XENOLITHS OF POLYMICTIC BRECCIAS FROM KIMBERLITES OF THE YAKUTIAN DIAMONDIFEROUS PROVINCE

The polymictic breccias, extremely rare mantle conglomerates being the fragments of rocks and large mantle minerals cemented with fine-grained mass have been long provoking interest of petrologists worldwide. This work provides a comparative analysis of two xenoliths of polymictic rocks from kimberlite pipes of different age and productivity occurring in the Siberian craton. The similarity of the chemical composition of some minerals of polymictic breccias from kimberlites within different parts of the Siberian craton assumes possible formation of these minerals caused by the same factors in the prekimberlite period of these parts of the lithospheric mantle. A wide range of compositions and chaotic zoning of minerals, the presence of exsolution textures in orthopyroxene, ilmenite, sulfide, and kelyphite rims on garnet suggest that the fragments of the studied xenoliths are unbalanced. Sample SH18/20 is the first polymictic breccia showing asthenospheric melt-fluids sampling shallow depths of the spinel facies

LITHOSPHERIC MANTLE COMPOSITION AND STRUCTURE VARIATIONS UNDER THE SIBERIAN PLATFORM KIMBERLITE FIELDS OF DIFFERENT AGES

We discuss herein the differences both in composition and structure of the lithospheric mantle of the Siberian Platform under kimberlite fields of different ages. Having analyzed the pyrope garnets from kimberlites of different ages this study aimed to identify the genesis, extent and probable causes for the evolution of the composition and structure of the lithospheric mantle within various SP blocks. Acquired results revealed noticeable differences in the lithosphere thickness of different blocks for Middle Paleozoic and Mesozoic, as well as within the same time. It was found, that the greatest differences in the thickness of the lithosphere individual blocks are typical for the Middle Paleozoic. The results obtained for pyrope garnets from the Paleozoic sedimentary secondary collectors was used to get direct evidence on the existence of thick lithosphere blocks (230–240 km) under the southern margin of the Olenek uplift, north and south of the Kyutyungde trough, as well as under the western and eastern margins of the Udzhinsky uplift. Significant variations in the intensity and extent of silicate-type metasomatism were identified by investigating of pyropes from peridotites of the lithospheric mantle, sampled by kimberlites of both Middle Paleozoic and Triassic fields. Direct evidence indicates high prospects for discovering new fields of diamondiferous high-grade kimberlites of the Middle Paleozoic age in some areas of the Siberian Platform, primarily in its Arctic region

КСЕНОЛИТЫ ПОЛИМИКТОВЫХ БРЕКЧИЙ ИЗ КИМБЕРЛИТОВ ЯКУТСКОЙ АЛМАЗОНОСНОЙ ПРОВИНЦИИ

The polymictic breccias, extremely rare mantle conglomerates being the fragments of rocks and large mantle minerals cemented with fine-grained mass have been long provoking interest of petrologists worldwide. This work provides a comparative analysis of two xenoliths of polymictic rocks from kimberlite pipes of different age and productivity occurring in the Siberian craton. The similarity of the chemical composition of some minerals of polymictic breccias from kimberlites within different parts of the Siberian craton assumes possible formation of these minerals caused by the same factors in the prekimberlite period of these parts of the lithospheric mantle. A wide range of compositions and chaotic zoning of minerals, the presence of exsolution textures in orthopyroxene, ilmenite, sulfide, and kelyphite rims on garnet suggest that the fragments of the studied xenoliths are unbalanced. Sample SH18/20 is the first polymictic breccia showing asthenospheric melt-fluids sampling shallow depths of the spinel facies.Полимиктовые брекчии, крайне редкие мантийные конгломераты, представляющие собой обломки пород и крупных мантийных минералов, сцементированных мелкозернистой массой, вызывают устойчивый интерес у петрологов мира. В работе проведен сравнительный анализ двух ксенолитов полимиктовых пород из разновозрастных и разнопродуктивных кимберлитовых трубок Сибирского кратона. Близость химического состава ряда минералов полимиктовых брекчий из кимберлитов разных частей Сибирского кратона указывает на возможность формирования этих минералов под воздействием одних и тех же факторов в докимберлитовый период этих частей литосферной мантии. Широкий диапазон составов и хаотичная зональность минералов, наличие структур распада в ортопироксене, ильмените, сульфиде, келифитовые каймы на гранате предполагают неуравновешенность фрагментов изученных ксенолитов. Образец SH18/20 является первой полимиктовой брекчией, демонстрирующей опробование астеносферными расплав-флюидами небольших глубин шпинелевой фации

