Lithos 112S (2009) 541–552

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Spatial and temporal evolution of kimberlite magma at A154N, Diavik, NorthwestTerritories, Canada☆

S. Moss a,b,⁎, J.K. Russell a, R.C. Brett a, G.D.M. Andrews a,b

a Volcanology & Petrology Laboratory, Department of Earth & Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4b Mineral Deposit Research Unit, Department of Earth & Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

☆ Special Volume, LITHOS, Proceedings of 9th Interna⁎ Corresponding author. Earth and Ocean Science

Columbia, 6339 Stores Road, Vancouver, B.C., Canada V6fax: +1 604 822 6088.

E-mail address: smoss@eos.ubc.ca (S. Moss).

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.03.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2008Accepted 15 March 2009Available online 5 April 2009

Keywords:KimberliteVolcanologyEmplacementEruption ratesOlivine

The A154N kimberlite pipe is part of the Diavik kimberlite cluster in the Northwest Territories, Canada. Thepipe is filled by deposits that record a sequence of eruptive and intrusive events for which we haveestablished relative times and modes of emplacement. As such, this sequence of kimberlite deposits providesa unique opportunity to explore the connections between magma properties, pipe or conduit geometry anderuption style.Deposits within the A154N pipe represent four discrete volcanic events including: (1) an array of pre-cursordykes of coherent kimberlite that intrude the adjoining country rock (CK1); (2) pyroclastic depositsrecording the explosive eruption of A154N (PK1); (3) late dykes of coherent kimberlite intruding the maininfill of the pipe (CK2); and (4) a sequence of water-laid pyroclastic deposits that sourced from anotherkimberlite volcano (PK2). We use observations on polished slabs and thin sections to characterize the magmathat erupted to produce each deposit. Image analysis techniques are used to quantify olivine content(abundance and size distributions) and abundances of vesicles within deposits and within individualjuvenile pyroclasts. Our analysis shows that during the eruption cycle (CK1 to PK1 to CK2), olivine contentincreased and volatile content decreased.Lastly, we estimate physical properties for the magmas that produced each deposit. The magma propertiesare combined with 3-D models for the geometry of the conduit at the time of eruption to constrain the massflux and duration of each eruptive event. Steady-state assumptions for velocity and magma flux yield totaleruption durations of minutes to hours. The diversity of textures and compositions recorded in pyroclastickimberlite (PK1) and dykes of coherent kimberlite (CK1 and CK2) are a possible manifestation of separatedthree-phase flow involving kimberlite melt and crystals, and an ever-increasing proportion of exsolved CO2–

H2O fluid. Specifically, we suggest that the early dykes (CK1), pyroclastic kimberlite (PK1), and late-stagedykes (CK2) are representative of three separate regimes of kimberlite magma created by the ascent of avigorously degassing magma. We envisage this being composed of: (a) the gas-charged front of the magmacreated by decoupling the more buoyant gas phase from the silicate melt; (b) a transient, gas-rich body ofmagma inwhich later-stage exsolved fluids/gases are essentially coupled to the melt; and (c) a fluid-depletedtail of magma which remains buoyant within the lithosphere but ascends more slowly than the fluid andexsolving gas phases.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Studies of modern volcanic systems have shown that variations ineruption style typically relate to the composition and physicalproperties of the magma, the degree of magma overpressure, thetotal volume of magma, and vent geometry. For example, solid, liquidand gas ratios in erupting magmas have been shown to modulate the

tional Kimberlite Conference.s, The University of BritishT 1Z4. Tel.: +1 778 231 8078;

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style (i.e. effusive vs. explosive) and intensity (e.g. fountain height) ofbasaltic eruptions (e.g. Parfitt et al., 1995; Houghton and Gonnermann,2008). Moreover, low viscosity magmas can outgas efficiently andallow for a concomitant reduction of magma overpressure anderuption intensity (e.g. Stasiuk et al., 1996; Namiki and Manga,2008). Vent geometry can also be a significant factor in determiningeruption style and intensity (e.g. Wilson et al., 1980; Scandone andMalone, 1985; Bower and Woods, 1998).

Similar linkages between magma properties, vent geometry, anderuption style have not been made for kimberlite eruptions for severalreasons: (1) kimberlite eruptions have not been observed, and surfacefeatures and extra-crater deposits of even the youngest kimberlitevolcanoes are usually highly eroded or completely removed; (2)

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pristine samples of kimberlite melt are rarely preserved becausekimberlitic volcanic deposits are highly susceptibility to post-emplacement alteration and textural modification; and (3) kimberlitemagmas are volatile-rich and likely to have highly transient physicalproperties during transport and eruption. For example, due to the highvolatile contents estimated andmeasured for kimberlite magmas (e.g.Eggler, 1978; Price et al., 2000; Sparks et al., 2006, this issue) thevolumetric proportions of melt:solid:gas must evolve from melt-dominated systems at mantle conditions to gas-dominated systems atthe point of eruption (Gernon et al., this issue). Such variations willhave profound impacts on the physical properties of kimberlitemagmas and, hence, the mechanisms and styles of eruption.

Though conduit geometries of active volcanoes are difficult tomeasure, it is accepted that conduit morphologies are dynamic andsubject to change (Scandone, 1996). In conventional volcanologicalstudies, the data used to constrain unknown conduit geometries aremeasured heights of eruption columns from active systems or particlesize distributions in fallout deposits for unobserved systems. Incontrast, mining of diamondiferous kimberlite offers a uniqueopportunity to characterize conduit and vent geometries of anerupted volcano. For example, drill-core intercepts, downholegeophysical measurements, and open-pit surveys provide directobservations of subsurface contacts and, thus, constrain the 3-Dgeometry of kimberlite conduits.

The A154N kimberlite pipe in the Eocene Diavik kimberlite cluster(Graham et al., 1999) in the Northwest Territories, Canada, (Fig. 1a)comprises a sequence of extrusive and intrusive events for which the

Fig. 1. Location of the Diavik kimberlite field and internal units of A154N: (a) Kimberlite pipA154S pipes andmapped (solid lines) and inferred (dashed) dykes of CK1; (c) Inclined 3-Dmkimberlite dykes (CK1) (translucent); pyroclastic kimberlite (PK1); coherent kimberlite dyk(PK2).

relative timing, vent geometry and mode of emplacement are clearlyestablished. As such, it provides a unique opportunity to explore theconnections between magma properties, vent geometry and eruptionstyle.

