Liquid immiscibility in plutonic environments

Establishing if unmixing occurred in intrusions is difficult, as intrusive rocks are the fractional crystallisation products of the relevant eutectic for the bulk and interstitial liquids (Veksler and Charlier, 2015). If the two immiscible liquids did not segregate, there would be minimal traces of unmixing in the crystallised rocks as the two immiscible liquids would crystallise the same assemblages and the solidified products of the emulsion would be indistinguishable from the products of a homogeneous liquid with the same bulk composition. However, microstructural features have been suggested to record evidence of late-stage unmixing and two-phase separation within the residual liquids of solidifying gabbroic cumulates (e.g. Holness et al., 2011; Humphreys, 2011), and unmixing has been proposed as a mechanism for large-scale magma differentiation based on melt inclusions and whole

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rock geochemistry – provided unmixing occurs early enough during crystallisation (e.g. Jakobsen et al., 2011; Namur et al., 2012; VanTongeren and Mathez, 2012; Veksler et al., 2007). Examples of large-scale and interstitial unmixing raise important questions pertaining to an emulsion’s physical behaviour (e.g. nucleation, coarsening and migration).

A variety of approaches have been used in an attempt to identify whether the liquid line of descent of an intrusion crossed the silicate liquid immiscibility field. Pairs of compositionally different, but simultaneously trapped, melt inclusions (one Fe-rich, the other Si-rich) provide direct evidence of an emulsion in the liquid during mineral growth. Such melt inclusions in cumulus apatite have been analysed to irrefutably show evidence of silicate liquid immiscibility in the Skaergaard Intrusion (Jakobsen et al., 2005); the Sept Iles intrusion (Charlier et al., 2011); and the Upper Zone of the Bushveld Complex (Fischer et al., 2016; VanTongeren, 2018). Other authors have used geochemical datasets to infer that unmixing occurred, for example the presence of bimodal whole rock compositions in the Sept Iles intrusion (Namur et al., 2012), and bimodal patterns of trace element compositions such as REEs in apatite in the Bushveld Complex (VanTongeren and Mathez, 2012).

There is not necessarily a need to form fully segregated immiscible layers to significantly affect material transport and post-cumulus crystallisation in an intrusion (Veksler and Charlier, 2015). Even if the bulk magma is compositionally far from the miscibility gap, the interstitial liquid in the crystal mush may become sufficiently fractionated to unmix (Holness et al., 2011). If significant two-phase separation occurs in the immiscible interstitial liquid, the cumulus phases of the mush will be in local disequilibrium with the immediately adjacent liquid. Consequently, the migration of segregated immiscible liquids within the cumulate pile can be documented by inter-cumulus reactive microstructures and trace element mineral chemistry, e.g. TiO2 in plagioclase (Holness et al., 2011;

Humphreys, 2011).

Since the 1970s, immiscibility on the liquid line of descent has been hypothesised and debated for many intrusions, including: Skaergaard, Greenland (McBirney, 1975); Nain Complex, Labrador (Wiebe, 1979); Mull, Scotland (Philpotts, 1982); Duluth Complex, Minnesota (Ripley et al., 1998); Bjerkreim-Sokndal Intrusion, Norway (Wilson and Overgaard, 2005); Upper Zone of the Bushveld Complex, South Africa (Cawthorn, 2013; Fischer et al., 2016; VanTongeren and Mathez, 2012; VanTongeren, 2018); and Sept Iles, Canada (Namur et al., 2012; Namur et al., 2011). Below I outline the evidence for unmixing in several mafic layered intrusions that have helped shape the recent debate.

1.9.1 Skaergaard Intrusion, East Greenland

While the Skaergaard Intrusion has been influential in the silicate liquid immiscibility debate, here I present only a brief overview: a more detailed background summary of the Skaergaard is presented in Chapter 5 together with new observations of liquid unmixing in the intrusion.

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Holgate (1954) first hypothesised that the ferrobasaltic bulk magma in the Skaergaard Intrusion split into two immiscible conjugates from the behaviour of “quartzose xenoliths” in basic igneous magmas.

Subsequently, field evidence of Si-rich pods and lenses in the upper reaches of the intrusion have been cited as evidence of immiscible Si-rich liquid segregations (e.g. McBirney, 1989; McBirney and Nakamura, 1974; Naslund, 1984; Stewart and DePaolo, 1990). More recently, coexisting Si-rich and Fe-rich inclusions in cumulus apatite provided direct evidence of an emulsion in the bulk magma as the cumulus grains crystallised (Jakobsen et al., 2005). Studies investigating the liquid line of descent also demonstrate the occurrence of liquid immiscibility in the bulk magma (McBirney, 1975; McBirney and Nakamura, 1974; Veksler et al., 2007; Veksler et al., 2008a).

