Controls on the explosive emplacement of diamondiferous kimberlites: New insights from hypabyssal and pyroclastic units in the Diavik mine, Canada

Journal Pre-proof Controls on the explosive emplacement of diamondiferous kimberlites: New insights from hypabyssal and pyroclastic units in the Diavik mine, Canada

Madeline Tovey, Andrea Giuliani, David Phillips, Stephen Moss PII:

S0024-4937(20)30047-5

DOI:

https://doi.org/10.1016/j.lithos.2020.105410

Reference:

LITHOS 105410

To appear in:

LITHOS

Received date:

6 November 2019

Revised date:

16 January 2020

Accepted date:

3 February 2020

Please cite this article as: M. Tovey, A. Giuliani, D. Phillips, et al., Controls on the explosive emplacement of diamondiferous kimberlites: New insights from hypabyssal and pyroclastic units in the Diavik mine, Canada, LITHOS(2020), https://doi.org/10.1016/ j.lithos.2020.105410

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© 2020 Published by Elsevier.

Journal Pre-proof

Controls on the explosive emplacement of diamondiferous kimberlites: New insights from hypabyssal and pyroclastic units in the Diavik mine, Canada Madeline Toveya*, Andrea Giuliania,b, David Phillipsa, Stephen Mossc

Kimberlites and Diamonds (KiDs), School of Earth Sciences, The University of Melbourne,

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a

Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zurich,

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Clausiusstrasse 25, Zurich, 8092 Switzerland

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b

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Parkville, Melbourne, VIC 3010, Australia.

Terram Vero Consulting Inc., 2685 Dundas Street, Vancouver, British Columbia V5K 1R1,

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Canada.

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*Corresponding author; email, [email protected]

Journal Pre-proof Abstract Kimberlites are mantle-derived magmas that either crystallise as hypabyssal intrusions, erupt explosively after rapid ascent to the surface, or less commonly form lava lakes and flows, thereby creating texturally distinct kimberlite units. Efforts to fully understand the processes responsible for the explosive eruption of kimberlite magmas have been hindered by the widespread alteration and crustal contamination of most volcaniclastic kimberlites. To

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address this issue, we have undertaken a detailed petrographic and mineral chemical study of

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fresh (i.e. minimally altered) pyroclastic and hypabyssal kimberlites (HK) from the ca. 55-56

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Ma A154 North and South kimberlite pipes in the Diavik Mine (Lac de Gras, Canada). These localities host exceptionally fresh kimberlites and are therefore ideally suited to this study.

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Kimberlite emplacement at A154 North and South initiated with the intrusion of

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hypabyssal kimberlite (external dykes), and was followed by the explosive formation of kimberlite pipes and volcaniclastic kimberlite infill. Subsequent kimberlite magmas intruded

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the volcaniclastic kimberlite units forming multiple cross-cutting, internal dykes. The studied

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volcaniclastic units feature abundant rounded magmaclasts and massive textures, suggestive

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of primary deposits. These units are classified as pyroclastic kimberlites (PK). Pyroclastic and hypabyssal kimberlite units at Diavik exhibit subtle mineral compositional differences. Samples from both internal HK units and PK units feature identical compositions for liquidus olivine rims (Mg# = 90.5 ± 0.1 and 90.7 ± 0.2, respectively), with a marginally lower Mg# of 90.2 ± 0.2 in olivine rims from the external HK dykes. Similarly, early-formed chromite compositions are the same for internal HK and PK units (Cr# = 79.1 ± 3.4 and 78.3 ± 5.7; Mg# = 60.0 ± 1.3 and 60.0 ± 2.2), but, differ in the external HK units (Cr# = 86.9± 2.7; Mg# = 52.8 ± 1.9). The internal HK and PK units also exhibit lower carbonate contents than the internal HK units. These compositional differences indicate that the external dykes were probably derived from slightly different primitive melt

Journal Pre-proof compositions to those parental to the internal HK and PK units. Spinel evolutionary trends from chromite to magnesian ulvӧspinel-magnetite (MUM) compositions (Fe3+# = 47.2 ± 5.8 and 49.7 ± 9.3; Cr# = 25.7 ± 11.0 and 17.0 ± 14.0 for MUM) are indistinguishable in internal HK and PK samples. These results demonstrate that the primitive melt compositions and early magmatic evolution processes are identical for the internal kimberlite units, regardless of whether the kimberlite melts erupted explosively or were emplaced as shallow intrusions. However, magmaclasts in the PK units contain higher abundances of phlogopite (<52 vol. %)

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and lower quantities of carbonate (<4 vol. %) than the groundmass of the hypabyssal kimberlite samples (<2 vol. % and 25-65 vol. %, respectively). This indicates that the

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explosively erupted magmas featured higher H2 O/CO 2 ratios. In contrast, abundant

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carbonates, including dolomite, in the internal HK samples indicate that CO 2 , and therefore

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low H2 O/CO 2 ratios, were retained during the emplacement of this magma, which likely prevented phlogopite crystallisation. Lower K and Rb whole-rock compositions for internal

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HK samples compared to PK samples, are attributed to the removal of these components in

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late-stage kimberlitic fluids, as indicated by hydrothermal alteration of the adjacent

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volcaniclastic kimberlite units. The above results clearly rule out variations in primitive melt composition and melt evolution trajectories as a primary control on the explosive behaviour of the kimberlite magmas at Diavik. Our study also emphasises how volatile loss resulting from different emplacement styles can have a profound effect on the whole-rock compositions and petrography of kimberlite units. Controls on kimberlite explosivity at Diavik are likely due to external factors, such as local stress regimes, the availability of groundwater (i.e. phreatomagmatism) and differing magma supply rates. Keywords: Diavik; kimberlite; explosive emplacement; pyroclastic; volatiles

Journal Pre-proof 1

Introduction Kimberlites are volatile-rich (CO 2 and H2 O), SiO 2 -poor, deeply derived magmas (> 150

km), that ascend rapidly towards the Earth’s surface, entraining and transporting abundant mantle xenoliths and xenocrysts, including rare diamonds (Bussweiler et al., 2016; Giuliani and Pearson, 2019; Haggerty, 2017; Kjarsgaard et al., 2009; Le Roex et al., 2003; Mitchell, 1986; Soltys et al., 2016; Sparks et al., 2006). In the upper crust, these magmas form units featuring a variety of morphologies and textures that are indicative of emplacement processes

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ranging from explosive (i.e. abundant volcaniclastic particles and fragmented crystals) to intrusive (i.e. a crystalline groundmass devoid of volcaniclastic materials; Mitchell, 1986;

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Mitchell et al., 2019; Scott Smith et al., 2013; Sparks, 2013).

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The explosive emplacement of any magma is controlled by the catastrophic fragmentation

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of the melt phase due to either rapid volatile expansion (Dingwell, 1998a, b), or interaction with external water (Dellino et al., 2012; Kurszlaukis and Lorenz, 2008; Lorenz, 1975). In

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kimberlite magmas, volatile exsolution and expansion have also been linked to rapid magma

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ascent, whereby exsolution of a CO 2 -rich fluid phase at mantle depths increases magma

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buoyancy thus accelerating its ascent (Brett et al., 2015; Russell et al., 2012; Wilson and Head, 2007). As CO 2 solubility is higher in H2 O and SiO 2 -poor kimberlite melts compared to H2 O and SiO 2 -rich kimberlite melts (Brooker et al., 2011; Moussallam et al., 2016; Russell et al., 2012), the latter magmas will exsolve greater volumes of CO 2 , especially at shallow depths. Elevated H2 O and SiO 2 concentrations may either be inherited from the primary kimberlite melt1 (Moussallam et al., 2016), or develop during magma ascent due to assimilation of wall-rock material (i.e. mantle orthopyroxene and clinopyroxene, and crustal rocks; Gaudet et al., 2017; Russell et al., 2012; Stone and Luth, 2016). Roeder and Schulze

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In this study, we use the terms ‘primary’, ‘primitive’, and ‘parental’ as defined by Foley, S.F., Pintér, Z., 2018. Primary melt compositions in the Earth's mantle, Magmas Under Pressure. Elsevier, pp. 3-42.

Journal Pre-proof (2008) demonstrated that minor compositional variations for kimberlitic chromite may become more prominent as kimberlite melts evolve, leading to large compositional differences in late-stage spinel. Therefore, small differences in primitive melt composition or melt evolution trajectories may have a significant impact on the volatile concentrations of kimberlite melts at shallow depths, thereby influencing kimberlite emplacement styles. Unfortunately, efforts to constrain kimberlite melt compositions have been hindered by poor preservation of the primary mineral assemblages, resulting from post-emplacement

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hydrothermal alteration and weathering (Giuliani et al., 2014; Mitchell, 2013; Mitchell et al., 2019; Mitchell et al., 2009; Sparks et al., 2009; Sparks, 2013; Sparks et al., 2006). Olivine,

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one of the most abundant phases in kimberlites, is particularly susceptible to these processes,

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and is commonly replaced by serpentine. In addition, the volcanic edifices and extra-crater

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deposits that are used to analyse modern volcanic systems (e.g., Brown and Valentine, 2013),

Tappe et al., 2018).

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are seldom preserved in kimberlites as the vast majority are >40 Ma old (Heaman et al., 2019;

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Identifying the similarities and/or differences between the compositions of explosive and

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non-explosive kimberlite melts emplaced within the same pipe is crucial for determining whether melt composition controls explosive kimberlite emplacement. The A154 North and South kimberlite pipes of the Diavik Diamond Mine, Lac de Gras (LDG), Northwest Territories, Canada, comprise exceptionally fresh units of volcaniclastic (VK), including pyroclastic kimberlite (PK), and coherent kimberlite (CK; Moss et al., 2018b). These kimberlite units provide a rare opportunity to undertake detailed, comparative petrological studies to evaluate the role of melt compositions in the explosive emplacement of kimberlite magmas from the same locality. To trace the geochemical evolution of magmas parental to PK and CK units, we have combined detailed petrography with major element analyses of fresh olivine and spinel in representative samples from key units. These studies reveal for the

Journal Pre-proof first time that PK and internal HK units from the same locality exhibit similar olivine and spinel compositions, indicating that the melts derived from the same or similar primitive melt compositions and underwent similar evolution during ascent in the mantle. Therefore, the contrasting emplacement styles at Diavik are not related to differences in kimberlite melt compositions. 2

Geological setting and samples

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The Lac de Gras (LDG) kimberlite field of the Slave Craton is located approximately 300

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km northeast of Yellowknife in the Northwest Territories, Canada (Figure 1a). Kimberlitic

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magmatism occurred in LDG between 75 and 45 Ma forming over 300 pipes (Sarkar et al.,

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2015). These include the four highly diamondiferous A21, A418, A154 North (A154N) and A154 South (A154S; Figure 1b) localities (Moss et al. 2018a), which form the main

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kimberlitic bodies of the Diavik diamond mine. These kimberlites were emplaced at ca. 55-

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56 Ma into faulted muscovite-biotite monzogranites (2599-2580 Ma) that were intruded by multiple diabase dykes (2.02-2.03 Ga) and overlain by soft sediments (Amelin, 1996; Barton,

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1996; Graham et al., 1999; Kjarsgaard, 2002; Moser and Amelin, 1996; Moss et al., 2018b).

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Multiple units of volcaniclastic kimberlite (VK) and smaller volume units of coherent kimberlite (CK) infill the Diavik kimberlite pipes, indicating a complex multi-stage kimberlite emplacement history (Figures 1d and 2). According to Moss et al. (2018a, b), formation of the A154N and A154S kimberlite pipes (the focus of the current study) was preceded by the emplacement of a precursor dyke named Duey’s (Figures 1d and 2f). Duey’s dyke intruded along a pre-existing fault named Duey’s fault (fault set I), cross-cutting a second shallowly dipping fault set (fault set II) and two pre-existing diabase dykes during emplacement (Figure 1d). Explosive activity followed the emplacement of these dykes, excavating and partially infilling the A154N kimberlite pipe with three texturally massive volcaniclastic units PK1-N, PK2-N and PK3-N (Figures 1d, 2a and 2b). These pyroclastic

Journal Pre-proof units are separated by, and overlain by reworked, mud-rich, weakly bedded volcaniclastic kimberlite (MRVK1-N and MRVK2-N; Figure 1d) that formed during two period of quiescence. A laminated mudstone, stratigraphically above PK3-N indicates a crater lake likely developed after the emplacement of this unit and before the formation of unit MRVK2N. Unit MRVK2-N was later overlain by pyroclasts from a nearby kimberlite that produced a graded volcaniclastic kimberlite unit at the top of the pipe (PK4-N; Figure 1d). Finally, multiple structurally massive and homogeneous coherent kimberlites intruded the

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volcaniclastic units in the A154N pipe forming an irregular shaped unit (CK-N; Figures 1d and 2d; Moss et al., 2018a; Moss et al., 2018b).

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After the emplacement of Duey’s dyke (Figure 2f), explosive activity also formed and

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infilled the A154S pipe with massive volcaniclastic kimberlite (unit PK-S). Fragments of

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layered volcaniclastic kimberlite (LVK), and some mud (MUD; Figure 1d) were entrained during the emplacement of PK-S. Reworking of the surface eruption products and sediments

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then resulted in the formation of an overlying unit of massive to chaotic mud-rich VK

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(MRVK-S) that infilled the rest of the pipe (Figure 1d). As with A154N, the VK units of

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A154S are crosscut by multiple closely related, structurally massive coherent kimberlite intrusions along the pipe margins (CK-S; Figures 1 and 2c; Moss et al., 2018a; Moss et al., 2018b). Additional dykes were also emplaced external and adjacent to the A154S kimberlite pipe separated by a few meters of granite (e.g., unit U381). However, due to a lack of visible field relationships, the relative emplacement age of these dykes is unknown. Entrained wood and soft surface sediments recovered from the LDG kimberlite pipes indicate minimal surface erosion (< ~300 m) after kimberlite emplacement (Nowicki et al., 2004) and emplacement of the kimberlite units into a wet, terrestrial environment (Hook et al. 2014; Sweet et al. 2003). This study focuses on two volcaniclastic and four coherent kimberlite drill-core samples from the A154N and A154S kimberlite pipes (Table 1; Figures 1 and 2). The volcaniclastic

Journal Pre-proof samples derive from units PK1-N (A154-U190; no. 2 in Figures 1d and 2b) and a melt-rich area within PK3-N (GTH-75_15; no. 1 in Figures 1d and 2a) of A154N. The latter units predominantly feature a homogenous massive structure, similar to typical Kimberley-type pyroclastic kimberlites, and contain abundant fragmented olivine macrocrysts (>500 µm; 2836 vol. % of rock; Table 2) indicative of high energy emplacement processes. In addition to broken olivine, the VK units contain abundant rounded magmaclasts, but, rarely feature unconsolidated mud as layers or clasts, suggesting a primary origin for the units (Figures 2a

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and 2b; Figure 3a; Supplementary Figure 1b). Consequently, samples from these units are termed pyroclastic kimberlites (PK).

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Coherent kimberlites were sampled from Duey’s dyke (Duey’s A154S_01; no. 6 on

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Figures 1d and 2f), a dyke adjacent to the A154S kimberlite pipe (A154-U381; no. 5 on

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Figures 1d and 2e), CK-S (04GTH-78_03; no. 4 on Figures 1d and 2d) and CK-N (A154U384; no. 3 on Figures 1d and 2c). These coherent kimberlite units have sheet-like (Duey’s

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and U381) and irregular morphologies formed by the intrusion of multiple dykes (CK-S and

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CK-N); feature homogeneous inequigranular textures devoid of flow banding except unit

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U381; and feature fresh euhedral to rounded macrocrystic olivine (9 vol. % to 38 vol. %; Table 2) with minor fractures (Figures 2 and 3; Supplementary Figure 1a). These characteristics are typical of sub-surface intrusive magmas. Consequently, these units are termed hypabyssal kimberlite (HK) to distinguish them from extrusive coherent kimberlite, which is common in the nearby Ekati Diamond Mine. Hypabyssal kimberlite samples are categorised into two groups: i) dykes external to the main kimberlite pipes that include sample Duey’s A154S_01 (Figures 1d and 2f; no. 6), and A154-U381 (Figure 2e; no. 5) and are termed external HK samples; and ii) irregular intrusions that cross-cut the volcaniclastic units and were emplaced after the formation and infill of the main kimberlite pipes (internal

Journal Pre-proof HK units). The internal HK samples studied include 04GTH-78_03 from unit CK-S (Figures 1d and 2d; no. 4), and A154-U384 from unit CK-N; (Figures 1d and 2c; no. 3). The homogeneous nature of the Diavik PK and HK units (except U381) and the preservation of fresh olivine in the selected, representative samples are considered ideal for comparative petrographic and mineral chemistry analysis of explosive PK and intrusive HK. Flow banding in external dyke U381 has resulted in both spinel-rich and spinel-poor bands (Figure 2e; Supplementary Figures 1e and 1f). Mineral modal abundances have been

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completed for both spinel-rich and spinel-poor bands to ensure that results for this sample are

Methodology 3.1

Petrography and modal analysis

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3

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representative of the whole unit (Supplementary Table S1).

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The studied PK and HK samples were characterised using microscope and scanning

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electron microscope (SEM) petrography, electron microprobe (EMP), major element mineral

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chemistry and whole rock geochemistry (major and trace elements). After an initial petrographic study of hand samples and thin sections using a standard polarised microscope,

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detailed examination was undertaken on carbon-coated, polished thin sections using a Philips (FEI) XL30 environmental SEM featuring an OXFORD INCA-Xmax80 energy dispersive spectrometer (EDS) at the University of Melbourne. Analytical conditions during acquisition of back-scattered electron (BSE) images included an accelerating voltage of 15 kV and beam size of <5 µm. Modal abundances of olivine macrocrysts, and groundmass constituents were quantified by point counting on hand specimen photographs and BSE images, respectively, using the JMicroVision 1.2.7 software. Olivine macrocryst modal abundances were point counted from a single scan of each hand specimen that encompassed one elongate side of the drill-core. For the groundmass mineral modal abundances, three areas were analysed in each HK sample

Journal Pre-proof (0.04-0.7 mm2 ), while for the magmaclasts of PK sample GTH-75_15 we selected 4 areas (0.04-0.08 mm2 ). Selected areas were devoid of macrocrysts and xenoliths. A total of 400 points were included in the modal analysis of each area. The modal abundances have been corrected for mineral replacement where the identity of the replaced mineral is not ambiguous, for example, serpentinised monticellite was recorded as monticellite rather than serpentine. Results (Section 4.1; Supplementary Table S1) feature low uncertainty values and are therefore considered to be representative of volumetric fractions. Modal mineral

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abundances for PK sample A154-U190 were not determined due to extensive magmaclast

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serpentinization.

