Compositional variation of phlogopite in a marble sample

Size: px

Start display at page:

Download "Compositional variation of phlogopite in a marble sample"

Beatrix Esther Malone
5 years ago
Views:

1 Sonderdrucke aus der Albert-Ludwigs-Universität Freiburg KURT BUCHER Compositional variation of phlogopite in a marble sample Implications for geological thermobarometry Originalbeitrag erschienen in: Journal of metamorphic geology 6 (1988), S

J. metamorphic Geol. 1988, 6, 667-772 Compositional variation of phlogopite in a marble sample: implications for geological thermobarometry K.

2 J. metamorphic Geol. 1988, 6, Compositional variation of phlogopite in a marble sample: implications for geological thermobarometry K. BUCHER-NURMINEN, Department of Geology, University of Oslo, P.B. 1047, Blindern, N-0316 Oslo 3, Norway Abstract. The textural and compositional features of phlogopites in a contact-metamorphic dolomite marble inclusion in the BergeII intrusion (central Alps) and in a metasomatic reaction vein cutting through this marble suggest different origins for vein phlogopites: (a) High-Al vein phlogopite represents former marble phlogopite which has been compositionally modified by reaction with the vein forming fluid. (b) Low-Al vein phlogopite represents phlogopite precipitated from the vein forming fluid. As both types of vein phlogopite were in contact with the same vein forming fluid at the same time, low-al phlogopite most likely represents an equilibrium phlogopite composition, whereas high-al phlogopite does not. High-Al vein phlogopite retained its Al-content from the contact-metamorphic marble parent phlogopite and only underwent Fe Mg exchange with the metasomatic fluid. All the vein phlogopites studied are strongly enriched in Fe relative to marble phlogopite. The data may suggest in general that phlogopite Al/Si ratios may be retained from the conditions under which the phlogopites first formed, whereas the Mg/Fe-ratios may be substantially modified by exchange with other ferromagnesian solid phases and/or a metamorphic fluid at later stages in their metamorphic history. This may have significant effects on calculated pressures and temperatures from thermobarometers involving biotite. Key-words: marbles; non-equilibrium phlogopite compositions; reaction veins; thermobarometry. INTRODUCTION Under equilibrium conditions solid-solution phases such as amphiboles or micas nucleate and grow with a composition controlled by the coexisting phases and the pressure and temperature at the time of growth. Subsequent changes in pressure and temperature may require a continuous adjustment of their composition. Along a prograde P T path, for example, many of the earlier formed phases must continuously readjust their composition (by a number of conceivable mechanisms) in order to maintain equilibrium under changing conditions. The presence of growth zoned phases such as garnet in many low- to mediumgrade metamorphic rocks demonstrates that equilibrium phase compositions may not always be attained. In zoned refactory phases only the 'rim' composition may represent an equilibrium composition controlled by the matrix phases and P and T. The dimension of the equilibrium domains in rocks containing such phases may, therefore, be considerably smaller than grain dimensions. It is more difficult to detect the persistence of non-equilibrium compositions in unzoned matrix phases such as biotite or muscovite. Equilibrium phase compositions are often 'demonstrated' by using criteria which are necessary but not sufficient for equilibrium. Such criteria include, for example: no permitted large compositional variation for a given phase in a sample; no crossing tie-line relationships among mineral pairs from the same sample on chemographical representations; no bimodal distribution of a phase composition for phases showing continuous solid solution; etc. The criteria can strictly be used to demonstrate disequilibrium only. The absence of phase compositions which obviously violate equilibrium constraints does not prove that the phases actually possess equilibrium compositions. Some observations on the composition of phlogopite presented below suggest that compositional readjustment may not be complete in some geological processes. Compositional changes in previously formed phases that respond to changing external conditions may be considerably more complex than anticipated. 667

3 668 K. Bucher-Nurminen GEOLOGICAL OCCURRENCE OF SAMPLE MATERIAL The analysed samples were collected by drill from one of many marble inclusions in the tonalite of the Berge11 intrusion (Val Sissone, northern Italy). The dolomite marble shows abundant phlogopite-rich bands. The bands are typically one to several centimetres thick and probably represent a primary bedding. The characteristic assemblage is dolomite calciteforsterite (clinohumite) spinel phlogopite. The marbles were heated to about C by the intrusion at a pressure of about 2-3 kbar (e.g. Bucher-Nurminen, 1977; Trommsdorff & Evans, 1977). During cooling and uplift of the area the marbles were repeatedly fractured together with their tonalitic envelope. Silicarich hydrothermal solutions flowing along the extension fractures produced a complicated network of reaction veins. The particular tremolite calcite vein which provided the sample for this study (P7D1) formed at about C (Taylor & Bucher- Nurminen, 1986). Because the reaction veins formed from a contact-metamorphic phlogopite marble, the vein forming hydrothermal solution partly modified the composition of pre-existing phlogopites. These compositional variations will be discussed below. The local geology and the formation of the reaction veins have been discussed in some detail by Bucher-Nurminen (1981). TEXTURAL RELATIONSHIPS The textural relationships of the vein are shown in Fig. 1. The vein consists of two distinct parts. An outer part adjacent to unmetasomatized dolomite consists of tremolite, calcite and phlogopite. Here, the primary phlogopite-rich bands of the marble are present. This part of the vein, therefore, represents replaced dolomite. In the central part of the vein this banding is absent. It is interpreted as a sealed extension fracture. The mineralogy of the central part or zone is calcite + tremolite. Minimum fluid/rock ratios during vein formation were of the order of 10 and larger (Taylor & Bucher-Nurminen, 1986). The most significant mineralogical change in the vein is the disappearance of dolomite along a very sharp reaction front. The dolomite was replaced by tremolite + calcite according to the following reaction which went to completion: 5 Dolomite + 8 Si0 2(aq) + H 20 > tremolite + 3 calcite 7 CO 2. (1) Pre-existing contact-metamorphic marble phlogopite in the bands was not replaced by the TEXTURE OF VEIN P7D1 phi bands persist phl bands absent replaced dolomite marble 1 cm dolomite marble (dol+ cal+ fo+spl+phl phlogopite rich bands (primary bedding) tremolite -+ calcite N. \ Nt initial fracture Fig. 1. Textural relationships of vein P7D1. The true total thickness of the vein is about 5-6 cm. The exposed lateral extension exceeds 7 m.

Compositional variation of phlogopite 669 vein forming process, however, its composition was strongly modified by the hydrothermal solution (it is designated as vein phlogopite 0 for 'old').

4 Compositional variation of phlogopite 669 vein forming process, however, its composition was strongly modified by the hydrothermal solution (it is designated as vein phlogopite 0 for 'old'). In addition, new vein phlogopite precipitated from the vein forming solution. The new vein phlogopite occurs in thin bands parallel to the vein and preferentially close to the reaction front or to the fracture wall (it is designated vein phlogopite N for 'new'). In contrast, modified primary phlogopite (vein phl 0) occurs in bands nearly perpendicular to the vein. COMPOSITIONAL DATA Analytical procedure Forty-six phlogopites (averages of five point analyses per grain = 230 analyses) have been analysed by electron microprobe from the tremolite vein P7D1. The analyses were performed using the ARL SEMQ microprobe with four crystal X-ray spectrometers and an X-ray energy dispersive analyser (TN2000, Tracor Northern) at the University of Basel. The microprobe was operated (at 15 kv) with a sample current, beam size and counting time combination which optimized counting statistics and minimized potassium losses due to evaporation. Synthetic stoichiometric F-phlogopite was used as a reference standard for K, Mg, Al, F and Si. Synthetic and natural silicates and oxides were used for the elements Fe, Ca, Ti, Mn and Na. The standardization was repeatedly checked against natural silicates (biotite and hornblende) of well known composition. Raw data were processed and corrected by using Tracor Northern software. Phlogopite compositions The three types of phlogopite display very distinct compositions. Using stoichiometric phlogopite (K 2Mg6Al 2Si 60 22(OH)4) as a reference composition most of the compositional variation can be described by three exchange components: (a) AlAIMg_ I Si _ i(ts); (b) FeMg_ 1(FM); and, (c) Ti (T1). The specific exchange component involving Ti could not be identified. The number of Ti atoms per formula unit, based on 24 oxygens, was used to describe variations in the titanium content. TS (tschermak component) was calculated by subtracting the stoichiometric amount of tetrahedral Al (= 2) from the total number of tetrahedral Al. FM is the number of iron atoms per formula unit (or per reference composition). The unmetasomatized contact-metamorphic marble phlogopites are characterized by high TS and T1 and low FM. Marble phlogopites in the vein, which have been modified by the hydrothermal solution, showed a marked increase in FM relative to their unaltered marble counterpart. The increase is typically by a factor of 4-5. On the other hand, this phlogopite retains its high TS- and TI-content. New phlogopite, which precipitated directly from the vein forming solution, is considerably poorer in T1 than the two other types and has a TS content close to zero. This phlogopite is, however, also strongly enriched in FM compared to the marble phlogopite. Figure 2 shows the TS and T1 variation of phlogopite. It is evident that the two types of vein phlogopite are compositionally (and texturally) unrelated. Phlogopite (0) is indistinguishable from marble phlogopite with respect to TS and T1, phlogopite (N) is close to the reference composition. There is no compositional overlap with respect to TS and T1 between phlogopite (N) and the other groups. The TS versus FM diagram (Fig. 3) shows that high-al phlogopite in the vein is distinguished from its marble counterpart by a four- to five-fold FM-enrichment. Some phlogopite has an FM-content intermediate between the two groups. This phlogopite occurs at the immediate reaction front and is designated transitional phlogopite (vein phi T), its composition resulting from incomplete FM cr) ci 0 itdo piefot) 0 CI 0 CI W I. o Ti Fig. 2. TS and TI variation of four types of phlogopite (reference composition is stoichiometric phlogopite based on 24 anions). (0) Marble phi, (A) vein phi N, (*) vein phl 0, (0) vein phi T.

