Mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization: Evidence from kimberlite volcanic systems

Abstract

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface; the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption.