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ItemAge and Petrogenesis of Kimberlites and Related Rocks from Finland( 2022)Kimberlites are rare, small volume ultramafic igneous rocks found on every continent on Earth, with eruption ages spanning almost 3 billion years. These rocks are derived from the deepest magmas that reach Earth’s surface (>150-200 km) and provide unique insights into the nature of the convecting mantle. In addition, their cargo of entrained mantle xenocrysts (including diamonds) and xenoliths permit examination of the subcontinental lithospheric mantle (SCLM), while ‘deep’ diamonds give insights into the composition of the underlying asthenospheric mantle. Despite their significant scientific and economic importance, and decades of research, particularly in regions of southern Africa, North America and Siberia, questions remain as to the petrogenesis of kimberlites. Contention persists around the depth of origin of kimberlites, melting trigger(s) including tectonic settings, and the composition and evolution of kimberlite melts during their ascent. To provide new insights on these issues, this study presents a comprehensive petrographic, geochemical and geochronological investigation on samples from three occurrences of kimberlite and related magmatism in Finland, comprising the Lentiira-Kuhmo cluster of olivine lamproites, Kuusamo cluster of kimberlites and ultramafic lamprophyres (UMLs) and the Kaavi-Kuopio kimberlites. Finland represents an optimal location for testing various petrogenetic models, particularly regarding the links to geodynamic processes as the tectonic evolution of the Baltic Shield and its role in supercontinent cycles are well constrained. This work presents the first petrological account of the Kuusamo kimberlites, revealing that they represent highly differentiated magmas with scarce olivine macrocrysts and other mantle-derived xenocrysts. These characteristics contrast with the neighbouring Kaavi-Kuopio kimberlites, which are inferred to have crystallised from less differentiated magmas that were modified by mantle assimilation, as evidenced by correlations between the Mg# of xenocrystic (mantle-derived) olivine cores and the composition of magmatic olivine rims, spinel, and groundmass modal mineralogy. New radiometric ages show that at least ~100 Myr separates the emplacement of the Kuusamo kimberlites (~735-750 Ma) from those at Kaavi-Kuopio (~625-585 Ma). These new age data also indicate temporal overlap between ultramafic lamprophyre magmatism at Kuusamo and the eruption of olivine lamproites at Lentiira-Kuhmo (~1180-1220 Ma), some 100 km to the northeast. As part of this geochronological investigation, the robustness of Rb-Sr phlogopite, U/Pb perovskite and 40Ar/39Ar phlogopite dating methods were evaluated by applying multiple geochronometers to individual intrusions. It is evident that each radiometric system can yield both precise and accurate emplacement ages, with important caveats regarding best practice and interpretation. Radiogenic isotope data (Sr-Nd-Hf) indicates that the olivine lamproites and UMLs were contemporaneous, but have distinct source compositions. The highly unradiogenic Nd-Hf isotope compositions of the former are consistent with derivation from the metasomatised SCLM whereas the UML compositions suggest they were sourced from predominantly asthenospheric melts that were modified by (up to 15%) incorporation of enriched SCLM components. The Mesoproterozoic timing of their emplacement suggests that eruption of the olivine lamproites and UMLs was facilitated by the extensional regime associated with the separation of Baltica from Laurentia. The Kuusamo and Kaavi-Kuopio kimberlites were also emplaced at a time of supercontinent disruption. The Kuusamo eruptions occurred as the break-up of Rodinia was initiated, while the Kaavi-Kuopio rocks were emplaced as Rodinia break-up was completed, contemporaneous with the formation of the Central Iapetus large igneous province. In keeping with their petrographic disparities, the Sr-Nd-Hf isotopic composition of these kimberlites indicates that they were sourced from distinct source regions in the convective mantle. The homogenous composition of the Kuusamo rocks overlaps the prominent PREMA-like signature of kimberlites globally, whereas the Kaavi-Kuopio samples exhibit an extreme range in Hf isotope compositions with a temporal trend from PREMA-like towards lower epsilon Hf(i) values in younger kimberlites. Isotopic modelling suggests that this temporal enrichment of the kimberlite source region was due to increasing entrainment (of up to 10%) of subducted material. These findings are consistent with mounting evidence for subducted material being an important source ‘pollutant’ for kimberlites globally and a petrogenetic link with supercontinent cycles and/or the large mantle plumes that initiate supercontinent disintegration.
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ItemPetrological and geochemical constraints on the source to surface evolution and emplacement style of the Lac de Gras kimberlites, Canada( 2021)Kimberlites originate from the deepest-derived magmas on Earth and are characterised by ultrabasic, H2O and CO2-rich, and silica-poor compositions. These magmas entrain and transport mantle material (sometimes including diamonds) during their ascent to the Earth’s surface, before intruding the upper crust, or erupting explosively to form deep (2-3 km) conical diatremes. With the exception of one Quaternary occurrence, active kimberlite magmatism has not occurred since ~30 Ma, and surface deposits are often eroded. Assimilation of mantle material, crustal contamination and post-emplacement hydrothermal alteration modify the compositions of kimberlites during emplacement, hindering attempts to constrain original melt compositions. There is also uncertainty about the factors that control the emplacement style of these magmas and whether melt compositions have any influence. To improve constraints on the composition and evolution of kimberlite melts and their mode of emplacement, 30 coherent intrusive and extrusive kimberlites (CK), and two volcaniclastic kimberlites (VK) from the Lac de Gras (LDG) field, Northwest Territories, Canada were studied using petrographic and geochemical methods. Olivine rim and chromite compositions show that kimberlites at LDG derive from a range of primitive melt compositions. Increasing age-corrected Nd-Hf isotope ratios with time correlate directly with olivine rim Mg# [100xMg/(Mg+Fe2+)] compositions and inversely with chromite Ti# [100xTi/(Ti+Al+Cr)] compositions for central LDG kimberlites. These correlations indicate that melt compositional variations stem from partial melting of an evolving kimberlite source due to progressive assimilation of less refractory, deeply-subducted crustal material. These relationships are not observed when considering all the LDG kimberlites. This is attributed to decoupling of the kimberlite source and primitive melt compositions for all the LDG kimberlites by assimilation of laterally heterogenous mantle material, as indicated by a strong correlation between olivine rim and olivine core compositions, which are considered to be proxies for the compositions of primitive melt and entrained lithospheric mantle material, respectively. Different initial epsilon Nd and Hf, and olivine rim and chromite compositions for extrusive pipe-filling CK and intrusive kimberlite dykes from different LDG localities indicate derivation from different primary melt compositions. However, at some localities (e.g., Diavik), intrusive and extrusive kimberlites feature indistinguishable olivine and chromite compositions, indicating similar primitive melt compositions. These results indicate that primitive melt compositions may control the emplacement style of some, but not all, kimberlite magmas at LDG. Greater modal abundances of groundmass phlogopite and monticellite and lower groundmass abundances of carbonate for the extrusive versus intrusive kimberlites are attributed to greater volatile exsolution during the ascent and higher energy emplacement of the extrusive kimberlites. Greater SiO2, MgO and NiO, and lower incompatible element (i.e., TiO2, Nb, Ta, REE) whole-rock compositions for the extrusive versus the intrusive kimberlites cannot be explained by mixing lithospheric mantle or crustal compositions with reconstructed primitive kimberlite melt compositions, suggesting that these processes were not responsible for the different degrees of volatile exsolution evident in these kimberlites. Explosive emplacement of gas-rich magma excavated pipes at LDG prior to the emplacement of pipe-filling CK, which suggests that pipe-filling CK might reflect the waning stages of volcanic eruptions initiated by the explosive emplacement of a gas-rich dyke tip (VK) followed by the emplacement of melt-rich tails (pipe-filling CK). Further work is required to test the potential genetic relationship between CK and VK at LDG. Primitive melt composition, geological setting, the availability of water to trigger phreatomagmatic eruptions and/or magma segregation during ascent are suggested to influence the emplacement style of kimberlite magmas.
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ItemThe origin, composition, and evolution of the Kimberley kimberlites (South Africa)( 2019)Kimberlites are deeply derived (i.e., >150 km), small-volume igneous bodies that have been emplaced on all continents throughout the last 2.8 billion years. The typical volcanic expression of kimberlites is a deep irregular root zone and/or feeder dyke system connected to a regular steeply dipping and outwardly tapering pipe-like diatreme, which may be overlain by a crater and extrusive material (when not removed by erosion). The crater and diatreme facies contain pyroclastic rocks, which transition into coherent (sub-volcanic) rocks in the root zone. Kimberlites are of economic value as the major host of gem quality diamonds at the Earth’s surface. They also hold great scientific significance, as the deepest derived melts to reach the surface, with entrained mantle material that provides some of our best information on the structure, composition, and evolution of the sub-continental lithospheric mantle. However, despite over a century of dedicated research, numerous aspects of kimberlite petrogenesis remain poorly understood and contentious. One central issue that this project has addressed is the composition and evolution of kimberlite melts. The composition of kimberlite melts remain poorly constrained because: (1) rocks emplaced near the surface are prone to deuteric and hydrothermal alteration; (2) they have been contaminated by the physical incorporation of xenocrystic and xenolithic material; (3) their parental magmas have been modified by interaction with and partial assimilation of mantle and crustal material; and (4) they undergo syn-emplacement differentiation. Therefore, in this study exceptionaly ‘fresh’ rocks from the well studied kimbeley cluster (the type locality) were examined to gain further insights into kimberlite melt compositions. Better constraints on kimberlite melt compositions