670 K. Bucher-Nurminen reaction of marble phlogopite with the vein forming fluid phase. The last group of phlogopite (vein phi N) is again compositionally unrelated to the marble phlogopite.

5 670 K. Bucher-Nurminen reaction of marble phlogopite with the vein forming fluid phase. The last group of phlogopite (vein phi N) is again compositionally unrelated to the marble phlogopite. The compositional data (Fig. 3) also show that vein phlogopite (0) is enriched in FM over vein phlogopite (N). It may be concluded from Fig. 3 that phlogopite (0) preferentially incorporates Fe from a fluid of a given composition relative to phlogopite (N). Similar positive TS FM correlations (and consequently nonideal mixing on octahedral sites) have been described from biotite (e.g. Guidotti, Cheney & Guggenheim, 1977), from pyroxenes (e.g. Koons, 1984), and seem to be a general feature in Al Fe Mg silicates. Seifert (1978) observed, increasing Fe Mg disorder with increasing TS in anthophyllite/gedrite series amphiboles. Seifert suggested that an increasing Al-content on the M2 site of the amphibole results in changes in the geometry of all octahedral sites and consequently in changing site preference energies. The observations shown in Fig. 3 are evidence for similar relationships in biotites. Both types of vein phlogopite coexisted with the same fluid phase with a given Fe/Mg ratio. Recent calculations of the composition of this vein forming fluid showed that the fluid was strongly enriched in Fe relative to the solids (Bucher-Nurminen, 1987). Because TS-rich vein phlogopite (0) also has a higher FM content compared to vein phlogopite (N), nonideal Fe Mg mixing behaviour may decrease with increasing TS in phlogopite (biotite) (unless the excess free energy of mixing is negative). It is interesting to note that if one considers only the Fe Mg distribution between a fluid phase and phlogopite, all three types of phlogopite (M,N,O) may have been in equilibrium with a fluid phase of a constant composition. This is because marble phlogopite formed at a higher temperature than vein phlogopite and the Fe-enrichment of the fluid relative to the phlogopite decreases with a decrease in temperature (Schulien, 1975, 1980). However, the total composition of the fluid phase was very different. The fluid phase at the maximum temperature of contact-metamorphism in the marble was an internally derived relatively CO2-rich fluid with a very low content of dissolved aqueous silica. The vein forming fluid, in contrast, had its composition externally controlled, was very H 20-rich and was saturated with aqueous silica (Bucher-Nurminen, 1981). DISCUSSION Figure 4 summarizes the compositional features of the three textural types of phlogopite in sample P7D1. The textures and composition of marble phlogopite and phlogopite in a metasomatic reaction vein suggest two different origins r 00 cob cn Fe Mg _ 1 Fig. 3. TS and FM variation of four types of phlogopite (same phlogopite reference composition and symbols as on Fig. 2) Fe Mg Fig. 4: Idealized TS and FM compositions of four types of phlogopite (same reference phlogopite composition as on Fig. 2). In the compositional range of the sample the two types of vein phlogopite (N,0) define a straight line with a slope of +4. The indicated temperatures represent the temperature of contact metamorphism (600 C) and of hydrothermal vein formation (500 C), respectively.

