A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada Lucy Anne Porritt, James Kelly Russell, Hayley McLean, Gus Fomradas, and David Eichenberg
Abstract
The A418 kimberlite pipe, Northwest Territories, Canada, has a typical downward tapering morphology, has been explored to a depth of *600 m where the pipe has a diameter of *50 m, and is infilled by volcaniclastic deposits. The pipe-filling volcanic succession has a minimum volume of *6 9 106 m3 and comprises structurally diverse deposits including finely bedded, variably bedded and massive. The finely bedded volcaniclastic deposits are dominated by surge-like beds containing abundant ash aggregates. Petrographic and geochemical characteristics of the volcaniclastic deposits indicate little contamination by the country rock granite; thus, the pipe excavation phase of the eruption is not recorded by the infill. The lack of preserved within-pipe deposits associated with the excavation of the pipe requires an initial highly energetic explosive eruptive phase that completely cleared the pipe and dispersed material away from the vent. Subsequently, the eruption intensity waned allowing the pipe to progressively infill over time; the character of the pyroclastic deposits requires phreatomagmatic eruptive activity. In the upper reaches of the pipe, a massive poorly sorted deposit cross-cuts and buries the surge deposits with intercalated or gradational contacts between the two facies. The surge beds all dip inwards towards the massive deposit. Repetition of these lithological units at depth indicates a recurring sequence of phreatomagmatic surge deposits cross-cut and overlain by massive pyroclastic or debrisflow deposits, the latter forming during lulls in the phreatomagmatic eruptive activity. Keywords
Kimberlite geology Kimberlite eruption Whole rock geochemistry
Phreatomagmatic
Pipe-clearing
Pipe-filling
Introduction L. A. Porritt (&) J. K. Russell Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada e-mail:
[email protected] H. McLean G. Fomradas D. Eichenberg Diavik Diamond Mine, Yellowknife, NWT, Canada Present Address: L. A. Porritt School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Vancouver, Bristol BS8 1RJ, UK
The nature and style of kimberlite eruptions has been hotly contested since at least the late 1970s. This debate is largely fostered by the fact that there are no recent eruptions of kimberlite and, thus, our interpretations are based on the properties of the deposits themselves. At the core of this debate is the issue of whether kimberlite are mainly products of magmatic (Scott Smith 1999) or phreatomagmatic (Lorenz et al. 1999) processes. It is more likely that, as with
D. G. Pearson et al. (eds.), Proceedings of 10th International Kimberlite Conference, Volume 2, Special Issue of the Journal of the Geological Society of India, DOI: 10.1007/978-81-322-1173-0_7, Ó Geological Society of India 2013
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all other magma types, there is a spectrum of eruptive styles for kimberlite magmas (Sparks et al. 2006; Cas et al. 2008). Indeed this is broadly supported by the recognition of different types or classes of kimberlite (Skinner and Marsh 2004; Scott Smith 2008) where pipe morphology and infill are considered to reflect the influence of country rock geology and eruption style. The familiar steep-sided pipe morphology (80–85°) common in Southern African (Type 1) and Lac de Gras (Type 3) kimberlites suggests that the pipe dimensions reflect the strength of the country rocks, the volume of erupted magma and the longevity and intensity of the eruption. Type 3 pipes are smaller and likely result from smaller volumes of erupting magma giving shorter eruption durations. The abundance of residual country rock material found within the pipe infilling deposits reflects the efficiency of the eruption to excavate and clear the vent; highly energetic eruptions result in low abundances (Porritt and Cas 2011) and less energetic eruptions allow for high abundances (Porritt and Cas 2009). In many eruption models, pipe formation and pipe filling are considered to be concurrent with the eruptive activity excavating the pipe while also filling it (Lorenz et al. 1999; Scott Smith 1999; Kurszlaukis and Lorenz 2008). Other models suggest excavation occurs during the waxing and peak magma flux whilst filling occurring during the waning stage of the eruption (e.g. Sparks et al. 2006; Brown et al. 2009; Porritt and Cas 2009). Here we use the structures and properties of deposits preserved within the A418 pipe to reconstruct the eruption style and intensity of this volcano. The deposits are well preserved and exposed through diamond drilling and mining. The infilling volcanic succession is unique in that most of the deposits are pyroclastic in origin; there are no discernible hiatuses or sedimentary intercessions and many of the deposits are well bedded and feature abundant accretionary lapilli.
The A418 Pipe The A418 kimberlite pipe is a member of the Lac de Gras kimberlite field located on the northern Canadian Slave Craton (Fig. 1). The pipe is Eocene in age, 55–56 Ma (Amelin 1996; Moser and Amelin 1996) and was emplaced into late Archean granitoid rocks (Stubley 1998; Graham et al. 1999). Phanerozoic strata are not observed in the area, though clasts of marine shales and sandstones commonly preserved within the Lac de Gras pipes provide evidence that a 50–300 m thick sedimentary succession was overlying the Archaean basement at the time of kimberlite emplacement (e.g. Stasiuk et al. 1999; Sweet et al. 2003). Wood found within the A418 pipe and within the adjacent Ekati pipes, which are of a similar age, suggests that the
L. A. Porritt et al.
pipes were emplaced into a humid, temperate and terrestrial environment (Nassichuk and McIntyre 1996; Nowicki et al. 2004). The A418 kimberlite pipe is characterised by finely bedded deposits containing abundant ash aggregates (coated ash pellets and accretionary lapilli, cf. Brown et al. 2010). Ash aggregates form due to the adhesion of ash particles which typically involved condensed water (Durant et al. 2009, 2010) and are rarely reported in kimberlite pipes. Detailed drill core logging and mapping of surface exposure of the active open pit show the deposits to be continuous at depth. Here we use a combination of thin section and XRF analysis to provide the detailed componentry needed to constrain the origins of these deposits.
