Retention of radiation damage in zircon xenocrysts from kimberlites, Northern Yakutia

We have studied zircon xenocrysts from Mesozoic kimberlites from the Kuoika and Ary–Mastakh fields in Northern Yakutia. Zircon xenocrysts are assumed to originate from crustal rocks. Our SHRIMP (Sensitive High mass Resolution Ion MicroProbe) analyses yielded predominantly concordant U–Th–Pb ages (up to ~ 3570 Ma; Paleoarchean) that clearly predate kimberlite formation. The general U–Th–Pb concordance observed excludes notable disturbance of the zircon xenocrysts U–Th–Pb isotope system during kimberlite ascent and emplacement. In addition, zircon xenocrysts were found to be significantly more radiation-damaged than would correspond to damage accumulation only since the time of kimberlite formation. This observation first indicates that zircon crystals were sampled by the kimberlite magma at comparably shallow depths not exceeding 10–12 km. If, in contrast, zircon crystals originated from deeper levels of the Earth's crust, they would have been exposed to temperatures of 250–300 °C or more. This in turn would have caused long-term thermal annealing of the radiation damage, which was however not observed in our study. Second, our observation contradicts the hypothesis that high temperatures experienced by zircon xenocrysts during kimberlite ascent will cause notable structural reconstitution by short-term thermal annealing. Consequently, zircon crystals cannot have spent more than a few hours at temperatures exceeding ca. 700–800 °C, or more than a few days at temperatures exceeding ca. 500–600 °C. This in turn suggests that (i) temperatures of the ascending kimberlite magmas were rather moderate, and (ii) kimberlite ascent is a comparably short process followed by rapid cooling

Mg-Ilmenite from Kimberlites, Its Origin

The main regularities of the saturation of kimberlite rocks with the accessory mineral Mg-ilmenite (Ilm), the peculiarities of the distribution of Ilm compositions in individual pipes, in different clusters of pipes, in diamondiferous kimberlite fields, are considered as the example of studies carried out within the Yakutian kimberlite province (Siberian Craton). Interpretation of different crystallization trends in MgO-Cr2O3 coordinates (conventionally named “Haggerty’s parabola”, “Steplike”, “Hockey stick”, as well as the peculiarities of heterogeneity of individual zonal and polygranular Ilm macrocrysts made it possible to propose a three-stage model of crystallization Ilm: (1) Mg-Cr poor ilmenite crystallizing from a primitive asthenospheric melt; (2) Continuing crystallization in the lithospheric contaminated melt by MgO and Cr2O3; (3) Ilmenite subsequently underwent sub-solidus recrystallization in the presence of an evolved kimberlite melt under increasing oxygen fugacity (ƒO2) conditions

Vitimアルカリ玄武岩及びYakutianキンバライト岩(ロシア・シベリア地方)に由来するイルメナイト : 単斜輝石シンプレクタイトの鉱物学と地球化学

The ilmenite-clinopyroxene symplectites are rare among xenoliths from kimberlite and related rocks. We found similar ilmenite-clinopyroxene symplectite nodules in Miocene picrobasalts and Pleistocene basanites of the Vitim volcanic field. For a comparative study of ilmenite-clinopyroxene intergrowths from kimberlites and alkaline basalts, we combined the data on xenoliths from Yakutian kimberlites (Mary pipe, Kuoisk field, and Mir pipe, MaloBotuobinsk field) and Vitim alkaline basalts. Compositional and textural variations in symplectite ilmenites revealed likeness of their genesis from both the alkaline basalts and kimberlites. Clinopyroxene from alkaline basalts corresponds to low-Mg end of the megacryst trend. The beginning of ilmenite precipitation is marked by inflection of Ti-Fe trend in the variation diagram for clinopyroxene. Clinopyroxene from Mary symplectites is divided into two groups indicating different types of parental melts or repeating of injection pulses. Trace element variations in megacryst and symplectite clinopyroxene show similar patterns for symplectite clinopyroxene in both kimberlites and basalts with more depleted HREE for those from kimberlites. Experimental work of Wyatt (1977) had shown a possibility for eutectic precipitation of clinopyroxene and ilmenite after an clinopyroxene-only crystallization. From estimated residual melt fraction after the symplectite precipitation in the Vitim area, we suppose, that eutectic clinopyroxenes must be more evolved than observed ones. Ilmenite-clinopyroxene symplectites characterize the beginning stage of simultaneous mineral precipitation in both kimberlites and alkaline basalts. Transitions from symplectite to common intergrowths within a single sample give an evidence for their kinship. Thus, the ilmenite-clinopyroxene symplectite formation can not be explained by simple eutectic crystallization and calls for some specific conditions of melt fractionation.論文Articl

Primary Composition of Kimberlite Melt

The compositions (mineralogy, major- and trace-element chemistry of rocks and minerals, and Sr-Nd-Hf isotope systematics) of two kimberlite bodies, the Obnazhennaya pipe and the Velikan dyke from the Kuoika field, Yakutian kimberlite province (YaKP), which are close to each other (1 km distance) and of the same Upper Jurassic age, are presented. The kimberlites of the two bodies are contrastingly different in composition. The Obnazhennaya pipe is composed of pyroclastic kimberlite of high Mg and low Ti composition and is characterized by high saturation of clastic material of the lithospheric mantle (CMLM). The pyroclastic kimberlite contains rare inclusions of coherent kimberlite from previous intrusion phases. The Velikan dyke is represented by coherent kimberlite of relatively high Fe and high Ti composition, having neither mantle xenoliths nor olivine xenocrysts. The similarity of the isotopic geochemical characteristics for kimberlites from both bodies and their spatial and temporal proximity suggest that their formation is associated with the presence of a single primary magmatic source located in the asthenosphere. It is proposed that the asthenospheric melt differentiated into two parts: (1) a predominantly carbonate composition and (2) a carbonate–silicate composition, which, respectively, formed (a) low Fe and (b) Mg-Fe and high Fe-Ti petrochemical types of kimberlites. Both parts of the melt had different capabilities to capture the xenogenic material of the mantle rocks. The greater ability to destroy and, subsequently, capture CMLM belongs to the melt, which formed a high Mg type of kimberlite and which, according to the structural–textural classification, more often corresponds to the pyroclastic kimberlite. It is suggested that the primary kimberlite melt of asthenospheric origin is similar in composition to the high Fe, high Ti, coherent kimberlite from the Velikan dyke (in wt. %: SiO2–21.8, TiO2–3.5, Al2O3–4.0, FeO–10.6, MnO–0.19, MgO–21.0, CaO–17.2, Na2O–0.24, K2O–0.78, P2O5–0.99, CO2–12.6). It is concluded that the pyroclastic kimberlite contains only xenogenic Ol, whereas some of the Ol macrocrysts with high FeO content in the coherent kimberlite have crystallized from the melt. The similarity of Sr-Nd-Hf isotope systematics and trace element compositions for kimberlites of different ages (from Devonian to Upper Jurassic) in different parts of the YaKP (in the Kuoika, Daldyn and Mirny fields) indicates a single long-lived homogeneous magmatic asthenospheric source