Mantle Mush Compaction: a Key to Understand the Mechanisms of Concentration of Kimberlite Melts and Initiation of Swarms of Kimberlite Dykes |
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Authors: | GREGOIRE, M. RABINOWICZ, M. JANSE, A. J. A. |
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Affiliation: | 1 LABORATOIRE DE DYNAMIQUES TERRESTRE ET PLANÉTAIRE-CNRS UMR 5562, OBSERVATOIRE MIDI-PYRÉNÉES, UNIVERSITÉ PAUL SABATIER, 14 AVENUE E. BELIN, 31400 TOULOUSE, FRANCE 2 ARCHON EXPLORATION PTY LTD, 11 ROSLEY WAY, CARINE, PERTH, W.A. 6020, AUSTRALIA |
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Abstract: | Kimberlite pipes or dykes tend to occur in clusters (a few kilometresin diameter) within fields 3050 km in diameter. Theyare generally considered to originate from low degrees of partialmelting of carbonated peridotite within zones of ascending mantle.Numerical modelling shows that at the depth of formation ofkimberlite melts (>>200 km), mantle compaction processescan result in the formation of melt pockets a few tens of kilometresacross, with melt concentrations up to 7%. The initiation ofswarms of kimberlite dykes at the top of these melt pocketsis inevitable because of the large excess pressure between themelt and the surrounding solid, which exceeds the hydraulicfracturing limit of the overlying rocks. After their initiationat mantle depth the swarm of dykes may reach the surface ofthe Earth when the entire cratonic lithosphere column is inextension. We propose that kimberlite fields represent the surfaceenvelope of dyke swarms generated inside a melt pocket and thatkimberlite clusters represent the discharge of melt via dykesoriginating from sub-regions of the pocket. This model reproducesthe worldwide average diameter of kimberlite fields and is consistentwith the observation that some of the main kimberlite fieldsdisplay age ranges of c. 10 Myr. It is deduced that, at thescale of the Kaapvaal craton, different fields such as Kimberley,N. Lesotho and Orapa, dated at 8090 Ma, probably resultfrom synchronous melt pockets forming inside an ascending mantleflow. The same model could apply to the fields of the Rietfontein,Central Cape and Gibeon districts dated at 6070 Ma. Itis suggested that the same mantle flow that produced the Kimberley,N. Lesotho and Orapa fields migrated over 2030 Myr afew hundred kilometres westward to form the Rietfontein, CentralCape and Gibeon fields. KEY WORDS: kimberlites; mantle; compaction; convection; volcanism |
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Keywords: | : kimberlites mantle compaction convection volcanism |
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