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Received 31 Oct 2011 | Accepted 11 Apr 2012 | Published 15 May 2012 DOI: 10.1038/ncomms1842

Kimberlites are volatile-rich magmas from mantle depths of 150 km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme lls are pelletal lapilliwell-rounded clasts consisting of an inner seed particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when uid melts intrude into earlier volcaniclastic inll close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged particles are simultaneously uidised and coated by a spray of low-viscosity melt. A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites, kamafugites and melilitites.

The origin of pelletal lapilli in explosive kimberlite eruptions

T.M. Gernon1, R.J. Brown2, M.A. Tait3 & T.K. Hincks4

1 Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK. 2 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK. 3 Rio Tinto Limited, St Georges Terrace, Perth, Western Australia 6000, Australia. 4 Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Correspondence and requests for materials should be addressed to T.M.G. (email: [email protected]).

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Kimberlite melts ascend from the Earths mantle to the surface in a matter of hours to days1,2. Their diatreme-hosted deposits provide valuable insights into the dynamics of other vol

canic conduits and represent the main source of diamonds on Earth. Kimberlite diatremes are subject to a wide range of volcanic and sedimentary processes and interactions3,4, and in some cases, are host to exceptional fossil preservation5. Additionally, the xenoliths and xenocrysts they contain provide valuable information on the structure and composition of the deep subcontinental mantle6,7.

Most volcaniclastic kimberlites contain ubiquitous yet poorly understood composite particles termed pelletal lapilli1,812. These are dened as discrete sub-spherical clasts with a central fragment, mantled by a rim of probable juvenile origin9. Pelletal lapilli typically range in size from < 160 mm, and occur both as accessory components of pipe-lling volcaniclastic kimberlites and as the main pyroclast type in narrow, steep-sided pipes within the diatreme. These clasts have previously been attributed to incorporation of particles into liquid spheres in the rising magma8,13 and rapid unmixing of immiscible liquids14. However, these models fail to explain many aspects of their internal structure, composition and abundance in pyroclastic intrusions. Pelletal lapilli have been identied globally in a wide range of other alkaline volcanic rocks, including carbonatites15, kamafugites13,16, melilitites14,15 and orangeites17. They have also been referred to as tuffisitic lapilli15, spherical lapilli18, spinning droplets13,19 and cored lapilli20,21. Although pelletal lapilli are similar in appearance and structure to armoured (or cored) lapilli22, the latter are formed by accretion of moist ne-grained ash (as opposed to liquid melt) to the central fragment23,24. Pelletal lapilli share similar properties to particles formed during industrial uidised granulation processes, but such processes have not previously been considered in a geological context.

Fluidised spray granulation is widely used in industrial engineering to generate coated granules with specic size, density and physicochemical properties25,26. The mechanism involves continuous injection of atomisable liquids, solutions or melts into a powdery uidised bed27, which produces a dispersion of larger coated granules that are simultaneously dried by the hot uidising gas27,28. When gas ows upwards through particles, the point of minimum uidisation (Umf) occurs when the ow velocity (U) is sufficiently high to support the weight of particles without transporting them out of the system29. Umf is dened according to the semi-empirical

Ergun equation30:

= +

P h

28.8576

28.8612

29.0052

Botswana

PLI

Venetia

Letengla-Terae

Fig.2c

Lesotho

Basalt raft

PLI

Pyroclastic intrusion

MVK

MVK, sediment-bearing

Marginal VK breccias

3050 m elevation

Venetia K1 diatreme

Leteng Satellite diatreme

Fig.2a

22.4352

520 m elevation

Country rock breccia

Coherent kimberlite

29.3220 100 m

100 m

Figure 1 | Simplied geological maps of the Venetia and Leteng kimberlite pipes. (a) Venetia K1 is dominated by massive volcaniclastic kimberlite (MVK, see text for details) with subordinate marginal breccias, sediment-bearing volcaniclastic kimberlite and coherent kimberlite lithofacies. The approximately 15-m-wide pelletal-lapilli intrusion (PLI) occurs near the north-central margin of the pipe, where it cross-cuts MVK and is closely associated with numerous minor late-stage dykes. (b) The Leteng Satellite pipe is also dominated by MVK; pelletal lapilli are conned to a northern circular pipe approximately 100 m wide (modied after Palmer et al.36). Inset depicts location of the deposits in southern Africa.

. ( ) . ( )

where P/h is the pressure drop across a bed of height, h, g is the gas density, g is the gas dynamic viscosity, is porosity and xp

is the diameter of spherical particles. Fluidised spray granulation is a characteristically steady growth process, producing uniform well-rounded particles with a concentric layered structure28.

We present analyses of pelletal-lapilli occurrences from southern African kimberlites that are best explained by uidised spray granulation during emplacement. The physical properties of pelletal lapilli (e.g., uniformly coated concentric internal structure combined with a restricted particle size distribution), provide strong evidence that this process occurs when uid, volatile-rich melts are intruded as dykes into loose, granular deposits close to the diatreme root zone. This type of multi-stage intrusion will result in spatial and temporal variation in the structure and composition of pipe-ll and consequently, could inuence local diamond grade and size distributions.

ResultsVenetia K1 diatreme. Pelletal lapilli occur prominently in two of

the worlds largest diamond mines, Venetia (South Africa) and

(1)(1)

Leteng-la-Terae (Lesotho), both well-exposed, extensively surveyed11,31,32 and economically signicant localities. The Venetia K1 diatreme was emplaced during the late-Cambrian Period (c. 519 6 Ma)33 into metamorphic rocks of the neo-Archaean to Proterozoic Limpopo Mobile Belt (3.32.0 Ga). The diatreme (Fig. 1a) is dominated by massive volcaniclastic kimberlite (MVK; previously termed tuffisitic kimberlite breccia)12, a characteristically well-mixed lithofacies comprising serpentinised olivine crystals and a polymict range of lithic clasts32,34. The formation of MVK has been attributed to uidisation1,8,11, the scale and context of which is heavily debated34,35. Pelletal lapilli (Fig. 2a,b) are conned to a narrow (1015 m diameter), discordant and lenticular body near the northern margin of K1 (Fig. 1a). Although pipe-like, we refer to these features as pyroclastic intrusions to avoid confusion with the large-scale (0.51 km diameter) pipes or diatremes in which they occur. Field and drill-core data suggest that the intrusion is a steep-sided tapering cone, associated with numerous late phlogopite-rich dykes. The intrusion is characteristically structureless, clast- to matrix-supported, and poorly to moderately sorted. It contains abundant (90 vol%) coated lapilli-sized and very rare bomb-sized clasts (Fig. 2a,b), ranging in diameter from 0.2 to 100 mm (mean, x = 9.4 mm; Fig. 3a). These pelletal lapilli comprise a sub-angular lithic clast or olivine macrocryst as their core surrounded by a variably thick coating (generally < 1 cm); typically, this coating comprises olivinephlogopitespinel-bearing kimberlite with a heavily altered groundmass containing amorphous serpentine and talc. A concentric alignment of crystals is commonly developed in the coating around the core (Fig. 2b). In some cases, the pelletal coatings appear to have partially coalesced.

Leteng-la-Terae Satellite Pipe. The Leteng-la-Terae Satellite Pipe erupted during the Late Cretaceous Period (c.91 Ma11,36) through

150 1 1 75 1

m e e

r e e

g g

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5 cm

500 m

d e

2 cm 1 mm 100 m

Spherical layered structure

Outer layer, uniform coating

Core

Concentrically aligned inclusions (inner layer)

Figure 2 | Photographs of pelletal lapilli from southern African kimberlites and a synthetic analogue. (a) Exposure from the Venetia K1 pyroclastic intrusion showing concentrations of pelletal lapilli, which are characteristically well rounded. (b) Scanning electron microscope (SEM; backscattered-electron) image of a pelletal lapillus from (a), comprising a serpentinised olivine core and ne-grained rim comprising talc, spinel and numerous concentrically aligned micro-phenocrysts. (c) Hand specimen from Leteng showing circular-elliptical pelletal lapilli and crystals. (d) SEM image of an elliptical pelletal lapillus from (c); note the incorporation of smaller crystals into the rim. (e) SEM image of the matrix of (d) showing the inter-growth of void-lling serpentine (Se) and diopside (Di), an assemblage indicative of low-temperature hydrothermal alteration31.

(f) For comparison, a synthetic pharmaceutical granule produced by several stages of uidised granulation; crystalline sugar core surrounded by layers of glucose, talc, polymers and cellulose (after Jacob et al.38).

200 m

Lower Jurassic ood basalts of the Drakensberg Group. Pelletal lapilli occur within a steep-sided (~80), 100-m-wide circular intrusion. This cross cuts MVK and marginal inward-dipping volcaniclastic breccias (Fig. 1b)36, dening a nested geometry32. Pelletal lapilli are characteristically well rounded (Fig. 2c,d), ranging in size from 60 to 61 mm (x = 3.5 mm; Fig. 3a). Pelletal cores typically constitute mantle36 and crustal xenoliths, the most abundant being basaltic lithic clasts (85%) of presumed Drakensberg origin11. The rims to serpentinised olivines (Fig. 2d) typically consist of euhedral to subhedral olivine phenocrysts, very ne-grained chrome spinel, perovskite and titanite. The pore space is inlled by a secondary serpentine-diopside assemblage (Fig. 2e), which further from olivine clusters gives way to calcite11.

Discussion

The characteristics of observed pelletal lapilli (Figs 2 and 3) are indicative of uidised spray granulation27,28. This process generates well-rounded composite particles28, uniformly coated37 with layered concentrically aligned inclusions (Fig. 2f)38. For both deposits, data show a moderate to strong positive correlation between the cross-sectional area of the seed particle and that of the coating (Fig. 3b), suggesting a uniform coating process and underlying scale invariance. Particle growth rate generally increases with increasing particle diameter27, because of their greater surface area. However, in this instance, larger clasts have proportionally less rim material (gradient < 1; Fig. 3b). Larger clasts have higher inertia, requiring higher sustained velocities for uidisation, and experiencing increased abrasion at lower velocities (U < Umf). The circular-

elliptical geometry exhibited by pelletal lapilli (Figs 2 and 3c) suggests their formation is governed by surface tension1, a major variable in uidised spray granulation37. The presence of multiple rims and concentrically aligned phenocrysts in some pelletal lapilli (Fig. 2b)14 is suggestive of a systematic multi-stage layering process28.

Another key characteristic of spray granulation is the generation of a narrow particle size distribution26, partly due to the agglomeration of nes26,27. This is evidenced by the incorporation of small discrete rimmed crystals within larger pelletal rims (Fig. 2b,d). Although the Venetia and Leteng size distributions are not strictly narrow (Fig. 3a), the host and proposed source material (i.e., MVK, see Fig. 1) has a remarkably wide size distribution, with observed crystal and lithic inclusions ranging from 0.015 to approximately 800 mm (6 to 9.7 )32,34. Venetia MVK contains a high proportion of small olivine crystals (mode0.2 mm) with proportionally fewer larger lithic clasts (mode23 mm) resulting in a bimodal joint size distribution (Fig.6 in Walters et al.32). Lapilli sizes at Leteng and Venetia also show slight bimodality (Fig. 3a), but the size range is more restricted (0.0332 mm; 5 to 5 ), with a higher proportion of larger lapilli (Venetia mode = 5.7 mm) and a relative paucity of ne-grained particles ( < 0.5 mm; Fig. 3a).

To uidise and coat the largest observed pelletal lapilli in the intrusions, gas velocities must have reached ~45 m s 1 (Fig. 3d), broadly consistent with other estimates for MVK1,32,34. We emphasise, however, that the local velocity due to gas bubbles and jets is normally several times greater than the characteristic velocity of the bed29,39. Additionally, the tapered geometry gives rise to a circulating uidised system29, enabling a wide range of pelletal lapilli sizes to coexist in equilibrium. For Venetia, Umf of the maximum-size lapilli is approximately equal to the escape velocity Ue (the velocity at which particles escape from the system)40 of the mediansize lapilli (Fig. 3d). This implies there must be signicant local variation in gas velocity to sustain uidisation across the range of particle sizes observed, although retaining the smaller size fraction. For Leteng, median particle size is considerably lower (approximately 2 mm, Umf = 3 m s 1), suggesting greater variation in gas velocity, which can be explained by the wider vent diameter and more pronounced tapering. Clasts too large to become uidised will behave as dispersed objects34.

Given the high volatile contents required to generate melts of kimberlite composition (510 wt%)41,42, we argue that gas ow rates required to uidise the clasts are easily achievable during degassing of a kimberlite magma. Assuming a pyroclastic intrusion diameter of 10 m, and taking gas velocities of 12 m s 1 (for the mean of the Venetia distribution; see Fig. 3d) and 45 m s 1 (for the maximum size; see Fig. 3d), we would require gas ow rates on the order of 9423.5103 m3 s 1, respectively. Our previous calculations34 show that degassing in kimberlite root zones could result in gas mass ow rates as high as 3106 m3 s 1, so the above estimates seem conservative. Given these estimates, a hypothetical kimberlite dyke segment of conservative length, h = 10 km, breadth, b = 50 m and width, 2w = 2 m, containing 10% volatiles, could release

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(mm)

0.0625 0.25 1 4 16 64

104

0.001 0.01 0.1 1 10 100 1,000 104

Venetia: one clast (= 0.56%)

>64 mm

1,000

y = 0.91 * x(0.9) R2 = 0.84

y = 0.68 * x(0.9) R2 = 0.74

1:1 line

Cross-sectional area of rim (mm2 )

Leteng Venetia

100

Frequency (%)

0.1

0.01

Leteng Satellite Venetia K1

0.001

Major axis length,

Cross-sectional area of core (mm2)

101

102

103

104

10 cm

1 cm

1 mm

0.1 mm

Leteng Satellite Venetia K1

Frequency (%)

Particle size (m)

netia Leteng

Leteng

U mf

Ve Maximum

Mean

50th percentile

U e

Core maximum

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

103

102

101

100

101

102

103

Circularity

Gas velocity (ms1)

Figure 3 | Physical properties of pelletal lapilli from Leteng and Venetia. (a) Step plot showing the frequency (%) of lapilli versus lapilli size in phi () scale, where = Log2d, and d is the lapillus long-axis in millimetres. (b) The area of the rim is plotted against the area of the core for pelletal lapilli from both intrusions. (c) Histograms showing circularity for pelletal lapilli distributions from Leteng and Venetia (see methods). (d) Variation in the minimum uidisation velocity (Umf, equation (1)) and escape velocity (Ue)40 for crystals and lithic clasts, uidised by CO2 at 1,000 C (modied after Sparks et al.1).

Parameter values are s = 3,300 kg m 3, to represent olivine crystals and dense lithic clasts; voidage, mf = 0.5 and viscosity, = 4.6210 6 Pa s. The graph shows the gas velocities required to reach Umf and Ue for a range of characteristic particle sizes (shown) for Leteng and Venetia. Note that Umf of the maximum lapilli size Ue of the mean lapilli size. The window between Umf and Ue shows that a range of particle sizes can be supported (i.e., uidised), but not ejected.

sufficient gas volumes to uidise the entire intrusion ll for tens of seconds to several minutes. As such, a degassing dyke could sustain a gas jet for long enough to efficiently entrain a signicant amount of recycled pyroclasts. These results are not surprising, as comparable basaltic systems (e.g., persistently active volcanoes) can release large volumes of gas with broadly equivalent mass uxes over significant periods of time43, without necessarily erupting any signicant volume of degassed lava44.

We propose that uidised spray granulation occurs when a new pulse of kimberlite magma intrudes into unconsolidated pyroclastic deposits within the diatreme (Fig. 4a). The magma is transported through a dyke or system of dykes in the deep-feeding system, which at low-to-intermediate levels drive explosive volcanic ows45 within the tapered pyroclastic intrusion. At the interface between the dyke and conduit, intensive volatile exsolution results in the formation of a gas jet39, where velocities are sufficiently high (order of tens of metres per second)1,34 to uidise the majority of particles (Fig. 3d) and inhibit formation of liquid bridges between clasts28. Particles from MVK

are entrained into the jet because of the drag force exerted by the uidising gas (Fig. 4a)27. Degassing is accompanied by a continuous spray of low-viscosity melt into the gas jet region27. Melt droplets are provided by fragmentationthe catastrophic bursting of bubbles to form a gassy spray46. The fragmentation level (Fig. 4a) will vary depending on the tensile strength of the magma46, which will be inuenced by the ambient pressuretemperature conditions, magma rheology47 and magma water content1,46. As melt droplets are deposited on the hot particles, they produce a thin lm governed by surface tension, which dries rapidly to form a solid uniform coating27 (Fig. 4a,b). Most of the very ne ash ( < 500 m) is either agglomerated to the pelletal coatings2628 or elutriated by powerful gas ows1,31. Because of a combination of cohesion, high gas velocities and high uid pressures, a fracture develops and the uidised dispersion ascends turbulently through the diatreme ll with limited attrition and breakage (Fig. 4b). Fluidisation may be promoted by a sudden drop in pressure and corresponding increase in gas exsolution accompanying fracture development48. The lack of

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a b

Fine particles elutriated in

eruption column

Leteng Satellite

MVK

Venetia K1

MVK

Late-stage kimberlite melt intruded into granular diatreme fill

Fluidised gas-particle flow ascends rapidly through diatreme

~500 m

Figure 4 | Schematic showing the formation of pelletal lapilli in kimberlite diatremes. (a) A uid, volatile-rich melt is intruded into loose diatreme ll; intensive volatile exsolution produces a gas-jet, opening up a fracture within MVK deposits; inset: particles from MVK are entrained and uidised in the gas jet and uniformly coated by a spray of melt (red); ne particles are either agglomerated to pelletal coatings or elutriated by strong gas ows. (b) Driven by gas expansion and exsolution in the jet region, gas-particle dispersion ascends rapidly ( > 20 m s 1) and turbulently through the diatreme, and the eruption is abruptly ended.

segregation of large lithic clasts indicates a relatively rapid termina

tion of gas supply34.

Our observations from Venetia and Leteng can be explained by dyke intrusion resulting in explosive ow processes within a narrow conduit. However, we recognise that this model will not explain all occurrences of pelletal lapilli globally, and that other important granule-forming processes may operate during eruptions. An example might include the Hawaiian-style lava fountains at the surface, where it is common for melt and gas phases to coincide with crystals and entrained clasts49. It is not difficult to conceive situations in which such particles could be fully supported by the viscous drag of escaping volatiles, and simultaneously, coated by a spray of fragmented low-viscosity melt. This process would provide an opportunity for recycling of previously generated pyroclasts. However, our model (see Fig. 4) provides a mechanism for pelletal lapilli to form at depth in pyroclastic intrusions within the vent, consistent with eld relationships observed at Venetia and Leteng. In this model, pelletal lapilli are also expected to get erupted explosively and ejected during degassing (see Fig. 4b), producing deposits at the surface in which pelletal lapilli are volumetrically substantial.

Several mechanisms could lead to incorporation of pelletal lapilli into more typical vent-lling MVK, as observed in other pipes such as Leteng (main pipe), Wesselton, Lemphane, Liqhobong, Kao and Premier9,50. For example, pelletal lapilli ejected at the surface

will get deposited in marginal bedded regions, which are capable of subsiding to deep levels in the pipe during subsequent explosive bursts at depth5154 and gas uidisation of the pipe-ll34,55. Such large-scale uidisation processes are thought to promote thorough mixing of pre-existing pyroclastic material32,34 (including pelletal lapilli), as the vigorously uidised dispersion eectively erodes and entrains loose material from the marginal subsided strata in the pipe55. It is very likely that successive eruptive phases would disrupt and disaggregate pre-existing pyroclastic intrusions, and thereby mix assemblages of pelletal lapilli together with several phases of MVK. This model is supported by the presence of steep internal contacts in kimberlite pipe-lls, separating distinct eruptive units with variable particle-size distributions32,34.

Fluidised spray granulation may also help explain the welding of pyroclasts from low-viscosity magmas56 and complex transition zones between hypabyssal and diatreme-facies kimberlites12,57,58. Within the Venetia K1 intrusion, occasional coalesced lapilli boundaries suggest that clasts either agglomerated during circulation, or may have remained hot and partially molten during emplacement. Although sprayed kimberlite melt is likely to solidify rapidly upon contact with lithic lapilli, high magma-supply rates may lead to sustained high temperatures and the system becoming dominantly viscous with particle sintering and agglutination59. This would explain observed gradations to non-welded deposits and overlaps in texture and composition with adjacent pyroclastic deposits56.

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The origin of pelletal lapilli is important for understanding how magmatic pyroclasts are transported to the surface during explosive eruptions. Observed dierences in juvenile composition may signify a magma with a dierent mantle provenance, or one that had dierentiated at depth before ascent60. Any resulting compositional dierences may be signicant in terms of diamond grade (carats per tonne), size and quality. Recognising the structurally variable nature of the pipe-ll is also important for economic forecasting. For example, the Leteng pelletal lapilli intrusion has yielded a relatively high number of large diamonds (106215 cts), compared with the surrounding pipe-ll36.

Spray granulation requires a strong uidising gas ow, so our model sheds new light on the role and magnitude of uidisation in kimberlite volcanic systems35. Our constraints on gas velocity provide important new inputs into thermodynamic models of kimberlite ascent and eruption, estimates of gas budget, and possibly, even magma rheology. The ability to tightly constrain gas velocities is signicant, as it enables estimation of the maximum diamond size transported in the ow. Gas uidisation and magma-coating processes are also likely to aect the diamond surface properties.

Our observations also have important implications for understanding pyroclastic processes in conduits of active volcanoes (e.g., Ol Doinyo Lengai) where episodes of ash venting (commonly attributed to uidisation) have been related to changes in eruptive activity. In such settings, pressurised CO2 will ow through volcaniclastic deposits in the vent and crater on its way to the surface, and is likely to uidise some of the granular material while ejecting the ner particles. The gas source is dierent but gives rise to the same phenomena. Our results add support to the hypothesis that pelletal lapilli in other volcanic settings are formed within the diatreme as opposed to the eruption column21.

Most diatremes worldwide contain minor hypabyssal intrusions that cross-cut pyroclastic lithofacies1,9. When such melts penetrate loose granular deposits in the presence of rapid gas ows, we envisage that some degree of spray granulation is inevitable. On the basis of the abundance of pelletal lapilli in volcanic deposits worldwide1,9,10,1315, uidised spray granulation is likely a fundamental, but hitherto unrecognised physical process during volcanic conduit formation.

Methods

Sampling and microscopy. Hand specimens containing pelletal lapilli were collected from both pyroclastic intrusions (Fig. 1) and analysed petrographically using optical and scanning electron microscopy (HITACHI S-3500N). High-resolution digital photographs (scaled and oriented) were taken of polished slabs and bench exposures.

Particle-size distribution analysis. Particle-size distribution analysis was carried out following the technique outlined in Walters et al.32. Because there is naturallya size limit to observable particles at any scale, samples were analysed at several overlapping scales32. Individual pelletal lapilli, lithic fragments and serpentinised olivine crystals were manually identied and digitised in the Adobe Illustrator (CS4) graphics package, and the resulting bitmap images were then processed in the image-analysis soware package, ImageJ (developed by the US National Institute of Health; http://rsb.info.nih.gov/ij/download.html) following Gernon et al.11. This provided major and minor axis measurements, cross-sectional areas for cores and rims, and circularity values (dened as 4area/perimeter2; i.e., 1.0 indicates a perfect circle).

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Acknowledgements

This research was supported by De Beers Group Services UK, who facilitated accessto Venetia Mine for eldwork. We also acknowledge Leteng Diamonds (Pty) Ltd, in particular K. Whitelock and C. Palmer for onsite discussions and allowing access to the Leteng Diamond Mine. We would like to thank Steve Sparks, Mark Gilbertson, Matthew Field, James Head III, Kelly Russell and Hugh OBrien for their helpful discussions.

Fig. 2f (modied aer Jacob et al.38) is reproduced with the permission of Oxford University Press.

Author contributions

T.G. directed the research; T.G., R.B. and M.T. carried out the eldwork and sampling; T.G. and R.B. performed petrographic analysis and analysed particle-size distributions; T.G. and T.H. wrote the paper, T.G. draed the gures and T.H. assisted with plotting the maps. All authors discussed the results and contributed to the nal manuscript.

Additional information

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/

How to cite this article: Gernon, T. M. et al. The origin of pelletal lapilli in explosive kimberlite eruptions. Nat. Commun. 3:832 doi: 10.1038/ncomms1842 (2012).

NATURE COMMUNICATIONS | 3:832 | DOI: 10.1038/ncomms1842 | www.nature.com/naturecommunications

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Kimberlites are volatile-rich magmas from mantle depths of ≥150 km and are the primary source of diamonds. Kimberlite volcanism involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are 'pelletal lapilli'--well-rounded clasts consisting of an inner 'seed' particle with a complex rim, thought to represent quenched juvenile melt. Here we show that these coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We propose that pelletal lapilli are formed when fluid melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally scavenged particles are simultaneously fluidised and coated by a spray of low-viscosity melt. A similar origin may apply to pelletal lapilli in other alkaline volcanic rocks, including carbonatites, kamafugites and melilitites.

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Title

The origin of pelletal lapilli in explosive kimberlite eruptions

Author

Gernon, Tm; Brown, Rj; Tait, Ma; Hincks, Tk

Pages

832

Publication year

2012

Publication date

May 2012

Publisher

Nature Publishing Group

e-ISSN

20411723

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1038/ncomms1842

ProQuest document ID

1017693087

The origin of pelletal lapilli in explosive kimberlite eruptions

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