Journal of Petrology Advance Access published online on January 7, 2009
Journal of Petrology, doi:10.1093/petrology/egn076
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Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Received February 14, 2008; Revised typescript accepted December 9, 2008
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe intercumulus liquid of a crystal mush fills pore spaces, and typically solidifies to form overgrowths on cumulus grains and poikilitic post-cumulus minerals. If the liquid is immobile, solidification produces zoned intercumulus minerals, as a result of progressive fractionation of the residual liquid. Convection within the mush results in buffering of the liquid composition, and thus limits mineral zonation. For fully solidified cumulates, ‘fossil’ changes in liquid composition or porosity are difficult to identify. However, detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material. Ti contents of plagioclase overgrowths, in samples from the lowermost parts of the Skaergaard Intrusion, show strong variations with anorthite content. With decreasing XAn, Ti concentrations first rise and then fall, consistent with changing TiO2 contents of the intercumulus liquid during solidification. TiO2 in plagioclase decreases sharply at An55, reflecting local saturation of Fe–Ti oxides. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is limited. Other than slight reverse zones that may occur on the plagioclase margins, XAn falls continuously during crystallization. The reverse zoning is interpreted as the result of compaction-driven dissolution and reprecipitation of plagioclase. The continual decrease in XAn is exploited, together with back-scattered electron images of the cumulates, to produce calibrated images showing regions of progressive crystallization. This allows the regions crystallizing at each stage of solidification to be visualized. These images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain-boundary melt films. This approach can provide information about the progressive reduction of porosity during cumulate solidification.
KEY WORDS: residual liquid; cumulate; plagioclase; porosity; Skaergaard
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INTRODUCTION |
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The formation of cumulate rocks begins with capture of crystals into a mushy boundary layer on the margins of the intrusion. The mechanisms of accumulation may vary greatly, from in situ crystallization (e.g. McBirney & Noyes, 1979
), to deposition from currents (e.g. Irvine et al., 1998), or crystal settling (or flotation, e.g. Wager et al., 1960). However, the result is a crystal framework of grains with interstitial liquid. As the mush layer solidifies, the mush and interstitial liquid experience strong changes in temperature, and there is compositional evolution as crystallization proceeds. The nature of intercumulus crystallization depends on the mobility of the interstitial liquid and the permeability of the mush. For example, adcumulus-style crystallization should occur where there is high permeability, such that the evolving intercumulus liquid can maintain communication with liquid from the main magma reservoir by diffusion and/or compositional convection (e.g. Kerr & Tait, 1986; Tait & Jaupart, 1992). Perfect adcumulates would contain only primocryst minerals with essentially unzoned overgrowths, and no evolved phases (e.g. Morse, 1998). Conversely, in a system without circulation of the interstitial liquid, orthocumulus-style crystallization will occur. Pure orthocumulates would contain primocrysts with zoned overgrowth rims (on plagioclase primocrysts) and evolved intercumulus phases that correspond to crystallization from a trapped liquid (e.g. Wager et al., 1960; Morse, 1998).
Pure adcumulates and orthocumulates are idealized end-members of cumulus crystallization, and may occur only rarely, if at all, in nature (e.g. Wager et al., 1960; McBirney & Hunter, 1995; Grant & Chalokwu, 1998; Morse, 1998). First, the likelihood of convection within the mush will depend on the physical properties (density and viscosity) of the evolving interstitial liquid as well as the mush permeability. Therefore the style of crystallization may change between orthocumulus and adcumulus (or vice versa) during the course of solidification. For example, if the interstitial liquid passes through a density maximum during differentiation, this may allow compositional convection to be initiated (e.g. Sparks et al., 1984; Morse, 1988; Toplis et al., 2008). There is still disagreement over the changing density of the Skaergaard liquid, but some studies indicate that it may pass through a density maximum, although at differing stages of fractionation (e.g. Wager & Brown, 1968; Hunter & Sparks, 1987; Toplis & Carroll, 1995; Tegner, 1997). Differences in mush geometry may also result in variations in the occurrence or extent of movement of the interstitial melt (Bédard et al., 1992). Compaction of the cumulate pile will reduce the volume of the interstitial material crystallized but will not alter the compositional path taken by the liquid (Meurer & Meurer, 2006). Second, the preservation of zoned overgrowth rims will depend on species diffusivities relative to the cooling rate. Diffusion rates for a given element will vary between minerals, as well as with temperature. For very slow cooling rates, which are relevant for the Skaergaard Intrusion, only the most slowly diffusing species will retain a zoned profile caused by continuous differentiation of immobile interstitial liquid. Suitable tracer species can therefore be used to assess how the composition of the interstitial liquid changes with differentiation, and thus gain insights into the processes occurring in the mush during solidification.
For the Skaergaard Intrusion, the liquid line of descent of the crystallizing bulk magma has been estimated using either experimental petrology (e.g. Toplis & Carroll, 1995; Thy et al., 2006) or reconstructions based on bulk analyses of whole-rocks or mineral separates (Wager & Brown, 1968; McBirney, 1989; Jang & Naslund, 2001; Nielsen, 2004). However, relatively little attention has been paid to the compositional evolution of the interstitial liquid (e.g. Toplis et al., 2008). This study investigates the composition of the interstitial liquid by examining the concentrations of minor components and trace elements in plagioclase overgrowths and clinopyroxene oikocrysts, in rocks from the lower parts of the Layered Series of the Skaergaard Intrusion. Slowly diffusing components (e.g. CaAl–NaSi in plagioclase) do not re-equilibrate with the interstitial liquid on the timescales of cooling for the intrusion, and can therefore be used to distinguish periods of orthocumulate-style crystallization, which result in zoned overgrowths, from adcumulus-style crystallization with significant compositional convection, which results in a buffered liquid composition and unzoned overgrowths. Trace element compositional variations in the interstitial material are used to constrain further the compositional evolution of the residual liquid. XAn decreases more or less continuously during intercumulus crystallization, allowing back-scattered electron (BSE) images to be used to visualize the spatial distribution of the liquid at each stage of solidification. These results are discussed in the context of convection, compaction and compaction-driven dissolution–reprecipitation of plagioclase.
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GEOLOGICAL SETTING |
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The summary given here is drawn from several previous studies that have described the Skaergaard Intrusion in detail (e.g. Wager & Brown, 1968
; McBirney, 1989; Irvine et al., 1998). The intrusion is one of a series of Tertiary plutonic complexes that crop out on the east coast of Greenland (Nielsen, 1987). Roughly oval in plan view, it covers an area of 
80 km2 and has an estimated volume of 280 km3 (Nielsen, 2004). It cuts through Archaean gneiss basement, Cretaceous sediments and Tertiary basalts. The layered rocks were originally flat-lying in the centre of the intrusion, and have been tilted 10–20° to the SSE by regional post-solidification subsidence (Nielsen, 2004), so that >3·5 km of stratigraphy is currently exposed, with stratigraphically higher rocks exposed in the south.
The layered rocks are divided into three series (Fig. 1): the Marginal Border Series (MBS), which crystallized inwards from the walls of the intrusion; the Layered Series (LS), which crystallized upward from the floor; and the Upper Border Series (UBS), which crystallized downwards from the chamber roof. Each series is subdivided further on the basis of cumulus mineral assemblage (Wager & Deer, 1939). In the Layered Series, cumulus olivine is present throughout the Lower Zone (LZ), absent in the Middle Zone (MZ), and present again (though more ferric in composition) in the Upper Zone (UZ). The compositions of the cumulus minerals show gradual cryptic variation towards more evolved compositions with stratigraphic height, ascribed to closed-system fractional crystallization of the magma body (Wager & Deer, 1939). The MBS and UBS show equivalent fractionation trends in terms of bulk composition, mineral assemblage and cryptic variations. The Hidden Zone (HZ) belongs to the LS and is not exposed, but was sampled in part by the 1966 Cambridge Drill Core I and contains cumulus olivine and plagioclase. The core is 700 m long and extends from LZb (as defined by McBirney, 1989) 150 m into the Hidden Zone (Fig. 1).
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SAMPLES AND METHODS USED |
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The five samples in this study are taken from the Hidden Zone, LZa and the lower parts of LZb, from the 1966 Cambridge Drill Core I (Table 1). The bottom of the core is thought to be close to the base of the intrusion (Maaløe, 1976
). The rocks are primarily olivine–plagioclase cumulates, with intercumulus plagioclase, augite, inverted pigeonite, Fe–Ti oxides and minor biotite and apatite (Fig. 2). In the LZb samples, it is difficult to determine whether clinopyroxene is cumulus or intercumulus, because it initially has a poikilitic habit even when growing as a cumulus phase (Holness et al., 2007b, 2007c). Most samples contain at least one symplectite of Fe–Ti oxides with a silicate phase (e.g. Haselton & Nash, 1975; Stripp & Holness, 2006).
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Samples were examined optically and using a JEOL JSM-820 scanning electron microscope at the University of Cambridge. Plagioclase and pyroxene compositions were analysed using a Cameca SX-100 electron microprobe at the University of Cambridge. Major elements were analysed using a 15 kV, 10 nA beam; minor elements were analysed with a 100–200 nA beam. The beam was focused to a 2 µm spot, with peak counting times of 20 s for major elements and typically 40 s for minor elements. Typical analytical errors are given in Tables 2 and 3. To obtain representative compositions from all stages of crystallization, plagioclase grain boundaries and triple junctions were analysed as well as primocryst cores and post-cumulus overgrowths. For clinopyroxene, thin cusp-shaped protrusions at grain boundaries (Holness et al., 2007a
) and oikocryst margins were analysed in addition to the interiors of oikocrysts.
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PREVIOUS STUDIES OF SKAERGAARD PLAGIOCLASE |
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Plagioclase textures and zoning patterns have been studied in detail by Carr (1954
), Maaløe (1976) and Toplis et al. (2008). Cores are typically euhedral and have compositions that gradually become more albite-rich with increasing stratigraphic height (e.g. Maaløe, 1976; Tegner, 1997; Toplis et al., 2008). Many grains are strongly twinned. Crystallized melt inclusions are common, manifest as negative-crystal shaped patches of albitic plagioclase, occasionally associated with other minerals (Hanghøj et al., 1995; Fig. 3a). Cores may be unzoned or show oscillatory variations of amplitude 3 mol% An or less (Wager & Deer, 1939; Carr, 1954; Maaløe, 1976). Maaløe (1976) recognized two types of oscillatory textures: simple plagioclase are most common from the LZ upwards and typically have euhedral oscillatory zonation (Fig. 3b), whereas complex plagioclase are most common in the HZ and show strong, irregular internal resorption surfaces cross-cutting the oscillatory variations. The complex textures disappear above +30 m in the stratigraphy, and may be related to temperature variations experienced during crystal settling prior to the onset of convection (Maaløe, 1976) or to magma injection in the early life of the intrusion (Holness et al., 2007b). The oscillatory variations have been ascribed to variations in temperature (Wager & Deer, 1939; Maaløe, 1976) or pH2O (Carr, 1954) during magma convection. Maaløe (1976) also described ‘divided zoning’, which occurs in the MZ and UZ and comprises crystallographically controlled blebs of compositionally distinct plagioclase in the primocryst cores. This texture was ascribed to skeletal growth during supercooled crystallization (Maaløe, 1976).