6 Compositional variation of phlogopite 671 for vein phlogopite. (a) A TS-rich vein phlogopite representing former marble phlogopite (phi M) which has been compositionally modified by reaction with the vein forming fluid (vein phi 0). (b) A TS-poor phlogopite representing phlogoliite precipitated from a phlogopite-saturated vein forming fluid (vein phi N). This leads to the conclusion that because both types of vein phlogopite were in contact with the same vein forming fluid at the same time, vein phlogopite (N) may represent an equilibrium phlogopite composition, whereas vein phlogopite (0) does not. Furthermore, it is suggested that vein phlogopite (0) retained its Al-content from the contact-metamorphic marble phlogopite and exchanged only Fe for Mg with the metasomatic fluid. The higher FM-content of phlogopite (0) relative to phlogopite (N) is probably a consequence of decreasing non-ideal Fe Mg mixing in phlogopite with increasing TS-content. The compositional data (Fig. 4) show that there is no TS-component in the exchange vector which modified phlogopite (M). The exchange process for phlogopite (M) involves FM only and the composition of phlogopite (0) may represent an equilibrium FM-coordinate (for the given vein forming fluid at 500 C). Figure 4 also shows that the TS-coordinate for phlogopite (0) cannot be an equilibrium value. The TS-content of the marble phlogopite is controlled by the assemblage spinel + forsterite which buffers the tschermak component in phlogopite according to the equilibrium: MgAl 204 = Mg2SiO 4 + 2A1Si_ IMg_ (2) spinet olivine phlogopite The data may suggest, in general, that a phlogopite or a biotite which grew under equilibrium conditions with a given TS content tends to retain the originally acquired Al/Si ratio. However, its FM content may be modified by subsequent processes. Although this has been demonstrated only for the presented example, it seems feasible that this may also hold for others. As a consequence, biotite with a true equilibrium composition can only form if all phlogopite formed at earlier stages of the rock's reaction history completely dissolves and phlogopite then precipitates with the equilibrium composition under the new conditions. By analogy a similar behaviour may be predicted for white mica (muscovite celadonite series) or for sheet silicates in general (or possibly for any phase with TS exchange). In the specific reaction vein example the temperature of formation was about 100 C below the maximum temperature (formation of marble phlogopite M) and the vein forming process may have been of short duration (e.g. Mawer, 1987). Therefore, the persistence of metastable biotite compositions may not be surprising because the kinetics of the TS exchange reaction and slow intracrystalline diffusion may not permit equilibration at lower temperature and within a short time interval. In regional high-grade metamorphism the problem of metastable mineral composition may be less dramatic. On the other hand, the vein example shows that the FM exchange reaction between phlogopite and the fluid phase probably reached equilibrium. It is also important to remember that the major reaction in the vein formation was the replacement of dolomite by calcite and calcic amphibole. This discontinuous heterogeneous reaction went to completion leaving no metastable relics of dolomite. The inferred very high fluid/rock ratio associated with vein formation certainly favoured high reaction kinetics. It is concluded that the conditions during vein formation clearly favoured fast and complete reaction even at a temperature which did not exceed conditions of lower amphibolite facies and in a process of short duration compared to the time-scale of regional metamorphism. Furthermore, it is concluded that in sheet silicates (and possibly in other silicates as well) the TS content is fixed at the time of formation and the FM content is subject to subsequent modifications by FM exchange reactions during the subsequent P T history. The situation is complicated by the fact that TS and FM are not independent variables but rather that the two components are positively correlated. The compositional behaviour of phlogopite, as described above, may have significant consequences for geological thermobarometry. Calculated activities of the phlogopite component, using the ideal site mixing model proposed by Powell & Evans (1983) for average compositions of the three different phlogopite populations, are: marble phlogopite (M) = 0.67, vein phlogopite (N) = 0.69, vein phlogopite (0) = 0.52 (referenced to a standard state of unit activity for pure KMg3AISi30 10(OH)2 at any P and 7). Modified vein phlogopite (0) has a calculated phlogopite activity which is 25% lower than the calculated value for the

672 K. Bucher-Nurminen equilibrium vein phlogopite (N) (please note that this difference is for the two types of vein phlogopite which were in contact with the same fluid).

