Experimental differentiation of kimberlitic to carbonatitic melts: crystallization sequence and liquid line of descent

Olivine is the principal mineral of kimberlite magmas, and is the main contributor to the ultramafic composition of kimberlite rocks. Olivine is partly or completely altered in common kimberlites, and thus unavailable for studies of origin and evolution of kimberlite magmas. The masking effects of alteration, common in kimberlites worldwide, are overcome in this study of exceptionally fresh diamondiferous kimberlites of the Udachnaya-East pipe from the Daldyn-Alakit province, Yakutia, northern Siberia. The serpentine-free kimberlites contain large amount of olivine (~ 50 vol%) in a chloride-carbonate groundmass. Olivine is represented by two populations (olivine-I and groundmass olivine-II) differing in morphology, colour and grain size, and trapped mineral and melt inclusions. The large fragmental olivine-I is compositionally variable in terms of major (Fo85-94) and trace element concentrations, including H2O content (10-136 ppm). Multiple sources of olivine-I, such as convecting and lithospheric mantle, are suggested. The groundmass olivine-II is recognised by smaller grain sizes and perfect crystallographic shapes that indicate crystallisation during magma ascent and emplacement. However, a simple crystallisation history for olivine-II is complicated by complex zoning in terms of Fo values and trace element contents. The cores of olivine-II are compositionally similar to olivine-I, which suggests a genetic link between these two types of olivine. Olivine-I and olivine–II have oxygen isotope values (+5.6 ± 0.1 ‰ VSMOW, 1 std. dev.) that are indistinguishable from one another, but higher than values (+5.18 ± 0.28 ‰) in “typical” mantle olivine. These elevated values most likely reflect equilibrium with the Udachnaya carbonate melt at low temperatures and 18O - enriched mantle source. The volumetrically significant rims of olivine-II have constant Fo values (89.0 ± 0.2 mol%), but variable trace element compositions. Uniform Fo compositions of the rims imply absence of fractionation of the melt’s Fe2+/Mg, which can be possible in the carbonatite melt – olivine system. The kimberlite melt is argued to have originated in the mantle as a chloride-carbonate liquid, devoid of “ultramafic” or “basaltic” aluminosilicate components, but became olivine-laden and olivine-saturated by scavenging olivine crystals from the pathway rocks and dissolving them en route to the surface. During emplacement the kimberlite magma changed progressively towards an original alkali-rich chloride-carbonate melt by extensively crystallising groundmass olivine and gravitational separation of solids in the pipe.

Recent data suggest an active role for chloride-bearing alkali carbonatitic melts in the formation and evolution of kimberlites, whereas experiments indicate that chlorides could be responsible for liquid immiscibility during melting of carbonated mantle rocks. This study considers melting trends in kimberlite-related chloride- carbonate-silicate systems at a pressure of 5.5 GPa.The direction of these trends largely depends on the chlorine concentration in the system. All trends start with a Cl-rich carbonatitic liquid coexisting with crystalline phases. Melting of a peridotite-carbonate-chloride system containing 4.4 wt % Cl results in a gradual transition from a carbonatitic melt (5-7 wt % SiO2, MgO/CaO= 0.5-0.6, ~2 wt%Cl) at 1000-1100 oC through a Cl-rich carbonate-silicate melt (12-15 wt % SiO2, MgO/CaO =0.6, and up to 14 wt % Cl) at 1360-1400 oC towards a Cl-bearing ultrabasic carbonate- silicate melt (UCSM), i.e. kimberlite-like (26-29 wt % SiO2, MgO/CaO=1.5-2.8, and 6-4 wt % Cl), at 1500-1600 oC.This trend results from the specific behavior of chlorides, which are found to be stable crystalline phases up to about 300 oC above the solidus. In addition, the trend touches a miscibility gap at about 1450 oC, although immiscibility does not significantly influence the melt evolution. In contrast, the melting trend of a peridotite system with about 1.3 wt%Cl does not intersect the miscibility gap and proceeds from a carbonatitic melt (~5 wt % SiO2, MgO/CaO= 0.4-0.5, ~2wt % Cl) at 1100-1150 oC toward the UCSM (~25 wt % SiO2, MgO/CaO= 1.7-1.8, and 0.6-1•0 wt % Cl) at 1500- 1600 oC. Chloride-carbonate-silicate systems containing 17 wt % Cl show an abrupt transition from the chloride-carbonate liquid toward the UCSM because of the immiscibility gap between carbonate- silicate and chloride-carbonate melts. Melting relations in all studied chloride-carbonate-silicate systems are exclusively regulated by peritectic reactions between the silicate phases and carbonate constituents (mostly CaCO3) of the melts. Chlorides decrease the solidus temperatures of carbonated peridotites, but do not reduce the temperature interval for the transition from carbonate- to silicate-dominated melts. The experimental results suggest that Cl-rich carbonatite liquids preserved in diamonds and the Cl-rich kimberlites of the Udachnaya-East kimberlite pipe (Yakutia) could represent a linked system of chloride-rich liquids that evolved over a wide temperature interval during incipient melting and evolution of kimberlite magma from a carbonated mantle source containing 3-4 wt % Cl.

Two hundred years of magmatic history are documented by the lavas and tephra sampled from Kīlauea's historical summit eruptions. This paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near Kīlauea's summit caldera since the 17th century. Our results elucidate the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano. Prior to the 1924 summit crater collapse, a trend of increasing incompatible element and CaO and decreasing SiO<inf>2</inf> abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20th century, including during the current Pu`u `Ō`ō eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, relatively small body rather than a plexus of dikes and sills. Olivine fractionation is the dominant petrologic process within this reservoir. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in Kīlauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition. These compositional variations correlate with magma supply rate; both are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. However, Kīlauea's source compositions and partial-melting processes have varied only within a narrow range over the past 350 kyr.

Origin of primitive ultra-calcic arc melts at crustal conditions — Experimental evidence on the La Sommata basalt, Vulcano, Aeolian Islands

Conditions of diamond crystallization in kimberlite melt: experimental data

We present, major element geochemical data for ilmenite grains obtained from heavy mineral concentrate of diamondiferous Majhgawan kimberlite clan diatreme in Central Indian Diamond Province (CIDP) in Panna District of Madhya Pradesh, India. The chemical composition of 148 ilmenite grains suggests different compositional trends when plotted over “Haggerty's parabola” and as seen in MgO-Cr2O3 bivariant plots. The study indicates that the ilmenite crystallized in three stages: the first stage where Cr - poor ilmenite is crystallized from protokimberlitic or kimberlitic melt and forms the base of Haggerty's parabola on MgO-Cr2O3 plots; the second stage ilmenite is rich in MgO and Cr2O3 -represented by left branch of Haggerty’s parabola-might have formed by interaction between melt and lithosphere; the third stage ilmenite is formed by sub-solidus recrystallization in an evolved kimberlite melt due to oxidation and is reflected in the right branch of Haggerty’s parabola in MgO-Cr2O3 plots. The various trends in the ilmenite composition from Majhgawan pipe are attributed to conditions prevailing during ilmenite crystallization in a kimberlite melt ascending through the lithospheric mantle. These geochemical features indicate a genetic link between ilmenite and the host kimberlite melt.

High-Mg carbonatitic melts in diamonds, kimberlites and the sub-continental lithosphere

Experimental constraints on orthopyroxene dissolution in alkali-carbonate melts in the lithospheric mantle: Implications for kimberlite melt composition and magma ascent

Slab–mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle

Origins of kimberlites and carbonatites during continental collision – Insights beyond decoupled Nd-Hf isotopes

