Formation of “Tuffisitic Kimberlites” by phreatomagmatic processes

Abstract

The rock type “Tuffisitic Kimberlite” (TK) occurs in the deeper diatremes of some Southern African, Canadian and Siberian kimberlite pipes and has been considered a unique rock type related to a particular emplacement process. The key features of this rock type are its fragmental nature, its massive, well-mixed appearance and a specific matrix mineralogy characterized by the presence of serpentine and microlitic clinopyroxene and the absence or scarcity of carbonate. Historically, all these features were thought to be related to a highly specific intrusive–extrusive magmatic emplacement process. In this process, the expansion of magmatic volatiles drives the fluidization of a pre-existing, shallow-crustal, vertical magma reservoir (the “embryonic pipe”) and its consequent evolution into the final diatreme after breakthrough to the surface. The specific matrix mineralization is explained by the dissociation of carbonate into CO 2 and CaO. While the expanding CO 2 drives the fluidization process, CaO reacts with SiO 2 released from xenolith and olivine alteration to form microlitic clinopyroxene. With the phreatomagmatic process chain we offer an alternative model that can readily explain this specific rock type. Ongoing explosions in a downward penetrating root zone excavate a pipe consisting of a rather regular, cone-shaped diatreme underlain by the irregular root zone. At this stage the pipe in its majority is infilled by warm to hot pyroclastic tephra. During thermohydraulic explosions in the root zone the expansion of water vapor homogenizes the overlying diatreme tephra and mixes it with the adjacent country-rock lithologies. This result in the massive, well-mixed volcaniclastic rock type typical for TKBs. Post-emplacement hydrothermal alteration and mineralization under epi- to mesothermal conditions are considered responsible for the observed specific matrix mineralogy. Carbonate is frequently present in coherent root zone rocks but is rare or absent due to its dissolution in hot hydrothermal fluids ascending from the lower diatreme, which in turn precipitate their dissolved carbonate higher up in the cooler, bedded upper diatreme region. Clinopyroxene also grows under epi- to mesothermal conditions. For the crystallization of serpentine a X(H 2O):X(CO 2) ratio of 20:1 is required, which suggests both a high availability of groundwater and a low abundance of CO 2 after and likely during emplacement. Isotope studies (H, C, O, Sr) of matrix minerals suggest participation of groundwater in the formation of these minerals.