210, 33-48. Funding for this research was provided by grants from the U.S. N.S.F. to R.J. Bodnar (Grant EAR-0125918) and Lithosphere Fluid Research Laboratory.
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