Mantle metasomatism refers to the interaction between mantle melt, fluid, and mantle rock. It not only affects the physical and chemical properties of the lithospheric mantle but also plays an important role in the process of metal and gem mineralization. In order to explore the nature and evolution of metasomatism in the lithospheric mantle of the Mengyin area in the eastern part of the North China Craton, this paper combines the previous data of garnet inclusions in diamonds and analyzes the major and trace elements of garnet xenocrysts in the Shengli No. 1 kimberlite pipe from the EPMA and LA-ICP-MS experiments. The experiments show that the garnet xenocrysts of the Shengli No. 1 kimberlite pipe are mainly lherzolitic and harzburgitic garnets. The content of Zr and TiO2 in some garnets are low, which are the characteristics of depleted garnets. Conversely, another group of garnets display high Zr and TiO2 contents, indicative of high-temperature melt metasomatism. When comparing the Ti/Eu ratio of the depleted garnets to that of the primary mantle, a significantly lower value is observed. Additionally, the (Sm/Er)N value undergoes minimal changes, while the Zr/Hf value exceeds that of the primary mantle. These characteristics are indicators of carbonatite melt metasomatism. Garnets that are affected by high-temperature melt metasomatism exhibit low (Sm/Er)N content, a significant variation in the Ti/Eu ratio, and a Zr/Hf value greater than that of the primary mantle. These characteristics indicate the influence of kimberlite melt metasomatism. Garnets impacted by carbonatite melt metasomatism display a strong sinusoidal distribution pattern of rare earth elements (REE) and are often found as lherzolitic garnet xenocrysts and garnet inclusions in diamond. On the other hand, garnets influenced by kimberlite melt metasomatism exhibit a slight sinusoidal REE distribution pattern in harzburgitic garnets and a slight sinusoidal REE distribution or a flat pattern from medium rare earth elements (MREEs) to heavy rare earth elements (HREEs) in lherzolitic garnet xenocrysts. Based on these findings, it is evident that there are at least two types of metasomatism occurring in the lithospheric mantle of the Mengyin area in the eastern part of the North China Craton. The first type involves the metasomatism of early carbonatite melt to the mantle peridotite. Garnets formed under this condition exhibit high Sr and LREE contents, as well as low Zr, Hf, Ti, Y, and HREE contents, indicating depletion characteristics. The second type entails the metasomatism of late kimberlite melts affecting the mantle peridotite. Garnets formed under this process display high Zr, Hf, Ti, Y, and HREE contents.

H2O–CO2 solubility in low SiO2-melts and the unique mode of kimberlite degassing and emplacement

Towards composition of carbonatite melts in peridotitic mantle

<p>Orogenic garnet peridotites exhumed in ultrahigh-pressure-ultrahigh-temperature terranes represent windows into material transfer in deep subduction zones. Multiphase solid inclusions (MSI) trapped in garnet proved to be important tracers of metasomatism by crustal-derived fluids. Our study of the MSI from the Saxothuringian basement in the Bohemian Massif, European Variscan Belt, allowed identifying the source and evolution of the liquids metasomatized the mantle rocks. As the MSI could not be re-homogenized due to a high content of volatiles, their bulk composition was estimated considering the proportions, phase densities and chemical composition of the constituent minerals.</p><p>The MSI occur in an annulus at garnet rim of garnet lherzolite and harzburgite, and throughout garnet in garnet pyroxenite. The major phases of the MSI include amphibole, barian mica and carbonate (dolomite, magnesite). Minor phases are clinopyroxene, orthopyroxene, garnet II, spinel, apatite, monazite, thorianite, graphite, pentlandite, scheelite and sulphides. The proportion of hornblende systematically decreases from pyroxenite and close harzburgite and lherzolite to more distal mantle rocks, where clinopyroxene and garnet II occur instead. By contrast, the amount of barium-bearing phases (barian mica, Ba-Mg carbonate norsethite, barian feldspar) and carbonates increases in the same direction.</p><p>Major element composition of garnet pyroxenite, including enrichment in alkalies and barium, approaches carbonate-silicate melts similar to kimberlites.  Trace element signatures indicate that it is a rare example of low-degree supercritical liquid derived from a mixed crust-mantle source frozen in the mantle. The MSI hosted by garnet in pyroxenite represent a residual solute-rich liquid after high-pressure fractional crystallization of the parental melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO<sub>2</sub>, Cl, F, P). The MSI in peridotites allow tracing the changes of this metasomatizing liquid during its reactive infiltration into peridotite through silicate crystallization as well as interaction with mantle minerals distinct in lherzolite and harzburgite (garnet±clinopyroxene). The liquid evolved from more silicic, solute-rich to more diluted carbonate-rich, with gradual enrichment in LILE (K, Ba) and volatiles (CO<sub>2</sub>, Cl) and LREE fractionation, similar to evolution of kimberlitic to carbonatitic melts through differentiation by fractional crystallization.  </p><p>Here we demonstrate that the MSI trapped in garnet can be used as a unique tool for tracing chemical evolution of the liquids metasomatizing the mantle wedge. Importantly, these results are valid even in the case of the interaction of the trapped material (MSI) with the host garnet, as this potential contamination mainly concerns Al, Si and Cr while majority of the other elements used for petrogenetic implications remained unaffected</p>