A variety of petrological and geochemical studies indicate the presence of intercumulus unmixing in the middle to upper reaches of the intrusion. This has been argued from the presence of reactive intercumulus microstructures including mafic symplectites, fish-hook pyroxenes, polycrystalline olivine rims on cumulus phases and serrated grain boundaries (Holness et al., 2011). Mineral chemistry (specifically TiO2 in plagioclase) has been used to constrain the spatial distribution and differential migration of interstitial immiscible liquids within the Skaergaard Intrusion (Humphreys, 2011). Overall, the debate surrounding silicate liquid immiscibility in Skaergaard is not whether unmixing occurred, but rather, when (e.g. Holness et al., 2011; Jakobsen et al., 2005; McBirney and Nakamura, 1974;

McBirney and Naslund, 1990; Veksler et al., 2007; Veksler et al., 2008a).

1.9.2 Sept Iles Intrusion, Canada

The Sept Iles layered intrusion (some 80 km in diameter) crystallised from a ferrobasaltic bulk magma (Namur et al., 2015b) that followed a tholeiitic liquid line of descent, with two major magma influxes (Namur et al., 2010). Charlier et al. (2011); and Namur et al. (2012) suggest, based on the bimodal bulk-rock compositions of the ferrogabbros, that unmixing was a significant differentiation process during the crystallisation of one of the three megacyclic units of the Layered Series. The physical separation of the Fe-rich and Si-rich liquids resulted in the formation of alternating bands of leucocratic and melanocratic gabbros on the 5-20 m scale, with the melanocratic bands comprising major P-Ti-Fe deposits. Evidence for unmixing is further supported by co-existing Fe-rich and Si-rich melt inclusions in apatite, which have a homogenisation temperature of 1100–1060°C (Charlier et al., 2011), implying unmixing occurred below this temperature interval. The stratigraphic distribution of particular symplectite structures (attributed to a reaction between cumulus primocrysts and an Fe-rich immiscible liquid following loss of the buoyant Si-rich conjugate (Holness et al., 2011)) are also used to suggest that unmixing occurred during progressive crystallisation (Namur et al., 2012).

1.9.3 Bushveld Complex, South Africa

The Rustenburg Suite of the Bushveld Complex, is the largest layered intrusion on Earth, covering over 66,000 km² and comprising mafic-ultramafic gabbroic cumulates, overlain by granites (Eales and

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Cawthorn, 1996; Wager and Brown, 1968). The cumulates formed in an open system with multiple magma mixing and replenishment events (Cawthorn, 2015). There is a close association between oxide-rich and phosphate-oxide-rich layers in the Complex. A number of different processes have been proposed for the origin of this layering, ranging from dynamic processes of crystallisation of a boundary layer and periodic overturn (Tegner et al., 2006), to silicate liquid immiscibility (e.g. VanTongeren and Mathez, 2012). Reynolds (1985); Von Gruenewaldt (1993) suggested that the oxide-phosphate layering in the upper third of the Bushveld Complex formed from an immiscible Fe-rich liquid based on textural and compositional relations. Sharp changes in the concentration of REE in apatite have been used to infer intrusion-wide segregation in the upper third of the Complex (VanTongeren and Mathez, 2012).

However, Cawthorn (2013) disputed the conclusions drawn from the apatite REE patterns (VanTongeren and Mathez, 2012) and instead suggested that the apatite geochemistry formed as a result of the trapped liquid shift effect, whereby cumulus crystals re-equilibrated with an evolved interstitial liquid. Incomplete separation of two immiscible conjugates in the upper third of the Bushveld Complex is hypothesised by Fischer et al. (2016) based on the compositions of apatite melt inclusions. Fischer et al. (2016); and Yuan et al. (2017) suggested that the partial segregation of immiscible liquids in the crystal mush could form the cyclic melanocratic (oxide-phosphate layers) to leucocratic layering observed in the upper third of the Bushveld Complex.

In the Rustenburg Suite are Fe-rich ultramafic pegmatites. These are dominated by ferro-augite, olivine, magnetite and ilmenite. The coarse-grained isolated bodies have a variety of forms, including tubes, discs and complex branching morphologies that cut the stratigraphy (Cawthorn et al., 2000; Reid and Basson, 2002; Scoon and Mitchell, 1994; Viljoen and Scoon, 1985). They range from metres to a few kilometres in scale and have sharp contacts with the host gabbro. The whole rock composition of these Fe-rich pegmatites is analogous to the composition of an Fe-rich immiscible liquid, leading to suggestions that they formed from unmixing and replacement of anorthosites (Reid and Basson, 2002;

Scoon and Mitchell, 1994). However, the nature of the processes forming these pegmatitic bodies is controversial (Veksler and Charlier, 2015). Further occurrences of such bodies are presented in Chapter 5.

1.9.4 Stillwater Complex, Montana, USA

The Stillwater Complex is a mafic-ultramafic layered intrusion, with significant mineral deposits, that formed in an open system, with multiple magma injections (McCallum et al., 1980). A major feature of the Complex is the presence of anorthosite sheets, each several hundred meters thick (Hess and Smith, 1960; Raedeke, 1982). Cumulus plagioclase from the anorthosite sheets host crystallised multiphase melt inclusions, which contain traces of Fe-rich liquids. Consequently, it has been suggested that either the anorthosite sheets crystallised from a Si-rich immiscible liquid, or that immiscibility occurred in compositional boundary layers around the crystallising plagioclase (Loferski and Arculus, 1993).

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