7 672 K. Bucher-Nurminen equilibrium vein phlogopite (N) (please note that this difference is for the two types of vein phlogopite which were in contact with the same fluid). The effect on the Powell & Evans (1983) barometer is relatively small (about 0.85 lnk units for their equilibrium (2), corresponding to a pressure difference of about 1 kbar). However, for Fe Mg exchange thermometry the effects are more serious. Using the garnet biotite thermometer as an example, the calculated temperature difference may be of the order of 50 (545 C (recalculated) and 600 C (measured) at 4 kbar). In the example, the calibration of Ferry & Spear (1978) was used together with mineral compositions from Bucher-Nurminen & Droop (1983) (sample Al26). The temperature difference was calculated for a biotite with a measured TS content of 0.6 and a FM content of It is assumed here that the FM TS correlation seen in Fig. 4 is linear and independent of the magnitude of FM. The example shows that the non-ideal mixing properties of FM in biotite (which is dependent on TS) may have significant effects on calculated temperatures using FM exchange thermometry. In vein sample P7D1 very clear textural and compositional relationships made it possible to distinguish easily between different generations of stable and metastable phlogopite. This may not always be the case, however, and in regional metamorphic metapelites, homogeneous biotite may contain a TS component which was fixed at the time of biotite formation and an FM component which may have been strongly modified after biotite formation. ACKNOWLEDGEMENTS I would like to thank Steve Larter for his comments on the manuscript. I also thank John M. Ferry and Tim J.B. Holland for the constructive reviews which helped to improve this contribution. This paper is No. 30 in the series of the Norwegian Lithosphere Project (IL?). Support from a NAVF Grant Nr. D /109 (Norway) and from a SNF Grant (Switzerland) is acknowledged. I also thank M. Frey and H. Schwander, from the University of Basel, for providing microprobe facilities. REFERENCES Bucher-Nurminen, K Die Beziehung zwischen Deformation, Metamorphose und Magmatismus im Gebiet der Bergeller-Alpen. Schweizerische Mineralogische und Petrographische Mitteilungen, 57, Bucher-Nurminen, K The formation of metasomatic reaction veins in dolomitic marble roof pendants in the BergeII intrusion (Province Sondrio, Northern Italy). American Journal of Science, 281, Bucher-Nurminen, K The composition of hydrothermal fluids responsible for silicate reaction veins in dolomitic marbles. In: Chemical Transport in Metasomatic Processes, (ed. Helgeson, H.C.), pp , Reidel, Dordrecht, Holland. Bucher-Nurminen, K. & Droop, G.T.R The metamorphic evolution of garnet cordierite sillimanite gneisses of the Gruf-Complex, Eastern Pennine Alps. Contributions to Mineralogy and Petrology, 84, Ferry, J.M. & Spear, F.S Experimental calibration of the partitioning of Fe and Mg between garnet and biotite. Contributions to Mineralogy and Petrology, 66, Guidotti, C.V., Cheney, J.T. & Guggenheim, S Distribution of titanium between coexisting muscovite and biotite in pelitic schists from Northwestern Maine. American Mineralogist, 62, Koons, P.O Implications to garnet clinopyroxene geothermometry of non-ideal solid solution in jadeitic pyroxenes. Contributions to Mineralogy and Petrology, 88, Mawer, C.K Mechanics of formation of goldbearing quartz veins, Nova Scotia, Canada. Tecton- physics, 135, Powell, R. & Evans, J.A A new geobarometer for the assemblage biotite muscovite chloritequartz. Journal of Metamorphic Geology, I, Schulien, S Determination of the equilibrium constant and the enthalpy of reaction for the Mg 2 Fe 2 + exchange between biotite and a salt solution. Fortschritte der Mineralogie, 52, Schulien, S Mg Fe partitioning between biotite and a supercritical chloride solution. Contributions to Mineralogy and Petrology, 74, Seifert, F Equilibrium Mg Fe 2 + cation distribution in anthophyllite. American Journal of Science, 278, Taylor, B.E. & Bucher-Nurminen, K Oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids, Bergell aureole, Northern Italy. Geochimica et Cosmochimica Acta, 50, Trommsdorff, V. & Evans, B.W Antigorite ophicarbonates: contact metamorphism in Valmalenco, Italy. Contributions to Mineralogy and Petrology, 62, Received 12 June 1987; revision accepted 15 March 1988

Reactions take place in a direction that lowers Gibbs free energy

Reactions take place in a direction that lowers Gibbs free energy Metamorphic Rocks Reminder notes: Metamorphism Metasomatism Regional metamorphism Contact metamorphism Protolith Prograde Retrograde Fluids dewatering and decarbonation volatile flux Chemical change vs