This study aims at understanding parental melt compositions and evolutionary history of mantle-derived kimberlitic magmas, using unaltered Udachnaya-East kimberlite as an example. Recent advances in theoretical, experimental and melt inclusion research strongly suggest that the mantle is highly heterogeneous on a small scale, but this heterogeneity is effectively obscured by the blended nature of most erupted magmas. Thus, the original compositions of individual mantle-derived melt batches that supposedly reflect their respective mantle sources are in fact averaged, owing to mixing of melts en route to the surface. The exception may be occasional low degree partial melts that erupt with little or no mixing with subsequent melt fractions. However, such melts are particulary prone to reaction with country rocks along the pathways to ascent, and are also very rare among erupted rocks. Among all known erupted mantle-derived magmas, kimberlites offer the deepest probes into the convecting subcontinental mantle, derived from the lowest degrees of melting. Such origins make kimberlites most suitable for characterising primitive (undepleted by previous melting) mantle assemblages with their likely enrichment in the volatile elements, and thus with the lowest solidus temperatures. On the other hand, the large amount of lithospheric and crustal xenoliths in kimberlites and their typically high degree of alteration, can significantly affect interpretations that can be drawn from bulk rock analyses. This study attempts to overcome such problems related to alteration of kimberlites and presents detailed petrographic, mineralogical, chemical, and isotope data on exceptionally fresh kimberlite samples from the diamondiferous Udachnaya-East pipe (Daldyn-Alakit region, Siberia). I demonstrate that the Udachnaya-East rocks have radiogenic isotopic compositions, petrographic features and major and trace element geochemistry typical of group-I kimberlites. However, unlike common kimberlites, the studied samples show no primary or secondary serpentine, and thus are essentially anhydrous (< 0.5 wt% H2O), but CO2-rich (10-11 wt%). In contrast with other kimberlites worldwide, the Udachnaya-East samples are uniquely enriched in chlorine and alkalies (2.3-3.2 wt% Cl, 2.6-3.7 wt% Na, and 1.6-2.0 wt% K). Enrichment in CO2, Cl and alkalies is expressed in the essentially alkali-carbonate (shortite, zemkorite) and alkali-chloride (halite, sylvite) composition of the kimberlite groundmass. These minerals cement olivine phenocrysts and form round segregations (nodules). Radiogenic isotope compositions (Nd, Sr, and Pb) of the chloride, chloride - alkali carbonate, carbonate and oxide-silicate constituents in the groundmass of the Udachnaya-East kimberlite effectively show the coexistence of these phases in the closed system since kimberlite emplacement ~ 347 Ma. Complementary to insights into radiogenic isotope composition of the parental mantle source of the Udachnaya-East kimberlite, my study explores stable isotope compositions of the kimberlite groundmass (O, C and S isotopes), chloride-carbonate nodules (O and C isotopes) and two populations of olivine (O isotopes). Detailed study of zoning and composition of the groundmass olivine-II demonstrates very complex fractionation of the ultramafic primary kimberlite melt. Additional constraints are provided by olivine-hosted inclusions of cogenetic minerals, fluid and melt. The wide compositional interval shown by the cores of olivine-II (Fo86-93) reflects either crystallisation from different melt batches, or re-equilibration (in terms of Fe-Mg) with different mantle lithologies. I report the discovery of previously unknown inclusions of high-Ca pyroxene in the olivine-II cores. They formed in the diamond stability field (45-50 kb) at temperatures of 900-1100oC, from a melt with a trace element composition resembling that of the kimberlite groundmass. The inferred P-T conditions correspond to the lower part of lithosphere beneath the Siberian craton. I consider that a prolonged evolution of the kimberlite magma by olivine crystallisation was responsible for a build-up of abundances of alkalies, chloride, carbonate, and sulphate components. As a result, the residual kimberlite magma acquires an essentially non-silicate composition, but high in CO2, Cl, and alkalies. This magma crystallises at low temperatures (<650-750oC), and undergoes chloride-carbonate liquid immiscibility at ~600oC. I propose that significant amounts of alkali chlorides and carbonates in the Udachnaya-East kimberlite are pristine magmatic components inherited from the kimberlite parental/primary magma. This enrichment may be responsible for the kimberlite low liquidus temperatures, low viscosities, and rapid ascent.