Electron microprobe analysis of olivine and spinel

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Olivine and spinel were selected for major and minor element analysis using a polarised

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microscope. This method eliminates bias towards strongly zoned grains that would otherwise be unavoidable when employing BSE images for grain selection, ensuring that olivine cores

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with similar Mg# compositions to the olivine rims are also included. Selected olivine grains

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were then examined using a SEM to identify compositional zones, and olivine cores, rims and

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rinds (following the terminology of Giuliani, 2018) were selected for analysis. In olivine grains without a high contrast in BSE response between the core and rim, two points were selected for analysis, one in the centre, and one at the margin of each grain. Analysis of spinel grains include one spot per grain with electron microprobe transects completed for some larger grains (> 25 m) to clarify the details of compositional zoning. In PK samples, the olivine and spinel grains analysed comprised a mixture of grains hosted in magmaclasts, and individual grains hosted by the interclast matrix. Olivine and spinel major and minor element compositions were quantified at the University of Melbourne using a Cameca SX-50 Electron Microprobe (EMP), with a beam acceleration voltage of 15kV, beam current of 35nA, beam diameter of 2 µm and counting

Journal Pre-proof times of 20-40 s on two background positions either side of the peak position. During spinel traverses counting times were halved. Analysed standards included natural (e.g., San Carlos olivine, wollastonite and periclase) and synthetic materials (e.g., titanium oxide and aluminium oxide). Detection limits for major-element analysis of olivine and spinel are available in Supplementary Tables S2 and S3 respectively. EMP results were screened to eliminate analyses with major-oxide totals <98 wt. %, or >102 wt. %, after Fe3+ was calculated by stoichiometry and included in the spinel totals. For olivine grains with no

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zoning in BSE images, points at the grain margin were allocated as cores, rims or rinds, using the following procedure: rim if Mg# (100×Mg/(Mg+Fe2+)) is within 1𝜎 (standard deviation)

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of the average rim Mg# measured for that sample, and the NiO concentration is <0.3 wt. %;

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rind if NiO concentrations are <0.3 wt. %, but Mg# value is > 1𝜎 of the average rim

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composition; and core in any other case. The core analyses of grains for which a core composition had been already obtained were subsequently removed from the dataset to

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eliminate repeat measurements. Only 17 of 188 olivine analyses were reassigned adopting

Bulk-rock analysis

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3.3

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this procedure.

Samples were prepared for whole-rock major and trace element analysis following two stages of crushing. After initial crushing in a jaw crusher, chips of xenolith fragments and chips containing large xenocrysts were removed. The remaining chips were pulverised in an agate ring mill to a fine powder. Glass disks were prepared for major-element whole-rock analysis by mixing 0.6 g of sample powder with 4.1 g of flux comprising 57 % tetraborate and 43% metaborate and an ammonium iodide tablet. This mixture was then heated and quenched using a F-M4 fusion bead casting machine to create each disk. The major-oxide compositions of each glass disk were then analysed using a Spectro XEPOS energy dispersive x-ray fluorescence (XRF) spectrometer at The University of Wollongong with a

Journal Pre-proof detection limit of <1 wt. % for all oxides, which was calibrated using natural standards including USGS andesite, basalt and diorite (Jochum et al., 2016). Additional details of the standards and preferred values used for this work are detailed in Supplementary Table S4. Whole-rock CO 2 compositions and loss of ignition (LOI) were measured by ALS Geochemistry using HClO 4 digestion and a CO 2 coulometer, and heating the samples to 500 °C and 1000 °C respectively. These results were compared to XRF results for each of the samples to verify the accuracy of the analysis and returned combined totals between 99.1 and

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100.5 wt. % (Supplementary Table S5).

Whole-rock trace element compositions of the samples were analysed at the University

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of Melbourne using an Agilent 7700x quadrupole inductively coupled plasma mass

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spectrometer. Powdered samples were dissolved in two stages in a clean laboratory. Initial

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dissolution involved mixing 0.2 g of sample powder with a mix of hydrofluoric acid, nitric acid, and pure water, which was left on a hot plate overnight. In stage two, the undissolved

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material was transferred to dissolution bombs where more hydrofluoric acid was added to the

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sample, and the samples were stored in an oven overnight at a temperature of 180 °C for

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dissolution. In total, 8.8 g of acid were added to the sample powder (dissolution by a factor of 360). Standards (USGS and CRPG basalts, and CRPG diabase; Supplementary Table S4) and blanks used for calibrations were subjected to the same process. 4

Results

The PK and HK samples are predominantly fresh, except for PK sample A154-U190. This sample exhibits fresh olivine and spinel, but, serpentinised and carbonatized late-stage groundmass mineral phases. Consequently, petrographic analysis of the groundmass of PK sample A154-U190 was not possible, however, the preserved macroscale features of this sample (e.g. magmaclast shape and size) are recorded in section 4.1. Olivine and spinel

Journal Pre-proof chemistry results, and whole-rock compositional data for all the samples (including PK sample A154-U190) are presented in sections 4.2 and 4.3 respectively. 4.1

Petrography

In the following petrographic descriptions, ‘PK groundmass’ refers to the magmatic phases preserved within the magmaclasts in PK sample GTH-75_15, whereas ‘PK matrix’ refers to the material hosting the magmaclasts in all the PK samples. The groundmass

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mineralogy for both PK and HK samples are summarised in Table 2, with additional data

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available in Supplementary Table S1. Modal mineral abundances in all the samples are

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recorded as percentages of the volume occupied by the groundmass. PK samples feature abundant non-vesicular, rounded magmaclasts (<0.1 mm to ~1.5

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cm), macrocrysts of olivine, lesser phlogopite, garnet and clinopyroxene, and country rock

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fragments of granite and lesser mudstone hosted in a carbonate and serpentine-rich matrix (Table 2). Matrix serpentine and carbonate can be anhedral (PK sample A154-U190), or form

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ribbon-like segregations with minor brucite, hosting <25 µm inclusions of euhedral spinel

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(PK sample GTH-75_15; Table 2; Figure 3a).

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Rock fragments and macrocrysts hosted by the PK matrix can also occur as cores in magmaclasts surrounded by single complete or incomplete crystalline rims in both PK samples (Figures 2a, 2b and 3a; Supplementary Figure 1b; Table 2). Cored and uncored magmaclasts commonly feature euhedral microphenocrysts of olivine (>100 µm; Table 2; Figures 2a, 2b and 3a), which are partially to completely serpentinised in PK sample A154U190 (Supplementary Figure 1b), but, exceptionally fresh in PK sample GTH-75_15. Fresh olivine is a common feature in Fort à la Corne pyroclastic kimberlites (FPK), as recognised and named after their type locality in Fort à la Corne, Canada (Scott Smith et al., 2013; Skinner and Marsh, 2004). Similarly to FPK magmaclasts, the well-preserved magmaclasts in PK sample GTH-75_15 are also devoid of diopside. However, calcite, a major constituent of

Journal Pre-proof FPK magmaclasts is rare in the PK groundmass at Diavik (<4 vol. % of the groundmass) much like the Kimberley-type pyroclastic kimberlites of Kimberley, South Africa (Scott Smith et al., 2013; Skinner and Marsh, 2004). In addition to calcite, zoned, bladed to fibrous phlogopite (35-51 vol. %; ~5 µm), monticellite partially to completely pseudomorphed by serpentine (12-34 vol. %; <35 µm), interstitial serpentine (2-22 vol. %), spinel without atolls (7-14 vol. %; ~10 µm), equant apatite (3-11 vol. %; <10 µm), perovskite (3-9 vol. %; ~5 µm with rare crystals up to 30 µm) and interstitial brucite (<1 vol.%) form the PK groundmass

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(Supplementary Table S1; Figure 3b). These phases are predominantly euhedral to subhedral except for serpentine and brucite which feature anhedral morphologies (Figure 3b).

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In contrast to the PK samples, the HK samples are devoid of magmaclasts, and feature

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rare granite, eclogite, dunite and lherzolite fragments (Figure 2c). These rock fragments as

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well as macrocrysts of fresh olivine, phlogopite, garnet and clinopyroxene are hosted in a homogeneous groundmass (Table 2; Figures 2c-f, 3c-h; Supplementary Figure 1a), except in

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external HK sample A154-U381 which features spinel rich and poor bands (Figure 2e;

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Supplementary Figures 1e and f; Supplementary Table S1). The groundmass of all the HK

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samples contains spinel (<19 vol. %; up to 50 µm), which can feature atolls with serpentine and brucite-filled lagoons, very minor perovskite (<1 vol %; ~20 µm), and tabular, concentrically zoned phlogopite (<2 vol. %; up to 50 µm; Figures 3f and g; Supplementary Figure 1d; Supplementary Table S1). Monticellite is absent from the external HK samples (Figure 3h), but, comprises <12 vol. % of the groundmass in internal HK sample A154-U384 and 43-48 vol. % of the groundmass in internal HK sample 04GTH-78_03 (Supplementary Table S1; Figure 3f). In both internal HK samples, monticellite is commonly pseudomorphed by serpentine, or less frequently dolomite (Figures 3b and f). In contrast to monticellite, apatite (10-15 µm) is more abundant and equant in the groundmass of the external HK samples than the internal HK

Journal Pre-proof samples (<10 vol. % compared to 2-5 vol. %, respectively) where it can feature elongate and rosette textures, and grain sizes up to 50 µm (Figure 3f; Supplementary Table S1). Similarly, carbonate (calcite and dolomite) comprises up to 65 vol. % of the groundmass in the external HK samples, and only up to 50 vol. % of the internal HK groundmass (Supplementary Table S1). In the external HK samples, carbonates occur as laths of discrete subhedral calcite enveloped by interstitial dolomite (14-32 vol. %), and occasionally host inclusions of spinel and apatite (Figures 3g and h). In the internal HK samples both dolomite (4-32 vol. % in

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sample A154-U384 and <2 vol. % in sample 04GTH-78_03) and calcite are interstitial phases (Figures 3d and f; Supplementary Table S1). In addition to carbonate, serpentine (up to 53

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vol. %) and brucite (<16 vol. %), which likely crystallised during serpentinisation, are

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interstitial groundmass phases in the HK samples (Figures 3d, f and h; Supplementary Table

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S1). Both carbonate and serpentine commonly form segregations in the Diavik HK samples (Figure 3c; Supplementary Figure 1c and d). These segregations can be rounded or anhedral,

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host calcite and dolomite (Supplementary Figure 1c) and, in rare cases, inclusions of tabular

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zoned phlogopite (Supplementary Figure 1d).

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The most significant petrographic differences between the Diavik samples include the larger groundmass grain sizes in HK samples than PK magmaclasts (<50 µm compared to <25 µm), and the groundmass modal mineral abundances and morphologies of carbonate and phlogopite. Carbonate, particularly dolomite, is more abundant in the HK samples than the fresher PK sample GTH-75_15, whereas phlogopite is much more abundant in sample GTH75_15 than the HK samples. Segregations of carbonate and/or serpentine are common in the Diavik HK samples (Figure 3c; Supplementary Figure 1c and d) but are not observed in PK sample GTH-75_15.

Journal Pre-proof 4.2

Mineral chemistry

Olivine and spinel compositions for all the samples are described in groups (external HK, internal HK, and PK) and shown in Figures 5, 6, 8 and 9 with extended datasets available in Supplementary Tables S2 and S3. In the PK samples, olivine and spinel compositions have been analysed in both grains hosted by magmaclasts, and isolated grains in the interclast matrix. Olivine

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4.2.1

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Olivine in the Diavik samples are occasionally complexly zoned, featuring up to four,

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compositionally distinct, concentric zones; however, other grains are unzoned. From the centre to the crystal margin, olivine zones are referred to as core, internal zone, rim and rind

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(Figure 4). In this study a total of 120 olivine cores, 49 olivine rims and 19 olivine rinds were

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analysed. For external HK, internal HK and PK samples, 46, 43 and 31 olivine cores, and 15, 19 and 15 rims were analysed respectively. Olivine rinds either did not crystallise or were not

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preserved in the PK samples and were only analysed in the HK samples. Seven rinds from

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external HK grains, and 12 rinds from internal HK grains were measured.

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Olivine cores are typically anhedral with irregular, sharp to smooth boundaries. Olivine rim and rind crystallisation results in euhedral habits for phenocrysts (Figure 4a), but subhedral and rounded habits for macrocrysts (Figure 4c). Olivine cores in Diavik HK and PK samples have compositions that range between 87.2 to 93.2 Mg#, 0.14 to 0.47 wt. % NiO, 0.06 to 0.20 wt. % MnO, and ≤0.19 wt. % CaO (Figures 5 and 6), which overlaps with granular peridotite (dark blue dashed field in Figure 6) and megacryst (yellow dashed field in Figure 6) olivine compositions in kimberlites worldwide. Within this range, olivine cores from external HK samples feature the lowest average Mg# (90.3 ± 1.2, 1σ; Figures 5a, b and g, and Figure 6), compared to average Mg# values of 90.9 ± 1.0 and 91.4 ± 0.9 for internal HK and PK samples, respectively (Figures 5c, d, g and Figure 6). Maximum Mg# values for

Journal Pre-proof olivine cores are similar for all three rock types (93.2 for external HK, 92.9 for internal HK, and 92.9 for PK samples; Figures 5 and 6). Fe-rich olivine cores (Mg# <90), which are particularly abundant in the external HK samples, show increasing MnO (0.07 to 0.20 wt. %), and decreasing NiO concentrations (0.43 to 0.14 wt. %) with decreasing Mg# at relatively constant and low CaO contents (0.02 to 0.08 wt. %; Figure 6). Internal HK and PK olivine core compositions are comparable to those of the HK and VK olivine core compositions analysed by Brett et al. (2009; Supplementary Figure 2), whereas the core compositions of

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external HK olivine in this work extend to lower Mg# values.

In all samples, olivine rims host inclusions of titanium magnesium aluminous

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chromites (TiMAC), whereas magnesian- ulvöspinel magnetite (MUM) spinel, ilmenite and

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rutile only occur in olivine rims in the HK samples. Olivine rim compositions in all samples

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are defined by near constant Mg# values, and a range of NiO (0.07 to 0.36 wt. %), MnO (0.09 to 0.22 wt. %) and CaO concentrations (0.02 to 0.27 wt. %; Figures 5 and 6). Average rim

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Mg# for internal HK (90.5 ± 0.1; n= 19) and PK (90.7 ± 0.2; n=15) samples are within

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uncertainty of one another (Figure 5g and Figure 6) but are lower in external HK samples

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(90.2 ± 0.2 (1); n=15). These values are marginally lower than previously observed by Brett et al. (2009) for internal HK and VK olivine rims (averages of 91.1 ± 0.2 and 91.3 ± 0.3, respectively; Supplementary Figure 2), possibly due to interlaboratory bias. Multiple repeat analyses of San Carlos olivine during the same analytical session as the Diavik olivine provided consistent Mg# values within uncertainty of that accepted for the San Carlos olivine grains in our lab (90.2), thus providing confidence in the current results. Olivine rinds feature sharp, occasionally irregular contacts with olivine rims, a variable thickness, and are often missing along one or multiple edges of olivine grains (Figures 4c and d). Olivine rinds are only observed in the HK samples and display decreasing NiO (0.01 to 0.41 wt. %) with increasing Mg# (90.7 to 96.7) approaching grain boundaries

Journal Pre-proof (Figures 5a, b, c and d and Figure 6). Olivine rinds in the internal HK samples reach a higher Mg# of 96.7 compared to 91.8 for external HK olivine (Figure 6). MnO and CaO contents in the rinds overlap with, and extend above, those in the rims, with MnO and CaO concentrations up to 0.26 and 1.24 wt. %, respectively (Figures 6b and c). Similar rind compositions have been observed in other Lac de Gras (Bussweiler et al., 2015; Lim et al., 2018) and worldwide kimberlites (Giuliani, 2018). Spinel

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4.2.2

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Spinel in the Diavik samples features both gradational zoning and sharp contacts (Figure

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7). Compositions were measured for 35 individual grains in the external HK samples, 20 grains in the internal HK samples and 27 grains in the PK samples. Transects were completed

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for 8 spinel grains (3 in external HK samples, 2 in internal HK samples and 3 in PK samples).

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The results are summarised in Figures 8 and 9 with extended datasets in Supplementary Table S3.

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Spinel analyses have been compositionally classified into xenocrystic, early magmatic

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chromite, pleonaste and MUM-spinel. Xenocrystic spinel is defined by TiO 2 <1 wt. %, based

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on comparisons with xenocrystic spinel compositions in kimberlites worldwide (Roeder and Schulze, 2008) and spinel in Diavik mantle xenoliths (Figure 9; Aulbach et al., 2007b). Magmatic chromite grains have Cr2 O 3 concentrations of >45 wt. %, Al2 O3 concentrations of <20 wt. %, and TiO 2 concentrations between 1 and 8 wt. %. Pleonaste spinel grains have Al2 O 3 and Cr2 O3 compositions of ≥20 wt. % and <10 wt. %, respectively, and MUM-spinel grains have Cr2 O3 and TiO 2 compositions of <45 wt. % and >15 wt. %, as defined by Mitchell (1986). The cores of three PK spinel grains and one internal HK spinel grain have Mg# and Fe3+# [=100×Fe3+/(Fe3++Cr+Al)] compositions that fall within the xenocrystic field. Two distinct compositions of early magmatic chromite can be discerned in the Diavik samples

Journal Pre-proof (Figure 9). Chromite compositions in PK and internal HK samples are similar, featuring an average Cr# [=100×Cr/(Cr+Al)] of 78.3 ± 5.7 (1σ; n = 23) and 79.1 ± 3.4 (n = 12), and Mg# of 60.0 ± 2.2 and 60.0 ± 1.3, respectively. In contrast, chromite in external HK samples exhibits a higher Cr# of 86.9 ± 2.7 (n = 16), and lower Mg# (52.8 ± 1.9) at similar TiO 2 contents and Fe3+/Fe2+ ratios (Figure 9). Spinel compositions in internal HK and PK Diavik samples evolve from chromite to MUM-spinel along trend 1 (T1) of Mitchell (1986), which is typical of archetypal kimberlites

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worldwide (Figures 8c-f and Figure 9; Roeder and Schulze, 2008). In comparison, external HK spinel evolves from chromite to pleonaste compositions along trend 3 (T3), before

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acquiring MUM-spinel compositions (trend 7 of Roeder and Schulze (2008); Figures 8a-b, 9a

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and 9f). A similar evolution is shown by spinel from the Tli Kwi Cho PK unit in the Lac de

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Gras field (Roeder and Schulze, 2008). Pleonaste spinel grains in external HK samples (n = 3) have an average Cr# of 3.5 ± 1.4 and Mg# of 76.7 ± 7.8 (Figure 9).