Geology Pipe Morphology At the present-day surface, the A418 pipe is roughly ovoid in shape, has a diameter of *125 m and a surface area of *11,500 m2. The pipe has been delineated to *600 m depth by drilling, where it tapers to *\100 m in diameter (Figs. 2, 3). The observed pipe volume is *6 9 106 m3; an uncertain volume of the original volcanic edifice has been removed due to glacial erosion but is likely on the order of *1.2–2.5 9 106 m3 as estimated by projecting the pipe upwards by 100–200 m, respectively. Observations in the open pit show the irregular but steeply dipping pipe walls (*88°) to be scalloped and smoothed with little to no country rock breccia along the contact. The dominant country rock comprises a two mica granite containing xenoliths of older metaturbidites, this is cross-cut by Proterozoic dolerite dykes in the vicinity of the pipe (2590–2580 Ma, Stubley 1998) (Fig. 1). The A418 pipe is dominantly filled with volcaniclastic deposits comprising four main lithological units: (1) finely bedded ash aggregate-rich lapilli tuff (FBK); (2) massive, poorly sorted mudclast and olivine-rich lapilli tuff (MK); (3) variably bedded (diffusely to well bedded) lapilli tuff which is transitional in character between 1 and 2 (VBK); and (4) massive kimberlitic mudstone (MUD). Minor coherent kimberlite (CK) and pyroclastic kimberlite rich in coated olivine grains or juvenile lapilli (PK) are observed in drill core; however, these are not very continuous and the emplacement origins are undetermined.
FBK The FBK is a diffusely to well bedded, ash aggregate and olivine-rich volcaniclastic kimberlite. Bedding occurs at millimeter to centimeter scale and is defined by distinct
A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada Water Kimberlite Field Palaeozoic Cover Proterozoic Cover Archean Craton
110° 15’
A154N A154S
66˚ N
Jericho Field
Lac de Gras Field
64° 30’
Fig. 1 Maps showing the location of the Diavik Diamond Mine. a The regional geology and modified from Stubley (2004) b the local geology and location of kimberlite pipes in the Diavik mine plan (from Stubley 1998)
99
A418
DDMI 64˚ N
YELLOWKNIFE
Two-mica granite
100 km
(a)
114˚W
110˚W
Tonalite to quartz diorite with abundant pegmatites Metaturbidites Kimberlite in mine plan
East Island, Lac de Gras
(b)
MK MUD
FBK
9200 L
PK VBK
9100 L MK CK 9000 L Sea Level
8900 L
FBK
8800 L WEST
Dolerite dyke Fault
1 km
9400 L
9300 L
A21
NORTH
Fig. 2 The pipe morphology and internal geology of the A418 pipe taken from the Diavik Diamond Mine 3D model. No vertical exaggeration
variations in grain size and componentry (i.e. vol%, Fig. 4). The individual beds are moderately well sorted and preserve depositional structures such as grading and cross bedding. The major components, in order of abundance, are olivine grains (broken to unbroken, fresh to moderately altered, \0.5–10 mm), ash aggregates typically defining bedding (1–5 mm) comprising coated ash pellets, ash pellets and accretionary lapilli (using the terminology of Brown et al. 2010) including broken rim fragments, and black kimberlitic mudstone clasts (1–40 mm, with fluidal morphologies). Minor components include garnet and chrome diopside (kimberlite indicator minerals) and mantle xenoliths, clasts of volcaniclastic kimberlite, granite clasts, metasediment clasts and wood fragments. Crustal lithic fragment abundance ranges from\5 to 10 %, and is dominated by kimberlitic mudstone. Two stratigraphic units of FBK occur within the pipe, the uppermost is exposed within the open pit. Here the bedding is steeply dipping (*45° but up to 80°) inwards towards the within-pit exposure of MK (Fig. 3), minor slumping and syn-depositional faulting are observed. A second FBK unit is also observed at depth within the pipe (Fig. 2) where the fine-scale bedding and steep dips persist; however, the bedding orientations here cannot be determined from the unoriented or vertical drill core. The finely bedded character of this lithology and the pervasive abundance of ash aggregates suggest transport and deposition by dilute pyroclastic density currents (surges) and fall combined with minor resedimentation. This strongly supports a pyroclastic phreatomagmatic origin.
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L. A. Porritt et al.