are pivotal if we are to move toward a comprehensive understanding of the petrogenesis of these enigmatic rocks. The studied samples derive from the Kimberley cluster (South Africa), which lies within the Western terrane of the Kaapvaal carton. This cluster constitutes the type locality of kimberlites, containing five major kimberlite pipes (The Kimberley mine, De Beers, Dutoitspan, Wesselton, and Bultfontein), numerous smaller pipes, and abundant dyke/sill complexes (e.g., Benfontein, Wesselton Floors, Wesselton Water Tunnels). The Kimberley cluster has been dated by various geochronological techniques yielding emplacement ages of ~80-90 Ma. To provide new insights into the composition and evolution of kimberlite melts, a detailed petrographic study of sub-volcanic (hypabyssal) coherent kimberlites was conducted. This included the investigation of mineralogy, mineral zonation, inclusion populations (mineral, melt, and fluid), and textural relationships between phases, utilizing a range of microscopy techniques. This petrographic data formed the basis of targeted geochemical analysis by electron microprobe. A study of olivine compositions across multiple intrusions of the Kimberley cluster shows that olivine, more than any other mineral, provides the most complete record of kimberlite evolution. This study showed that pre-ascent metasomatism of the lithosphere by kimberlite melts is wide-spread, and that so-called ‘xenocrystic’ olivine is not directly representative of the wider lithosphere due to metasomatism of the conduit by previous pulses of kimberlite magmatism. The composition of kimberlitic liquidus olivine overlaps that of olivine from other mantle-derived carbonate-bearing magmas (orangeites, ultramafic lamprophyres, melilitites), with low Mn/Fe and Ca/Fe, and moderate Ni/Mg ratios. It is suggested that these compositions are typical of olivine in equilibrium with melts derived from carbonate-rich peridotite sources. Compositional zonation patterns indicte that olivine crystallises throughout magma ascent and that its crystallisation continues after emplacement into the upper crust. Magmatic olivine (i.e. crystallised from the kimbelrite magma) displays distinct generations of crystallisation, with increasing Mg, Ca, and Mn contents interpreted as the result of fractional crystallisation and increasing oxygen fugacity (fO2). The stability of olivine at sub-solidus conditions implies that secondary melt inclusions cannot trap primitive melts, but rather evolved residual fluids. Although olivine provides a wealth of information on early kimberlite melt evolution and metasomatism of the surrounding lithosphere, details about the later stages of kimberlite melt evolution are evident in other magmatic groundmass phases. Therefore, detailed petrographic and mineral chemical studies were undertaken on all mineral consistents from a suite of samples from the exceptionally fresh De Beers dyke to determine the crystallisation sequence. In turn, this data yielded insights into melt evolution from the perspective of multiple different magmatic phases. The early stages of kimberlite crystallisation (i.e., olivine, Cr-spinel, Mg-Ilmenite, rutile) are defined by decreasing Mg/Fe ratios. This subsequently reverses (i.e., increasing Mg/Fe) during later groundmass crystallisation, which is attributed to increasing fO2. Comparison with published data shows that the melt parental to early crystallising phases in this dyke are indistinguishable from those in the root-zone intrusions of the Kimberley cluster, meaning that not all dykes are the crystalisation product of magmas that underwent pre-emplacement fractionation. To gain additional insights into the very late stages of kimberlite melt evolution further detailed petrographic and mineral chemical studies were conducted on late-stage groundmass phases (i.e., apatite and mica) from samples of different root zone intrusions and dyke/sill complexes in the Kimberley area. Despite the early crystallising phases (i.e., olivine, Cr-spinel, Mg-ilmenite) being compositional indistinguishable in dykes/sills and root zone kimberlites, the compositions of apatite appear to be controlled by the style of magma emplacement. Apatite from dykes/sills is Si-rich and Sr-poor, whereas apatite in root zone intrusions show the opposite features. The high Si content of apatite in dykes/sills is attributed to the coupled incorporation of silica and a carbonate ion for phosphorus, reflecting higher CO2 contents in the melts parental to dykes/sills. The high Sr content of apatite in root zone intrusions likely requires crystallisation from, or overprinting by, hydrous fluids. These features indicate that dyke/sill kimberlites have higher CO2/H2O ratios than the magma that produced root zone intrusions. This is consistent with petrographic observations, whereby dykes/sills are enriched in carbonates, may contain dolomite, and have lower abundances of serpentine, mica, and monticellite than root-zone kimberlites. These differences in CO2/H2O ratios of the crystallised melt are attributed to differences in emplacement style, whereby a rapid decrease in pressure in root zone kimberlites leads to exsolution of a (CO2-rich) fluid phase, possibly caused by breakthrough to the surface. The knowledge gained through detailed petrographic and mineral chemical studies led to the development a new quantitative method for reconstructing the composition of kimberlite melts. This model allowed for constraints to be placed on the composition of primitive kimberlite melts and their evolution, as they incorporate and assimilate xenocrystic material, undergo fractional crystallisation, and post emplacement alteration. The results of this modelling indicate that the melt parental to the Bultfontein kimberlite was transitional between silicate and carbonate. This composition is consistent with experimental constraints on the amount of CO2 that can be dissolved into kimberlite melts in the upper crust. The reconstructed primitive melt composition is in equilibrium with asthenospheric source rocks. Based on constraints from experimental studies, Kimberley kimberlites could have been produced by ~0.5% melting of carbonated lherzolite in the upper asthenosphere (i.e., 6.0-8.6 GPa and ~1400-1500 degrees Celsius).