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As with chromite, MUM-spinel in the external HK samples is compositionally distinct

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from MUM-spinel in the internal HK and PK samples featuring an average Fe3+# of 60.2 ±

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2.3 (n = 22), compared to 47.2 ± 5.8 (n = 22) in internal HK spinel and 49.7 ± 9.3 in PK spinel (n = 25). MUM-spinel in the external HK samples has a lower average Cr# (7.3 ± 3.3) than spinel in the internal HK (25.7 ± 11.0) and PK samples (17.0 ± 14.0) at similar Mg# values (external HK = 57.0 ± 2.7; internal HK = 60.7 ± 3.3; PK = 64.1 ± 4.2) and TiO 2 contents (Figure 9). 4.3

Whole-rock compositions

Major and trace-element whole-rock compositions of the Diavik kimberlites are summarised in Figure 10, with the full dataset available in Supplementary Table S5. The PK and internal HK samples feature some compositional similarities with SiO 2 , MgO, and Ni concentrations of 33-36 wt. %, 32-35 wt. %, and 1489-1553 ppm, and 30-32 wt. %, 35-37 wt. %, and 1470-

Journal Pre-proof 1485 ppm respectively (Figure 10b), which are significantly higher than those in the external HK samples (16-24 wt. %, 22-26 wt. % and 806-814 ppm). In contrast, the external HK samples feature the highest average abundances of TiO 2 (1-2 wt. %), CaO (17-20 wt. %) followed by the internal HK samples, with the lowest abundances found in the PK samples (Figure 10c). CO2 concentrations are also highest in the external HK samples (15-26 wt.%), but, are similar for the internal HK and PK samples (4-8 wt. % and 6-9 wt. %). All the samples feature MnO, P2 O5 and Na2 O concentrations of <1 wt. %. Concentrations of Al2 O3

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(1.1-2.2 wt. % Figure 10a), and H2 O (2.6-5.5 wt. %) are variable without systematic differences among the three groups.

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Incompatible trace element concentrations are lower in the PK samples than the HK

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samples, except for K and Rb (Figures 10e and f). Both the external HK and internal HK

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samples feature similar concentrations of K 2 O (0.47-0.50 wt. % and 0.30-0.34 wt.%) and Rb (21-44 ppm and 32-39 ppm), which are much lower than the concentrations of K 2 O and Rb in

Discussion

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5

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the PK samples (0.92-2.16 wt. %, and 64-92 ppm respectively; Figures 10a and e).

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The samples analysed in this study represent pyroclastic and hypabyssal kimberlite units that are characterised by different emplacement styles. Larger groundmass grain sizes in the HK samples compared to the PK magmaclasts (<50 µm versus <25 µm) result from rapid quenching of the latter. This difference in grain size, combined with the different mineralogical assemblages observed for the PK magmaclasts and HK groundmass at Diavik, indicate that magmaclasts in the PK samples are melt-bearing pyroclasts (clasts that crystallised from the transporting magma), rather than autoliths (clasts entrained from previously emplaced kimberlite pulses), and are therefore representative of the sampled PK melt. Minimal alteration of the majority of these samples enables a comparison of mineral compositions and groundmass mineral assemblages for both explosive and intrusive

Journal Pre-proof kimberlite magmas. Here we use these data to examine the factors that may have controlled the emplacement of these kimberlite units, including the composition and evolution of primitive kimberlite melts. 5.1

Derivation of internal HK and PK units from analogous primitive melts

Olivine and chromite are widely considered to be liquidus phases in kimberlite melts (Abersteiner et al., 2017a; Bussweiler et al., 2015; Dalton et al., 2020; Fedortchouk and

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Canil, 2004; Giuliani, 2018; Lim et al., 2018; Mitchell, 1986, 2008; Soltys et al., 2018)

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Therefore, the Mg# of olivine rims, in combination with chromite compositions, can be used

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as proxies for primitive melt compositions. Olivine rim compositions from the same pipe in the Diavik and previously studied kimberlites (Bussweiler et al., 2015; Giuliani, 2018; Lim et

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al., 2018; Soltys et al., 2018) feature relatively constant Mg# values. Conversely, chromite

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compositions often differ between kimberlite units within the same pipe but are indistinguishable within a single kimberlite unit (van Straaten et al., 2008).

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Average olivine rim Mg# values, and chromite compositions for Diavik internal HK

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and PK samples are indistinguishable within uncertainties (Figures 5g, 6 and 9), suggesting

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very similar compositions for internal HK and PK primitive melts. In contrast, olivine rims and chromites in the external HK samples feature lower average Mg# values compared to the other Diavik samples. Chromite Cr# is higher in the external HK samples than the internal HK and PK samples (Figure 9b). In addition, only external HK samples contain spinel with zoning from chromite to Al-rich pleonaste (Figure 8 and 9). Different olivine and spinel compositions in the external HK samples compared to the other Diavik units, combined with the different groundmass mineralogy and carbonate petrography of the external HK samples compared to the internal HK samples (Fig. 3), indicate that at least two different primitive melt compositions contributed to the kimberlite units in the A154N and A154S pipes. This

Journal Pre-proof finding differs from that of Moss et al. (2009) who suggested that the external HK, PK and internal HK units of Diavik A154N had the same primitive melt composition. The external HK units feature higher abundances of olivine cores with relatively low Mg# compositions, similar to those of African megacrysts, compared to the internal HK and PK units (Figure 6). This suggests that the melts forming the external HK units likely entrained and assimilated more metasomatised mantle prior to the crystallisation of kimberlitic olivine and chromite than the internal HK and PK melts (see Giuliani et al., in

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press). This might be due to sampling deeper, more enriched mantle material beneath the Lac de Gras field (e.g., Griffin et al., 1999), compared to that sampled by the PK and internal HK

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melts. Increased melt Fe and Ti concentrations caused by the entrainment and assimilation of

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enriched mantle material would facilitate rutile, ilmenite and MUM spinel crystallisation

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leading to higher abundances of these minerals as inclusions in olivine rims and higher whole-rock TiO 2 concentrations for the external HK compared to the internal HK and PK

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units. Variable carbonate abundances and higher whole-rock CO 2 concentrations in the

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external HK units than the internal HK units may reflect differences in the behaviour of

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volatiles in these units at shallow depths resulting from different primitive melt compositions. Similar primitive melt compositions for the internal HK and PK units demonstrate melt composition was not the principal control on the explosive emplacement of kimberlites at Diavik. Therefore, the emplacement mechanism of kimberlites may be determined at shallower depths; i.e. later in the evolution history of the kimberlite melt, as discussed in the following section. This finding is consistent with the observations of Lim et al. (2018), who observed similar olivine rim Mg# for the extrusive coherent unit of the Grizzly kimberlite (91.5 ± 0.2), and the hypabyssal unit of the Koala kimberlite in the nearby Ekati Diamond Mine (91.4 ± 0.2) and olivine studies for other Ekati and Diavik kimberlites (Brett et al., 2009; Bussweiler et al., 2015; Fedortchouk and Canil, 2004). Consequently, the range of

Journal Pre-proof emplacement styles recorded for many of the LDG kimberlites are unlikely to have directly resulted from variable primitive melt compositions. 5.2

Contrasting late-stage evolution of melts parental to internal HK and PK

Zoning of spinel in kimberlite reflects well defined evolutionary trends, which makes this mineral ideal for examining the evolution of kimberlite melt compositions from early chromite crystallisation to the late formation of titanomagnetite (Mitchell, 1986; Roeder and

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Schulze, 2008). These trends are controlled by the composition and evolution of the primitive

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kimberlite melt, as well as the co-crystallisation of other mineral phases (Roeder and Schulze,

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2008). For example, phlogopite crystallisation may prevent the formation of pleonaste spinel, due to preferential partitioning of Al into mica (Pasteris, 1980). There are no major

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differences between the late-stage MUM spinel compositions in the internal HK and PK

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Diavik samples (Figure 9e). This indicates that the evolution of these melts was indistinguishable during spinel crystallisation.

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Although the Al content of late-stage MUM spinel and whole-rock Al2 O 3

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concentrations for the internal HK and PK samples are similar (1.8-1.9 wt.% compared to

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1.4-1.9 wt. %; Figure 10a), the PK magmaclasts feature much higher abundances of phlogopite than the groundmass of internal HK samples (34-52 vol. % compared to <2 vol. %; Supplementary Table S1). Phlogopite is considered to crystallise after spinel in kimberlites (e.g. South Africa, Canada and Finland; Abersteiner et al., 2017b; Mitchell, 2008; Soltys et al., 2018), suggesting different late-stage evolution pathways for the PK and internal HK melt compositions. In the following sections we discuss why kimberlites with similar primitive compositions and evolutionary trends (to the point of MUM-spinel crystallisation) experienced such dramatically different late-stage evolutionary paths, resulting in K2 O-rich PK units (up to 2.2 wt.%; Figure 10b) containing magmaclasts with ~40 vol. % phlogopite

Journal Pre-proof and <4 vol. % carbonate (Supplementary Table S1), and K 2 O-poor internal HK units (K 2 O = 0.30-34 wt.%) with <2 vol. % phlogopite and 25-51 vol. % carbonate (including up to 32 vol. % dolomite). Similar petrographic differences have been documented previously for phlogopite-bearing, H2 O-rich extrusive coherent kimberlites and dolomite-bearing, H2 O-poor dykes in the Lac de Gras (LDG) field (Armstrong et al., 2004; Nowicki et al., 2008), as well as phlogopite-bearing ‘tuffistic’ (i.e. pyroclastic) kimberlites and dolomite-bearing hypabyssal kimberlites in Kimberley, South Africa (Mitchell et al., 2009). There are three

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possible scenarios that might explain the compositional differences between melts parental to PK and internal HK units at Diavik: 1) addition of K 2 O and H2 O after spinel crystallisation in

Assimilation of crustal material

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5.3

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PK melts; 2) loss of CO 2 from PK melts and/or; 3) loss of K 2 O and H2 O from HK melts.

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Assimilation of K2 O-rich hydrous crustal and/or mantle material (i.e. phlogopite) is a process sometimes invoked to account for elevated K 2 O contents in kimberlite melts (Kjarsgaard et

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al., 2009; Le Roex et al., 2003). Addition of SiO 2 during mantle/crustal assimilation has also

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been argued by some authors to decrease CO 2 solubility, which may trigger explosive

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kimberlite emplacement (Bussweiler et al., 2016; Gaudet et al., 2017; Russell et al., 2012; Stone and Luth, 2016). The internal HK samples analysed in this study are typically K 2 Opoor (0.30-0.34 wt.%; Figure 10a). Therefore, given the similar primitive composition and early (mantle) evolution of the PK and internal HK melts constrained by olivine and spinel geochemistry, it seems unlikely that the elevated K2 O concentrations required for the crystallisation of abundant phlogopite in the PK magmaclasts was due to the assimilation of mantle material. This is consistent with the paucity of phlogopite-rich material in the mantle xenoliths entrained in the Diavik kimberlites (Aulbach et al., 2018; Moss et al., 2018a) compared to other localities such as the Kimberley area in South Africa (e.g. Fitzpayne et al., 2018; Giuliani et al., 2016; Grégoire et al., 2002).

Journal Pre-proof In contrast to mantle xenoliths, crustal xenoliths of granodiorite and mudstone analysed in the local A418 Diavik kimberlite, and three Ekati kimberlites (Fox, Lynx and Panda) are K 2 O-rich (granodiorite = 2.4-2.8 wt. %; mudstone = 2.9-9.7 wt. %, respectively; Graham et al., 1999; Nowicki et al., 2008). To test whether crustal assimilation can shift kimberlite melts from internal HK to PK compositions, we carried out mixing calculations using whole-rock compositions of internal HK and PK samples and the above crustal rocks (Supplementary Table S6). In this calculation the internal HK units are considered

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representative of pristine kimberlite melts unaffected by crustal contamination. Variable proportions of crustal material were mixed with the internal HK starting composition until a

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target composition of 0.92 wt. % K2 O was reached (i.e. the bulk composition of PK sample

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GTH-75_15, the freshest PK). The calculation results show that assimilation of 37.9 %

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granodiorite or 13.2 % mudstone is required to increase the K 2 O concentration from 0.32 wt. % (the average K 2 O composition of internal HK samples) to 0.92 wt. % (Supplementary

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Table S6). Assimilation of mixtures of granodiorite and mudstone yields intermediate results.

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Assimilation of crustal material also elevates the SiO 2 , Al2 O 3 and Na2 O concentrations well

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above levels observed in the PK sample (Supplementary Table S6). Furthermore, the A154N kimberlite pipe only features rare crustal xenoliths (Moss et al., 2018b). Therefore, it is concluded that assimilation of crustal material is unlikely to be solely, if at all, responsible for the elevated K 2 O concentration and H2 O/CO 2 ratios of the PK units compared to the internal HK samples, and could not have been the main trigger for the explosive emplacement of some kimberlite magmas at Diavik. 5.4

Volatile loss

Degassing is a prominent process in all magma types. In kimberlites, the timing and depth of volatile degassing is poorly constrained, with some authors suggesting volatile exsolution starting as deep as at the kimberlite source (e.g., Wilson and Head, 2007), others

Journal Pre-proof preferring degassing during ascent through the mantle (e.g., Russell et al., 2012), and some authors favouring shallow magma degassing in the upper crust (e.g., Brooker et al., 2011; Sparks et al., 2006). Here we discuss the timing of degassing at Diavik and the effect this may have had on the composition and petrology of the Diavik kimberlites. Olivine rims have an indistinguishable Mg# composition in the internal HK and PK samples (Figures 5 and 6). Provided that the olivine- melt Mg-Fe distribution coefficient (KD) is also affected by melt CO 2 and H2 O content (e.g., Dalton and Wood, 1993; Toplis, 2005),

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the composition of olivine rims in the internal HK and PK samples require similar volatile as well as major element concentrations in their parental melts. In other words, if any volatile

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component was lost during melt ascent prior to olivine rim crystallisation, this process would

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the observed petrographic differences.

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similarly affect the magmas that generated the internal HK and PK units and cannot explain

The significantly greater abundance of groundmass carbonates in internal HK samples

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(25-50 vol.% comprising up to 32 vol. % dolomite) compared to PK magmaclasts (<4 vol.%

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with no dolomite) could indicate that CO 2 degassing was more prominent during the

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emplacement of PK melts than internal HK melts, which is consistent with previous studies (e.g., Mitchell et al., 2009; Sarkar et al., 2011). CO2 degassing could also trigger monticellite crystallisation (Abersteiner et al., 2017b); monticellite is more abundant in the PK magmaclasts (12-34 vol. %) than the groundmass of internal HK sample A154-U384 (<11 vol. %), but not internal HK sample 04GTH-78_03 (43-47 vol. %; see below). Given that the PK and internal HK spinels show an identical compositional evolution, CO 2 degassing of the PK melt probably started after spinel, but before monticellite and phlogopite crystallisation (i.e. in the upper crust at, or just before, emplacement). CO 2 degassing would increase the H2 O/CO 2 ratio of the residual PK melt (Figure 11a). This is in agreement with the model of Moussallam et al. (2016), which highlights that loss of CO 2 is unlikely to affect H2 O

Journal Pre-proof solubility at upper crustal pressures (100-350 MPa). Higher H2 O/CO 2 ratios in the melt phase are consistent with the crystallisation of hydrous minerals (i.e. phlogopite) in the PK. The formation of phlogopite, a major sink of Mg, combined with earlier CO 2 loss, would have supressed dolomite crystallisation (Figure 11a). The carbonate-poor mineralogy of the magmaclasts in PK sample GTH-75_15 and the absence of diopside differs from that observed in both Kimberley-type (diopside-rich and carbonate-poor) and Fort a la Corne-type (carbonate-rich and diopside-poor) pyroclastic

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kimberlites (e.g. Scott Smith et al., 2013; Skinner and Marsh, 2004). The unusual nature of the PK magmaclasts is likely due to preferential partitioning of Ca into monticellite (<34 vol.

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%), apatite (<11 vol. %) and perovskite (<10 vol. %). Formation of abundant monticellite

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rather than diopside may be due to lower SiO 2 concentrations than in other kimberlites due to

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limited assimilation of crustal material, as indicated by the mixing calculation above. Similarly to the PK magmaclasts, internal HK sample 04GTH-78_03 contains

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abundant monticellite (43-47 vol. %), and low abundances of dolomite (<2 vol. %),

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indicating substantial CO2 degassing of unit CK-S. In contrast to the PK magmaclasts,

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internal HK sample 04GTH-78_03 also features high-Mg olivine rinds, low abundances of phlogopite (<2 vol. %) and, therefore, a low whole rock K 2 O composition (0.30 wt. %). Unfragmented, euhedral olivine, and a homogeneous crystalline groundmass in internal HK sample 04GTH-78_03, demonstrate that unit CK-S was not emplaced explosively. These textures indicate that CO2 loss, via degassing or fluid separation, during the emplacement of this internal HK magma occurred during groundmass crystallisation and was probably passive (Figure 11b). Crystallisation of olivine rinds and monticellite significantly reduced the Mg available in the evolving melt, which may have prevented abundant phlogopite formation in internal HK sample 04GTH-78_03, thus leaving residual fluids enriched in H2 O and K (Figure 11b). The low concentration of K 2 O in this sample (0.30 wt. %) suggests that

Journal Pre-proof at least some K was lost during the migration of residual deuteric fluids (Figure 11b). This scenario, whereby both CO 2 and H2 O were lost to residual fluids during intrusive emplacement of the magmas parental to unit CK-S, from which internal HK sample 04GTH78_03 derives, is supported by extensive serpentinization and carbonation of the pyroclastic rocks of unit PK-S surrounding internal HK unit CK-S (Figures 1d and 11b). Although more difficult to constrain, it is possible that a similar process also affected the other internal HK units at Diavik, thus contributing to the lower K and Rb contents of the internal HK samples

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compared to the PK samples (Figure 10). This suggestion is at odds with the incompatible trace element enrichment of HK samples compared to PK samples at Diavik and in the wider

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Lac de Gras (LDG) field, because expulsion of ash particles formed from residual PK melts

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would have reduced the incompatible trace element concentrations of PK, but not HK, units

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(Nowicki et al., 2008). However, melt inclusion studies from the LDG kimberlites and other kimberlites worldwide show higher alkali contents in primary melt inclusions hosted by

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magmatic phases than in bulk kimberlite rocks (Abersteiner et al., 2017a; Abersteiner et al.,

What triggered explosive kimberlite eruption at Diavik?

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5.5

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2019; Giuliani et al., 2017; Kamenetsky et al., 2013).

In summary, it is likely that the contrasting petrographic and geochemical features of the internal HK and PK samples resulted from different shallow degassing and volatile loss histories. Copious CO 2 degassing in the PK magmas triggered by their explosive emplacement caused an increase of H2 O/CO 2 in the residual melt and stabilised phlogopite at the expense of dolomite (Figure 11a). Potassium was incorporated into crystallising phlogopite during this process. In the internal HK samples the formation of Mg-rich phases, including olivine rinds, monticellite and/or dolomite prevented phlogopite crystallisation, which caused K and Rb to build up in the residual fluid phases. Migration of these K-rich deuteric fluids into country rocks or adjacent kimberlites could explain the significantly lower

Journal Pre-proof K and Rb contents of the internal HK samples compared to the PK samples (Figure 11b). Expulsion of ash particles (comprising solidified residual PK melt) from the PK units at LDG could have depleted the incompatible element concentrations in these units compared to HK units (e.g., Nowicki et al., 2008). Therefore, enrichment of K and Rb in the PK units compared to the HK units requires a specific process (i.e. loss of the HK deuteric fluids). The above discussion highlights that the different petrographic and geochemical features of the internal HK and PK units may be the result of contrasting emplacement styles

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rather than different primitive melt compositions or compositional evolutions. There are no obvious processes that could have changed the H2 O/CO 2 compositions of the PK and internal

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HK melts during ascent, because mantle and crustal assimilation were either negligible or

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affected these magmas in similar ways resulting in similar spinel and whole-rock Al2 O3

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compositions. Therefore, other factors must control the emplacement mechanisms of kimberlites, such as the availability of external water at shallow crustal levels (i.e.

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phreatomagmatism), local stress regime and/or variable magma supply rates.

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Explosive phreatomagmatic eruptions occur when juvenile magma encounters an

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external water source. Phreatomagmatic deposits are typically identified by abundant fine, angular ash particles, and accretionary lapilli, and at a larger scale by surge-like bedding (e.g., Dellino and Kyriakopoulos, 2003; Porritt et al., 2013). Entrained coniferous wood and soft sediments highlight a wet and temperate environment during the emplacement of the Diavik kimberlites (Graham et al., 1999). Large scale grading of unit PK4-N and MRVK2-N at A154N has been interpreted by Moss et al. (2008) to result from hydraulic sorting of pyroclastic deposits in a crater lake, indicating that groundwater was present during the late infill of the A154N kimberlite pipe. Abundant accretionary lapilli, pyroclastic deposits with surge-like beds, and finely bedded pyroclastic and volcaniclastic deposits also occur in the nearby A418 kimberlite pipe and have been attributed to alternating magmatic and

Journal Pre-proof phreatomagmatic processes (Porritt et al., 2013). Therefore, interaction between the A154N PK magma and groundwater could have triggered an explosive eruption. In contrast, the rounded ash particles, unfragmented magmaclasts, and a lack of accretionary lapilli in the PK samples studied here (Figure 3a; Supplementary Figure 1b), are inconsistent with a phreatomagmatic cause for the explosive emplacement of unit PK1-N. At the same time, White and Valentine (2016) have shown that petrographic textures may not be a reliable

Conclusions

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6

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explosive activity at A154N cannot be totally discounted.

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identifier of phreatomagmatic eruptions; consequently, phreatomagmatic control for the

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The A154 North and South kimberlites of Diavik were emplaced in multiple stages, producing kimberlite pipes infilled with a mixture of shallow explosive and intrusive units.

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We compared the mineral chemistry of olivine and spinel in two minimally altered samples

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from pyroclastic kimberlite units in the A154N kimberlite pipe to four samples of hypabyssal kimberlite from two external dykes and two late-stage, internal hypabyssal units (one from

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A154S and one from A154N). Compared to the internal HK samples, the external HK

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samples exhibit higher TiO 2 , CaO and CO 2 whole rock concentrations and lower chromite and olivine rim Mg# compositions. These results indicate that at least two melt types with marginally different primitive compositions contributed to the Diavik kimberlites. A higher proportion of Fe-rich olivine cores in the external HK than internal HK units indicate that the difference in melt compositions likely resulted from variable assimilation of metasomatised mantle material prior to spinel and olivine crystallisation. These small variations in primitive melt composition became exaggerated with melt evolution and may have influenced volatile solubility at shallow depths. The similar compositions of the PK and internal HK units demonstrates that explosive kimberlite emplacement was not controlled by primitive melt compositions. Furthermore,

Journal Pre-proof spinel and olivine compositions and mixing calculations indicate that explosive kimberlite emplacement was also not influenced by the assimilation of mantle or crustal material. The compositional evolution of the HK and PK magmas diverged in the upper crust and produced higher H2 O/CO 2 ratios in the PK melts than HK melts. Evolution to higher H2 O/CO 2 ratios in the PK melts was probably triggered by abundant CO2 degassing. In contrast, the internal HK magmas lost alkalis to residual deuteric fluids. These results indicate that the late evolution of kimberlite magmas at Diavik was controlled by the emplacement mechanisms rather than

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vice versa, and that emplacement styles are not a function of melt composition. It is concluded that the explosive emplacement of kimberlites at Diavik, and perhaps elsewhere,

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was not controlled by melt composition, and that the emplacement style played a fundamental

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role in controlling the volatile content of the kimberlites. Explosive emplacement of the

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Diavik kimberlites was most likely triggered by increased availability of groundwater, changes in the local stress field, and/or variable magma supply rate. Future studies should

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target broadly coeval hypabyssal and pyroclastic kimberlites from other localities to assess

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whether our findings at Diavik are consistent with kimberlites globally, or alternative ly,

mechanisms. 7

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whether different primitive melt compositions can result in different emplacement

Acknowledgements

MT acknowledges funding from the University of Melbourne through the George Sweet Trust and Baragwanath Scholarship, which enabled travel to Canada to collect the samples used in this paper. We thank Kari Pollock, John Carlson, Dominion Diamond Mine Incorporated, Diavik Diamond Mine Incorporated PLC and Rio Tinto PLC for provision of the Diavik samples, and permission to publish these results. We also thank Graham Hutchinson for the support he provided with the electron microprobe. We are grateful to Tom Nowicki, Ashton Soltys, Hayden Dalton and Angus Fitzpayne who provided insightful

Journal Pre-proof discussions during and preceding the writing of this manuscript; and Janine Kavanaugh and Yannick Bussweiler for thoughtful reviews. This research was funded by the Australia Research Council through a DECRA fellowship awarded to AG (grant n. DE-150100510). Declaration of interests The authors declare that they have no known competing financial interests or personal

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Figure 1: (a) Location map of the Diavik Diamond Mine, (b-c) Diavik kimberlites and local faulting, and (d) simplified cross sections of the A154 North (A154N) and A154 South (A154S) kimberlite pipes modified from Moss et al. (2018b). In b, italicised font indicates emplacement ages of 54 Ma to 56 Ma for the Diavik kimberlites (Amelin, 1996; Barton, 1996; Moser and Amelin, 1996). In d, white labels mark the main units of A154S and A154N as identified by Moss et al. (2018b) highlighting the units infilling the A154 North kimberlite pipe with ‘N’ and units infilling the A154 South kimberlite pipe with ‘S’. Multiple units of the same lithology are differentiated with a number that reflects the relative timing of emplacement for each unit (e.g. PK1-N was emplaced before PK3-N). These units are colour coded in dark purple for external hypabyssal kimberlites (HK), black for internal HK and

Journal Pre-proof greyscale for volcaniclastic kimberlites and unconsolidated mud. External HK dyke U381 (samples A154-U381; no. 5) occurs 2-3 m to the southwest of the A154S kimberlite pipe but is not marked due to the small volume of this unit. Numbered stars 1-6 highlight sample localities GTH-75_15, A154-U190, A154-U384, 04GTH-78_03, A154-U381 and Duey’s A154S_01, respectively. Dark blue and cyan colouring in d highlight areas of extensive alteration and melt-rich domains, respectively. Abbreviations are as follows: PK = pyroclastic kimberlite; CK = coherent kimberlite, LVK = layered volcaniclastic kimberlite; MRVK =

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mud-rich volcaniclastic kimberlite; MUD = unconsolidated mud.

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Figure 2: Hand specimen photographs of hypabyssal (HK) and pyroclastic kimberlite (PK)

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from Diavik (a-f). (a) PK sample GTH-75_15 from unit PK3-N (1), (b) PK sample A154-

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U190 from unit PK1-N (2), (c) internal HK sample A154-U384 from unit CK-N (3), (d) internal HK sample 04GTH-78_03 from unit CK-S (4), (e) external HK sample A154-U381

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(adjacent to A154 South; 5), and (f) external HK sample Duey’s-A154S_01 (unit CK1-N; 6).

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xenolith; Ol = olivine.