Fig. 3 Map of the A418 open pit at the 9320 m level showing the distribution of lithologies, dips of bedding and locations of the pit photos (a–c), white lines in (a) indicate bedding planes and white arrows in (c) indicate dip direction of beds
MK
Two stratigraphic units of MK are found within the pipe (Fig. 2). The uppermost forms a downward tapering deposit The MK is a massive poorly sorted mudstone clast- and which appears to cross-cut and overlay the finely bedded olivine-rich volcaniclastic kimberlite (Fig. 4). Diffuse layer- kimberlite, though contacts are gradational and not well ing is also observed and can be seen in the within-pit expo- preserved. A second body of MK occurs deeper within the sure. Major components in order of abundance comprise: pipe underlying the VBK and overlying the lowermost unit olivine grains (20–50 %, broken to unbroken, fresh to altered, of FBK (Fig. 2). The eruptive and depositional processes \0.5–20 mm); mudstone clasts (10–20 %; 1–500 mm often for these lithological units are somewhat ambiguous, as is having fluidal morphologies); juvenile pyroclasts (\10 %, whether they represent the same depositional event or not. olivine grains with thin to thick coats of coherent kimberlite); Recycling of earlier kimberlite deposits through mass ash aggregates scattered throughout (\10 %, 1–5 mm, coated wasting or ongoing vent-in-vent eruption is suggested by ash pellets, ash pellets and accretionary lapilli, including the massive poorly sorted nature of the deposits and the broken rim fragments); volcaniclastic kimberlite clasts entrained clasts of volcaniclastic kimberlite. (4–100 mm, sub-rounded to fluidal, often containing ash aggregates) and granite clasts (5–450 mm, angular to subrounded). Minor components include kimberlite indicator VBK minerals and mantle xenoliths, metasediment clasts, dolerite clasts and wood fragments. Lithic fragment abundance is The VBK is transitional in character between the FBK and MK comprising variable thickness beds from mm to metre 10–20 % and is dominated by kimberlitic mudstone.
A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada
101
Fig. 4 Polished slabs showing examples of FBK (a); MK (b); and a photomicrograph showing an accretionary lapillus (c) (scale bar = 1 mm)
scale. The thin beds resemble the bedding in the FBK and are also considered to be surge deposits, whereas the thicker beds are massive and poorly sorted, and appear to be matrix supported debris flows or pyroclastic density current deposits. Underground exposure of VBK in the mine workings has revealed sub-vertical bedding, indicating that the whole sequence is likely steeply bedded at depth. The VBK unit appears to represent a transition from intense phreatomagmatic surge deposition to a period dominated by sedimentary reworking accompanied by periodic explosive activity.
MUD MUD is a black to dark grey, massive and silt to fine sand containing low, but variable, abundances of kimberlite components including olivine and other indicator minerals. Faint outlines of ash aggregates are occasionally observed. Several MUD deposits occur along the upper pipe margins (Fig. 2a) and as large domains (or mega clasts) within the pipe at depth. The composition of MUD is highly variable; however, there is always a pervasive kimberlitic component. Fluidal black mudstone clasts found within MK and FBK appear to be composed of the same (i.e. kimberlite derived) material. It is unclear whether this material is related to the eruptive sequence at A418 or whether it represents pre-existing kimberlitic mud associated with the surrounding volcanic centres, though the occurrence of
clasts of MUD within all other lithologies supports the latter suggestion. The fluidal shape of MUD clasts within the other lithologies suggests that the mud was wet at the time of incorporation; it is likely that the interaction of magma and these wet sediments was the driving force behind the phreatomagmatism (White 1991).
Ash Aggregates The dominant pyroclasts found within the A418 pipe are ash aggregates (Fig. 4c; Gilbert and Lane 1994; Brown et al. 2010). The ash aggregates are pervasive within the volcanic succession, although they are less abundant in the MK relative to the FBK. Within certain horizons of the FBK, they may constitute up to 80 % of the deposit. The most common type of ash aggregate is the coated ash pellet (using the terminology of Brown et al. 2010); these have an unstructured ash pellet at the core coated by thin layer of finer grained ash. Ash aggregates occur in a variety of different volcanic settings but typically form through electrostatic attraction and moist adhesion of ash particles (e.g. Schumacher and Schmincke 1991, 1995; Gilbert and Lane 1994; Brown et al. 2010; Durant et al. 2010). In conventional pyroclastic deposits, ash aggregates are not uncommon and generally indicative of magma–water interactions; however, they are not commonly documented in kimberlite deposits (Kurszlaukis and Barnett 2003; Porritt and Cas 2009; van Straaten et al. 2009). They may be
102
uncommon in kimberlites because most kimberlite bodies comprise deposits formed and preserved deep within the conduit. In contrast, ash aggregates generally form in eruption plumes, ash clouds and density currents and are typically deposited away from the vent. At A418, however, ash aggregates occur deep within the conduit within beds that appear to be in situ. In these kimberlitic deposits, coated ash pellets typically range from 1 to 5 mm in diameter, though may be [12 mm, and the internal grain size of the ash components of the coated pellets vary considerably (Fig. 4c). The ratio of the diameter of the core to the width of the rim of the coated pellets is variable but averages approximately 10:1. In decreasing abundance, other ash aggregates present are: ash pellets which are similar to coated ash pellets but do not have the finergrained ash rim, accretionary lapilli with multiple finegrained rims (Fig. 4c); and cored ash pellets with a lithic fragment at the core. The ash aggregates are found in beds or deposits which themselves have variably sized components including a fine-grained matrix. The ash aggregates can be the largest components within the beds but may also be equal to or smaller than the average grain size of the host deposit. Often the black fluidal mudstone clasts present within the deposits are found to contain abundant ash aggregates on closer inspection. Broken coated ash pellets are seen in several horizons indicating they were hard and brittle at the time of incorporation into the deposit, and that reworking of the aggregates occurred. In other horizons, remnant disaggregated ash pellets occur which indicate that they were soft and likely moist at the time of deposition. In general, the ash aggregates are sub-spherical and typically do not show any signs of deformation through compaction.