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The margins of the primocrysts are marked by one or more reverse zones, which are succeeded by normally zoned post-cumulus overgrowths that may be locally embayed (Maaløe, 1976
; Fig. 3a). The composition of the overgrowth rims also becomes more albitic with increasing height in the intrusion (Toplis et al., 2008). |
MINERALOGICAL COMPOSITIONS |
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Plagioclase
Major elements
Plagioclase compositions from the samples studied span a wide range of compositions (Table 2; Electronic Appendix 1, available at http://www.petrology.oxfordjournals.org), from An74 to An30. The different textural features correspond to distinct major element compositions. The cores of cumulus grains are An-rich, typically varying from An60 to An70. The average core composition becomes slightly more albitic with increasing stratigraphic height, in agreement with previous studies. Oscillatory zoned cores show muted compositional variations of 3–6 mol% anorthite, in agreement with the measurements of Carr (1954
) and Maaløe (1976). Post-cumulus overgrowth rims are more sodic than the cores, with compositions typically An57–50. Occasionally, reverse zoning of 5 mol% anorthite is observed (An50–55). The most evolved compositions (An56–31) are found at at grain boundaries, or at three-grain triple junctions. There is no systematic difference in overgrowth composition between samples from different stratigraphic horizons.
Minor elements
Minor elements in plagioclase include Fe, Ti, Mg and K. Plagioclase compositions from Skaergaard show strong variations in minor element concentrations.
- K2O contents correlate negatively with anorthite content. The upper part of the trend is sharp and well defined; however, the compositions are scattered to lower K2O values for a given XAn (Fig. 4a). In particular, this is true for the post-cumulus overgrowths (An55–50) and cores (An65–60).
- Ti contents vary strongly, from 0·01 to 0·133 wt % TiO2. TiO2 concentrations increase from An75 to An60, then increase more rapidly to a maximum of 0·133 wt % at An55 (Fig. 4b). Post-cumulus overgrowths record a sharp drop in TiO2 from 0·133 wt % to 0·03 wt % over a small range in XAn (An55–50). At An50, TiO2 continues to decrease but the gradient shallows, and the most evolved rims show consistently low TiO2 contents.
- MgO concentrations are low, typically <0·05 wt % (Fig. 4d). Plagioclase cores record a wide variation of MgO contents, between 0·019 and 0·045 wt %; the most An-rich cores have slightly lower Mg concentrations. The post-cumulus overgrowths show decreasing MgO from 0·045 wt % to 0·02 wt % as anorthite content decreases from An60 to An50. The most evolved rims contain very low MgO concentrations, typically <0·02 wt %.
- FeO compositions are scattered but decrease steadily with falling XAn (Fig. 4c). There is no clear pattern of rising and falling concentrations.
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errors. Inset shows theoretical growth (continuous lines, G) and diffused (dashed lines, D) compositional profiles. The growth profiles are estimated on the basis of likely changes in temperature, liquid composition and XAn during fractionation, with Dpl from Bindeman et al. (1998Plagioclase traverses
Core to rim traverses through individual plagioclase crystals show similar compositional variations in major and minor elements (Fig. 5), but can provide further information about the progressive changes in composition. XAn decreases rimwards from the core (
An60–70) to an overgrowth rim of An45–55. In some traverses, there is a rim plateau at An50, consistent with the observations of Toplis et al. (2008). K2O concentrations increase rimward to 0·35 wt %, and FeO typically shows a gradual, continuous decline towards the rim. TiO2 concentrations gradually increase rimwards, then strongly decrease just inside the rim, starting at An55. In detail, the profiles show that the drop in TiO2 may coincide with a slight increase in XAn. MgO concentrations show a clear decrease rimward.
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In some traverses, the outer margins of the grains show compositional reversals. XAn and FeO reach a minimum and increase once more just inside the rim, whereas K2O contents reach a maximum and then decrease. TiO2 concentrations continue to fall, whereas there is no discernible change in MgO.
Clinopyroxene compositional variations
Clinopyroxene compositions are more Ca-rich than the crystallization trend defined by Brown et al. (1957) and Brown & Vincent (1963) but are consistent with the observed subsolidus trend defined by Nwe (1976) for rocks from the Lower Zone. Pyroxenes from each sample plot within a restricted range of En contents (Wo44–54En46–55; Table 3; Electronic Appendix 2), but do not show systematic compositional variation with increasing stratigraphic height. TiO2 correlates weakly with Al2O3 (Fig. 6a) and Na2O (Fig. 6b), but not with other elements. MnO decreases slightly with increasing Mg-number (Fig. 6c). Mg and Fe contents do not vary spatially, but compositional profiles across towards the outermost margins of clinopyroxene oikocrysts (Fig. 7) show that Al and Ti contents typically decrease slightly towards oikocryst rims and towards cpx–plag–plag triple junctions, consistent with previous observations of oikocryst growth (Claeson et al., 2007).
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MINOR COMPONENTS RECORD CHANGES IN INTERCUMULUS LIQUID COMPOSITION |
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In agreement with many previous studies (e.g. Wager et al., 1968
; Maaløe, 1976; Naslund, 1984), the major element compositions of plagioclase overgrowth rims at Skaergaard show a continuous, gradual decrease of XAn towards the margins. This reflects the decrease of anorthite content with decreasing temperature and increasing differentiation of the melt. In practice, major-element diffusion in plagioclase is too slow to allow equilibrium to be maintained between crystal and liquid, so decreasing XAn is a reliable indicator of increasing differentiation. Slowly diffusing trace elements should also record changes to the composition of the intercumulus liquid during differentiation. The TiO2 content of plagioclase overgrowths increases to a maximum and then declines as XAn decreases (Fig. 8), suggesting that the intercumulus liquid passes through a maximum TiO2 content, probably related to the onset of Fe–Ti oxide crystallization in the mush pore space. This is consistent with the sequence of experimental liquid compositions of Thy et al. (2006), which showed a Ti maximum at the onset of Fe–Ti oxide crystallization, and with studies of lunar plagioclase (Steele et al., 1980; Smith & Brown, 1988) in which Ti concentrations in plagioclase are inferred to give an indication of the Ti content of the liquid. In contrast, the MgO content of plagioclase overgrowths decreases continuously, perhaps reflecting continual crystallization of intercumulus clinopyroxene crystallization (or depletion of MgO as a result of fractionation in the bulk magma).
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The geometry of clinopyroxene oikocryst growth can be highly irregular (Claeson et al., 2007
). However, compositional profiles show that, in general, the margins are depleted in Ti and Al relative to the centres of the oikocrysts (Fig. 7). This is consistent with removal of Ti and Al components from the liquid during crystallization of Fe–Ti oxides and plagioclase, respectively.
Fe, Mg compositional variations in plagioclase
TiO2 concentrations in plagioclase show strong variations that are inferred above to result from changes in the liquid Ti content. MgO contents of post-cumulus overgrowths show decreasing MgO and XAn, consistent with Mg depletion of the liquid as a result of clinopyroxene oikocryst growth. Only limited variation of FeO is seen in plagioclase (Fig. 4). The observed Fe variation in the overgrowth rims typically comprises a steady decrease with XAn. However, a rise and then steeper decrease, similar to the trend for TiO2, would be expected as a result of local saturation in Fe–Ti oxides. The muted FeO variations cannot be the result of compositional convection within the crystal mush, because the TiO2 contents are not buffered (see subsequent discussion). Fe partitioning into plagioclase has been shown to increase with oxygen fugacity (e.g. Phinney, 1992; Wilke & Behrens, 1999). However, although the fO2 of the Skaergaard Intrusion is thought to have decreased during differentiation (e.g. Frost & Lindsley, 1992; Toplis & Carroll, 1996), the change of DFepl with fO2 is very small at the quartz–fayalite–magnetite buffer (QFM) and below (Phinney, 1992). Therefore changes in fO2 cannot explain the limited variation in Fe. Despite the paucity of Fe partitioning data, Fe is known to occur in plagioclase as Fe3+ and minor Fe2+, substituting for Al3+ or Ca2+ (Longhi et al., 1976; Smith & Brown, 1988; Lundgard & Tegner, 2004). The observed negative correlation of Fe with XAn is consistent with published datasets, which show that the Fe content of plagioclase decreases with anorthite content and with crystallization temperature (e.g. Smith, 1983; Smith & Brown, 1988; Tegner, 1997).
A likely explanation for the relatively insignificant variation of Fe and Mg in plagioclase compared with Ti is the difference in cation diffusivity. Diffusion will act to reduce variations in minor components within the crystal, at a rate dependent on temperature and species diffusivity, and only very slowly diffusing species will closely track the changing melt composition. In plagioclase, the coupled substitution CaAl–NaSi is very slow (5·4 x 10–22 m2/s at 1200°C, Grove et al., 1984) and in practical terms, diffusion of the initial anorthite profile will be minimal. Ti4+ diffusion is likely also to be very slow, because the activation energy for diffusion increases with ion charge (e.g. Hofmann & Magaritz, 1977; Jambon, 1982), so compositional variations in TiO2 can easily be preserved. Experimental data for Mg and Fe diffusion in plagioclase are limited, but the diffusivity of Mg in An95 has been measured at 10–19 m2/s at 1200°C (LaTourrette & Wasserburg, 1998), a little faster than that of Sr at the same temperature (Cherniak & Watson, 1994; Giletti & Casserly, 1994), and much faster than CaAl–NaSi. The characteristic diffusion lengthscales (L = 
4Dt) for Mg and CaAl–NaSi support this argument. On the basis of the estimated cooling time of the Skaergaard Intrusion (34 kyr, Gettings, 1976), at a constant temperature of 1100°C the diffusion lengthscale for CaAl–NaSi interdiffusion is 10 µm whereas that for Mg is 510 µm (diffusion coefficients from Grove et al., 1984; LaTourrette & Wasserburg, 1998). Plagioclase overgrowths typically have apparent thicknesses of 200–300 µm, so assuming that the diffusivity of Fe is similar to that of Mg, any initial variations in Mg and Fe will be significantly dampened by diffusion. Eventually, the compositions will tend towards the equilibrium compositional profiles for Mg and Fe (Fig. 4), which will reflect only changes in partition coefficient related to the smooth normal zoning of the underlying XAn profile (Zellmer et al., 1999; Costa et al., 2003).
Compositional variations in clinopyroxene
Fractional crystallization of intercumulus clinopyroxene should produce significant Mg, Fe zoning in the oikocrysts, but no variation is observed. This is probably due to rapid Mg–Fe diffusion during cooling, or partial re-equilibration with the interstitial liquid during growth (e.g. Barnes, 1986). The available diffusion data show that Mg–Fe diffusion is approximately two orders of magnitude faster than that of Al (Sautter et al., 1988; Anovitz, 1991; Dimanov & Sautter, 2000). This is consistent with the observed decrease in diffusivity for cations with small ionic radius (van Orman et al., 2001); Ti diffusion should therefore also be slower than that of Fe–Mg. The lack of Mg–Fe zonation and presence of Al–Ti zoning is therefore consistent with variations in species diffusivity. Al in clinopyroxene decreases with increasing melt fractionation (LeBas, 1962; Loucks, 1990), and the correlation between Ti and Al reflects charge balancing (Brown et al., 1957; Kushiro, 1960). The decrease in Al and Ti observed towards the margins of oikocrysts and towards cpx–plag–plag triple junctions is therefore consistent with decreasing Ti (and/or Al) in the intercumulus melt during its solidification.
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MODELLING TiO2 CONCENTRATIONS IN THE INTERCUMULUS LIQUID |
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Variations in the TiO2 contents of plagioclase are interpreted as the result of changing TiO2 in the residual intercumulus liquid. Therefore, the TiO2 contents of plagioclase can be used to reconstruct the liquid composition and hence investigate crystallization of the interstitial liquid. Changes in mineral chemistry during intercumulus crystallization are caused by changes in temperature and melt composition. Further complicating factors include the possibility of large-scale compositional convection within the crystal mush (e.g. Morse, 1986
) or melt loss through compaction (Meurer & Boudreau, 1998a). It is therefore assumed initially that there is no compaction, no compositional convection within the mush, and only minimal diffusive exchange within the interstitial liquid. The validity of these assumptions is discussed below. Important parameters for the model include the initial TiO2 content of the interstitial liquid, the initial porosity of the mush, the partition coefficient for Ti in plagioclase (DTipl) and the bulk distribution coefficient for TiO2 (which depends on the modal proportions of the crystallizing intercumulus material as a function of temperature). The values of these parameters are discussed below.
Initial TiO2 content of the interstitial liquid
The bulk composition of the parental liquid that crystallized to form the Skaergaard Intrusion has been debated in many previous studies. The parental liquid composition has been estimated from chilled marginal rocks (Wager & Brown, 1968; Hoover, 1989), from the chilled margins of associated dyke swarms (Brooks & Nielsen, 1990) and from mass-balance models (Nielsen, 2004) based on bulk-rock analyses (McBirney, 1989). Experimental studies have recently used samples of the dykes (Thy et al., 2006) or synthetic equivalents of them (Toplis & Carroll, 1995). Estimates have TiO2 contents in the range 2·05–2·92 wt % (Brooks & Nielsen, 1990), 2·35–2·72 wt % (Hoover, 1989), and 3·09 wt % (Nielsen, 2004). The modelling presented here uses phase relationships derived from experimental studies (Toplis & Carroll, 1995; Thy et al., 2006). The TiO2 content of the parental liquid is assumed to have an initial value of 2·05 wt %, equivalent to the composition of the experimental starting material used in these studies.
Ti (plagioclase–melt) partition coefficient, DTipl
To reconstruct the TiO2 content of the interstitial melt (m), the partition coefficient for Ti in plagioclase (DTipl = CTipl/CTim, where C is the concentration) must be known. Data on DTipl from the literature are few, in part because of poor counting statistics for typical electron microprobe analyses of Ti. Bindeman et al. (1998) reported dependence on both temperature and XAn, with an Arrhenius relationship. However, the experiments reported by Bindeman et al. (1998) were performed in air and are therefore not appropriate for Skaergaard magmas. Bédard (2006) also reported similar Arrhenius equations for regressions of data compiled from previous studies, but with very low R2. In this study, DTipl is calculated by linear regression of the experimental glass and plagioclase compositions of Thy et al. (2006), such that
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Initial porosity of the mush
The initial porosity pi of the mush is defined as the liquid-filled pore space present immediately following crystal deposition. Crystallization will reduce the porosity over time, to the residual porosity pr (Morse, 1986). In the absence of compaction, pi will be the same as the proportion of trapped liquid, such that pr = pi F, where F is the fraction of liquid remaining during progressive fractional crystallization. The initial porosity of a crystal mush has been estimated from experiments (e.g. Jackson, 1961; Finney, 1970; Philpotts et al., 1999) and numerical models (e.g. Jerram et al., 1996; Saar et al., 2001; Rudge et al., 2008), whereas the trapped liquid fraction has been calculated from geochemical arguments (e.g. Irvine, 1980; Tegner et al., in preparation).
Numerical modelling shows that the initial porosity of the crystal framework will be influenced by the rate of accumulation (Blumenfeld et al., 2005), crystal shape and aspect ratio (e.g. Williams & Philipse, 2003; Rudge et al., 2008) and preferential alignment of crystals (Saar et al., 2001). Interconnected frameworks of plagioclase crystals form experimentally at very high porosities (75 vol.%, Philpotts et al., 1999). Random packings of monodisperse spheres give a maximum theoretical packing density of 0·64 (equivalent to a minimum porosity of 36%). Packings of more elongate particles have higher porosity (Rudge et al., 2008); however, polydisperse grain packings will have lower porosity relative to monodisperse packings of the same grain shapes (e.g. Bezrukov et al., 2001).
Several researchers have estimated the trapped liquid fraction from the concentrations of incompatible elements in cumulate rocks. Irvine (1980) estimated 50–58% trapped liquid in the marginal picrites of the Muskox Intrusion. For Skaergaard specifically, Henderson (1970) determined a trapped liquid fraction of 15–24% in samples from LZa from bulk P contents. Similarly, Tegner et al. (in preparation) proposed trapped liquid fractions that decrease from 28–47% in LZa to 4–5% in MZ and UZa. Compaction is minimal in the lower parts of the Lower Zone (Tegner et al., in preparation), which suggests an initial porosity equivalent to the trapped liquid fraction, 15–47%. The initial porosity of a crystal mush can also be estimated from the textures of glassy cumulate nodules. Studies by Tait (1988) described cumulate nodules from Laacher See, Germany with 15–35 vol.% glass. Holness & Bunbury (2006) reported glass contents of 25–40% for amphibole-bearing nodules from Kula, Turkey.
Based on these previous studies, the initial porosity of the polydisperse olivine + plagioclase ± clinopyroxene mush at Skaergaard, prior to compaction and intercumulus crystallization, is taken to be 35%.
Crystallization of the intercumulus material
Because there is no systematic difference in plagioclase overgrowth composition between samples, previously published data from HZ, LZa and lower LZb plagioclase are pooled and considered together. Thus, the modelling will produce an ‘average’ result for the lowermost parts of the Skaergaard Intrusion. The fractional crystallization (FC) model presented here has three stages. The first stage is effectively a period of ‘primocryst’ growth, which takes into account that the primocrysts are deposited into an ‘LZ’ liquid that is differentiated from the Skaergaard parental liquid. Olivine and plagioclase are assumed to be crystallizing in constant proportions (70% plagioclase and 30% olivine; Toplis & Carroll, 1996; Table 4) from the parental liquid, which had 2·05 wt % TiO2. Textural observations show that the most evolved primocryst cores have composition 
An60; therefore this pre-cumulus stage of the calculations ends when the plagioclase composition reaches An60 (see below). When this occurs, TiO2(m) is 3·30 wt % (Table 4).
The second stage describes intercumulus crystallization prior to saturation of Fe–Ti oxides. This stage begins with 3·3 wt % TiO2 in the interstitial liquid, at An60, and assumes 55 vol.% overgrowth of plagioclase + 45 vol.% crystallization of clinopyroxene oikocrysts (Toplis & Carroll, 1996). Ti is strongly partitioned into the liquid, and is therefore described by a low bulk partition coefficient (DTiB). The point at which the melt becomes saturated in Fe–Ti oxides (the beginning of phase 3) is determined from the experimentally derived constraint of Toplis & Carroll (1996):
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; Toplis et al., 2008). Ti is compatible in the bulk crystallizing assemblage, and has a high DTiB. For all phases, DTi is calculated using partition coefficients from Bédard (2005) for olivine and Hart & Dunn (1993) for clinopyroxene, with DTipl from Thy et al. (2006) as previously described (Table 4).
For all three stages of the calculation, the proportion of liquid remaining (F) is quantified as a function of temperature using an empirical calibration (Toplis & Carroll, 1996):
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- For each value of F, calculate temperature using equation (3).
- Calculate XAn using equation (4), and hence calculate DTipl.
- Calculate TiO2(m) in the liquid using the Rayleigh fractional crystallization (FC) equation CL/C0 = f (D – 1), and the bulk distribution coefficient for Ti. It should be noted that f is distinct from F in equation (3) and represents the fraction of liquid remaining within each stage of the calculation, such that f is reset to zero at each new stage. DTiB changes at each stage (e.g. when Fe–Ti oxide saturation is reached).
- Calculate TiO2 in plagioclase from steps (2) and (3).
1115°C, to An38 at 1035°C. Table 4 summarizes the calculation process and the values of the coefficients used.
Figure 8 shows the results of the fractional crystallization (FC) calculations described above. Despite the simplicity of the model, the calculated plagioclase Ti trend matches the shape and composition of the analytical data well. At the point of oxide saturation, the liquid reaches a maximum calculated TiO2 content of 4·5 wt %, in good agreement with the experimental liquid compositions of Thy et al. (2006). The onset of Fe–Ti oxide crystallization causes a strong decrease in calculated plagioclase Ti content. These features support the interpretation that the Ti compositional variations in plagioclase can be explained by changes to the composition of the interstitial liquid. TiO2(m) can also be estimated using the Ti contents of intercumulus clinopyroxene, which crystallizes at the same time as the plagioclase overgrowth rims. Assuming a constant DTipx of 0·4 (Table 4), TiO2(m) is estimated at 0·2–2·9 wt %. Although the highest TiO2 contents are not recorded, the range is consistent with the range of TiO2(m) calculated from plagioclase compositions.
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DISCUSSION |
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Although the simplified model produces a trend that is similar to the data, in detail there are aspects of the data that are not matched by the calculated trend, as follows.
- The calculations show that the rock is fully solidified (zero residual porosity) at An34, whereas the most evolved plagioclase compositions analysed reach An30. The modelled final plagioclase has no TiO2 at An35, whereas the data show 0·01–0·02 wt % TiO2.
- The drop in TiO2 between An50 and An55 is accompanied by a slight increase in XAn (Fig. 4b). The calculated FC trend does not match this, with continual decrease in XAn.
; McBirney & Naslund, 1990; Morse, 1990; Toplis & Carroll, 1996; Ariskin, 2003; Thy et al., 2006), and the compositional evolution of the intercumulus liquid is even less well understood. Furthermore, very few experimental data are available at evolved plagioclase compositions (An50 and below; Thy et al., in preparation). Despite these limitations, experimental calibrations are still probably the best means of estimating crystallization conditions.
The observation of slight reverse zoning associated with decreasing TiO2 (Fig. 4b) is interpreted as equivalent to the reversals described by Maaløe (1976), whose analyses also showed decreasing TiO2. Similar features have also been observed by Shimizu (1978) for Skaergaard and other mafic intrusions (e.g. Kiglapait troctolites, Morse & Nolan, 1984; Harp Lake, Emslie, 1980). At Harp Lake, pyroxene traverses also showed decreasing Al contents towards the grain margins (Emslie, 1980), as observed at Skaergaard. The reversals in XAn cannot be explained by interaction with the overlying magma reservoir, because this would result in a buffering of the TiO2 content to higher values, instead of the continual decrease that is observed. Instead, the reverse zoning is interpreted as the record of compaction-driven resorption and reprecipitation of plagioclase. Resorption of unfavourably oriented plagioclase has been described by several workers (e.g. Maaløe, 1976; Nicolas & Ildefonse, 1996; Meurer & Boudreau, 1998b). The resorbed material, which has higher XAn than the crystallizing grains, is redeposited in more favourable orientations (Maaløe, 1976) and results in reverse zoning. The TiO2 content of the crystallizing plagioclase continues to fall during reverse zoning, which demonstrates that intercumulus crystallization of Fe–Ti oxides continues during resorption of plagioclase.
Compositional convection within the crystal mush
Plagioclase profiles from throughout the Lower Zone in Skaergaard were reported by Toplis et al. (2008). They also observed outer rims with constant or slightly reversed plagioclase compositions at An50–55, and interpreted these as evidence of compositional convection within the mush. They argued that the residual liquid passed through a density maximum following crystallization of Fe–Ti oxides, resulting in gravitational instability of the residual liquid. In other words, intercumulus crystallization initially occurs in situ, and is followed by a period of crystallization where the liquid is mobile, allowing buffering of the plagioclase composition (Toplis et al., 2008). The decrease of TiO2 in reversely zoned plagioclase confirms that the reversals occur after Fe–Ti oxide saturation. However, the strong decrease of TiO2 in plagioclase during formation of these intermediate rims means that crystallization must be in situ. Any overlying melt circulating by convection must be less evolved than the interstitial liquid; therefore convection would buffer the interstitial liquid to more calcic, but also more Ti-rich compositions. The plagioclase compositional traverses (Fig. 5) show that reverse zoning is always accompanied by decreasing TiO2, and therefore that there can only have been minimal chemical communication with the main magma reservoir.
Fe–Ti oxides make up only 2 vol.% of the samples studied, and hence about 5 vol.% of the intercumulus material (given an initial porosity of 35%). The oxide-rich regions are heterogeneously distributed, leaving parts of each sample that are rich in oxides whereas other areas contain none. The decreasing plagioclase TiO2 concentrations show that there can have been only minimal communication with the main magma reservoir. However, there must have been at least some local millimetre- to centimetre-scale diffusive or convective exchange within the residual liquid, for the plagioclase overgrowths to record decreasing TiO2 even in oxide-free areas. Similar conclusions have been drawn for granitic systems from the distribution of cuneiform alkali feldspar pockets (Bryon et al., 1996). Further constraints on the lengthscale for such exchange are not possible from these data, given the two-dimensional nature of the sections.
Spatial distribution of residual liquid during crystallization
Although the fractional crystallization calculations described above are dependent on the choice of partition coefficient and liquidus calibration, the results can be used to visualize the evolving spatial distribution of residual liquid in the crystallizing cumulates. The key observation is that, other than the slight reverse zoning observed between An55 and An50, intercumulus crystallization results in a more or less continuous decrease in the anorthite content of plagioclase. For plagioclase, the greyscale intensity of a back-scattered SEM image correlates linearly with anorthite content (Ginibre et al., 2002), because the intensity is related to mean atomic number of the sample. BSE SEM images, which contain electron microprobe spot analyses, were therefore calibrated for anorthite content by correlating the greyscale with known XAn, using the public domain image processing programme ImageJ (Rasband, 1997–2008). Good linear correlations were produced, commonly with R2 > 0·98 [for a discussion of the use of BSE images for studying zoning profiles see Ginibre et al. (2002)]. A series of thresholds (corresponding to >An60, An60–55, An55–50, etc.) was then applied to each calibrated photograph, generating a set of black and white images showing only the regions at the required anorthite content (e.g. An50–55). Finally, each image was ‘despeckled’ to remove noise and increase clarity.
Figure 9 gives an example set of such images. As expected, the overgrowth rims initially form parallel to the euhedral growth faces of the primocryst cores, with crystallization ceasing locally when two growing margins meet (Fig. 9). Any slight reverse zoning between An50 and An55 is not a problem for this method because this material is all included within one threshold bracket. The thresholded images also provide information about the very last stages of solidification. In particular, the most evolved plagioclase compositions (<An45) commonly occur at the margins of triangular pockets and along plagioclase–plagioclase grain junctions (Fig. 9). These are heterogeneously distributed even within a single thin section, and typically represent a small or insignificant volume proportion of each image. The distribution of these evolved margins suggests that in the final stages of solidification, the residual liquid occupies pore corners and thin films along grain boundaries (Fig. 9). This is consistent with the suggestion of Morse & Nolan (1984) and observations of grain boundary melt films in partially crystalline cumulate nodules (Holness et al., 2007a).
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Effects of compaction
During solidification, the crystal mush porosity can be reduced by mechanical compaction (adjustment of pore spaces), or by ‘chemical compaction’ (dissolution and reprecipitation), as well as by crystallization. The plagioclase textures and reverse zoning described above provide evidence for ‘chemical compaction’. Dissolution of more An-rich plagioclase causes the residual liquid to become more Ca-rich, and therefore results in reverse zoning in areas of reprecipitation. Chemical compaction should not affect the volume of interstitial material because the sum of the grain areas remains constant (Meurer & Boudreau, 1998b
), but would affect its spatial distribution. Mechanical compaction should not result in a modification of the compositional trend of the residual melt unless there is significant interaction with more evolved liquids that may be squeezed out of the underlying layers. Such interaction would result in lower concentrations of both Ca and Ti in the melt, and hence plagioclase with lower TiO2 and lower XAn. This is in contrast to convective circulation with the overlying magma reservoir (or overlying cumulates in the case of a very thick mush), which would buffer the liquid to higher TiO2 and higher XAn. The main effect of mechanical compaction will be to reduce the volume of interstitial material relative to a region of mush that had not lost any residual liquid.
A major difficulty in quantifying changes in porosity is in determining when ‘closed-system’ intercumulus crystallization begins—where is the boundary between essentially adcumulus and essentially orthocumulus behaviour? At the magma–mush interface, (open-system) exchange between the intercumulus liquid and the magma reservoir is possible. Compositional convection is unlikely to occur at this stage unless the density of the interstitial liquid is less than that of the overlying magma body (Toplis et al., 2008). However, diffusive exchange is possible, and could be enhanced by convection in the main magma body. After some time, dependent in part on the crystal accumulation rate, the interstitial liquid in the mush may become isolated from the magma reservoir. At this point, no chemical exchange between intercumulus liquid and external reservoir is possible (although convective circulation within the mush may still occur) and closed-system (orthocumulus-style) crystallization can occur. Possible mechanisms for sealing the mush could include deposition of modally distinct layers with a strong contrast in permeability produced by finer grain size or variations in crystal shape, or the formation of an adcumulate ‘hardground’ (Morse, 1986; Petersen, 1987; Mathez et al., 1997; Holness et al., 2007c). Alternatively, at lower temperatures, diffusion rates in the interstitial liquid may be effectively too slow to alter the compositions of phases crystallizing some distance below the mush interface. This study implicitly assumes that the transition between adcumulus- and orthocumulus-style crystallization happens early in the intercumulus crystallization. However, the match between the FC calculations and the observed trend suggests that this assumption is reasonable for the lower parts of the Layered Series of the Skaergaard Intrusion.
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CONCLUSIONS |
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The minor element concentrations of intercumulus plagioclase overgrowths show strong variations that can be explained by changes in the composition of the residual liquid during fractional crystallization of the interstitial liquid. In particular, the onset of intercumulus Fe–Ti oxide crystallization causes melt Ti to decrease, resulting in falling TiO2 contents in plagioclase. Decreased Al and Ti at the margins of clinopyroxene oikocrysts are also consistent with expected changes in melt composition. However, faster diffusing species (e.g. Mg, Fe) show little variation because of the prolonged cooling history. There is no systematic difference in plagioclase composition between samples from HZ to LZb, suggesting that they follow a similar crystallization path. Fractional crystallization calculations indicate that the Fe–Ti oxide saturation occurs after
30% intercumulus crystallization. Constant or slightly reverse XAn in plagioclase rims is explained by compaction-driven resorption of unfavourably oriented plagioclase grains, during continued in situ intercumulus crystallization. The continual decrease of XAn during solidification means that BSE images can be used to visualize the spatial distribution of melt during solidification. The most evolved plagioclase compositions correspond to the last to crystallize, which are triangular pockets and films along grain boundaries. This approach should be useful in understanding the manner in which porosity is reduced during mush solidification.
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SUPPLEMENTARY DATA |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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The author was supported by a Junior Research Fellowship from Trinity College, University of Cambridge. Chris Hayward is thanked for his assistance with electron microprobe analyses. The manuscript was improved by constructive reviews from Troels Nielsen, Tony Morse, Jean Bédard and an anonymous reviewer, and by editorial handling by Marjorie Wilson. Marian Holness and John Maclennan are also thanked for helpful discussions and comments on an earlier version of the manuscript.
*Telephone: +44 (0)1223 333433. Fax: +44 (0)1223 333450. E-mail: mcsh2{at}cam.ac.uk
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