Microsymposium 36, MS032, 2002

DIATREMES AND KIMBERLITES 1: DEFINITION, GEOLOGICAL CHARACTERISTICS ANDASSOCIATIONS: J. W. Head1 and L. Wilson2, 1Dept. of Geol. Sci., Brown Univ., Providence, RI 02912 USA,2Lancaster University, Lancaster, UK, james_head@brown.edu.

Introduction: Diatremes and kimberlites are un-usual and enigmatic features and rock types that havebeen approached from a variety of disciplines and per-spectives as evidenced by definitions and discussions inthe recent Encyclopedia of Volcanolgy [1]. From a geo-physical standpoint [2], Jeanloz defines kimberlite as"An explosively emplaced volcanic rock that is origi-nally a fluid-rich ultramafic in overall composition andtypically contains many xenoliths." According to Jean-loz, kimberlites represent explosive eruptions that ex-tract xenoliths very rapidly and at least in some caseserupt from great depths. The presence of diamond insome kimberlites indicates a source at pressures of atleast 4-5 GPa, the pressure at which diamond is stablerelative to graphite at mantle temperatures; this corre-sponds to depths of ~100-150 km. From a petrologicalstandpoint [3], Rutherford and Gardner define kimber-lite as "A very low silica igneous rock rich in volatilesthat erupts explosively from sources in the upper man-tle. Commonly contains mantle xenoliths, occasionallycontains diamonds." From a mineral deposit standpoint,White and Herrington [4] define kimberlite as "A por-phyritic alkalic peridotite containing abundant phenoc-rysts of olivine (commonly altered to serpentine or car-bonate minerals) and phlogopite in a fine-grainedgroundmass of calcite, olivine, and phlogopite with ac-cessory minerals. Kimberlite is the main host rock fordiamonds." Finally, Vesperman and Schmincke definediatremes as [5]: "Funnel-shaped breccia pipes thatreach as much as 2500 m in depth. Diatremes arethought to form by hydrovolcanic fragmentation andwall rock collapse. Diatremes may underlie maars andgrade at depth into dikes." This range of definitionsillustrates the unusual properties of diatremes and kim-berlites. We here summarize the characteristics of thesefeatures to provide the basis for developing models forthe ascent and eruption of kimberlites and the formationof diatremes and associated features.

Background: Early studies of kimberlites [6]showed that they occur both as 1) carrot-shaped verticalintrusions (pipes or diatremes) and 2) as tabular dikesknown as fissure kimberlites, but their connections werenot fully appreciated until the classic analyses of Daw-son and Hawthorne [7] who established the basic prin-ciples of kimberlite magmatism by recognizing [6]: "1)the existence of hot mobile kimberlite magmas, 2) thatsuch magmas could undergo differentiation, 3) the oc-currence of pyroclastic and epiclastic kimberlites, 4)that diatremes with increasing depth are gradational intononbrecciated hypabyssal kimberlites, 5) the existenceof kimberlitic sills." From this time on, kimberliteswere recognized as "volatile-rich ultrabasic magmaswhose evolution and emplacement can be described interms of standard differentiation, intrusion and extrusionprocesses"..."and diatremes are only a particular mani-festation of a more general magmatic style..." [6]. Therelationships between the major components of a kim-berlite magmatic system (Fig. 1) include effusive rocksand crater, diatreme, and hypabyssal rocks. These three

components also have three textural genetic groups ofrocks, each associated with a particular style of mag-matic activity.

Crater Facies Kimberlites: These include lavas,pyroclasitic rocks and epiclastic rocks. Kimberliticmagmas rarely produce lava flows but typically formpyroclastics, which, where studied in detail [8], displayfour types of deposits (oldest, lowest, to youngest): 1)basal breccias, 2) poorly stratified coarse pyroclastics(these tuffs and tuff breccias contain fragments of kim-berlite, country rock, and mantle-derived xenoliths ce-mented by pyroclastic material like overlying tuffs), 3)well-stratified tuffs (alternating layers of coarse lapilli-sized tuffs and laminae of finer ash-sized tuffs), and 4)epiclastic lacustrine deposits. Graded beds and deposi-tional features seem to be absent leading to the inter-pretation that the tuffs are primarily airfall [3]. Fluviatilereworking of tuffs in crater lakes produces epiclastickimberlites. Volumes of pyroclastics are small and theyare typically confined to craters and to thinly beddedtuff-rings; magmatic upwelling and magma-filled con-duits do not follow pyroclastic eruptions. Erosionquickly follows, but marginal downfaulting may pre-serve rim facies. Similarities of this model to hydrovol-canic tuff ring formation models [9] exist.

