Epigenetic aragonite in a sheared lherzolite xenolith from the Udachnaya kimberlite pipe

<p>Here we report the first finding of the high-pressure polymorph of calcium carbonate (aragonite) in the interstitial space of a sheared lherzolite xenolith from kimberlites of the Udachanaya diamond deposit (Siberian craton, Russia). Xenoliths with a sheared texture are the deepest mantle rocks sampled by kimberlite magma from 180-230 km depth. According to experimental data, aragonite is the high-pressure polymorph of calcium carbonate, which is stable at upper mantle pressure and temperature. Thereby aragonite is used as a reliable geobarometer in studies of magmatic and ultrahigh-pressure metamorphic rocks.<br>Aragonite was determined by Raman spectroscopy study. The Raman bands at 208 cm<sup>-1</sup>, 702-706 cm<sup>-1</sup> and 1462 cm<sup>-1</sup> are the identification features of aragonite. Chemical analyses of aragonite were obtained by scanning electron microscope with an energy dispersive system. Some analyses were verified by electron microprobe as well. The concentration of SrO in aragonite ranges from 0.5 to 8.8 wt.%. Aragonite has a Na<sub>2</sub>O concentration of 0.1-1.1 wt.%.<br>Aragonite (up to 100 µm) is the most common subordinate mineral from the interstitial space of this xenolith. It occupies on average 70 vol.% of the interstitial space. Aragonite grains consist of three chemically distinct zones. The first zone (core) is characterized by a low content of SrO (<1.5 wt.%) and low Mg# (~15). The second zone has roughly the same SrO but noticeably higher Mg# (~50). The third zone (rim) contains much higher concentration of SrO (up to 8.81 wt.%) and high Mg# (~50).<br>Sheared peridotite are located in the lithospheric mantle significantly below the aragonite-calcite equilibrium line. In particular, the investigated peridotite equilibrated at 1350°С and 69 kbar (~215 km). The presence of zoned aragonite from this peridotite means that this rock has been infiltrated by metasomatic agent. Numerical calculations reveals that such zoning can be preserved for 1 year at 1300°С (~equilibrium temperature of sheared peridotites) and for 10 years at 1000°С (~temperature of kimberlite magma at subsurface conditions). The short preservation time of zoning in aragonite (1-10 years) proves that aragonite could be formed immediately prior to kimberlite magmatism or after the capturing of the xenolith by kimberlite magma. Using adiabats of kimberlite magma and P-T parameters of aragonite stability in the upper mantle, aragonite in the studied sample was formed at the depth range of 80-215 km.<br>As the preservation time of zoning in aragonite is noticeably short (taking into account high temperatures), the best candidate for the role of an agent, which infiltrated the xenolith, is a primitive kimberlite melt of the Udachnaya pipe. The high percentage (70%) of aragonite in the interstitial space of the studied sheared lherzolite xenolith proves that such primitive kimberlite melt had carbonatitic composition. Our results show that not only different silicate-rich melts, but also carbonate or cabonated silicate melts might play a key role in mantle modifications. Carbonate melts are very suitable diamond-forming media and may support the idea of a genetic link between some diamonds and kimberlite magmatism.<br>This study was supported by the Russian Science Foundation (grant No 18-77-10062).</p>

Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaiá-II kimberlite, Alto Paranaíba Province, southeast Brazil

ABSTRACT The Indaiá-I and Indaiá-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaiá-I is classified as an archetypal group-I kimberlite, Indaiá-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaiá-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaiá-II. Mineral reactions observed in Indaiá-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12–2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaiá-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaiá-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaiá-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaiá-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaiá-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaiá-I and Indaiá-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaiá-II is a satellite blind pipe linked to the main occurrence of Indaiá-I.

Incipient stages of transformation of round natural diamonds under dissolution in Fe-S melt at high pressure

Research subject. The article presents the results of a microscopic and photogoniometric study of natural rounded diamonds of tetraghexahedral habit from the kimberlite pipe “Internationalnaya” (Yakutia). The diamonds was partially dissolved in a sulphur-containing iron melt (sulphur content of 15–30 wt %) at 4.5 GPa and 1450ºС.Methods. The experiments were carried out on a multi-puncheon apparatus of a “split-sphere” type in high-pressure solid-phase cells made of refractory oxides ZrO2, CaO, MgO using a cylindrical shape graphite heater. The crystals were studied using an MBS-10 optical microscope with a photo camera, and a Jeol JSM-6510LV scanning electron microscope. A goniometric study of diamond crystals was carried out by a photo method in a cylindrical chamber. It was found that when a sulphur content was 15 wt %, diamond crystals of tetrahexahedral habit were transformed into a curved shaped octahedroids with morphological features similar to natural diamonds found in kimberlites. When the sulphur content was 23–30 wt %, the rate of dissolution of diamonds in the Fe-S melt sharply reduced, while the diamond surface at the micro level became covered with numerous etching hillocks, whose sidewalls have surfaces similar to flat-faced {111} form. Dissolution of the rounded diamonds in the Fe-S melt at high pressure occurred by a “normal” mechanism, that is perpendicular to the surface of the dissolving crystal through trigonal dissolution layers, while a tangential-layered mechanism played a minor role.Conclusion. The natural diamond crystals could underwent dissolution in the mantle before they were captured by kimberlite magma. Two fundamentally different types of homomorphic and typomorphic features of the dissolution forms observed on natural diamonds can be determined, namely: on one side, those associated with storage in mantle before the crystals were captured by the kimberlite magma, and on the other side, with the kimberlite process itself. The presence of octahedral diamonds with parallel (trigonal) striation in kimberlite deposits may indicate on a high degree of diamond preservation due to relatively insignificant effect of the kimberlite magma. This, undoubtedly, should help to decipher the diamond genesis and, possibly, improve the mineralogical criteria used in diamond exploration.

Morphological characteristics of diamond crystals arising as the result of dissolution in Fe-Ni-S MELT at high pressure

The first results on the dissolution of flat-faced diamond crystals of octahedral habit in Fe-Ni-S melt at 3,5 GPa and 1400 С are presented. It has been established that as a result of dissolution, flat-faceted diamond crystals are transformed into curve-faced individuals with morphological features similar to kimberlite diamonds. It is concluded that similar forms of natural diamonds could have been formed in reducing domains of the Earths mantle before entering the kimberlite magma.

Dating Kimberlites: Methods and Emplacement Patterns Through Time

Key to deciphering the origin and tectonic setting of kimberlite magmatism is an accurate understanding of when they formed. Although determining absolute emplacement ages for kimberlites is challenging, recent methodological advances have contributed to a current database of >1,000 precisely dated kimberlite occurrences. Several profound findings emerge from kimberlite geochronology: kimberlites were absent in the first half of Earth history; most kimberlites were emplaced during the Mesozoic; kimberlite magma formation may be triggered by a variety of Earth processes (deep mantle plumes, subduction of oceanic lithosphere, continental rifting); and enhanced periods of kimberlite magmatism coincide with supercontinent breakup.

Petrogenesis of a Hybrid Cluster of Evolved Kimberlites and Ultramafic Lamprophyres in the Kuusamo Area, Finland

Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0·8 ± 0·1; Th/Nb = 0·07 ± 0·01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe2+/(Fe2+ + Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 ± 4·1 Ma (2σ), which is considerably older than the ∼750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ∼100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors.