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RESEARCH PRODUCT

FORMATION OF PHREATOMAGMATIC MAAR–DIATREME VOLCANOES AND ITS RELEVANCE TO KIMBERLITE DIATREMES

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ABSTRACT Studies of maars and diatremes suggest a specific process in their formation. Magma rises along a fissure and contacts ground– or surface derived water. The resulting phreatomagmatic eruptions give rise to base surge and air–fall deposits consisting of juvenile and wall–rock material. Spalling of the wall–rocks enlarges the fissure into an embryonic vent. At a critical diameter of the vent large-scale spalling at depth and slumping near the surface gives rise to a ring–fault of large diameter and subsidence of the enclosed wall–rocks and overlying pyroclastic debris. This subsidence leads to a maar crater at the surface. Fluidization processes are active in the narrow vent and in fractures and faults inside the surrounding structure which subsides en masse. Various features of kimberlite diatremes seem to be consistent with this model. They extend into fissures along which hot kimberlite magma rose. The diatremes, however, indicate emplacement by a cool gas phase, probably steam. Indicators for subsidence along ring–faults may be large diameter of diatremes, slickensides on walls, saucer–shaped structures, subsided “floating reefs”, concentration of xenoliths from specific horizons within certain areas, and zoning of diatreme rocks. It is suggested that formation of kimberlite diatremes may have been influenced by non-juvenile water.