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The numbers on each image correspond to the stars on Figure 1. M = magmaclast; X =

Figure 3: Plane polarised light microphotographs (a, c, e and g) and SEM back-scattered electron (BSE) images (b, d, f and h) highlighting the petrographic features of the Diavik pyroclastic (PK) and hypabyssal kimberlite (HK) samples. (a) Poorly sorted, clast-supported texture of PK sample GTH-75_15 defined by crustal rock fragments (X), magmaclasts (M) and ash-sized clasts of serpentine and spinel (A), hosted by a serpentine (Srp) and calcite (Cal) matrix; (b) detail of a magmaclast in sample GTH-75_15 with abundant phlogopite (Phl) and monticellite replaced by serpentine (Mnt*); (c) olivine (Ol) phenocrysts, and carbonate (Cb) segregation in internal HK sample A154-U384; (d) detail of groundmass

Journal Pre-proof composition, including calcite and dolomite, in sample A154-U384; (e) carbonate segregations, olivine micro-phenocrysts and abundant spinel (Spl) in the groundmass of internal HK sample 04GTH-78_03; (f) detail of the groundmass of internal HK sample 04GTH-78_03 showing euhedral monticellite grains pseudomorphed by serpentine; (g) micro-phenocrysts of olivine (Ol), carbonate laths and atoll-shaped spinel grains hosted in a serpentine mesostasis in external HK Duey’s-A154S_01; and (h) detail of the carbonate-rich groundmass of external HK sample A154-U381 featuring zoned carbonate grains with a

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calcite core and dolomite (Dol) rim as well as interstitial dolomite. Ap = apatite; Brc =

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brucite.

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Figure 4: (a-b) Examples of olivine phenocryst and (c-d) macrocryst zoning in the Diavik

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kimberlites (samples 04GTH-78_03 and Duey’s-A154S_01, respectively) as SEM back-

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scattered electron (BSE) images (a and c) and schematic diagrams (b and d).

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Figure 5: (a-f) Mg# vs NiO (wt. %) bivariate plots of Diavik olivine from (a-b) external

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hypabyssal kimberlite (HK) samples, (c-d) internal HK samples and (e-f) pyroclastic kimberlite (PK) samples. Vertical lines (continuous for rims, dotted for cores) highlight the average Mg# composition for each group. Grey markers (‘complete data set’) show all the data acquired in this study divided into cores, rims and rinds. (g) Box and whisker plot highlighting the range, upper quartile, median and lower quartile of olivine rim Mg# compositions, and average olivine core Mg# for Diavik samples. Mg# = 100×Mg/(Mg+Fe).

Figure 6: (a-c) Mg# vs NiO, MnO and CaO in olivine from the Diavik kimberlites. Vertical lines (continuous for rims, dotted for cores) highlight the average Mg# composition for each group. Dashed and dotted fields indicate the compositional ranges of olivine in megacrysts

Journal Pre-proof (yellow), sheared peridotites (pale blue) and granular peridotites (dark blue) in kimberlites worldwide (Giuliani, 2018 and references therein). (d) Box and whisker plot featuring the range, lower quartile, median, and upper quartile of olivine rim Mg# compositions for each kimberlite group; the average composition of olivine cores is shown with a star for comparison. PK = pyroclastic kimberlite; HK = hypabyssal kimberlite.

Figure 7: (a and c) SEM back-scattered electron (BSE) images and (b and d) schematic

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diagrams showing examples of spinel zoning with sharp (solid lines) and gradational (dashed lines) contacts in the Diavik hypabyssal kimberlite (HK) samples. Arrows highlight the

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approximate evolution of spinel in (a-b) external HK sample A154-U381from chromite to

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pleonaste compositions and (c-d) internal HK sample A154-U384 from chromite to MUM-

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spinel compositions.

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Figure 8: Al-Fe3+-Cr plots showing spinel compositional data for the Diavik kimberlites.

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Dashed lines highlight the compositional evolutionary trend for each sample. Solid lines

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indicate evolution trends T1 to T8 for spinel in igneous rocks as defined by Roeder and Schulze (2008). PK = pyroclastic kimberlite; HK = hypabyssal kimberlite.

Figure 9: Compositional variations in spinel grains from the Diavik kimberlites: (a) Cr 2 O 3 wt. % vs TiO 2 wt. %, (b) Mg# vs Cr# [=100×Cr/(Cr+Al)], (c) Fe3+/Fe2+ vs TiO 2 wt.%, (d) Mg# vs Fe3+# [=100×Fe3+/(Fe3++Cr+Al)], and (e-f) Fe3+-Cr-Al ternary plot. In (f) inferred evolution trends of spinel from external hypabyssal kimberlite (HK), internal HK and pyroclastic kimberlite (PK) samples of Diavik are compared to global spinel compositional trends T1 to T8 for igneous rocks (Mitchell, 1986; Roeder and Schulze, 2008). The dashed,

Journal Pre-proof shaded fields include global xenocrystic spinel compositions (Roeder and Schulze, 2008), including spinel data for the Diavik peridotites (Aulbach et al., 2007a).

Figure 10: (a) Al2 O3 vs K 2 O, (b) SiO 2 vs Ni, (c) TiO 2 vs CaO and, (d) CeN/YbN vs Th bivariate plots, and (e) a primitive mantle (P.M.) normalised spider diagram and (f) chondrite-normalised (N) REE diagram of whole-rock compositions for Diavik internal hypabyssal kimberlites (HK), external HK and pyroclastic kimberlites (PK). Trace elements

oo

f

were normalised using the chondrite and primitive mantle compositions of Sun and McDonough (1989). Shaded fields highlight whole-rock compositions of hypabyssal

pr

kimberlites (HK) and extrusive coherent kimberlites (CK) from the Ekati property in the Lac

Pr

e-

de Gras field (Nowicki et al., 2008).

Figure 11: Schematic diagram showing the evolution of melt compositions for (a) pyroclastic

al

kimberlite (PK) units and (b) internal hypabyssal kimberlite (HK) units of the A154N

rn

kimberlite pipe during shallow emplacement. (a) Degassing of CO 2 in PK increases the melt

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H2 O/CO 2 ratio thus triggering phlogopite crystallisation. Incompatible trace elements are less abundant in the whole rock compositions of the PK units due to melt fragmentation, ash production and expulsion. A possible phreatomagmatic trigger for explosive emplacement of the PK magmas is also considered. (b) In comparison, less degassing in internal HK results in a lower H2 O/CO 2 melt composition and Mg-carbonate forms instead of groundmass phlogopite. Fluids enriched in H2 O and CO 2 as well as fluid-mobile K and Rb are lost slowly, through cracks in the adjoining lithologies.

Journal Pre-proof Table 1: List of Diavik kimberlite samples used in this study. Location

Sample No.

Unit

A154N

GTH75_15 A154U190 A154U384 04GTH78_03 A154U381 Duey’s A154S_01

PK3-N PK

Pipe infill

PK1-N PK

Pipe infill

CK-N

Internal HK

Irregular

CK-S

Internal HK

Irregular

A154N A154S

Morphology

DrillCore Depth (m) 465.5465.65 273273.25 64.9-65

Depth from Surface (m) 448

No. on Figure 1 1

619

2

N/A

3

rn

al

Pr

e-

pr

oo

f

445.56- 580 4 445.8 Adjacent N/A External HK Dyke 122N/A 5 to A154S 122.25 Between Duey’s Internal HK Dyke N/A – 626 6 A154N Dyke Grab and A154S sample A154N = A154 North, A154S = A154 South, PK = pyroclastic kimberlite, HK = hypabyssal kimberlite.

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A154N

Classification

Journal Pre-proof Table 2: Summary of the petrographic characteristics and groundmass modal mineral abundances of the Diavik kimberlite samples.

(>0.5 mm)

04GTH78_03

A154U190

GTH75_15

Olivine (modal abundance in vol. % of the whole sample) Phlogopite

32

14.8

37.8

36.3

28

9.75

x

x

Clinopyroxene

Rx

Garnet Magmaclasts (<1.5 cm) Country rock fragments Mantle xenoliths

f

x

Zoned carbonate segregations Carbonate + serpentine segregations Fresh and pseudomorphed olivine Serpentine

Pr

Brucite Calcite Dolomite Apatite

al

Groundmass and magmaclasts* (average modal abundances in vol. % of the groundmass)

e-

Serpentine segregations

pr

Olivine micro-phenocrysts (>100 µm) Segregations

rn

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Perovskite

x x

Rx

Rx x x

M

x

x

x

x

x x

x x

x x

x Rx

x

Rx x x

M

x

2.0

<1

1.6

51.0

10.0

43.4

8.8

<1

12.6

8.9

29.4

21.7

26.5

17.0

31.6

19.6

<1

6.4

6.2

2.9

2.5

4.5

45.3

x

22.1

<1

<1

<1

x

40.7

<1

<1

<1

x

7.6

Fresh and pseudomorphed monticellite Phlogopite

Rx Rx

oo

Clasts

PK

A154SU384

Macrocrysts

Internal HK

Duey’s A154S_0 1 A154U381

External HK

<1

x

10.7 <1

x

1.5

6.1

Spinel 16.5 7.7 7.7 13.4 x *for PK samples, these modal analyses refer to the groundmass within the magmaclasts. ‘x’ = common occurrence; ‘Rx’ = rare occurrence, ‘M’ = only found in magmaclasts ; HK = hypabyssal kimberlite, PK = pyroclastic kimberlite.

11.2

Journal Pre-proof

Highlights:

Melts parental to the Diavik kimberlites had at least two primitive compositions.



Hypabyssal and pyroclastic kimberlites have the same early melt evolutions.



Kimberlite emplacement style is independent of primary melt compositions.



The late evolution of kimberlite magmas is controlled by the emplacement

f



rn

al

Pr

e-

pr

Emplacement style is likely controlled by external factors, not melt composition.

Jo u



oo

mechanism.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11