Cordierite formation during the experimental reaction ...

Contrib Mineral Petrol (2014) 168:1063 DOI 10.1007/s00410-014-1063-x

ORIGINAL PAPER

Cordierite formation during the experimental reaction of plagioclase with Mg‑rich aqueous solutions J. Hövelmann · H. Austrheim · A. Putnis 

Received: 8 April 2014 / Accepted: 3 September 2014 © Springer-Verlag Berlin Heidelberg 2014

Abstract  The reaction between plagioclase (labradorite and oligoclase) and Mg-rich aqueous solutions was studied experimentally at hydrothermal conditions (600–700 °C, 2 kbar). During the experiments, plagioclase grains were readily converted to cordierite and quartz within 4 days. The cordierite crystals had well-developed polyhedral shapes, but showed skeletal internal morphologies suggesting that the initial growth occurred fast under high-driving-force conditions. In pure MgCl2 solutions (0.5–5 M), plagioclase dissolution and cordierite precipitation were spatially uncoupled indicating that Al was to some extent mobile in the fluid. Cordierite crystals formed at 700 °C showed orthorhombic symmetry, whereas those formed at 600 °C dominantly persisted in the metastable hexagonal form suggesting a strong increase in Al, Si ordering speed between 600 and 700 °C. The thermodynamic evolution of the fluid–solid system ultimately resulted in stabilization of Ca-rich plagioclase as demonstrated by partial anorthitization of unreacted plagioclase grains. Cordierite was also observed to form when Mg was added to a potentially albitizing Na-silicate-bearing solution. In that case, cordierite precipitation appeared to be more closely coupled to plagioclase dissolution, and secondary alteration of remnant plagioclase grains did not occur most likely due to armoring of the plagioclase by the cordierite overgrowth. The Communicated by J. Hoefs. J. Hövelmann (*) · A. Putnis  Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany e-mail: j.hoevelmann@uni‑muenster.de; [email protected] J. Hövelmann · H. Austrheim  Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, 0316 Oslo, Norway

fast reaction rates observed in our experimental study have potential implications for Mg-metasomatism as a rockforming process. Keywords  Cordierite · Mg-metasomatism · Anorthitization · Al,Si ordering · Experimental petrology · Cordierite-orthoamphibole rocks

Introduction Plagioclase feldspars are among the most abundant minerals in the Earth’ crust, and their interaction with saline aqueous solutions can play an important role in controlling alkali and alkaline earth metal concentrations in crustal fluids. Previous experimental work has been mostly focused on determining cation exchange equilibria between plagioclases and Na–Ca–Cl solutions (O’Neil and Taylor 1967; Orville 1972; Schliestedt and Johannes 1990; Berndt and Seyfried 1993; Shmulovich and Graham 2008). A number of experiments have also been performed with Na–K–Ca– Cl fluids of seawater salinity and plagioclase-bearing mineral mixtures to mimic hydrothermal alteration processes at mid-ocean ridges (e.g., Seyfried et al. 1988; Berndt et al. 1989). In nature, a common result of the interaction of plagioclase with saline fluids is albitization, i.e., the replacement of plagioclase by almost pure albite. Regional scale Na-metasomatism often involves extensive albitization transforming whole rocks into albite-dominated assemblages. Classic field examples include the Bamble Sector in SE-Norway (Engvik et al. 2014; Nijland et al. 2014 and references therein) and the Curnamona-Province in S-Australia (Oliver et al. 1994; Clark et al. 2005) where metasomatic albitization affected a wide range of rock

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compositions over many square kilometers and is spatially related to significant mineral and ore deposits. Hydrothermal experiments have demonstrated that albitization reactions can take place within weeks at 500– 600 °C (Hövelmann et al. 2010; Norberg et al. 2011), suggesting that incomplete reactions in nature are likely the result of lack of fluid. Albitization is an example of feldspar–fluid reactions that involve the presence of a Na-rich fluid. However, in nature, metasomatic fluids are not confined to Na-rich fluids, and infiltrations of Ca-, K-, Fe-, Si- or Mg-rich fluids have all been proposed to account for chemical transformations of rocks (e.g., Schreyer 1977; Roddy et al. 1988; Manning 1997; John and Schenk 2003; Dalstra and Guedes 2004; Jöns and Schenk 2004; Beinlich et al. 2010; Ferrando 2012). As yet, there have been very few experimental studies investigating the reactivity of plagioclase in the presence of fluids rich in elements other than Na, Ca, and K. Notable exceptions are studies on plagioclase–CO2 reactions that were conducted to estimate the CO2 storage potential of plagioclase-rich rocks (Hangx and Spiers 2009; Munz et al. 2012). In this contribution, we have performed a series of hydrothermal experiments to study the reactions between plagioclase and Mg-rich saline solutions. Evidence for the existence of Mg-rich brines in the mid-to-lower crust comes from both direct fluid inclusion data (Ferrando et al. 2009) and indirect mass transfer calculations (e.g., Demény et al. 1997; Engvik and Austrheim 2010). High-salinity Mg–Cl fluids (up to 28 wt% NaCleq, Ferrando et al. 2009) are, for example, thought to have caused crustal scale Mg-metasomatism of Alpine granitoids resulting in widespread occurrences of Mg-rich high-pressure rocks (e.g., whiteshists, leucophyllites) (Demény et al. 1997; Barnes et al. 2004; Ferrando 2012). In this specific case, it is assumed that the metasomatic fluids were produced during dehydration of serpentinites (e.g., in a subduction-zone setting) and subsequently infiltrated the continental crust along high-strain zones. More generally, however, several other sources of Mg-rich fluids are also conceivable, including dehydration of Mg-rich evaporitic sediments (Gebauer et al. 1997) and albitization of mafic plutonic rocks (Rubenach and Lewthwaite 2002). Our experiments were performed under P–T conditions representative for hydrothermal reactions occurring at mid-crustal levels. The reaction products were characterized in detail using scanning electron microscopy (SEM), electron microprobe analysis (EMPA), Raman spectroscopy, and X-ray Diffraction (XRD) to gain insights into the reaction mechanism and kinetics. Our experimental observations are complemented by geochemical calculations and implications for metasomatic processes in nature are discussed.

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Materials and methods Experimental procedures We used gem-quality crystals of labradorite (Ab39An60Or1; Nuevo Casas Grande, Mexico) and oligoclase (Ab73An23Or4; Minas Gerais, Brazil) as starting material for the experiments. The same material has been used for albitization experiments performed by Hövelmann et al. (2010). Fragments of both specimens were crushed and sieved to obtain size fractions of either 200–500 or 500–1,000 µm. Before use, the material was thoroughly washed in doubly deionized water (resistivity >18 mΩ cm−1) and dried at 100 °C. Solutions were prepared by dissolving calculated amounts of reagent grade MgCl2•6H2O (Sigma-Aldrich) into deionized water in order to obtain concentrations of 0.5, 2, or 5 M. For each experiment, a gold capsule (inner diameter: 2.8 mm; length: 2.5 cm) was filled with 15–25 mg plagioclase grains plus fluid (Table 1). The capsule was then welded shut, weighed, and placed in an oven at 80 °C overnight. The capsule was re-weighted in order to check for leaks. It was then loaded into a standard horizontally mounted cold-seal pressure vessel. Experiments were performed at a temperature of either 600 or 700 °C and a pressure of 2 kbar. The target temperature (±5 °C) and pressure (±0.1 kbar) were reached within 2–3 h. Experimental durations ranged from 4 to 22 days. Experiments were quenched isobarically using compressed air. Room temperature was reached within 10 min. The capsules were then carefully opened, dried at 80 °C in an oven overnight, and the sample material extracted. We also performed similar experiments to study the effect of Mg added to a potentially albitizing fluid (see also Hövelmann et al. (2010)). For these experiments, weighed amounts of MgCl2•6H2O and NaCl were added to a Na-silicate-bearing solution (0.35 M NaOH + 0.43 M SiO2(aq); prepared using a reagent grade solution (SigmaAldrich) based on 13.8 % NaOH, 25.9 % SiO2, and 60.3 % H2O). The plagioclase-to-fluid mass-ratio in these experiments was about 1:2. All experimental runs performed in this study are summarized in Table 1. The reacted samples were divided for further analytical work. Part of the material was fixed onto aluminum stubs using Leit-C tabs and then observed in the SE mode of an SEM to study the external morphologies of the grains. Another part was embedded into epoxy resin and polished to make cross sections through the grains. These grain mounts were used to make BSE images and to determine the chemical composition of the reacted material using SEM and EMPA, respectively. The rest of the material was finely ground and used for XRD analysis.

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Contrib Mineral Petrol (2014) 168:1063 Table 1  Summary of experimental run conditions Exp.

Starting solid

Reactant solution

Added reagents

Run conditions

Plagioclase

Mass added (mg)

Grain size (µm)

Composition

Mass added (mg)

MgCl2·6H2O (mg)

NaCl (mg)

T (°C)

P (bar)

Time (days)

2 3 4 5 6 7 8 9 10 11

Labradorite (An60) Labradorite (An60) Labradorite (An60) Labradorite (An60) Oligoclase (An22) Oligoclase (An22) Labradorite (An60) Oligoclase (An22) Labradorite (An60) Labradorite (An60) Labradorite (An60)

14.23 14.34 24.69 18.30 26.86 21.76 24.45 24.10 7.94 7.60 9.87

200–500 200–500 200–500 200–500 200–500 200–500 200–500 200–500 500–1,000 500–1,000 500–1,000

5 M MgCl2 5 M MgCl2 2 M MgCl2 2 M MgCl2 2 M MgCl2 2 M MgCl2 0.5 M MgCl2 0.5 M MgCl2 Na-silicatea Na-silicatea Na-silicatea

14.23 17.36 27.33 25.83 27.59 23.81 24.85 25.03 15.04 14.88 17.23

– – – – – – – – 3.13 2.65 2.89

– – – – – – – – 2.55 2.44 2.64

600 700 600 700 600 700 600 600 700 600 600

2,000 2,000 2,000 2,000 2,000 2,000 2,000 2,000 2,000 2,000 2,000

4 5 4 8 4 4 4 4 13 22 5

12

Labradorite (An60)

9.41

500–1,000

Na-silicatea

24.48

2.83

2.44

700

2,000

5

1

a

  solution was composed of 0.35 M NaOH and 0.43 M SiO2(aq)

Analytical methods

Results

Backscattered (BSE) and secondary electron (SE) images were collected with a JEOL JSM 6460 LV scanning electron microscope (SEM) equipped with an energy- dispersive X-ray (EDX) detector for qualitative elemental analyses. Quantitative chemical analyses of the major and minor elements Si, Ti, Al, Fe, Cr, Mg, Ca, Na, and K were carried out using a Cameca SX100 electron microprobe (EMP). For all analyses, a counting time of 10 s was used at an acceleration voltage of 15 kV and a beam current of 10 nA. Standardization was carried out on synthetic element oxides and natural minerals. All analyses were normalized to oxygens per formula unit assuming stoichiometric mineral compositions. Single micro-Raman measurements were performed using a Horiba Jobin–Yvon XploRA confocal Raman spectrometer and the 638 nm line of a He–Ne laser as excitation source. The scattered light was collected in a 180° backscattering geometry and analyzed with a charge-coupled device (CCD) detector after being dispersed by a grating of 1,200 grooves/mm and passed through a 100-µm entrance slit. All spectra were collected using a 100 × objective, a confocal aperture of 500 µm, and an acquisition time of 2 × 60 s. X-ray powder diffraction (XRD) analyses were performed using a Phillips X’Pert PW3040 X-ray diffractometer equipped with a scintillation counter detector. Measurements were made with a CuKα1 radiation source operating at 45 kV and 40 mA. Data were collected in the 2θ range between 29 and 30° using a step size of 0.01° and a counting time of 6 s per step.

Morphological and textural and features of the run products Experiments with pure MgCl2 solutions Run products of experiments with pure MgCl2 solutions (Exp. #1–8) revealed abundant prismatic crystals with hexagonal shapes (Fig. 1). These were clearly identified as cordierite (Mg2Al4Si5O18) by Raman spectroscopy. Experiments performed at 700 °C (Exp. #2, #4, #6) resulted in the formation of spherical, polycrystalline cordierite aggregates. These were several tens to hundreds of µm large and consisted of numerous (>100) intergrown crystals radiating from the center (Fig. 1a). Individual cordierite crystals were typically smaller than 50 µm. In contrast, cordierite aggregates formed at 600 °C (Exp. #1, #3, #5, #7, #8) were composed of markedly fewer, but larger (up to 250 µm) crystals (Fig. 1b). Furthermore, the degree of crystal aggregation decreased with decreasing MgCl2 concentration of the starting solution. Most cordierite crystals showed well-developed, polyhedral external morphologies with smooth faces. The basal faces were commonly characterized by spiral steps (Fig. 1c). In a few instances, however, these faces contained large holes reaching more than 10 µm in diameter (Fig. 1d). Moreover, SEM images of cross sections revealed a large number of pores inside the crystals (Fig. 1e, f). In all experiments, numerous idiomorphic quartz crystals, up to 20 µm in size, were formed concomitantly with cordierite (e.g., Fig. 1c). However, quartz was notably more abundant in the run products from the oligoclase experiments (Exp. #1-4, #7) than in those of labradorite experiments (Exp. #5, #6, #8).

