Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation

The Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified. Key observations are that the olivine-rich lithofacieshas a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution of lithofacies points to magmaticco-intrusion, and flow segregation driven by fundamental rheological differences between the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presenceof such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during and after intrusio

Water in cratonic lithosphere : calibrating laboratory-determined models of electrical conductivity of mantle minerals using geophysical and petrological observations

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q06010, doi:10.1029/2012GC004055.Measurements of electrical conductivity of “slightly damp” mantle minerals from different laboratories are inconsistent, requiring geophysicists to make choices between them when interpreting their electrical observations. These choices lead to dramatically different conclusions about the amount of water in the mantle, resulting in conflicting conclusions regarding rheological conditions; this impacts on our understanding of mantle convection, among other processes. To attempt to reconcile these differences, we test the laboratory-derived proton conduction models by choosing the simplest petrological scenario possible – cratonic lithosphere – from two locations in southern Africa where we have the most complete knowledge. We compare and contrast the models with field observations of electrical conductivity and of the amount of water in olivine and show that none of the models for proton conduction in olivine proposed by three laboratories are consistent with the field observations. We derive statistically model parameters of the general proton conduction equation that satisfy the observations. The pre-exponent dry proton conduction term (σ0) and the activation enthalpy (ΔHwet) are derived with tight bounds, and are both within the broader 2σ errors of the different laboratory measurements. The two other terms used by the experimentalists, one to describe proton hopping (exponent r on pre-exponent water content Cw) and the other to describe H2O concentration-dependent activation enthalpy (term αCw1/3 added to the activation energy), are less well defined and further field geophysical and petrological observations are required, especially in regions of higher temperature and higher water content.The SAMTEX data were acquired through funding provided by the Continental Dynamics program of the U.S. National Science Foundation (grant EAR0455242 to RLE), the South African Department of Science and Technology (grant to South African Council for Geoscience), and Science Foundation Ireland (grant 05/RGP/GEO001 to AGJ) plus financial and/or logistical support provided by all members of the SAMTEX consortium. JF was initially supported by an IRCSET grant to AGJ for the TopoMed project (TopoMed: Plate reorganization in the western Mediterranean: Lithospheric causes and topographic consequences) within the European Science Foundation’s TOPOEUROPE EUROCORES (http://www.esf.org/activities/eurocores/ running-programmes/topo-europe.html), and subsequently by an SFI PI grant (10/IN.1/I3022) to AGJ for IRETHERM (www.iretherm.ie).2012-12-1

Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)

Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust — principally from ~22 to 28km — as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’ — inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.Clough and Mykura Fund of the Geological Society of Edinburgh; Timothy Jefferson Fund of the Geological Society of Londo

Phase petrology, geochemistry and evolution of the ultrabasic-carbonatitic Blue Hills complex (Southern Namibia)

The ultrabasic, hypabyssal Blue Hills complex in Southern Namibia comprises a suite of highly silica undersaturated magmatic rocks, which are investigated by microprobe and geochemical bulk-rock analyses. The parental magma of these rocks formed by buffered melting at temperatures of 1100-1270 degrees C in a magma chamber at a depth of 100-110 km. During its ascent, magmatic differentiation and loss of volatiles led to the genesis of more differentiated rock types of the ultrabasic suite culminating in the formation of carbonatites and a pegmatite. On the basis of their geochemistry, we describe the relationships between these rocks and present a model for the formation of the complex. In addition, a short description of the petrography and the phase relations is presented together with a comparison with kimberlites and other ultrabasic rocks

On the volcanology of the Gibeon Kimberlite Field, Namibia

Kimberlite volcanism in the Upper Cretaceous Gibeon Kimberlite Field, southern Namibia, consisting of at least 42 diatremes and a number of associated dykes, is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the Field. The volcanology of the diatremes and dykes as well as their structural setting is reported here. Because of the paleohydrogeological setting, and since juvenile kimberlite occurring in dykes, intrusive plugs, and spherical lapilli is devoid of vesicles, a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes. Karoo dolerite, basalt and sediment xenoliths in the diatremes provide evidence for the former extent of Karoo strata at the time of eruption

The Blue Hills Intrusive Complex in southern Namibia - relationships between carbonatites and monticellite picrites

Geochemical and petrological investigations were carried out on ultrabasic silicate and carbonatite magmatic rocks from the Blue Hills Intrusive Complex (southern Namibia). From detailed field mapping and geomagnetic traverses we propose a model for the complex build-up of this hypabyssal laccolith. The bulk comprises mica-olivine-carbonate picrite, with numerous small offshoots into the surrounding Nama shales which are metamorphosed at the contacts to the intrusive rock. The mica-olivine-carbonate picrite is intruded by small lenses and massive layers of monticellite picrite. Locally, a phlogopite-carbonate picrite occurs, that cuts through both the monticellite picrite and the mica-olivine-carbonate picrite. Late carbonatite dykes and sills and a carbonate-phlogopite-apatite pegmatite represent the last stage of the Blue Hills magmatism. Phlogopite-carbonate picrites are regarded as the parental and nearly primary magmas. They may be formed by partial melting in the dolomite stability field at a depth of 100 to 110 km and at a temperature of 1100-1250 degrees C. Fractional crystallisation of mostly olivine led to formation of the volatile-rich, mica-olivine-carbonate picrites, which were the first to intrude into the present position of the Blue Hills Intrusive Complex. Part of the phlogopite-carbonate picrite magma intruded undifferentiated, while part of it suffered devolatilisation and oxidation on its way to the surface which led to formation of the monticellite picrites. Both magma types intruded the hypabyssal mica-olivine-carbonate picrite. The late carbonatites most probably are residual liquids, i.e. the result of almost complete solidification of magmas Like phlogopite-carbonate picrites. The carbonatitic liquids were removed from the crystal mush by filter pressing. Pegmatitic veins are the late stage residual fluids or melts from within the Blue Hills body

Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development

Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe