The Hebridean Igneous Province plumbing system: A phase equilibria perspective

LITHOS 348-349 (2019) 105194

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The Hebridean Igneous Province plumbing system: A phase equilibria perspective Gautier Nicoli*, Simon Matthews Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 May 2019 Received in revised form 30 August 2019 Accepted 30 August 2019 Available online 3 September 2019

The architecture of magmatic plumbing systems and the behaviour of the crystal mush underlying active volcanoes can be assessed by examining the crystal chemistry and microstructure preserved in solidified magma bodies, i.e. dykes, sills, and layered intrusions. Combining field and petrographic observations of the Little Minch Sill Complex in Scotland, with analyses of its mineral chemistry, demonstrates that these shallow tabular intrusions formed by two distinct magma pulses, a cargo-rich magma and a cargo-free magma, in a relatively closed system. The co-genetic nature of the crystal cargo and its carrier liquid permits the application of phase-equilibria modelling to estimate the pressures and temperatures of crystallisation and sill emplacement. Our calculations using thermodynamic modelling software (Perple_X) suggest the presence of a of magma chamber at ~25 km depth and the formation of sill and dyke network at ~3e16 km below the surface. Our new estimates of the pressures of magma storage and crystallisation are similar to those found for the Iceland and the rest of the British Tertiary Igneous Province, suggesting the behaviour of magmas in the crust and the architecture of the plumbing system are comparable. This apparent temporal consistency supports the use of preserved intrusive bodies as a means for understanding and constraining processes occurring in the upper crust beneath active volcanoes. Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.

Keywords: Hebridean Igneous Province Sill Crystal cargo Olivine Phase equilibria modelling

1. Introduction Assessing the behaviour of magmas under active volcanoes is critical for our understanding of the timing and the frequency of eruptions (e.g. Caricchi et al., 2014; Thomas and Neuberg, 2014; Wilson et al., 1980). However, information about the extent of magmatic plumbing systems is scarce, and we have limited knowledge of where magma is stored and how it moves under active volcanoes. Such information can be retrieved by conducting geophysical surveys (e.g. Hudson et al., 2017; White et al., 1996; Woods et al., 2018) and analysing mineral phases contained in lava flows and tephra (e.g. Burney, 2015; Hartley et al., 2018; Neave and Putirka, 2017; Stock et al., 2018; Thomson and Maclennan, 2012). A recent review by Maclennan (2019) highlighted the challenges in using erupted material (i.e. crystal cargo, xenoliths) to assess the depth of the plumbing system and the processes magmas undergo within it. Firstly, the lava randomly samples only a tiny fraction on the magmatic system, potentially obscuring the relationship between the crystal phases and melt. Secondly, volcanic crystal cargoes offer little bearing on the architecture of crustal magma storage (e.g. sills and laccoliths). Geophysical surveys (e.g. Greenfield et al., 2016; * Corresponding author. E-mail address: [email protected] (G. Nicoli). https://doi.org/10.1016/j.lithos.2019.105194 0024-4937/Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.

Lees, 2007) reveal only the large scale structure of the plumbing systems, and do not constrain their internal architecture. Hence, if we aim to address the issue of the behaviour of magma under active volcanic provinces, we need to understand the origin of the crystal cargo and composition of the crystals in lava (e.g. Helz, 1987; Jones et al., 2014; Welsch et al., 2014). Alternatively, magmatic plumbing systems may be studied by surveying solidified magmatic intrusions (sills, dykes and layered intrusions). When combined with petrographic observations, crystal chemistry is powerful tool to unravel plumbing and recycling process in both eruptive and intrusive products (e.g. Jerram et al., 2003, 2009; Jerram and Martin, 2008; Martin et al., 2010). The generalisation of microstructural studies (e.g. Holness et al., 2006; Holness et al., 2015; Holness et al., 2017a, 2017b; Jerram et al., 1996; Nicoli et al., 2018; Vukmanovic et al., 2018) have permitted pre- to syn-solidification mechanisms to be unravelled in a series of well-studied mafic intrusions. Among them, the Little Minch Sill Complex in Scotland, part of the British Tertiary Volcanic Province, offers a direct window into the fluid dynamic mechanisms that govern the solidification processes of tabular intrusions. The magma that formed Little Minch Sill Complex contained olivine and plagioclase phenocrysts previously interpreted to originate from a dislocated mush in a deep magma chamber at the base of crust (Gibb and Henderson, 2006). A previous study of the Skye Main Lava Series (SMLS) (Font et al., 2008) made

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isotopic measurements on crystal cargos in erupted materials and shallow intrusions to investigate plumbing and storage depths in the British Tertiary system, arguing for magma storage at different depths. Since the sheets were emplaced at an upper crustal level (Schofield et al., 2016) and the behaviour of the crystal cargo during solidification has been well-established (Holness et al., 2017a; Nicoli et al., 2018), the chemical equilibrium between the carrier liquid and the crystal cargo can be investigated, which will, in turn be used to place constraints on the depth of the crustal magmatic plumbing system. At high-temperature conditions, there is a strong geological feedback between metamorphic processes (i.e. equilibrium-based solid phase mineral reactions) and igneous processes (i.e. melt crystallisation and extraction) as local equilibrium (Guevara and Caddick, 2016; Nicoli et al., 2017) is likely to be attained in magmatic processes. Therefore, since the petrological context of the Little Minch Sill Complex is so well constrained, phase equilibria modelling (Connolly, 2009) may be used with the bulk-rock compositions to provide information on the evolution of pressure and temperature conditions of the magma. These pressure and temperature estimates may then be used to constrain the extent of the magmatic plumbing system. 2. Geological setting The Little Minch Sill Complex in Scotland is part of the Hebridean Igneous Province (Emeleus and Bell, 2005; Kent and Fitton, 2000) (Fig. 1a), which formed during the early break-up of Pangea and the opening of the North Atlantic Ocean at 50e60 Ma. Outcrops of the Little Minch Sill Complex are mainly located on the northern part of the Isle of Skye, the Trotternish Peninsula, and on the Shiant Isles (Fig. 1b). The sills are emplaced into Mesozoic sediments (Gibb and Gibson, 1989) and have thicknesses varying from a couple of meters to >100 m. These intrusions are commonly composite and originated from a single magma source (Gibson, 1988; Gibson and Jones, 1991; Gibb and Henderson, 1996; Schofield et al., 2016; Holness et al., 2017a). By analysing the Sr- and Nd-isotope ratios of the sills,

Foland et al. (2000) demonstrated at least 95% of the magma forming the Little Minch Sill Complex originated from the mantle, with only ~5% crustal contamination from the amphibolite-facies to granulitefacies basement. The magma feeding the tabular intrusions was alkaline picrite to picritic basalt in composition, varying according to the proportion of crystal cargo entrained (Gibb and Henderson, 1996, 2006). The crystal cargo contained mostly olivine grains (phenocrysts and clusters) and minor plagioclase (Holness et al., 2017a; Nicoli et al., 2018). Using field observations and seismic data, Schofield et al. (2016) showed that composite sills were formed via two distinct magma pulses characterised by two compositional endmembers: a cargo-free pulse and a cargo-rich pulse (Holness et al., 2017a, 2017b; Nicoli et al., 2018). The timing of these pulses varies from one intrusion to another. From these pulses results three main lithologies (Gibson and Jones, 1991) (Fig. 1b): - Picrite: olivine >40%, plagioclase, clinopyroxene. Prime examples are the Shiant Isles Main sill lower picrite (~25 m thick) (Drever and Johnston, 1959; Gibb and Henderson, 1989, 1996), the Kilbride Point sill (~3 m thick) (Gibson, 1988). Picritic magma emplacement is achieved via flow sorting and concentration of the largest particles in the centre of the flow. Such a mechanism requires neither particle settling nor convecting magma (Holness et al., 2017a). - Picrodolerite: olivine: 10e20%, plagioclase, clinopyroxene. The picrodolerite occurs at the bottom and top margins of some crinanitic sills (Holness et al., 2017a; Nicoli et al., 2018). The basal part constitutes an accumulation sequence of settled olivine grains from the crystal cargo that occurred in two steps: during the early post-emplacement period, the biggest olivine grains and clusters from the incoming crystal cargo settled to form a fining-upwards sequence. The smallest particles remained in suspension, which then grew in a vigorously convecting magma to form a slower progressive accumulation characterised by a coarsening-upwards tail. The reader is referred to the study cases

Fig. 1. (a) Simplified geological map of Scotland showing the extend of the Hebridean Igneous Complex. Central complexes: 1. Cuillin; 2. Rum; 3. Ardnamurchan; 4. Mull; 5. Arran. (a) Simplified map of the Isle of Skye and the Shiant Isles. Most of the Little Minch Sill Complex crops out along the coast of the Trotternish Peninsula. SH: Shiant Isles Main Sill, MT: Meall Tuath sill, CI: Creagan Iar sill, KP: Killbidre Point sill; DR: Dun Raisburgh sill; IN: Inver Tote (Gibson, 1988).

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of the Shiant Isles Main Sill (Holness et al., 2017a) and the Dun Raisburgh sill (Nicoli et al., 2018) for details. - Crinanite: olivine <5%, plagioclase, clinopyroxene, minor analcime. A prime example of this is the 3 m thick Inver Tote crinanite sill (Gibson and Jones, 1991). 3. Petrography The Little Minch Sill Complex is characterised by mineral assemblages containing a variable amount of olivine, plagioclase, clinopyroxene (augite), Cr-spinel with minor amounts of interstitial analcime, magnetite, apatite and zeolite (Gibb and Henderson, 1996). Olivine in picrite and picrodolerite samples occurs as phenocrysts and clusters (Fig. 2a). Olivine crystal aggregates can be indicative of crystal recycling and accumulation (Jerram et al., 2003), which might be the result of mixing different crystal populations from different magma chambers, rather than a simple crystallising sequence (e.g. Jerram et al., 2009; O'Driscoll et al.,

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2007). In the case of the Little Minch Sill Complex, crystal size distribution and chemical analyses (Gibb and Henderson, 2006; Holness et al., 2017a; Nicoli et al., 2018) argue for latter scenario, with crystallisation in a single intrusion. The olivine grains in the picrite and picrodolerite are characterised by a distinct internal normal zoning with a magnesium rich core and an iron rich rim (Fig. 2b,c). The core often contains Crspinel mineral inclusions (Gibb and Henderson, 1996) (Fig. 2bed). Trails of fluid inclusions indicate the presence of healed cracks (Fig. 2d), interpreted as CO2 degassing during magma ascent (Roedder, 1965). The rim of the olivine grains, conserves an pseudoeuhedral shape, indicating growth onto a recycled core from the primitive crystal cargo, in a melt rich-environment (Fig. 2d,e). In the LMSC, synneusis is the most likely mechanism for the formation of particle aggregates (Fig. 2a) because the coarsening upwards sequence and the crystal morphology (i.e. clusters, rafts or chains) indicate growth in the convecting sill (Holness et al., 2017a). The presence of particles in suspension (e.g. olivine, plagioclase) in the

Fig. 2. (a) olivine cluster from the Meall Tuath sill fining-upwards sequence in crossed polarised light (2 m stratigraphic height). BSE imaging (b-e) of the olivine iron content, increases from dark blue to yellow colours. Arrows indicate the presence of melt inclusions. (b) Killbride sill sample (2 m stratigraphic height) showing gradual zoning in olivine. Note that some olivine grains are affected by late weathering (c) Olivine from the Meall Tuath sill accumulation sequence (1 m stratigraphic height) showing a distinct core/rim boundary. The dashed lines highlights the melt inclusion-rich zone of the olivine, mainly in the core. (d) Melt inclusion trail in olivine from Creagean Iar sill accumulation sequence (0.5 m stratigraphic height). (e) Olivine cluster in the accumulation sequence of the Meall Tuath sill (1 m stratigraphic height). The dashed lines highlight the faces along which grains are amalgamated together, (f) Crinanitic sample from the Inver Tote sill (1.5 m stratigraphic height) in in crossed polarised light. Dashed line: ophitic olivine.

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liquid affect the fluid dynamic properties of the magma (i.e. Stokes velocity) during the early stage of the sill formation, as has been observed in the Shiant Isles Main sill (Holness et al., 2017a) and the Dun Raisburgh sill (Nicoli et al., 2018). Altogether, this information indicates that most of the olivine cores present in picrite and picrodolerite units of the LMSC belong to the original crystal cargo carried along with the melt during the magma ascent (Gibb and Henderson, 1996) (see following section). In the absence of crystal cargo, both augite and olivine have an ophitic texture as they crystallise from the carrier liquid following the formation of the rigid plagioclase framework (Fig. 2f). Throughout the entire sill population, augite is interstitial and plagioclase mostly occurs as euhedral and randomly oriented grains (Gibson, 1988; Holness et al., 2017a; Nicoli et al., 2018). Note that if true for the LMSC, other olivine-rich intrusions may result from different emplacement mechanisms. 4. Bulk rock chemistry Variations in bulk-rock chemistry in the Little Sill Minch Complex have been extensively studied by Gibson (1988, 1990), Gibb and Henderson (1996, 2006) and Latypov (2003). The reader is referred to Gibb and Henderson (2006), who provided a full, detailed description on the overall chemical variation in the Shiant Isles Main sill. To summarise, when plotted against bulk-rock MgO (wt%) content, whole-rock major and trace elements for the three different lithologies define linear trends from evolved bulk-rock compositions (i.e. crinanitic magma) towards a more primitive magma, characterised by Fo83 (Fo ¼ Mg# ¼ Mg/[Mg þ Fe2þ]). Fig. 3 shows variations in bulk-rock CaO (wt%) and Cr (ppm) content as a function of the maficity, i.e. FeO þ MgO (Fig. 3). We used data from Gibb and Henderson (2006) and representative compositions from the sills in Trotternish Peninsula (Gibson, 1990). CaO content vs FeO þ MgO (Fig. 3a) defines a negative trend towards more primitive compositions. The higher CaO concentrations in the crinanites arise from the abundance of plagioclase and augite. Cr vs FeO þ MgO (Fig. 3b) defines a positive trend mirroring the CaO vs. FeO þ MgO trend. High bulk-rock chromium concentrations in picrite and picrodolerite from the fining-upwards sequence are due to the presence of Cr-spinel in inclusion in Mg-rich olivine cores. Picrite samples contained more olivine phenocrysts and plot towards higher FeO þ MgO values. The range of composition of the picrodolerite samples is greater than those of the picrite. Picrodolerite samples from the fining-upwards sequence (Holness et al., 2017a) contain a greater amount of olivine crystal cargo and therefore plot towards higher FeO þ MgO values. Samples from the coarseningupwards sequence contain olivine grains that grew postemplacement in equilibrium with the convecting carrier liquid (Fig. 3a). Gibb and Henderson (2006) made estimates of the carrier liquid composition, PicPL (Mg# 0.52), PdolPl1 (Mg# 0.52) and PdolPl2 (Mg# 0.53) and an evolved crinanite, EV (Mg# 0.36), based on different assumptions; all plot within the field of crinanite compositions. EV represents the last liquid remaining the sill at the end of the crystallisation sequence. The calculations subtracted olivine and Cr- spinel phenocrysts from the bulk-rock composition in the proportions they were observed. These trends emphasise that the majority of the bulk-rock chemical variability observed in the Little Minch Sill complex arises from variable amounts of an olivine and spinel rich crystal cargo (high FeO þ MgO values and high Cr content), in a carrier liquid of largely uniform composition. 5. Mineral chemistry We compiled mineral chemistry data from Trotternish Peninsula (Gibson, 1988) and the Shiant Isles (Gibb and Henderson, 1996)

Fig. 3. Bulk rock compositions as a function of maficity (FeO þ MgO, wt%). (a) CaO content (wt%). (b) Cr content (ppm). Data from Gibson (1990) and Gibb and Henderson (2006). Calculated carrier liquids: PicPL, PdolPl1, PdolPl2, EV e evolved crinanite (Gibb and Henderson, 2006). Arrows points towards Fo83 and Cr-spinel. F: fining-upwards sequence, C: coarsening-upwards sequence (Holness et al., 2017a; Nicoli et al., 2018).

to characterise its variability between the different lithologies, summarised in Fig. 4. Picrite and picrodolerite samples contain olivine characterised by an Mg-rich core (Fo80e84), and rims characterised by Fo40e70 and Fo55e75 respectively (Fig. 4a). However, olivine compositions in the criananite only range from Fo40 to Fo65. Using kd [Fe/Mg]mineral/[Fe/ Mg]liquid ¼ 0.3±0.03 (Roeder and Emslie, 1970) it can be shown that the cores of olivine phenocrysts are in equilibrium with a liquid characterised by Mg# ¼ 0.52e0.65 (Fig. 4b), similar to the estimated primitive liquid composition (Gibb and Henderson, 2006). The mean CaO contents of augite grains are within one standard deviation of one another: the picrite, picrodolerite and crinanite are characterised by CaO values of 19.5e21.5 wt%, 18e21.5 wt% and 17.5e21 wt% respectively (Fig. 4d). Using kd [Fe/Mg]mineral/[Fe/ Mg]liquid ¼ 0.25 ± 0.05 (Putirka, 1999), it can be shown that augite crystals characterised by Mg# values of 0.6e0.85 are in equilibrium with a liquid with a Mg# of 0.36e0.52 (Fig. 4d). The range of average plagioclase composition is the same in all three type of lithology, ~An40e70 (An ¼ Ca/[Ca þ Na]) (Gibson, 1988), with the crystals displaying normal zoning from anorthite rich core to albite rich rim. Gibson (1988) indicated that in some sills, e.g. Inver Tote crinanite sill, some plagioclase shows oscillatory zoning, usually interpreted to reflect changes in the rates of

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Fig. 4. Mineral compositions. (a) Olivine composition (Forsterite ¼ Mg / [Mg þ Fe2þ]). (b) Olivine-Liquid equilibria. The grey zone represent the evolution of the carrier liquid during fractional crystallisation (FC) in the sill (c) Augite CaO content (wt%). (d) Augite-Liquid equilibria. The horizontal bars in panel (a,c) indicate the average mineral composition ±1s. Data from Gibson (1988) and Gibb and Henderson (1996). Numbers in bracket indicate the number of analyses. PicPl, PidolPl1, PidolPl1 and EV after Gibb and Henderson (2006). Kd: partion coeffciant between mineral and melt.

crystallisation and diffusion in the magma chamber (Bottinga et al., 1966; Ginibre et al., 2002). Cr-Spinel inclusions within olivine phenocrysts (Fo80e84) in the Shiant Isles Main Sill are characterised by a Cr2O3 content of 18 to 30 wt% (Gibb and Henderson, 1996). 6. Thermodynamic modelling Field and petrographic observations combined with geochemical data (e.g. Gibb and Henderson, 2006; Holness et al., 2017a; Nicoli et al., 2018) strongly supports a closed system evolution of the Little Minch Sill Complex. Fluid dynamic processes in the sill are argued to allow the generation of crystal cargo-rich magma and a cargo-free magma by crystal settling (Holness et al., 2017a). Since there is a well understood co-genetic relationship between the liquid and crystal cargo (Gibb and Henderson, 2006) (Fig. 4), the mineral chemistry is likely to preserve a record of the pressure and temperature conditions at which the sill solidified and the crystal cargo formed. To estimate these parameters we modelled the evolution of the parental magma composition in a P-T space (Fig. 5) using the PicrPL, PdolPl1, PdolPl2 and Evolved Crinanite (Gibb and Henderson, 2006), and samples SC598 (picrite) and SC518 and SC492 (picrodolerite) (Gibb and Henderson, 1996, 2006). We used Perple_X (version 6.7.5 -Connolly, 2009) to perform the calculations, with the solution models for mafic and ultramafic magmas of Jennings and Holland (2015) in a Cr2O3-Na2O-CaO-FeOMgO-Al2O3-SiO2 system (Nicoli et al., 2018). Using the Cr2O3 content of the Cr-spinel inclusions in olivine (18e30 wt%) and olivine phenocryst core compositions (Fo82e84) along with bulk rock compositions from the picritic part and the fining-upwards sequence of the picrodolerite from the Shiant Isles Main Sill, (Gibb and Henderson, 1996; Gibson, 1988), we estimated the conditions of formation of the crystal cargo at an average 1170 ± 10  C and 7.2 ± 0.3 kbar (~25 km) (Fig. 5a).

To estimate the emplacement conditions of the sill, we used the bulk-rock composition of the carrier liquids along with augite CaO content and Mg# (17.5e21.5 wt% and 0.6e0.85 respectively) and olivine rim compositions (Fo30e75), arguing for both phases crystallising from the carrier liquid in the sill. Crystallisation temperature of the sills is 1065 ± 30  C (Fig. 5b). The depth of equilibration has estimated at ~10 km (2.7 ± 1.9 kbar). 7. Discussion 7.1. Modelling igneous rock with phase equilibria modelling Our study shows that when well-constrained, many magmatic systems can be understood using the principles of metamorphic petrology. In calculating the conditions of crystal cargo formation, we implicitly assume closed system crystallisation and remobilisation of the melt and crystals in the same proportion as exists in the deep magma chamber. The cogenetic relationship between the melt and crystals is difficult to ascertain in many eruptions: often, erupted material samples a large range of phenocrysts and primitive mineral compositions are interpreted to come from unmixed primary melts, carried by a mixed evolved liquid (e.g. Danyushevsky et al., 2004; Faure and Schiano, 2005; Maclennan, 2008). However, some olivine core compositions in primitive Icelandic basalts are in equilibrium with remaining melt or the surrounding crystal mush (e.g. Thomson and Maclennan, 2012). In the case of magma emplacement and storage in the crusts, it is likely that similar mechanisms applied with a crystal cargo being stored in the carrier liquid for a long time, subsequently mobilised by disruption of the crystal mush (e.g. new batch of magma, slurry). An all too common issue when dealing with rocks containing an initial crystal cargo is what the whole rock data actually represents

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Fig. 5. Pressure and temperature estimates. (a) Equilibrium conditions between Cr-spinel inclusions in olivine grains and Mg-rich olivine core using picrite and picrodolerite samples from the Shiant Isles Main Sill (Gibb and Henderson, 1996). Solid line: spinel chrome content (wt%); dashed line: olivine forsterite content. (b) Equilibrium condition between clinopyroxenes and olivine rims using carrier liquid compositions from Gibb and Henderson (2006). The grey area indicates dry solidus conditions. Solid line: spinel chrome content (wt%); dashed line: olivine forsterite content. Average P-T conditions and error taken from midpoint and range. Detail phase diagrams in supplementary information.

(e.g. Martin et al., 2010; Davidson et al., 2007 and references therein), particularly in olivine rich systems (e.g. Keiding et al., 2011). However, in addition of showing equilibrium between the carrier liquid and the crystal cargo, our thermodynamic calculations also predict similar mineral volume fractions to the observed crystallinity in the picritic sill, when the groundmass composition is in equilibrium with olivine core compositions at the inferred P-T conditions (see following section). In conclusion, such thermodynamic calculations may be applied when a magmatic system is already extremely well constrained by structural and petrographic observations and chemical analyses. Though our method provides information on the structure of the upper plumbing system within the crust, 0e35 km, it cannot assess the processes happening at mantle depths.

7.2. Properties of the magma during ascent and emplacement In the 25 m thick Shiant Isles Main sill lower picrite, the olivine mode is >40 vol%. Gibb and Henderson (2006) estimated that 10% of the olivine crystallised from the carrier liquid after settling of the crystal cargo; therefore the observed volume of olivine at a specific stratigraphic height, 4ol, is given by.

  4ol ¼ 4original þ 0:1 1  4original where 4original is the volume of olivine corrected from overgrowth (Holness et al., 2017a). The original volume of crystal cargo in the incoming magma, 4cargo, is then obtained by integrating 4original values from the lower half of the sill to the total thickness of the intrusion. In the Shiant Isles Main sill lower picrite, 4cargo is ~10 vol%. Two end-member scenarios were modelled, using phase equilibria calculations, to investigate the rheological behaviour of the magma during its ascent and emplacement at shallow crustal levels. The first scenario deals with a magma containing no crystal cargo (Fig. 6a). The composition of the carrier liquid was obtained averaging carrier liquid compositions (PicrPL, PdolPl1 and PdolPl2) from Gibb and Henderson (2006). For this composition, bulk Mg#

0.52, Cpx is in equilibrium with the carrier liquid, kd_cpx ¼ 0.2e0.3. For the second scenario, the composition of the magma was obtained by mixing the carrier liquid composition with 10 vol% of Fo83 olivine composition from Gibb and Henderson (1996). For this composition, bulk Mg# 0.61, Olivine core (Fo80e84) is in equilibrium with the carrier liquid, kd_ol ¼ 0.3e0.34. For spheric-like olivine phenocryst such as the one in picritic sill (Fig. 4a,b, in Holness et al., 2017a), particle cohesiveness and mechanically stable random loose packing is achieved at crystal volume fractions between 45 vol% and 55 vol% (Dong et al., 2006; Holness et al., 2017a; Vigneresse and Tikoff, 1999; Yang et al., 2007). For elongated particles such as plagioclase grains, Philpotts and Dickson (2000) and Philpotts et al. (1998, 1999) showed that randomly oriented particles form mechanically coherent frameworks at 25 vol% to 30 vol%. Nicoli et al., 2018 previously deduced a similar packing fraction (25 vol%) for randomly oriented plagioclase in the Dun Raisburgh sill. Such thresholds represent two extreme packing end-members. In reality, the olivine population in the LMSC (Fig. 2), contains both phenocrysts and glomerocrysts, which might decrease the packing threshold below 45 vol%, even for subspherical particle (Jerram et al., 1996, 2003). In the case of the crystallisation of cargo-free liquid (Fig. 6a), plagioclase is the first phase to crystallise. This sequence of crystallisation suggests water is absent during decompression and emplacement of the sill at shallow crustal levels (Neave and Putirka, 2017). The mechanically coherent framework (25e30 vol% of solid) forms at ~1150  C, prior or synchronous to the crystallisation of olivine and augite. These two phases then crystallise from the interstitial liquid, which is consistent with their ophitic habits observed in thin sections of the crinanite. In the case of the crystallisation of cargo-rich magma (Fig. 6b), olivine and a small proportion of spinel are the first phases to crystallise. During ascent, the magma starts crystallising plagioclase (Fig. 6b). The path followed by the magma reaches the conditions required for the formation of a mechanically coherent framework (45e55 vol% of solid) at 1100e1150  C. This also correspond to the onset of augite, which crystallises from the interstitial liquid (Holness et al., 2017a, 2017b).

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Fig. 6. Volume of melt. (a) Magma composition: average carrier liquid. Plagioclase in excess (b) Magma composition: average carrier liquid þ15 vol% Fo83 olivine. Olivine in excess. MCF: Mechanically coherent framework indicating the volume of crystal present in the magma (Dong et al., 2006; Holness et al., 2017a; Nicoli et al., 2018; Philpotts et al., 1998, 1999; Philpotts and Dickson, 2000; Vigneresse and Tikoff, 1999; Yang et al., 2007). The arrow indicate the P-T path followed by the magma, according the P-T estimates (see Fig. 5). The dashed regions correspond to the P-T estimates from Fig. 5. See supplementary information for the full phase diagram.

Despite being ideal scenarios, our modelling demonstrates that the amount of crystal cargo controls the crystallinity of the mush, which then might influence the physical properties of the magma during emplacement in the upper crust. During its ascent, the magma will cross different rheological thresholds (e.g. Holness et al., 2017a; Philpotts et al., 1998, 1999) leading to a more mechanically coherent framework of plagioclase and olivine grains, stopping its propagation further towards the surface. The ascent and shapes of shallow intrusions might also be controlled by changes in the rheology of the host rock (Magee et al., 2012). 7.3. Little Minch Sill Complex's plumbing system In this study, we used phase equilibria modelling to investigate the architecture of the plumbing system responsible for the formation of the igneous complex on the Isle of Skye. The welldocumented bulk-rock and mineral chemistry of the Little Minch Sill Complex (Gibb and Henderson, 1996, 2006; Gibson, 1988; Gibson and Jones, 1991) combined with microstructural information and fluid dynamic models (Holness et al., 2017a; Nicoli et al., 2018), constitute a reliable dataset to quantify the depth of the successive magma reservoirs underneath the Skye Main Lava Series (SMLS). The SMLS covers most of the superficies of the Isles of Skye (Fig. 1b). Melting of the mantle has been argued to have occurred between 60 and 112 km depth, in the garnet-lherzolite stability field, for temperatures ranging from 1400 to 1500  C (Hole and Natland, 2019; Scarrow et al., 2000; Scarrow and Cox, 1995; Thompson, 1974; Thompson et al., 1980). The magma feeding the SMLS lava then fractionated at the base of the crust at 9 kbar, while a late generation of dykes, the Preshal More basalt (PMB) crosscut the SMLS at shallower depths, from 5 kbar upwards. Olivine hosted Cr-Spinel inclusions in equilibrium with Fo82e84 olivine cores indicate a crystallisation depth of 25e27 km, similar to previous pressure estimates for low Cr# number spinel, 9e10 kbar (Hole, 2018; Hole and Natland, 2019). The estimated temperature of