Our analysis of this sequence of deposits suggests that transientmagma properties arising from separated three-phase flow involvingkimberlite melt, crystals, and an exsolved CO2–H2O fluid provide afundamental control on the style of kimberlite eruption and emplace-ment. Specifically, we expect and observe the volumetric proportionsof the fluid phase to: (1) increasewith ascent; but (2) to decreasewithtime during the eruption due to decoupling of the gas and the silicatemelt within the flow. We also expect the fraction of solids (i.e. crystalsand xenoliths) to concentrate at the base of the ascending plug ofkimberlite magma during ascent. These processes can account for thediversity of textures and compositions in pyroclasts and within dykesof coherent kimberlite.

2. Field mapping

Fieldmapping in the openpit at Diavik and logging of drill-core hasidentified a minimum of 4 discrete volcanic (e.g. 3 eruptive and 1intrusive) events in the A154N pipe (Fig. 1b and c), including: (1) anarray of dykes (CK1) intruded into country rock exposed at the present-day surface (415 meters above sea level) during mining; (2) massivepyroclastic deposits within the pipe (PK1); (3) sub-surface intrusionsof coherent kimberlite (CK2) intruded into PK1. The sequence is cappeddisconformably by an externally-sourced pyroclastic kimberlite

es at Diavik, East Island of Lac de Gras, NWT, Canada; (b) Plan view of the A154N andodel of A154N looking E, showing five separate units of kimberlite, including: pre-cursores (CK2); re-sedimented volcaniclastic kimberlite (RVK); upper pyroclastic kimberlite

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deposit (PK2) captured by and preserved within the vent to the A154Nvolcano (Moss et al., 2008). Here, we describe the macroscopic andgeometric properties of the individual deposits, the textures andcomponents therein, and their spatial context and relative timingwithin the A154N kimberlite volcano.

2.1. Early dykes (CK1)

An array of 5–150 cm-widedykes (CK1) of yellow-brown, carbonate-altered kimberlite is observed in the A154N open pit, striking ∼038° forover 800 m through the adjacent country rock (Figs. 1b and 2a). TheA154N and A154S kimberlite pipes cross-cut and truncate the center ofthe known strike length of the dyke array; there is no evidence of CK1

cross-cutting the fragmental kimberlite within either pipe. The contactrelationship between CK1 and the A154N pipe is evidence of a cleargenetic linkage to the subsequent pipe formation and pipe-fillingprocesses within A154N: CK1 precedes the formation of A154N;formation of A154N follows the CK1 dyke; pipe-filling is synchronouswith and immediately follows pipe formation. This implies that CK1

Fig. 2. Field and hand sample photographs of pre-cursor CK1 dykes at A154N: (a) thin dykes othrough CK1 dyke, partially-filledwith coherent kimberlite (adjacent to hammer), adjacent toCK1 featuring prominent tension cracks adjacent to dyke margin; (d) polished sample of Ccarbonate filled amygdules; (e) polished sample from interior of CK1 showing coarse (N1 c

dykes are the earliest emplacement event at the volcano. CK1 is within adownward-tapering zoneof vertical fractures in the country rock,whichnarrows to 6mwide at a depth of 245m(170masl). The dyke is variablythick (5–150 cm), vertically and horizontally discontinuous, andcommonly splits into multiple, thin (5–10 cm wide) dykes (Fig. 2);many of the vertical fractures do not contain kimberlite.

The dyke is variably competent; most of the uppermost exposures(i.e. N300 m asl) consist of yellow-brown mud with abundant,resistant indicator minerals (e.g. ilmenite, garnet). In contrast, deeperintersections of the dyke comprise a brown, dense rock with fine,internal fractures filled by serpentine. Portions of the dyke are rich inmiarolitic cavities and carbonate-filled amygdales (25% by volume),and flow-banding is locally evident adjacent to contacts withsurrounding country rock (Fig. 2d). Competent sections of the dykegenerally comprise minor amounts (∼5–15%) of olivine (1–10 mm)set in a groundmass of spinel, serpentine and carbonate. Garnet andclinopyroxene macrocrysts (1–10 mm) and ilmenite megacrysts (3–20mm) are also abundant. The smaller size fractions of olivine and thegroundmass of the dyke are pervasively altered, and are commonly

f CK1 exposed on 10 m bench-faces in open pit from 210–180 m asl; (b) exposed sectionaltered fracture zone (projecting intowall) void of kimberlite; (c) widest intersection ofK1 collected from contact in (c), showing well-developed flow-banding, and abundantm) ilmenite, garnet and relatively low olivine content (10–12%).

Fig. 3. Distribution and properties of PK1 within A154N: (a) 3-D model of A154Nshowing sampling locations (stars) along drillhole traces within PK1; (b) polished slabof PK1 showing granite xenolith (gx), mud-rich kimberlite fragment (k.f.), juvenilekimberlite pyroclast (jp), and abundant olivine crystals (ol); (c) underground bench-face with bolt-and-screen covering showing homogeneous distribution of granitexenoliths (gx) in massive, poorly-sorted pyroclastic kimberlite; (d) close-up of inset in(a) showing upper-contact between PK1 and overlying RVK and PK2 (translucent).Dashed outline indicates approximate contact between RVK and overlying PK2.

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completely replaced by carbonate, serpentine, and Fe-oxides. Countryrock xenoliths of granite and biotite schist are ubiquitous, butrepresent a small volume of the dyke (b5%), and are typicallyconcentrated at or near dyke margins.

2.2. Pyroclastic kimberlite (PK1)

The A154N pipe is largely infilled by a N500 m thick sequence(5.0×106 m3) of massive, poorly-sorted, fragmental kimberlite(Fig. 3a; PK1) which is interpreted as primary pyroclastic kimberlite(Graham et al., 1999; Moss et al., 2008). The contact with overlyingbedded, re-sedimented volcaniclastic kimberlite (RVK) is sharp,reflecting a change in componentry (mud-rich matrix) and a changein depositional style to small-volume, high-energy sub-aerial and sub-aqueous debris flows (Moss et al., 2008). The eruption fromwhich PK1

was deposited left a conduit that was only partially filled and, thus, anopen hole at least 200 m deep (Moss et al., 2008). The open hole wasfloored by a hummocky topography upon which subsequent sedi-mentation of crater walls (RVK) and other tephra (PK2) occurred(Fig. 3d).

The PK1 comprises mainly liberated olivine crystals and juvenilepyroclasts of quenched kimberlite (1–30 mm in diameter) supportedby a heterogeneous matrix of serpentine, carbonate, and variableproportions of ash-sized grains of olivine, quenched kimberlite, anddis-aggregated shale and unconsolidated mud (Fig. 3b). Olivinecrystals are commonly surrounded by thin rims or selvages ofcrystallized kimberlite magma. Accidental lithic fragments of countryrock (granite, biotite schist, grey shale, black shale) up to 30 cm indiameter, and accessory clasts (2–30 cm) of mud-rich, fragmentalkimberlite of unknown origin occur throughout PK1 (Fig. 3c). Mantlexenoliths (0.5–6 cm) of garnet lherzolite, wherlite and eclogite arecommon, as are derivative xenocrysts from disaggregation of mantleperidotites (e.g. garnet, clinopyroxene, strained and recrystallizedolivine). The PK1 shows variable amounts of clast and matrixalteration; alteration minerals include serpentine, carbonate andmagnetite.