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Fig. 1  Run products from experiments with pure MgCl2 solutions. a SEM-SE image of spherical cordierite (Crd) aggregates formed in a 5 M solution at 700 °C. Individual cordierite crystals show euhedral prismatic shapes. Co-precipitation of abundant quartz (Qtz) can be seen on the lower left of the image. b Cordierite aggregates formed in a 5 M solution at 600 °C. Lower temperatures tended to produce aggregates with fewer and larger individuals. c Higher magnification image of a cordierite aggregate showing spiral steps on basal faces of individual cordierite crystals and growth of euhedral quartz crystals. d Hexagonalshaped cordierite crystal showing large pores on the basal face. e–f SEM-BSE images of cross sections through cordierite aggregates revealing high intracrystalline porosities

In experiments performed with 0.5 and 2 M MgCl2 solutions (Exp. #3-8), remnants of the original plagioclase were partly replaced by a secondary plagioclase (Fig. 2). SEM surface images showed that the secondary plagioclase formed flat layers around the original plagioclase so that the angular morphologies of the grains were retained (Fig.  2a). EDX analyses demonstrated that the newly formed plagioclase is richer in Ca and Al, but poorer in Na and Si compared to the original plagioclase (Fig. 2b). Commonly, large holes with diameters of several tens of µm were present in the secondary plagioclase layer (Fig. 2c). In some areas where the replacement layer was interrupted, elongated, parallel oriented growth islands of the secondary plagioclase could be seen on the surface of the original plagioclase (Fig. 2d). BSE images of cross sections through partly replaced plagioclase grains revealed sharp compositional fronts between cores of the parent and rims of the product plagioclase (Fig. 2e, f).

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In general, it was observed that the amount of plagioclase in the run products decreased with both increasing MgCl2 concentration of the starting solution and increasing reaction temperature. After reaction with the 5 M MgCl2 solution (Exp. #1, #2), only a few tiny remnants of the original plagioclase were left. These were fully enclosed in aggregates of cordierite and did not show any signs of compositional alteration. Experiments with Na‑silicate‑bearing solutions Experiments with Na-silicate-bearing solutions (Exp. #9–12) also resulted in the formation of cordierite aggregates consisting of several well-developed, polyhedral crystals (Fig. 3). The degree of crystal aggregation was typically lower than in pure MgCl2 solutions, but again higher at 700 °C (Exp. #9, #12) than at 600 °C (Exp. #10, #11) (Fig. 3a, b). In general, cordierite crystals formed in

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Fig. 2  Replacement features of remnant plagioclase (Pl) grains from experiments with pure MgCl2 solutions. a SEM-SE image of a plagioclase grain partly replaced by a secondary plagioclase. The secondary plagioclase forms a flat layer around the original plagioclase maintaining its angular morphology. b SEM-EDX analyses of the original and secondary plagioclase shown in (a). The secondary plagioclase is characterized by higher Al and Ca and lower Na and Si concentrations. c Partly replaced plagioclase grain overgrown by some cordierite crystals. Large holes are visible in the secondary plagioclase, and small euhedral quartz crystals were deposited on top of the external plagioclase surface. d Higher magnification image of a partly replaced plagioclase grain showing oriented growth of the secondary plagioclase. e–f SEM-BSE images of cross sections through partly replaced plagioclase grains revealing sharp compositional interfaces between the original (darker cores) and secondary plagioclase (brighter rims)

the Na-silicate-bearing solutions were characterized by much lower internal porosities compared to those formed in pure MgCl2 solutions. Locally, however, significant variations in porosity could be observed (Fig. 3c). In most cases, the inner regions of the cordierite aggregates exhibited relatively high porosities. These zones were typically surrounded by almost pore-free regions with thicknesses of up to 100 µm. Small remnants of the original plagioclase were frequently enclosed in the cordierite aggregates. These showed no signs of compositional alteration. As in the experiments with pure MgCl2 solutions, cordierite formation was accompanied by precipitation of abundant quartz. In addition, small aggregates of fibrous crystals were formed on the surface as well as within the pores of the cordierite crystals (Fig. 3d). This phase was identified as mullite (Al6Si2O13) by means of Raman spectroscopy.

Chemical compositions of reaction products Chemical compositions of the cordierites as well as the original and secondary plagioclases are reported in Table 2. Cordierite crystals formed in the MgCl2 solutions (Exp. #1–8) showed no systematic changes in composition with reaction temperature or MgCl2 concentration of the starting solution. Moreover, no significant compositional differences were detected between cordierites formed in experiments with labradorite (Exp. #1–4, #7) and those formed in experiments with oligoclase (Exp. #5, #6, #8). Minor amounts of Na2O (0.21 ± 0.14 wt%) and CaO (0.52  ± 0.13 wt%) were detected in all analyzed grains yielding an average composition that corresponds to (Mg1.94Na0.04Ca0.06)Σ=2.04Al4.00Si4.99O18. The total weight percentages were commonly below 98 wt %, indicating the presence of volatiles (probably H2O) in the cordierite

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Fig. 3  Run products from experiments with Na-silicatebearing solutions. a SEM-SE image of a cordierite aggregate formed at 700 °C. The degree of crystal aggregation was typically lower than in pure MgCl2 solutions. b Two intergrown polyhedral cordierite crystals from an experiment at 600 °C. c SEM-BSE image of a cross section through a cordierite aggregate. The inner zone reveals a relatively high porosity, whereas the outer zone appears pore-free. Small remnants of the original plagioclase are fully enclosed by the cordierite. d Higher magnification image of a porous cordierite zone showing inclusions of quartz and mullite (Mul)

structure. Cordierite crystals formed in the Na-silicatebearing solutions with added MgCl2 (Exp. #9–12) typically had significantly lower CaO concentrations (0.13  ± 0.04 wt%) compared to those formed in pure MgCl2 solutions. Their average composition corresponds to (Mg1.93Na0.07Ca0.01)Σ=2.01Al3.88Si5.10O18. The compositions of the pristine labradorite and oligoclase are approximately An39Ab60Or1 and An22Ab74Or4, respectively, when expressed in terms of mol % of the anorthite (An; CaAl2Si2O8), albite (Ab; NaAlSi3O8), and orthoclase (Or; KAlSi3O8) components. Minor element oxides in the original labradorite include MgO, FeO, and TiO2, whereas the original oligoclase contains minor amounts of FeO only. The reaction rims that formed around both labradorite and oligoclase during experiments with 2 and 0.5 M MgCl2 solutions were consistently enriched in the anorthite component and depleted in the albite and orthoclase components. In addition, all minor element oxides that could be detected in the pristine plagioclases were below the detection limit in the reaction rims. The reaction rims were increasingly enriched in the anorthite component with increasing reaction temperature as well as with decreasing MgCl2 concentration in the starting solution. At 600 °C and in 2 M MgCl2 solution, anorthite contents increased from 60 to 82 mol% in the labradorite experiment (Exp. #3) and from 22 to 41 mol% in the oligoclase experiment (Exp #5). Increasing the reaction temperature to 700 °C, but keeping the MgCl2 concentration of the starting solution constant, resulted in further increases to 92 (labradorite experiment,

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Exp. #4) and 51 mol% An (oligoclase experiment, Exp. #6). On the other hand, decreasing the initial MgCl2 concentration to 0.5 M, but keeping the temperature constant, produced reactions rims with 87 (labradorite experiment, Exp. #7) and 61 mol% An (oligoclase experiment, Exp. #8). Structural states of the newly formed cordierites Raman spectroscopy and XRD were used to investigate the state of Al,Si ordering in the newly formed cordierite crystals. It is known that cordierite can occur as two polymorphs, i.e., in a high-temperature hexagonal form (stable at T > 1,450 °C) and a low-temperature orthorhombic form (stable at T