1170 ± 10  C is close to the eruption temperature for the SMLS, 1235 ± 35  C at surface conditions (Gibb and Henderson, 2006; Hole and Natland, 2019), and the plagioclase-in temperature, 1226  C (Hole and Natland, 2019 and references therein). The depth preserved by the mineral assemblage crystallising from the carrier liquid (i.e. augite and ophitic olivine), 3e16 km, is consistent with the maximum estimate used by Gibb and Henderson (2006), 13 km. The temperature preserved, 1065 ± 30  C, is close to solidus conditions (~1000  C) for primitive magma. In the southern part of the Isle of Skye, the Cuilin Central Complex, is characterised by a rhythmic layered gabbros and crosscut by basaltic cone-sheets (Bell et al.,1994; Brandriss et al., 2014; Hepworth et al., 2018). Sr isotopes indicate the crinanitic magma (crystal cargopoor and low Mg#) is genetically linked to the magma involved in the formation of the most differentiated cone-sheet found in the Cuillin Central Complex (Bell et al., 1994). Bell et al. (1994) argue that the cone-sheet magma evolved from a long-lived magma chamber at 10e12 km depth. This similar to the average depth of the sill complex emplacement (~10 km). The most primitive cone-sheet are interpreted to originate from the partial melting of the mantle, within the spinel peridotite stability field, at 30e35 km depth (Bell et al., 1994; McKenzie and Bickle, 1988), which is similar to our estimates. Collectively, this information allowed us to recreate the evolution of the plumbing system underneath the Isles of Skye at ca. 60 Ma (Fig. 7). Melting of the mantle induced by the mantle plume now beneath Iceland occurred within the garnet stability field. The magma produced ascended and ponded underneath the crust at ~30 km. Substantial cooling within these magma chambers allowed the formation of a more evolved liquid and a crystal mush composed of olivine and minor spinel and plagioclase. These crystals constitute the crystal cargo. The magma (carrier liquid þ variable amount of crystal cargo) then ascended towards the surface, accumulating in the upper crust (i.e. sill, dyke and layered intrusion) or erupting (SMLS). The temperature of the magma in the plumbing system is ~1200  C. At this temperature, the magma contains between 0 and

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Fig. 8. Compilation of recorded depth in the Hebridian Igneous Province and Iceland. 1: compilation by Maclennan (2019); 2: Neave and Putirka (2017), clinopyroxene-liquid barometer; 3: Hartley et al. (2018), melt inclusion and olivine-plagioclase-melt-augite barometer; 4: this study, phase equilibria modelling; 5: Bell et al. (1994), cone-sheet emplacement; 6: Upton et al. (2002), olivine-spinel equilibrium; 7: Magee et al. (2012), minimum depth estimates; 8: Preston et al. (1998), minimum depth estimates, 9: Tantrigoda (1999), 10: Lindsley et al. (1969); Williams (1971). Shallow intrusions (sill, dyke and layered intrusions) occurs between 7 and 12 km depth.

Fig. 7. The Little Minch Sill Complex plumbing system. SMLS: Skye Main Lava Series. Dry solidus (Takahashi and Kushiro, 1983) and stability field of aluminium rich phases (i.e. garnet, spinel and plagioclase) (Gasparik, 1985)for ultramafic rocks.

~15 vol% of crystal cargo (see below). This model is similar to the model suggested by Font et al. (2008), but does not require temporary storage between 15 and 25 km depth.

The architecture of magmatic plumbing system was, therefore, similar beneath the whole Hebridean Igneous Province. Partial melting of the mantle occurred at depths of 60e120 km (Hole, 2018). After the magma ponded at the base of the crust at ~25 km depth (Font et al., 2008), and crystallised some of its crystal cargo, it ascended towards the surface and was stored in intrusive bodies (i.e. sills) at 3e15 km depth. 7.5. A proxy for active volcanoes?

7.4. The Hebridian Igneous Province The Hebridean Igneous Province forms a ~400 km NW-SE trending magmatic province on the west coast of Scotland. The Province includes several central igneous complexes, located from south to north in the Isle of Arran, the Isle of Mull, the Ardnamurchan peninsula, the Isle of Rum and the Isle of Skye (Emeleus and Bell, 2005). Their emplacement and distribution at ca. 60 Ma is linked to the presence of a long-lasting mantle hotspot, currently beneath Iceland, during the opening of the North Atlantic Ocean (Kent and Fitton, 2000). In this section, we compare the estimated depths of magma storage in the Little Minch Sill Complex with the other magmatic complexes from the Hebridean Igneous Province. Fig. 8 shows that similar depths of shallow storage are preserved in Skye, Adnarmurchan, Mull and Arran. Magma storage at, or near, the Moho is inferred for Skye and Rum. In Rum, Upton et al. (2002) estimated the crystallisation depth of Fo93e87 olivine from a picritic magma at 25e30 km. The minimum emplacement depth of the ~58 Ma cone sheet swarm of Ardnamurchan was estimated at 5 km (Magee et al., 2012). Preston et al. (1998) estimate the depth of emplacement to the Loch Scridain Xenolithic Sill Complex on the Isle of Mull at 7e10 km. Aeromagnetic survey over the Isle of Arran (Tantrigoda, 1999) showed the presence of cone-sheet structures at an average depth of 6.5 km. However, the emplacement could have been deeper prior erosion. Crystals from the Mull Plateau Lava Formation (MPLF) record various equilibrium depths, from the base of the crust (35 km) to the deep lithospheric mantle (>120 km) (Kerr, 1994).