2.3. Coherent kimberlite (CK2)

Multiple intrusions of olivine-rich (N40%), dense, coherentkimberlite (CK2) are observed in the country rock and PK1 up to15 m asl (Fig. 4b), but not the uppermost portion of PK1 or overlyingRVK (Fig. 4d); we infer this to imply that the late intrusions did noterupt at surface. In several cases, vertical CK2 dykes have intrudedalong the margin between the pipe wall and the PK1 deposit. Withinthe PK1 deposit, CK2 form both sharp edged sheets having minoralteration halos (b1 cm) and thin (b1 cm), aphyric ‘fingers’ ofcrystalline kimberlite.

CK2 consists of abundant, fresh, sub-rounded to euhedral olivinecrystals (0.03–10 mm) enclosed by a groundmass of spinel, serpen-tine, apatite, and minor carbonate and perovskite. Garnet andclinopyroxene xenocrysts are minor, and ilmenite and phlogopitemacrocrysts are rare. Granite xenoliths, and xenocrysts of quartz andaltered feldspar are locally abundant (N10%), and clasts of lithified PK1

are uncommon but also present as inclusions within CK2.

2.4. Re-sedimented volcaniclastic (RVK) and pyroclastic (PK2) kimberliteinfill

PK1 is capped by ∼100 m of re-sedimented volcaniclastickimberlite (Fig. 1c, 3d; RVK). This unit is critical as it defines a hiatusof unknown length in the eruption sequence at A154N followed bydeposition of pyroclastic kimberlite (PK2) from an exotic source (Mosset al., 2008). The relative timing of CK2 and the emplacement of RVKand PK2 is unknown, and, thus, RVK and PK2 will not be consideredfurther in this present study.

The field relationships described above rigorously constrain thesequence of volcanic and subvolcanic events at theA154Npipe (Fig.1c)and allow for an investigation of the temporal evolution of the physicalcharacteristics of magma(s) responsible for different facies within thesame volcanic succession. Although the relative timing betweenevents is well-constrained, the absolute time elapsed between eventsis unknown. Previous Rb–Sr (phlogopite) dating of PK1 and PK2 inA154N yielded dates of 56.0+/−0.7 Ma (Amelin, 1996; Moser andAmelin, 1996). Rb–Sr dating of both pyroclastic and coherent rocksfrom adjacent economic pipes at Diavik (A154S, A21, A418) show ageranges from 55.7+/−2.1 Ma to 54.8+/−0.3 Ma (Barton, 1996;Heaman et al., 2004). The level of precision in these dates precludesspeculation on the time intervals between events (e.g. PK1 vs. PK2) orthe order inwhich each pipe (Fig. 1) was emplaced. Moreover, furtherage dating is not undertaken in this study. However, based on theinclusion of lithified PK1 within CK2, we infer that the emplacementof CK1 and PK1 was followed by at least a short hiatus before theemplacement of CK2.

3. Componentry

Our characterization of kimberlite samples uses image analysis toquantify abundances and size variations of olivine, vesicles, amyg-dales, miarolitic cavities, and primary and secondary carbonatephases within the groundmass. We use these attributes to uniquelycharacterize the individual samples of coherent (CK1 and CK2) and

Fig. 5. Olivine size distributions: (a) area % vs. olivine size (as mm2) for early dyke(dashed lines; CK1) and late dykes of coherent kimberlite (solid lines; CK2).

Fig. 4. Distribution and properties of CK2 within lower section (0 to −200 m asl) ofA154N: (a) 3-D model of CK2 of A154N (stars indicate sample locations); (b) drillcoreshowing sharp contact between CK2 and PK1; (c) polished sample of crystal-rich CK2;(d) polished thin-section of CK2 showing round to euhedral olivine crystals set in a fine-grained groundmass.

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fragmental kimberlite (PK1). Outlines of olivine crystals andpyroclasts are manually traced at two scales (polished slabs, 3000–10,000 mm2; polished thin sections, 27×46 mm) using a digitizingpad and Adobe Illustrator™ and analyzed using ImageJ™ to calculateparticle areas. These data sets are then normalized to the largestscale of observation. For the pyroclastic deposit (PK1) we also useimage analysis techniques to characterize the individual juvenilekimberlite pyroclasts. Thus, we are able to report estimates of modalolivine content and pyroclast size. We also have recorded texturalrelationships between olivine, vesicles, carbonate phenocrysts, andgroundmass.

3.1. Olivine crystal abundances

Olivine crystal contents for CK1 and CK2 were estimated forsamples collected from three levels in the open pit (315, 275, and230 m asl), and from drill-core intercepts of two separate dykes,respectively. Modal abundance of olivine crystals increases towardsthe centre of CK1, and crystals are rare to absent at the margins; themargins are volumetrically dominated by carbonate amygdales andmiarolitic cavities (Fig. 2d). Conversely, olivine crystals are evenly-distributed in CK2 dykes except at the immediate (b1 cm wide)margins of the dyke where modal olivine content decreases to b25%and crystal sizes are generally b3 mm. Therefore, for comparison,samples were collected from the relatively crystal-rich interiorportions (N5 cm from the dyke margins) of both CK1 and CK2,respectively.

Results from image analysis of slabs and thin sections from tworepresentative samples of CK1 and five from CK2 are expressed asmodal area % olivine vs. size (mm2), and shown in Fig. 5. Samples ofCK1 contain a truncated size range of olivine crystals (0.2–100 mm2)relative to CK2 (0.03–100 mm2), and have lower overall modal

abundances of olivine (∼5–15% vs. 40–58%) (Fig. 5). The difference inolivine crystal size ranges for CK1 and CK2 may be due to alteration;smaller-sized olivine crystals (0.03–0.2 mm) in CK1 are ubiquitous,but are commonly pseudomorphed by serpentine. As a result, theboundaries between pseudomorphs and groundmass are difficult todistinguish, and preclude these grains from accurate measurementusing image analysis. Lower overall abundances in CK1 relative to CK2,however, cannot be due to alteration alone; coarser olivine sizes(N0.2 mm) in CK1 are generally fresh, and smaller-sized olivinecomprises a relatively small fraction of the total abundance (Fig. 5) ofolivine in fresh CK2. Therefore, this discrepancy is interpreted to resultfrom intrinsic differences in the amount of olivine in CK1 vs. CK2.

3.2. Juvenile pyroclasts from PK1

To assess the crystal content of the magma parental to PK1, wehave examined the population of juvenile pyroclasts comprisingcrystalline kimberlite+/−olivine crystals (JPs). Twenty sampleswere collected at ∼10 m intervals throughout a 200 m verticalsection of the PK1 (Fig. 3a); ten or more JPs were chosen from eachthin section for analysis based on observable groundmass miner-alogy and lack of alteration. The size (e.g. area) of individualpyroclasts and their olivine crystal contents (area %) werecalculated by analyzing for total pyroclast area (mm2) and thearea occupied by olivine in each clast; the difference is the area ofcrystallized kimberlite melt. Pyroclast size vs. olivine crystalcontents (modal %) are shown in Fig. 6a and b, in which pyroclastsare subdivided into groups based on observations of texture andmineralogy. In Fig. 6a, JPs are sub-divided into vesiculated (v), non-vesiculated (non-v), carbonate microphenocryst-bearing (i.e. cal-cite or dolomite; carb), and vesiculated and carbonate micro-phenocryst-bearing (v+carb). In Fig. 6b, JPs are sub-dividedaccording to textural relationships between olivine crystals andsurrounding crystallized kimberlite melt: (1) pyroclasts contain-ing ‘multiple’ olivine crystals (M-type) in a groundmass ofcrystallized kimberlite magma; (2) single olivine crystals withselvages of crystallized kimberlite (S-type); (3) single olivinecrystals with partial selvages of crystallized kimberlite (PS-type);and (4) pyroclasts cored by a single olivine crystal (b70% of totaljuvenile pyroclast area) surrounded by other, smaller olivines andcrystallized kimberlite magma (C-type). Juvenile pyroclasts range

Fig. 6. Parametric analysis of juvenile pyroclasts from PK1 of A154N: (a) olivine content (%)vs. size (mm2) of juvenile pyroclasts containing: vesicles (v), groundmass carbonate (carb),both (v+carb), or that are vesicle-free (non-v). Shaded regions denote olivine to meltrelationships that are not observed; (b) olivine content (%) vs. size (mm2) for texturalsubgroups of juvenile pyroclasts, including: pyroclasts with multiple olivine crystals (M),selvages ofmelt on olivine crystals (S), olivine grains with partial selvage (PS), and cored (C)pyroclasts. Shaded bands indicate measured modal olivine (%) for CK2 and CK1; solid linesindicate observational limits of olivine content vs. size for M; dashed lines indicate projectedlimits. Note the similarity of observed sizes of juvenile pyroclasts and olivine crystalpopulations (Fig. 5). Absence of smallest juvenile pyroclast sizes attributed to alteration andelutriation processes (double-arrow); (c) histograms of components within texturalsubgroupsof juvenilepyroclasts (vesiculated, carbonate-bearing,N30%olivine,b30%olivine).

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in size from 0.02 to 30 mm2 and have olivine contents of 0–90%and minimum sizes of 0.03 mm2. Neither small JPs with highcrystal:melt ratios, nor large JPs with low crystal: melt ratios areobserved (shaded regions; Fig. 6a). The combined modal % olivinecrystals in JPs is 43.7%.

3.3. Textural evidence of volatile content

In this section, we describe and quantify textures indicating thepresence of an exsolved fluid phase; their abundance is used as aproxy for the relative fluid content of the magma at the time oferuption. Vesicles indicative of the presence of an exsolved fluid phaseare found in all deposits; they are differentiated by the amount ofinfill, and include: ‘vesicles’ denoting spherical to sub-spherical gascavities; ‘miarolitic cavities’ denoting gas cavities partially lined bylate-stage minerals (e.g. carbonate) in intrusions; ‘amygdales’ denot-ing gas cavities that are essentially filled by late-stage minerals (e.g.carbonate, serpentine). At the slab and thin section scale, CK1 containsmiarolitic cavities, abundant lath-shaped groundmass carbonate, aswell as spherical to sub-spherical patches of serpentine+carbonatewithin the groundmass. These domains are distinguishable fromserpentinized olivine grains by remnants of fresh olivine withinserpentinized grains and by the orientation of serpentine fibersperpendicular to the outer edge of the domains. It also featuresextensive post-emplacement carbonate alteration of olivine andgroundmass phases (e.g. Fig. 2d). Many miarolitic cavities withinCK1 are lined or partially filled with carbonate crystals (i.e. calcite,dolomite).

Juvenile pyroclasts from PK1 range in size from 0.02 mm2 to30 mm2, and ∼14% of pyroclasts contain vesicles or amygdales filledwith carbonate and/or serpentine (Fig. 6c). Vesicle contents rangefrom 0 to 20% by area (i.e. 0–40% of non-olivine groundmass orcrystallized melt). Textural sub-types of JPs show restricted sizeranges for vesiculated (0.25–32 mm2) and carbonate-bearing JPs(0.5–4 mm2) (Fig. 6c). Vesicles occur as both void spaces and asamygdales, infilled with carbonate and serpentine (Fig. 7a). The infillsare mineralogically and texturally diverse, occurring as: granularcarbonate, serpentine, euhedral carbonate-linings with serpentineinfills, or fibrous or botryoidal serpentine linings with carbonatecrystals in their interiors. The majority of vesiculated juvenilepyroclasts do not contain groundmass or phenocrystic carbonate,while the majority of groundmass carbonate-bearing juvenile pyr-oclasts do not contain vesicles (Fig. 7a). In contrast to CK1 and PK1, lateCK2 magmas contain minimal groundmass carbonate, minimalpatches of serpentine and/or carbonate (Fig. 4c,d) in the groundmass,and rare miarolitic cavities.

4. Discussion and interpretation

Our investigations reveal important variations in mineralogy,texture and componentry between CK1, PK1 and CK2. In this section,we discuss the origins of variations in olivine crystal size, olivinecrystal abundance, and the inferred proportions of gas and/or fluid.We then combine these observations with inferred physicalproperties of kimberlite magma to estimate possible mass fluxesinvolved in each emplacement event and attempt to constrain thedynamics of kimberlite emplacement in the A154N pipe.

4.1. Variations in olivine content with time

Variations in the olivine crystal contents (area %) of the CK1 andCK2magmas are substantial (Table 1).We interpret these variations asfirst-order differences between early (CK1) and late (CK2) emplacedmagmas. Modal estimates (area %) of olivine contents also vary widely(0–90%) for all sizes of JPs in PK1. Despite this variability, there is nodetectable sorting of JPs by size or type within PK1. Moreover, JPs from

Fig. 7. Photomicrographs of select juvenile pyroclasts (JPs) from PK1: (a) JP containing olivine (ol), spinel (spl) serpentine+/−carbonate filled amygdules (V/A) in serpentinegroundmass; (b) JP with carbonate laths (carb), olivine (ol) and spinel (spl) in serpentine and carbonate groundmass; (c) JP with b30 area % olivine content (ol) in spinel (spl) andserpentine groundmass; (d) JP with N50 area % olivine content (ol), and thin, olivine-free quenched-melt rim.

Table 1Summary of component properties in crystallized magma of units in A154N.

Unit CK1 PK1 CK2

Number of samples 4 211 5Sample size (mm2) 5200–12,000 0.02–100 3619% olivine crystals 5–15 5–44 40–58% vesicles 0–60 0–40 0–2Olivine sizes (mm) 0.2–10 0.03–6 0.03–10Vesicle sizes (mm) 0.2–5 0.1–1 –

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the same thin section and possessing similar groundmass mineralogy(e.g. spinel, serpentine, perovskite) and grain size can have highlyvariable olivine contents. Possible explanations for the variation inolivine content within JPs are discussed below and include: (1) PK1

hosts JPs derived from multiple eruptions; (2) single batches ofmagma can have intrinsically heterogeneous olivine contents (i.e.total modal %); or (3) single batches of magma having a constantolivine content (i.e. vol.%) can have variable olivine size distributions(0.01–100 mm2).

The PK1 is a massive deposit and the lack of any cross-cutting,pyroclastic kimberlite deposits suggests that it results from a singleeruption. To determine if the variations in the olivine contents of JPsare a function of the modal % or the crystal size distribution of theolivine in the original magma, we have sub-divided JPs into fourtextural classes (Fig. 6b). These subdivisions (M; S; PS; C) allow us tofilter out the influence of single crystals (i.e. crystal size) inmeasurements of JPs which show high crystal: melt ratios; i.e. if anindividual crystal is dominantly responsible for the observed olivinecontent (e.g. S, PS, or C).

Each sub-type has an olivine content expected of the texturalrelationship between olivine and surrounding crystallizedmelt (e.g. S N50% olivine). The presumed low viscosity of kimberlite melt (cf. Sparkset al., 2006) likely influences its interaction with crystals duringfragmentation, and may determine the selvage thickness (i.e. S) and

wetting angles between crystals and melt (i.e. PS) observed in JPs.Observations of the M sub-type, therefore, likely represent the closestapproximation of the crystal: melt ratios present in the pre-eruptivemagma responsible for PK1. However, multiple olivine (M) pyroclasts ofall sizes (0.02–80mm2) still show large ranges in olivine crystal content(i.e. % low— % high for a given size=30–50%).

Primary elutriation of fines during eruption and deposition andsecondary alteration can also have an effect on pyroclast diversity.Smaller pyroclasts are less likely to be preserved; higher surface areato volume ratios of smaller pyroclasts encourage size reduction oreven complete destruction due to alteration, and the finer pyroclastsare also more susceptible to elutriation and loss during eruption(Fig. 6b). Moreover, thin selvages of crystallized kimberlite magma

548 S. Moss et al. / Lithos 112S (2009) 541–552

around large olivine crystals are also susceptible to corrosion andpossible eradication due to alteration. Preserved JPs, therefore, likelyrepresent a sample bias towards crystal-rich magmas and may over-estimate the olivine content.

Larger pyroclasts (N100 mm2) and cognate lithic fragments ofcoherent kimberlite are rarely observed, but the few that are foundhave high olivine crystal content similar to that seen in CK2. The lackof large, crystal-free juvenile pyroclasts could imply that either themagma was entirely crystal-rich or that the inferred low viscosityand high gas contents of erupting kimberlite magma leads toefficient disaggregation of larger clots of kimberlite magma whenabundant crystal surfaces are not available to provide surfacetension. However, the presence of smaller JPs having low olivinecontents (Fig. 6b), combined with an inferred loss of fine ash-sizedJPs of quenched kimberlite melt provides some evidence forfragmentation of a crystal-poor magma. Therefore, if the observedJPs are representative of the starting eruptive magma, then thecombined modal % of olivine in JPs (43.7%) likely represents amaximum in olivine content.

Our reconstruction of olivine contents for these three kimberlitedeposits constrains the olivine contents of the correspondingmagmas.These data suggest that the erupting magmas increased in olivinecontent with time: CK1 (% Ol)bPK1 (% Ol)bCK2 (% OL) (Table 1). Wesuggest that the textural andmineralogical diversity of pyroclast typeswithin a single pyroclastic deposit (i.e. PK1) is a result of hetero-geneous olivine distributions and crystal:melt ratios within theerupting magma, rather than representing products of differentmagma batches. Furthermore, our results suggest that the variableolivine content relative to the melt fraction inferred for PK1 (5–44%),may be more representative of explosively erupted kimberlite magmathan that observed in CK2 (40–58%).

4.2. CO2 and H2O variation with time

The sequence of kimberlite deposits exposed at A154N documentsubstantial variations in the proportions of volatiles (i.e. exsolved anddissolved) during emplacement or eruption. For example, the earliestphase of eruption represented by the CK1 dyke contains groundmasscarbonate, abundant carbonate-serpentine (i.e. CO2 and H2O)domains or pools, and abundant miarolitic cavities. These featuresimply the following for CK1: (1) the presence of an exsolved CO2–H2Ofluid phase physically coupled to the melt (e.g. miarolitic cavities) atthe time of eruption; and (2) the presence of dissolved CO2 within themelt which allowed for crystallization of groundmass carbonate.

Juvenile pyroclasts from PK1 record variable amounts of volatiles.Juvenile pyroclasts can contain 0–20% vesicles, however, vesicles arepresent in only 14% of the measured JPs within a limited size range(0.5–20 mm2). The small proportion of vesiculated JPs may be theresult of elutriation of low-density particles during eruption andemplacement, or gas-escape from small JPs with high melt tempera-tures and low melt viscosities (Cas et al., 2008). Moreover, the smallsize of kimberlite pyroclasts (all b5 cm) enables an efficient release ofgas during fragmentation in the erupting system. However, singlesizes of JPs contain both vesiculated and non-vesiculated textures.Some JPs also contain both lath-shaped carbonate laths andcarbonate-serpentine domains (Fig. 7). The diversity in vesiclecontent found within JPs of PK1 of all sizes implies that a singlemagma experienced variable degrees of exsolution prior to disruptioninto discrete fragments. The occurrence of groundmass carbonate,carbonate laths and carbonate-lined or filled vesicles in the same JP israre (1 sample); the presence or absence of groundmass carbonateand carbonate laths may be due to a heterogeneity in CO2 solubilityreflective of subtle compositional differences within the ascendingmagma (Brooker et al., 2001).

Late dykes (CK2) contain only rare carbonate-serpentine domains,minor miarolitic cavities and never contain microphenocrysts of

carbonate minerals. The lack of textural evidence for an exsolved fluidphase could imply either efficient post-emplacement de-gassing ofthe dyke, or exsolution and escape of gases during its ascent. However,the crystal-rich nature and presumed rapid-cooling time for thin(b1m thick) dykes would severely inhibit the vertical movement and/or removal of exsolved gases from a static dyke. Therefore, we suggestthat CK2 had minimal exsolved fluid when it was emplaced; theimplication is that the majority of fluid exsolution occurred duringtransport.

Taken together, these variations in the mineralogical and texturalexpression of CO2 and H2O in deposits having an established sequenceof emplacement tell us about the relative proportion of both exsolvedand dissolved fluid phases present in erupting kimberlite magmas:CK1 XCO2+H2ONPK1XCO2+H2ONCK1 XCO2+H2O.

4.3. Fluid phase separation in kimberlite

These observations lead to several possible hypotheses regardingemplacement of kimberlite magmas (CK1, PK1, and CK2) at A154Nthrough time: (1) as three magma pulses distinct in space and timeorigins related only by the path they take to the near-surface; (2) asdistinct magma pulses originating from the same location in themantle at different times; or (3) as a single, separated three-phaseflow originating at the same time from the same location.

We submit that different physical attributes (% solids, gases, etc.)coupled with clear differences in emplacement style (fissure dykes,explosive eruption, passive intrusion), and known relative timingsuggest CK1, PK1, and CK2 are genetically-related in space, time, andcomposition.

The presumed low viscosity and high volatile contents (Price et al.,2000) in kimberlite melts implies that buoyant and coalescing gasbubbles may be able to ascend at different velocities than kimberlitemelt (e.g. Vergniolle and Jaupart, 1990). In this case, gas-streamingmay develop, and the ascending magmas can be described as an“open-system” (sensu Cashman and Mangan, 1994). During open-system degassing, gas bubbles can migrate from deeper to shallowerparts of a body of magma (Gonnermann and Manga, 2007; Namikiand Manga, 2008).

If CK1, PK1 and CK2 are derivatives of a single magma pulse, cleardifferences in exsolved gas proportions in the magma suggest gas wasefficiently moving through the kimberlite melt in open-systembehavior. However, phase separation was not efficient enough tosuppress explosive eruption; high amounts of exsolved gas and abreach of the present-day surface suggest CK1 was likely explosive,followed by the clearly explosive PK1 event. This implies thatexsolving gas was still coupled to the melt (e.g. Parfitt et al., 1995).This could result from fast ascent rates which would reduce thedifferences in rise velocity between bubbles and the kimberlite melt(Gonnermann and Manga, 2007). Alternatively, high volatile contentscould support continuous production of a fluid phase during ascent tothe point of eruption, whether or not the fluid phase had begunseparating earlier during transport. Furthermore, fresh, unalteredrocks and juvenile pyroclasts from each unit (CK1, PK1, CK2), containminerals that may reflect the primary volatile content of themagma atthe time of emplacement (e.g. H2O in serpentine, CO2 in calcite), andsuggests incomplete exsolution of the volatiles. The increasinglyhigher crystal content with time (CK1 to PK1 to CK2) could also act toinhibit efficient out-gassing of exsolved phases (Klug and Cashman,1996).

We interpret the apparent variations in fluid contents at the timeof emplacement and eruption to reflect the efficiency of fluidexsolution and fluid separation within a single magma pulse duringtransport from mantle to surface. We also interpret observedvariations in olivine crystal content to reflect decoupling of the‘crystal cargo’ (Davidson et al., 2004) from its host melt. Thedecoupling of fluid and solids from the melt is facilitated by the

Fig. 8. Vent geometries for A154N and calculated mass fluxes: (a) Inclined profile viewof A154N delineated by 5 m horizontal slices showing smooth, irregular vent marginsand geometries near surface (black; 35 m depth), at the widest intersection (greydashed; 255m), and narrowest intersection (grey shaded; 615m); (b) outlines of cross-sectional areas of PK1 are shown at surface (35 m below surface), at maximum cross-sectional area (255 m), and minimum cross-sectional area (615 m). Outlines of ventgeometry also for CK1 and CK2 at 615 m below present-day surface; (c) calculatedrelationship between velocity vs. mass flux for CK1 (solid line) and PK1 (dashed line)eruptions, and intrusion of CK2 (dash-dot) based on estimated magma properties.Brackets indicate observational limits for least explosive (fire-fountaining) and most-explosive eruption (Plinian) styles. Bars indicate critical velocities necessary forturbulence in dykes given conduit geometry and estimated viscosity. A likelyprogression based on the nature of emplacement (fissure dyke, pipe-excavation,passive intrusion) is indicated by numbers (1,2,3). Expected eruption durations forconstant mass flux in PK1 are indicated for a range of velocities (arrow).

549S. Moss et al. / Lithos 112S (2009) 541–552

long ascent path (N200 km) and the high volatile content and lowviscosity (0–100 Pa s) of the kimberlite magma. Moreover, the settlingvelocities of particles (i.e. olivine crystals) in turbulent flows can behigher than those in still fluids (Wang andMaxey, 1993; Yang and Shy,2005). Combined, these processes resulted in an ascending magmacolumn that was strongly stratified. The ascending column of magmawould feature a gas-rich, relatively crystal-poor top or front followedby relatively fluid-depleted magma that increased in olivine contentwith depth. The eruption of the stratified column of ascendingmagmawould tap sequentially a gas-dominated magma, a less gas-richmagma showing a range of olivine contents, and lastly a crystal rich,largely degassed magma. This evolution in magma composition (e.g.proportions of gas:melt:solid) would greatly modulate the style (i.e.effusion vs. explosive) of emplacement.

4.4. Vent geometry and mass flux (Mf)

One advantage offered to volcanologists by mining of kimberlite isdirect observation of the shape and size of the volcanic conduit. Thecombination of drill-core intercepts, downhole geophysical measure-ments, and open-pit surveys at A154N provides a robust 3-D modelgeometry for the volcanic conduit (Figs. 1, 8a and b). From this model wecan deduce the shape of the vent/conduit system for each phase of theeruption, anduse these geometries to constrain associatedmassflux rates.

Here, we use the following model cross-sectional areas taken at615 m below present-day surface (−200 m asl) for CK1, PK1 and CK2 toconstrain our eruption model of the A154N kimberlite: CK1=500 m2;PK1=253m2; CK2=143m2. These intersections are used because theyrepresent theminimum cross-sectional areas that all kimberlitemagma(extrusive and intrusive) must have passed through. These conduitgeometries are combined with measured (e.g. olivine content) andinferred magma properties (e.g. ρmelt, XCO2+H2O) to provide first-orderestimates on mass fluxes for the different phases of the eruption.

The mass flux (Mf: kg s−1) of kimberlite magma through a dykeand supporting the eruption of CK1 is computed from:

Mf = h � l � ρB � μ ð4:1Þ

where h is the dyke thickness, l the dyke length, ρB the bulk density of theeruptingmixtureof gas, crystals andmelt, andμ thevelocityof themixture(Wilson and Parfitt, 2008). Bulk density of the magma is estimated from:

ρB = 1− nf − nx

� �� ρm + nf � ρf

� �+ nx � ρxð Þ ð4:2Þ

where ρm is the density of the melt, ρf and ρx are the densities of theexsolved fluid phase and the solids, respectively, and nf and nx are thecorresponding volume fractions.

For our estimates, we assume a ratio of H2O:CO2 of 1, andnf of 60%, 90%and 5% for CK1, PK1 and CK2, respectively. Assumptions of volumetric gasfractions are based on: i) estimates of maximum gas fractions for inflatedbutnon-disrupted(i.e. coherent; e.g.CK1)magmas(b65%;e.g.Melniketal.,2005); ii) conditions for magma fragmentation (70–99%; e.g. Sparks andWilson,1982;Melnik et al., 2005) capable of producingpyroclastic deposits(e.g. PK1); and iii) textural observations (Section 3.3) on intrusive rocks(e.g. CK2).Measuredmodal olivine abundance (Sections 3.1, 3.2) is used fornx. The model calculated densities include: 117 kg m−3 for the fluid phase(500 bars at 1200 °C), 2900 kg m−3 for the melt and 3300 kg m−3 forolivine crystals (cf. Russell et al., 2006). CalculatedρBof solid,fluid andmeltmixtures are 1238 kgm−3, 405 kgm−3, and2951 kgm−3 for CK1, PK1, andCK2, respectively.

Model values of mass flux are computed from the observed conduitcross-sectional areas and an assumed range of possible eruptionvelocities (Fig. 8c). Our discussion uses mass eruption fluxes anderuption velocities estimated for two end-member styles of explosiveeruption: Hawaiian (fire-fountaining) and Plinian (arrow brackets; Fig.8c). Minimum estimates of mass fluxes and eruption velocities for low-

intensity Hawaiian-style eruptions are 104 kg s−1 and 1m s−1 (Parfitt etal., 1995), respectively; Plinian style eruptions have maximum massfluxes of 109 kg s−1 and eruption velocities of 400 m s−1 (Schmincke,2004). An additional constraint is providedbyassuming that transport ofgas-rich kimberlite magmas is likely to be turbulent (Sparks et al., 2006;Head and Wilson, 2008). In order to sustain turbulent flow in anascending and erupting kimberlite magma, a critical Reynolds' number(Rec∼1–4×103; variablewithflowgeometry)must bemaintained (Rott,1990) requiring a minimum, critical velocity (vc):

vc =Recð Þ � ηdc � ρB

ð4:3Þ

550 S. Moss et al. / Lithos 112S (2009) 541–552

where Rec is the Reynolds number threshold for turbulent behaviourin dykes, η is magma viscosity, dc is characteristic dyke width or pipediameter, and ρB is the bulk density.

Assuming a Rec of 2000 (e.g. Campbell,1985),we have calculated thecritical velocity for turbulent ascent of a melt with a η of 10 Pa s andcharacteristic conduit widths of 0.5, 15 and 1 m for CK1, PK1, and CK2,respectively. Given the ρB calculated above, the corresponding criticalvelocities for sustained turbulentfloware 32.3, 3.3, and 6.8m s−1. Thesecalculated velocities, therefore, provide an independent constraint onthe minimum eruption velocities at A154N (Fig. 8c). These values are inaccordwith those estimated by Sparks et al. (2006) (4–16m s−1) basedon turbulent flow, buoyancy-driven crack-propagation, xenolith trans-port, and minimum velocities required to avoid freezing. Head andWilson (2008) suggest large average pressure gradients (60 kPa m−1)between the source and surface presumed for ascending kimberlitemagmas can lead to even higher minimum velocities (∼30–50 m s−1).These velocity criteria are shown as shaded parallelograms in Fig. 8c.

Given these constraints, several observations can be made regardingeruption behavior at A154N. First, theminimumascent velocities requiredfor turbulent transport of kimberlite magma demand relatively highminimummassflux rates. For example, if a constant velocityof 10ms−1 isassumed, as estimated for kimberlite magma moving through dykes(Sparks et al., 2006), minimum mass fluxes would range from 106 to106.5 kg s−1 (Fig. 8c), several orders of magnitude higher than a low-intensity basaltic fire-fountaining event (e.g. Parfitt et al., 1995; HoughtonandGonnermann,2008). Second, changingventgeometries require eithertransient eruption velocities or transient mass fluxes. If mass flux isconstant for the threeevents (CK1, PK1, CK2), velocitymust increaseat leastone order of magnitude in time from CK1 to PK1 (Fig. 8c) followed by asubsequent decrease in the transition from PK1 to CK2, due to changes inbulk density and conduit geometry. If the three emplacement events atA154N are part of a single eruption cycle, this implies a highly transienteruption style. The eruption of the A154N kimberlitewould have featuredboth waxing (CK1 to PK1) and waning (PK1 to CK2) stages in terms ofvelocityandormassflux.Wenote, however, thatneithervelocitynormassflux are likely to be steady-state parameters in kimberlite eruptions. Inaddition, conduit geometry likely changes during eruption, and mayincrease in cross sectional area and deepen as the eruption continues. Theimplication is that the observed conduit diameters are absolute maxima.

A more-likely, hypothetical velocity-flux path reflecting transientactivity in the emplacement at A154N is shown with numbers (1,2,3)in Fig. 8c. Initial eruption from a fissure dykewould likely beginwith ahigh eruption velocity and high mass flux (Mf) but, as the ventenlarged with time, velocity would decay. However, given that theequivalent-area circular radii of vents at A154N are relatively small(b60m), it is likely that the eruptions (CK1, PK1) remained in a highly-pressurized, “choked flow” state at 615 m below surface for theduration of the eruption, until very low magma supply rates werereached. In this state, gases leaving the vent would expand violentlyupward and sideways, as the pressure of the mixture adjusts toatmospheric pressure, maintaining high eruption velocities.