L. A. Porritt et al.
XRF Results In general, the chemical compositions of the MK samples show much less variability than displayed by samples of FBK and MUD (Fig. 5; Table 1). The MK samples show a slight enrichment in both Al2O3 and SiO2 compared to the average CK composition. The SiO2 and Al2O3 contents of the FBK and MUD are much more variable than the MK and form an apparent trend from CK to the average host rock granite composition. However, when considering the Na2O abundance in these samples, the influence of granite on the composition is negligible. The source of SiO2 and Al2O3 is not from incorporation of granite but more likely a surficial mudstone such as that invoked by Nowicki et al. (2008) to account for similar observations in the Ekati kimberlites. Importantly, this trend supports the observed low abundance of granite clasts within these deposits and does not indicate the presence of any finely comminuted granite. All of the samples are chemically depleted in incompatible elements such as Ti, Nb and V relative to average CK. The sole exceptions are two MUD samples having significantly elevated V contents. A slight enrichment in Ni compared to the average CK is observed in the MK samples (Fig. 5) with a more variable Ni content in the FBK and MUD samples. Elevated Ni content indicates an increased abundance of olivine, and depleted incompatible elements are thought to correspond to a loss of groundmass or melt component in the form of fine ash (Nowicki et al. 2008).
Discussion The Case for Phreatomagmatism
Geochemistry A total of 140 samples of MK, FBK and MUD were analysed for their whole rock major element chemical compositions by X-ray fluorescence (XRF). The majority (117) of samples collected from the 9285 level bench were chemically analysed by SRC Lab, Saskatoon. The remaining 23 samples are from drill core and were collected from varying depths. These were analysed for major and trace element abundance by XRF at ALS Minerals, Vancouver. An additional ten samples of granitic host rock from drill core were analysed by Acme Labs, Vancouver, for major and trace element abundance by XRF; the average is presented in Fig. 5 and shown in Table 1. The composition of the average coherent kimberlite (CK) from Ekati kimberlites in the Lac de Gras field taken from Nowicki et al. (2008) is also presented for comparison.
The volcaniclastic deposits infilling the A418 pipe, particularly the FBK, are characterised by fine-scale bedding and structures consistent with deposition by tractional sedimentation from dilute pyroclastic density currents (or surges), intercalated with volumetrically minor beds derived from fallout and resedimentation by debris flows. Perhaps the most distinctive feature of the well-bedded volcaniclastic deposits is the pervasive and abundant presence of ash aggregates. Though ash aggregates are not always indicative of phreatomagmatic activity (Watanabe et al. 1999; Brown et al. 2010) moisture is required to facilitate efficient aggregation of fine ash (e.g. Gilbert and Lane 1994; Costa et al. 2010). The combination of bed forms and ash aggregates strongly suggests the involvement of surface or groundwater during the eruption (i.e. phreatomagmatism). The ash aggregates also attest to the presence of sub-aerial dilute eruption clouds or plumes. These plumes would be
A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada 80
350
70
300
60
250
50
V (ppm)
SiO2 (wt %)
Fig. 5 Plots showing the XRF results of the A418 samples
103
40 30
200 150
20
100
10
50
0
0
5
10
15
0
20
0
5
Al2O3 (wt %) 4.0
15
20
300
3.5
250
3.0
Nb (ppm)
Na 2O (wt %)
10
Al 2O 3(wt %)
2.5 2.0 1.5
200 150 100
1.0 50
0.5 0.0
0 05
10
15
20
0
5
10
15
20
Al 2O 3(wt %)
Al2 O3 (wt %) 1.2
2000
1.0
1600
MK
0.8
Ni(ppm)
TiO 2 (wt %)
Legend FBK
0.6 0.4
1200
MUD Avg granite host
800
Avg CK (Nowicki) 400
0.2 0
0 0
5
10
Al2O 3(wt %)
within the pipe (expanding) and also above the ground surface (expanded). Previous models invoked for the phreatomagmatic eruption of kimberlite are based on maar-diatreme volcanoes and employ a progressive, incremental excavation process from the top downwards. In this model, the locations of the conduit excavating explosions move deeper throughout the eruption (e.g. Lorenz and Kurszlaukis 2007; Kurszlaukis and Lorenz 2008). The pipe forms through wall collapse as juvenile and accessory material is transported through the pipe fill via debris jets and ejected in small volume at the surface, creating space at the base of the pipe for the earlier deposits to collapse into. However, this mechanism does not allow for the efficient removal of the host rock during pipe formation and wall stabilization prior to the deposition of the pyroclastic infill, which are both key observations at A418. The paucity of granite within the deposits as shown macroscopically and through the geochemical signature of the rocks, indicate that the eruption/deposition of the pipefilling deposits must have occurred subsequent to a pipe
15
20
05
10
15
20
Al2O3 (wt %)
opening and clearing stage of the eruption. Deposits recording this earlier episode are not found within the pipe, however, the size of pipe and its smoothed and scalloped walls attest to a highly energetic event. The Surtseyan style of phreatomagmatic eruptions (e.g. Kokelaar 1983; Cole et al. 2001) has not previously been considered as a style for kimberlite eruptions, perhaps because the pipe forming and pipe-filling processes have always been considered to be concurrent and diatremes are not associated with Surtseyan volcanoes. Surtseyan eruptions are those where water gains access to the top of the vent (Kokelaar 1983), which rapidly transforms to magmatic activity as a tephra cone builds and the vent is sealed off from the water source (typically the ocean but can also occur within lakes). At A418, the presence of fine-scale, sub-vertical bedforms and abundant accretionary lapilli indicate that the vent was not filled with water during the formation of these deposits. However, it is more likely that some water or wet sediments (such as the MUD material) accessed the vent and interacted with the rising magma giving rise to a fuel–coolant interaction, producing a
104
L. A. Porritt et al.
Table 1 Whole rock chemical compositions of select samples from the A418 kimberlite pipe including major (wt.% oxides) and trace element (ppm) abundances and the loss on ignition (LOI) Label
LP-03
LP-05
LP-06
LP-16
LP-02
LP-09
Avg CK
Avg granite
Type
FBK
FBK
MK
MK
MUD
MUD
CK
Granite
34.7
40.8
53.7
37.5
Major elements (wt.%) SiO2
37.0
38.4
TiO2
0.41
0.73
0.37
0.44
Al2O3
2.87
4.58
3.50
2.13
Fe2O3(T)
6.97
6.11
7.05
7.31
0.55 11.7 5.27
MnO
0.12
0.09
0.08
0.12
0.05
MgO
31.60
21.40
26.70
31.30
3.23
29.68
72.57
0.81
0.93
0.18
4.92
1.83
14.91
6.64
8.65
1.43
0.11 21.6
0.18
0.02
34.87
0.39
CaO
4.83
6.86
2.28
4.51
0.69
7.13
9.99
0.95
Na2O
0.06
0.13
0.07
0.08
0.28
0.17
0.03
3.72
K2O
0.34
0.82
0.46
1.14
2.08
1.22
0.51
4.82
P2O5
0.18
0.31
0.19
0.27
0.10
0.42
0.60
0.24
Cr2O3
0.16
0.17
0.20
0.22
0.01
0.20
N/A
0.00
SrO
0.06
0.09
0.03
0.06
0.02
0.10
N/A
N/A
BaO
0.12
0.25
N/A
N/A
LOI
13.35
22.3
0.20
17.4
0.07
14.3
0.15
20.1
0.06
18.05
12.83
0.62
Totals
98.1
98.5
99.2
100.5
97.8
99.1
100.1
99.9
Selected trace elements (ppm) Ni
1225
711
1375
1530
43
896
1352.5
6.7
Nb
87.9
138
61.1
108
30.3
154.5
237.5
5.8
V
62
99
70
77
307
121
119
11.9
Avg CK is the average composition of Lac de Gras coherent kimberlite taken from Nowicki et al. (2008). Avg Gran is the average of ten samples analysed in this study
phreatomagmatic Surtseyan-like eruption style (Kokelaar 1986; White 1996; Schipper et al. 2011).
Eruption Model: Magmatic to Phreatomagmatic Transitions Pipe Clearing The paucity of granite country rock within the pipe fill at A418 indicates that the pipe was completely emptied during the eruption. The scalloped and smoothed pipe walls indicate that the excavation of the pipe reached an equilibrium status where the material exiting the vent/conduit was polishing the pipe walls rather than extensively eroding them. The pipe volume is *6 9 106 m3, but the total volume of excavated country rock would have been slightly larger to account for the now eroded upper portion of the pipe (*1.2–2.5 9 106 m3 extra). We assume that the volcanic eruption which excavated the pipe was rapid and continuous due to the abraded scalloped nature of the pipe walls and efficient removal of the country rock which reflect high mass fluxes. To provide an approximation of the time scale of the eruption we can
employ estimates of the rates of vent widening in plinian eruptions, which range from 4 cm/min (Varekamp 1993) to 33.3 cm/min (or 20 m/h; Macedonio et al. 1994). Assuming an initial start diameter of 1 m, vent widening at these rates to the current diameter of 125 m would take between 3 and 26 h; which is at the lower end of total eruption durations suggested by previous authors (Sparks et al. 2006; Lorenz and Kurszlaukis 2007; Porritt and Cas 2009). We can combine the above estimates of volume removed and vent widening rates into mass flux estimates to compare the scale of the volcanic eruption to that of historic eruptions. The total volume of erupted material (VE) is the sum of the volume of kimberlite erupted (VK) and the volume of granite erupted (VG). We know VG (6 9 106 m3), and can predict the total volume based on the proportion of granite in the eruptive products (a). VE ¼ VG þ ð1 aÞVK
ð1Þ 3
Assuming the density of granite at 2600 kg/m and the density of kimberlite at 2800 kg/m3 we can translate volume erupted to mass erupted (ME) using Eq. (2) (Fig. 6). ME ¼ 2600 VG þ 2800 ð1 aÞVK
ð2Þ
Volume erupted (V E) millions m3
A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada 500 400 300 200 100 0 0
10
20
30
40
50
Volume excavated (V G) millions m3
Log of mass erupted (ME) kg
13 Plinian eruptions 12
11
100 % 75 % 50 %
10
25 % 10 %
9 9
10
11
12
Log of mass excavated (MG) kg
Fig. 6 Plot of the total mass erupted against mass of granite excavated. Lines plotted to show the total mass erupted as a function of abundance of country rock in the erupted product ranging from 100 % (all country rock) to 10 % country rock, 90 % juvenile material. Verical dashed line indicates mass of granite excavated to form the A418 pipe
When plotted (Fig. 6), we can see that the products of this phase of the eruption had to comprise \10 % granite lithic fragments in order to be comparable in mass to the lower range of total erupted masses for plinian eruptions (2.0 9 1011 kg) given by Carey and Sigurdsson (1989). We then consider a range of mass eruption rates based on the total mass erupted, ME, and the duration based on the erosion rates. This gives a range of 1.44 9 106 to 1.47 9 107 kg/s (based on 100 to 10 % granite with duration of 3 h) or 1.67 9 105 to 1.78 9 106 kg/s (based on 100 to 10 % granite with duration of 26 h). These mass flux rates do not consider the gas fraction and are comparable to the lower end of the range of peak intensities of plinian eruptions, 1.6 9 106 to 1.1 9 109 kg/s (Carey and Sigurdsson 1989; Sparks et al. 2006). High mass fluxes during the waxing eruption likely prevent the significant interaction of kimberlite magma with any surface- or groundwater, and pressurise the vent preventing in-vent deposition of tephra and influx of groundwater.
Pipe Filling The earliest pipe-filling deposits record phreatomagmatic activity. In comparison with the established model for
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phreatomagmatic kimberlite eruptions (e.g. Lorenz 1993; Lorenz and Kurszlaukis 2007; Kurszlaukis and Lorenz 2008), at A418 the deposits clearly lack country rock lithics. This indicates that pipe excavation occurred prior to the formation of these deposits. A decrease in mass flux, during waning of the eruption enabled the interaction of the magma with water; ground and meteoric water, wet sediment (MUD material), or a combination. The decrease in flux marked the onset of pipe-filling by surges (FBK and VBK) and debris flows (VBK), and the formation of ash aggregates within the pipe occupied by a turbulent, low density eruption cloud of ash, lapilli and condensing water vapour. Bedding at depth within kimberlite pipes has been attributed to within-vent fluidisation and subsidence of beds from the crater, pyroclastic flows and resedimentation of crater rim deposits. Both the fluidisation (Sparks et al. 2006; Walters et al. 2006; Gernon et al. 2008, 2009) and debris-jet (e.g. Ross and White 2006; Lorenz and Kurszlaukis 2007; McClintock et al. 2009) mechanisms for kimberlite formation account for bedding within diatremes occurring through the formation of beds at the surface and then subsidence and inward rotation as more material is removed from below and deposited on top. The destruction of bedding occurs at depth and towards the centre of the pipe, where material is fluidised or recycled. Prolonged eruptions occurring in both these manners could theoretically cleanse the pipe of all of its country rock material via continued dilution of each newly deposited layer by juvenile material, recycling of this country rock depleted material and ongoing addition of juvenile material. However, at A418 we do not observe any increase in abundance of granite clasts with depth in the FBK as would be expected in such recycling cells. Also, the presence and relatively consistent abundance of mudstone clasts cannot be readily accounted for by such a model where only granite is selectively removed. Here we do not favour the subsidence of finely bedded material from the surface, but consider the bedding to be in situ. Steeply dipping beds are known to occur in phreatomagmatic deposits where surges have encountered obstacles, ramped up and over them, and plastered deposits against the obstacle (Cole et al. 2001; Brand and Clarke 2009) and a similar depositional process is favoured here. Dilute pyroclastic density currents are generated by explosions deep within the pipe, as they run-out upwards along the pipe walls they rapidly lose energy and deposit, adhering to the walls and gradually filling the pipe. As the pile of deposits builds it may have developed into a recycling cell similar to that proposed by Lorenz and Kurzlaukis (2007), with a central debris jet or vent; however, importantly this pipe-filling occurs after the distinct pipe-clearing phase. The origin of the MK is somewhat more enigmatic. The uppermost unit of MK appears to form a vent-like structure
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within the surrounding FBK, with the FBK bedding dipping towards the in-pit exposure of MK. Recycled clasts of FBK within the MK and the presence of juvenile lapilli could be indicative of a second explosive eruption through the existing pipe-filling volcanic pile, or represents the concomitant vent deposits of the FBK forming eruption. How this relates to the lower MK unit is uncertain. The MUD units which appear attached to the pipe walls are also enigmatic in their origin. Geochemical evidence indicates that this fine-grained material, though variable, contains a significant kimberlitic component. Fluidal clasts of MUD material are incorporated within the MK and FBK deposits indicating that this material, or at least an equivalent facies, was present within the pipe and immediate surroundings prior to the FBK forming eruption. This material is likely to have sloughed into the pipe as the pipeclearing activity waned, and probably provided the source of water to drive the subsequent phreatomagmatic activity. The best documented maar-diatreme eruption occurred in 1977 at the Ukinrek maars in Alaska (Kienle et al. 1980; Self et al. 1980) where two maars and a lava dome developed over the course of 10 days of volcanic activity. A total dense rock estimate for the juvenile material ejected during the explosive phases of the eruption is 0.5 9 106 m3 (Self et al. 1980). Estimates of average tephra production rates for the explosive phase of the Surtsey eruption in 1963–1964 are 40 m3/s (Thorarinsson 1965), which produced a *500 9 106 m3 of tephra over several months of activity. The minimum estimate of the volume of ejecta from the phreatomagmatic infill phase of the eruption of A418 is 6 9 106 m3. Using Ukinrek as a model, we estimate a minimum duration of the infill phase for the A418 eruption of 120 days. However, using Surtsey tephra production rates as a model, we can estimate production of the A418 tephra in *40 h. Despite this large range, due to differences in magma supply rate, water supply rate (both transitioned to effusive eruptions once they dried out) and volume for Ukinrek and Surtsey, eruption duration of days to months for the infilling phase of A418 is considered plausible.