The North Atlantic Igneous Province (NAIP) spreads from the Baffin Island in the West to the Hebridian Igneous Province in the East. To the West of the mid-Atlantic ridge, the ca. 55 Ma Skaegaard Layered Intrusion (Wager and Deer, 1939) intrudes the Precambrian basement at 5 to 9 km depth (Lindsley et al., 1969; Williams, 1971) (Fig. 8).Volcanism started at ca. 62 Ma and Iceland represents its current active part (Hole & Natland and references therein). Whilst Iceland offers excellent opportunities to study erupted volcanics and their crystal cargoes, outcrops of the intrusions constituting the underlying magmatic systems are often missing. Only a few sills crop out in the inactive and eroded east and west of the island (e.g. Burchardt, 2008). Our knowledge of magmatic plumbing systems in Iceland relies mainly on geophysical surveys (e.g. Greenfield et al., 2016; Woods et al., 2018) and petrological analysis of the erupted material (e.g. Maclennan, 2019). Application of Cpx-liquid and OPAM barometry on lavas from the Krafla & Theistareykir volcanic systems (Maclennan et al., 2001; Winpenny and Maclennan, 2011) showed their parental magmas were stored at variable depths between 5 and 30 km. Neave and Putirka (2017) used an updated calibration of the cpx-liquid barometer to estimate magma storage as deep as ~20 km depth, with the majority of shallow intrusions forming at 10e15 km depth (Fig. 8). Hartley et al. (2018) showed that the olivine from the 2014e2015 Holuhraun eruption equilibrated at 7.5 ± 2.5 km. These observations are consistent with the presence of shallow lenses and tabular bodies observed in seismic profiles
ttir et al., 1997; Maclennan, 2008). (Brandsdo

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Despite differences in the tectonic setting, and the composition and architecture of the crust, magma storage depths in the upper crust are similar in Iceland, in Greenland and in the Hebridian Igneous Province. This observation implies that the architecture of the shallow plumbing system that underlaid the North Atlantic Igneous Province centres, may be comparable to that beneath Iceland in the present day. This apparent consistency strongly supports the use of preserved magmatic bodies to investigate the processes occurring directly beneath active volcanoes in the upper part of the crust. Additionally, evidence for crystallisation of magmas close to the base of the crust in both the North Atlantic Igneous Province and Iceland suggests magmas arriving in the lower crust may have experienced similar magmatic processing during their ascent. 8. Conclusion Our study shows that phase equilibria may be applied to well characterised intrusions within igneous provinces, such as the Little Minch Complex, in order to place robust estimates on the depths of deep magma storage and the original depths at which sills, dykes and layered intrusions were emplaced. Estimates of the pressure and temperature of magma equilibration using the crystal cargo contained in the tabular intrusions outcropping on the Isle of Skye and the Shiant Isles suggests the presence of magma chamber at ~25 km depth and the formation of sill and dyke network at ~ 3e16 km below the surface. Pressure estimates using mineral-liquid barometers on erupted material from active volcanoes in Iceland show similar pressures and temperatures of magma equilibration in the upper crust. Since there are similarities in the pressures and temperatures of magma storage between Iceland (Kent and Fitton, 2000) and the Little Minch Sill Complex, exposed crustal plumbing systems in eroded igneous centres may be used to constrain the behaviour of magmas beneath active volcanoes. Acknowledgements The authors thank Andrew Kerr for handling the manuscript and Dougal Jerram and Malcolm Hole for their constructive reviews. This work was supported by the Natural Environment Research Council [grant numbers NE/M013561/1 and NE/N009894/1]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.lithos.2019.105194. Refrences Bell, B.R., Claydon, R.V., Rogers, G., 1994. The petrology and geochemistry of conesheets from the Cuillin Igneous Complex, Isle of Skye: evidence for combined assimilation and fractional crystallisation during lithospheric extension. J. Petrol. 35 (4), 1055e1094. Bottinga, Y., Kudo, A., Weill, D., 1966. Some observations on oscillatory zoning and crystallisation of magmatic plagioclase. Am. Mineral. 51 (5e6), 792e806. Brandriss, M.E., Mason, S., Winsor, K., 2014. Rhythmic layering formed by deposition of plagioclase Phenocrysts from influxes of porphyritic magma in the Cuillin Centre, Isle of Skye. J. Petrol. 55 (8), 1479e1510.
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