4.5. Eruption duration calculations

Given the mean density and volume of erupting kimberliteinvolved in the emplacement events, it is possible to determine theminimum eruption times for emplacement using:

t =Vk

Mf � 1ρB

ð4:4Þ

where t is time (in seconds), Vk is minimum volume of eruptingkimberlite (e.g. solid+melt+fluid), Mf is mass flux, and ρB is thebulk density.

The fully erupted volume through conduit CK1 is unknown, andtherefore eruption durations for this stage of emplacement at A154N

cannot be easily estimated. However, given properties of CK1

(Mf=107 kg s−1; v=33 m s−1; ρB=1238 kg m−3) eruption of asimilar volume found in PK1 (VPK1=5.0×106 m3) through the CK1

conduit would take ∼12 1/2 min. Given properties of the pipe-formingeruption of A154N (ρB=405.7 kg m−3; VPK1+Vgas=5.0×107 m3), theportion of the eruption responsible for the PK1 deposit would havelasted at least 16 1/2 h if the style were Hawaiian (Mf ∼105.2 kg s−1 andv∼3.3m s−1). If, however, upon leaving the geometric constraints of theconduit at A154N, the erupting mixture (i.e. mass flux) could becomefully inflated andmatchPlinian-style intensities (v∼400ms−1), thePK1

deposits could be produced in aminimumof∼8min (Fig. 8c). Given themean density and an assumed minimum volume of kimberlite in CK2

above 615 m depth (ρB=2951 kg m−3; VCK2=1.1×105 m3) theemplacement of the late dykes would take place in at least ∼14 min(at 106.5 kg s−1 and 1 m s−1).

Calculated emplacement times of minutes to hours are muchshorter in duration than those hypothesized by Sparks et al. (2006)(hours to days) based on magma supply rate limitations and inferredproperties of kimberlite magmas. However, kimberlite eruptionslikely eject more kimberlite than is preserved intra-vent (e.g. Sparkset al., 2006; Porritt et al., 2008); thus, the emplacement times forPK1 are likely a small fraction of the true, total eruptive times for theentire A154N system, and may be up to an order of magnitudelonger.

If extreme ascent velocities (e.g., 50 m s−1 vs. 1 m s−1) areassumed for end-member portions of a single magma pulse (gas-richvs. gas-depleted) interpreted to result from phase separation, amaximum total time interval between CK1, PK1, and CK2 can also becalculated. Gas-rich (∼50 m s−1) and gas-depleted (∼1 m s−1)portions of a single magma would ascend 200 km in approximately∼1 h and ∼55 h, respectively, leading to – at most – a time interval of∼2 days for the emplacement of CK1, PK1, and CK2.

5. Emplacement model for the A154N kimberlite

We interpret the three events at A154N (CK1, PK1, CK2) to recordemplacement of a separated three-phase flow involving kimberlitemelt, crystals and a proportion of exsolved CO2–H2O fluid (Fig. 9). Wesuggest the timing and nature of the emplacement style (fissure dyke,pipe excavation, intrusive dykes) is a direct result of varying physicalproperties (crystal, melt and gas content) of the magmas supplied tothe A154N context. First, a transient, low-volume, gas-charged (i.e.CO2, H2O) fissure eruption of olivine-poor kimberlite magmaemplaced CK1. Breach of the surface would immediately lower thepressure in the ascending magma, triggering rapid downward move-ment of a de-pressurization wavefront, allowing for exsolution ofvolatile phases (e.g. CO2 and H2O), wall rock erosion and therefore achange in the vent geometry.

Second, a sustained (i.e. high flux, long duration) eruption ofvariable gas and olivine content excavated the A154N pipe anddeposited the PK1. De-pressurization of the magma eruptingthrough the fissure dyke would initially accelerate the eruptionvelocity, lower the bulk density, and enhance magma fragmenta-tion. Late-exsolving gases within magma whose crystal contentwas increasing with time remain coupled to the melt, and rapidlyexpand as the magma approaches the surface, eventually over-pressurizing the system, thus leading to explosive excavation ofsurrounding country rock. The degree of overpressure in themagma prior to eruption directly determines the initial ventdiameter and geometry (Fig. 9). Wall rock erosion would initiate atzones of structural weakness or lithology contrasts within thecountry rock, explosively widening the fissure dyke where theseweaknesses intersect. This explosive vent-widening focuses theerupting mixture of gas, solids and liquid from the fissure dykeinto a circular vent, which becomes the path of least frictionalresistance. The sustained eruption proceeds to “drill” down into

Fig. 9. Progressive model for emplacement and eruption of A154N. Upper panel (a) shows the expected evolution (relative time) in olivine and volatile (CO2+H2O) contentsthroughout the eruption cycle. The lower panel (b) denotes the major changes in the nature (melt:solid:fluid ratios) of the erupting magma, and the size and shape of the conduit/feeder system. Panels schematically portray presence and proportions of fluid, olivine and carbonate content for CK1, PK1, and CK2. Stages include: (1) pre-cursor dyke and fracturesystem (CK1) is irregular in orientation, and contains intermittent intervals of coherent kimberlite of variable thickness and country-rock content; (2) PK1 emplacement initiallyexcavates a crater with width (w), and systematically excavates deeper (z) into the country-rock with time; (3) late, crystal-rich CK2 only ascends to 0 m asl (i.e., to within 415 mbelow the surface).

551S. Moss et al. / Lithos 112S (2009) 541–552

the country rock, efficiently excavating down to N750 m belowsurface.

Third, non-eruptive, olivine-rich and gas-poor coherent kim-berlite (CK2) was passively emplaced into the interior of PK1 andalong the contacts between PK1 and adjacent country rock. Theterminus of the intrusions within PK1 represent levels of neutralbuoyancy for the coherent kimberlite magmas: the mixture ofliquid, crystals and gas was not buoyant enough to rise to thesurface (ρmagmaNρcountry rocks), despite the relatively low densityand semi-permeable texture of the fragmental kimberlite it wasascending through. This event may have significantly post-datedthe eruption which formed the A154N pipe, but the time gapbetween events cannot be further constrained.

Acknowledgements

This research was funded by an NSERC Collaborative Research andDevelopment Grant (22R42415) held by J.K. Russell and sponsored byRioTinto Diavik Diamond Mine, Inc. (DDMI), entitled Kimberliteeruption dynamics: Implications for diamond distribution in the Diavikkimberlite. It is Mineral Deposit Research Unit publication number P-227. We gratefully acknowledge critical reviews by Lucy Porritt, RoyEccles, and Sonja Aulbach, as well as constructive conversations with

Barbara Scott-Smith, Gus Fomradas, Roger Young, and Bram vanStraaten.

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