Diatreme Facies Kimberlites: Underlying craterfacies kimberlites are carrot-shaped bodies with circularto elliptical cross-sectional areas that have vertical axesand steeply dipping margins that converge and terminateat depth in a root zone, where the diatreme expands,contracts, or splits up into an irregularly shaped multi-phase intrusion of hypabyssal kimberlite [6]. The com-monest rocks in the diatreme facies are tuffisitic kim-berlite breccias, containing abundant angular to roundedcountry rock inclusions (mostly a few cm down to mi-croscopic), and discrete and fractured grains of olivine,garnet and ilmenite mega and macrocrysts, set in a fine-grained matrix of microcrystalline diopside and serpen-tine. The matrix quickly undergoes alteration and re-placement by clays and secondary calcite [6]. Typically,one to three texturally distinctive varieties of tuffistickimberlite breccias are seen in diatreme zones.

Hypabyssal Facies Kimberlites: These are rocksformed by the crystallization of volatile-rich kimberliticmagma and exhibit igneous textures and effects ofmagmatic differentiation; often they contain sufficientcountry rock xenoliths to be called kimberlitic breccias.These occur as dikes and sills and form the root zones ofdiatremes (Fig. 1). Kimberlitic dikes are typically verti-cally dipping with 1-3 m widths, but can be up to 10 m[6], and commonly form swarms of parallel features.Most dikes are single intrusions and pinch out towardthe surface, thickening with depth; many show evidenceof flow differentiation, glassy selvages are absent, andcontact metamorphic effects are slight. Some dikes areobserved to expand along strike into lenticular features10-20 times the dike width and up to 100 m in length;these are termed "blows" and may represent the lower-most portions of root zone intrusions [1]. Erosion and

DIATREMES AND KIMBERLITES 1: CHARACTERISTICS: J. W. Head and L. Wilson

mining have enabled unprecedented studies of the three-dimensional and temporal relations of dikes to be made.Antecedent or precursor dikes form swarms similar toregional swarms, and are concentrated in the vicinity ofpipes, extending to levels well above the level that sub-sequent diatremes expand upwards, but not to their up-permost levels or to the land surface [6]. Contempora-neous dikes occur as offshoots from the main pipe intothe adjacent country rock, are usually short (~1 m) andoccur along joints or fractures; such dikes are very rareand do not occur in most diatremes. Internal dikes arecommon in most diatremes and root zones, but aresmall, rootless, sinuous and pinch out laterally and ver-tically, cross-cutting intrusions within the pipes but notextending into the surrounding country rock. Most haveno preferred orientation and may be localized at thedike-wall rock contact or at the contact between discreteintra-diatreme intrusions. Although some internal dikesoccur in only one phase of the pipe, several periods ofdike formation are suggested by the petrology: 1) un-evolved macrocrystal hypabyssal kimberlite, 2) aphani-tic kimberlites, 3) mica-rich varieties, or 4) calcite-richlate differentiates [6]. Subsequent, or cross-cutting dikes

are extremely rare, suggesting that the diatreme-formingevent is the closing stage of kimberlitic magmatism.Kimberlitic sills are relatively rare and appear to becontrolled by local structure and rock types. Plutonickimberlitic complexes are unknown [6].

References: 1) Encyclopedia of Volcanoes, H. Sig-urdsson, ed., Academic Press, 2000; 2) R. Jeanloz,Mantle of the Earth, in Encyclopedia of Volcanoes, 41,2000; 3) M. Rutherford and J. Gardner, Rates of MagmaAscent, in Encyclopedia of Volcanoes, 207, 2000; 4) N.White and R. Herrington, Mineral deposits associatedwith volcanism, in Encyclopedia of Volcanoes, 897,2000; 5) D. Vesperman and H-U Schmincke, Scoriacones and tuff rings, in Encyclopedia of Volcanoes, 683,2000; 6) R. H. Mitchell, Kimberlites, Plenum Press,New York, 442 p., 1986; 7) J. Dawson, ESR, 7, 187-214, 1971; J. Hawthorne, Phys. Chem. Earth, 9, 1-15,1975; 8) G. Mannard, GACP, 19, 15-21, 1968; 9) K.Wohletz and M. Sheridan, AJS, 283, 385-413, 1983; 10)J. Dawson and J. Hawthorne, JGSL, 129, 61-85, 1973.

Figure 1. Model of an idealized kimberlite magmaticsystem illustrating the relationships between crater,diatreme, and hyabyssal zones and facies rocks. Notto scale. From [6].