Summary The eruption of A418 is considered to have occurred in several phases: (1) a rapid and violent vent opening and clearing phase which removed the majority of the granite from the pipe and system, and shaped and smoothed the pipe walls. Deposits associated with this phase are not preserved so the style of the eruption is unknown though calculated mass eruption rates are comparable to Plinian eruptions. The duration of this phase is estimated to be between 3 and 26 h; (2) a more prolonged post-conduit
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forming phreatomagmatic phase which is recorded by the oldest preserved deposits, the lower FBK. The transition from pipe excavation to surge deposition is considered to represent the waning stage of the eruption where decreasing magma flux facilitated the explosive interaction of magma and water ± wet MUD sediment; deposition occurred within an open vent filled by a dilute eruption plume; (3) a third possible phase forming the crosscutting MK which contains clasts of the FBK. The total infill phase is estimated to have taken several days to months. Acknowledgments We thank the staff at Diavik Diamond Mine for all their logistical and geological help. The research was carried out with funds from an NSERC CRD grant. LP acknowledges the support of funding from the Canadian Bureau of International Education and a Marie Curie Outgoing Fellowship. We also thank Rich Brown, Stephen Sparks and Jenny Gilbert for their constructive reviews which have greatly improved this contribution.
References Amelin Y (1996) Report on Rb-Sr and U-Pb study of kimberlite samples VR44444A through VR44465A. Confidential report for Kennecott Canada Inc Brand BD, Clarke AB (2009) The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: from a Surtseyan to an energetic maar eruption. J Volcanol Geotherm Res 180:203–224 Brown RJ, Branney MJ, Maher C, Dávila-Harris P (2010) Origin of accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife. Geol Soc Am Bull 122:305–320 Brown RJ, Tait M, Field M, Sparks RSJ (2009) Geology of a complex kimberlite pipe (K2 pipe, South Africa): insights into conduit processes during explosive ultrabasic eruptions. Bull Volcanol 71:95–112 Carey S, Sigurdsson H (1989) The intensity of plinian eruptions. Bull Volcanol 51:28–40 Cas RAF, Hayman PC, Pittari A, Porritt LA (2008) Some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective, and some suggestions. J Volcanol Geotherm Res 174:209–225 Cole PD, Guest JE, Duncan AM, Pacheco JM (2001) Capelinhos 1957–1958, Faial, Azores: deposits formed by an emergent Surtseyan eruption. Bull Volcanol 63:204–220 Costa A, Folch A, Macedonio G (2010) A model for wet aggregation of ash particles in volcanic plumes and clouds. 1. Theoretical formulation. J Geophys Res 115:B09201 Durant AJ, Bonadonna C, Horwell CJ (2010) Atmospheric and environmental impact of volcanic particulates. Elements 6:235–240 Durant AJ, Rose WI, Sarna-Wojcicki AM, Carey S, Volentik ACM (2009) Hydrometeor-enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens. J Geophys Res Solid Earth 114:B03204 Gernon T, Gilbertson MA, Sparks RSJ, Field M (2009) The role of gas-fluidisation in the formation of massive volcaniclastic kimberlite. Lithos 112:439–451 Gernon TM, Gilbertson MA, Sparks RSJ, Field M (2008) Gasfluidisation in an experimental tapered bed: Insights into processes in diverging volcanic conduits. J Volcanol Geotherm Res 174:49–58
A Phreatomagmatic Kimberlite: The A418 Kimberlite Pipe, Northwest Territories, Canada Gilbert JS, Lane SJ (1994) The origin of accretionary lapilli. Bull Volcanol 56:398–411 Graham I, Burgess JL, Bryan D, Ravenscroft PJ, Thomas E, Doyle BJ, Hopkins R, Armstrong K A (1999) Exploration history and geology of the Diavik kimberlites, Lac de Gras, Northwest Territories, Canada. In: Gurney JJ, Gurney JL, Pascoe MD, Richardson SH (eds) VIIth international kimberlite conference. Red Roof Design, University of Cape Town, South Africa Kienle J, Kyle PR, Self S, Motyka RJ, Lorenz V (1980) Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting. J Volcanol Geotherm Res 7:11 Kokelaar BP (1983) The mechanism of Surtseyan volcanism. J Geol Soc London 140:939–944 Kokelaar P (1986) Magma-water interactions in subaqueous and emergent basaltic volcanism. Bull Volcanol 48:275–289 Kurszlaukis S, Barnett WP (2003) Volcanological and structural aspects of the Venetia kimberlite cluster—a case study of South African kimberlite maar-diatreme volcanoes. S Afr J Geol 106:165–192 Kurszlaukis S, Lorenz V (2008) Formation of ‘‘Tuffisitic Kimberlites’’ by phreatomagmatic processes. J Volcanol Geotherm Res 174:68–80 Lorenz V (1993) On the phreatomagmatic origin of kimberlite and lamproite diatremes. IAVCEI international volcanological congress, Canberra, Australia, Abstracts Lorenz V, Kurszlaukis S (2007) Root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar-diatreme volcanoes. J Volcanol Geotherm Res 159:4 Lorenz V, Zimanowski B, Buettner R (1999) Discussion on the formation of kimberlite pipes: the phreatomagmatic model. CEV Newsletter, IAVCEI, pp 11–17 Macedonio G, Dobran F, Neri A (1994) Erosion processed in volcanic conduits and application to the AD 79 eruption of Vesuvius. Earth Planet Sci Lett 121:137–152 McClintock M, Ross P, White JDL (2009) The importance of the transport system in shaping the growth and form of kimberlite volcanoes. Lithos 112:465–472 Moser DE, Amelin Y (1996) Report on Rb-Sr geochronology of kimberlite samples BDK and 9501 through 9504. Confidential report for Kennecott Canada Inc Nassichuk WW, McIntyre DJ (1996) Fossils from diamondiferous kimberlites at Lac de Gras, N.W.T.: age and palaeogeography. In: Le Cheminant AN, Richardson DG, Dilabio RNW, Richardson KA (eds) Searching for diamonds in Canada. Open File, Geological Society of Canada, pp 43–46 Nowicki T, Crawford B, Dyck D, Carlson J, McElroy R, Oshust P, Helmstaedt H (2004) The geology of kimberlite pipes of the Ekati property, Northwest Territories, Canada. Lithos 76:1–27 Nowicki T, Porritt L, Crawford B, Kjarsgaard B (2008) Geochemical trends in kimberlites of the Ekati property, Northwest Territories, Canada: insights on volcanic and resedimentation processes. J Volcanol Geotherm Res 174:117–127 Porritt LA, Cas RAF (2009) Reconstruction of a kimberlite eruption, using an integrated volcanological, geochemical and numerical approach: a case study of the Fox Kimberlite, NWT, Canada. J Volcanol Geotherm Res 179:241–264 Porritt LA, Cas RAF (2011) The influence of complex intra- and extravent processes on the deposit characteristics of the Koala
107
Kimberlite, NWT, Canada: volcanology, sedimentology and root zone processes. Bull Volcanol 73:717–735 Ross P-S, White JDL (2006) Debris jets in continental phreatomagmatic volcanoes: a field study of their subterranean deposits in the Coombs Hills vent complex, Antarctica. J Volcanol Geotherm Res 149:62 Schipper CI, White JDL, Zimanowski B, Buttner R, Sonder I, Schmid A (2011) Experimental interaction of magma and ‘‘dirty’’ coolants. Earth Planet Sci Lett 303:323–336 Schumacher R, Schmincke HU (1991) Internal structure and occurrence of accretionary lapilli; a case study at Laacher See Volcano. Bull Volcanol 53:612–634 Schumacher R, Schmincke HU (1995) Models for the origin of accretionary lapilli. Bull Volcanol 56:626–639 Scott Smith BH (1999) Near-surface emplacement of kimberlites by magmatic processes. CEV Newsletter, IAVCEI, pp 3–10 Scott Smith BH (2008) Canadian kimberlites: geological characteristics relevant to emplacement. J Volcanol Geotherm Res 174:9–19 Self S, Kienle J, Huot J-P (1980) Ukinrek Maars, Alaska, II. Deposits and formation of the 1977 craters. J Volcanol Geotherm Res 7:39 Skinner EMW, Marsh JS (2004) Distinct kimberlite pipe classes with contrasting eruption processes. Lithos 76:183–200 Sparks RSJ, Baker L, Brown RJ, Field M, Schumacher J, Stripp G, Walters A (2006) Dynamical constraints on kimberlite volcanism. J Volcanol Geotherm Res 155:18–48 Stasiuk LD, Lockhart GD, Nassichuk WW, Carlson JA (1999) Thermal maturity evaluation of dispersed organic matter inclusions from kimberlite pipes, Lac de Gras, Northwest Territories, Canada. Int J Coal Geol 40:1–25 Stubley MP (1998) Bedrock geology of the East Island area, Lac de Gras. Internal report prepared for Diavik Diamond Mines Inc Stubley MP (2004) Spatial distribution of kimberlite in the Slave craton, Canada: a geometrical approach. Lithos 77:683–693 Sweet AR, Stasiuk LD, Nassichuk WW, Catuneanu O, McIntyre DJ (2003) Palaeontology and diamonds: geological environments associated with kimberlite emplacement, Lac de Gras, Northwest Territories, Canada. Long abstracts 8th international kimberlite conference, Victoria, Canada Thorarinsson S (1965) The Surtsey eruption course of developments and the development of a new island. Surtsey Res Progr Rep 1:51–56 van Straaten B, Kopylova MG, Russell JK, Scott Smith BH (2009) Stratigraphy of the intra-crater volcaniclastic deposits of the Victor Northwest kimberlite, northern Ontario, Canada. Lithos 112S:488–500 Varekamp JC (1993) Some remarks of volcanic vent evolution during plinian eruptions. J Volcanol Geotherm Res 54:309–318 Walters AL, Phillips JC, Brown RJ, Field M, Gernon T, Stripp G, Sparks RSJ (2006) The role of fluidisation in the formation of volcaniclastic kimberlite: Grain size observations and experimental investigation. J Volcanol Geotherm Res 155:119–137 Watanabe K, Ono K, Sakaguchi K, Takada A, Hoshizumi H (1999) Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugendake, Unzen Volcano, Japan. J Volcanol Geotherm Res 89:95–112 White JDL (1991) Maar-diatreme phreatomagmatism in Hopi Buttes, Navajo Nation (Arizona), USA. Bull Volcanol 53:239–258 White JDL (1996) Impure coolants and interaction dynamics of phreatomagmatic eruptions. J Volcanol Geotherm Res 74:155–170