ВАРИАЦИИ СОСТАВА И СТРОЕНИЯ ЛИТОСФЕРНОЙ МАНТИИ ПОД РАЗНОВОЗРАСТНЫМИ КИМБЕРЛИТОВЫМИ ПОЛЯМИ СИБИРСКОЙ ПЛАТФОРМЫ

We discuss herein the differences both in composition and structure of the lithospheric mantle of the Siberian Platform under kimberlite fields of different ages. Having analyzed the pyrope garnets from kimberlites of different ages this study aimed to identify the genesis, extent and probable causes for the evolution of the composition and structure of the lithospheric mantle within various SP blocks. Acquired results revealed noticeable differences in the lithosphere thickness of different blocks for Middle Paleozoic and Mesozoic, as well as within the same time. It was found, that the greatest differences in the thickness of the lithosphere individual blocks are typical for the Middle Paleozoic. The results obtained for pyrope garnets from the Paleozoic sedimentary secondary collectors was used to get direct evidence on the existence of thick lithosphere blocks (230–240 km) under the southern margin of the Olenek uplift, north and south of the Kyutyungde trough, as well as under the western and eastern margins of the Udzhinsky uplift. Significant variations in the intensity and extent of silicate-type metasomatism were identified by investigating of pyropes from peridotites of the lithospheric mantle, sampled by kimberlites of both Middle Paleozoic and Triassic fields. Direct evidence indicates high prospects for discovering new fields of diamondiferous high-grade kimberlites of the Middle Paleozoic age in some areas of the Siberian Platform, primarily in its Arctic region.Обсуждается проблема различий состава и строения литосферной мантии Сибирской платформы под разновозрастными кимберлитовыми полями. Целью работы являлось выяснение характера, масштабов и вероятных причин эволюции состава и строения литосферной мантии различных блоков платформы на основании изучения пироповых гранатов из разновозрастных кимберлитов различных полей. Анализ полученных результатов показал наличие существенных различий в мощности литосферы различных блоков не только на разное время (среднепалеозойское и мезозойское), но и для одного времени, причем наибольшие различия мощности отдельных блоков литосферы характерны для среднепалеозойского времени. На основании изучения пироповых гранатов из осадочных коллекторов палеозойского возраста получены прямые признаки существования на донижнекарбоновое время блоков мощной литосферы (до 230–240 км) под южным обрамлением Оленекского поднятия (участки к северу и югу от Кютюнгдинского прогиба), а также под западным и восточным обрамлением Уджинского поднятия. Значительные вариации интенсивности и масштабов метасоматоза силикатного типа установлены на основании изучения пиропов из перидотитов разрезов литосферной мантии, «опробованных» кимберлитами полей как среднепалеозойского, так и триасового возраста. Полученные результаты прямо указывают на высокие перспективы выявления новых полей высокоалмазоносных кимберлитов среднепалеозойского возраста в ряде районов Сибирской платформы, в первую очередь ее арктических территорий

Atomistic model of diopside–K-jadeite (CaMgSi2O6–KAlSi2O6) solid solution

Atomistic model was proposed to describe the thermodynamics of mixing in the diopside–K-jadeite solid solution (CaMgSi2O6–KAlSi2O6). The simulations were based on minimization of the latticeenergies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions betweenCaMgSi2O6 and KAlSi2O6 and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T–X diagram for the system CaMgSi2O6–KAlSi2O6 at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-adeite (KAlSi2O6) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside–K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi2O6 in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member

Carbonate-silicate composition of diamond-forming media of fibrous diamonds from the Snap Lake area (Canada)

This study presents new data on the compositions of microinclusions in fibrous diamonds from the Snap Lake area in the eastern part of the Slave Craton (Canada). The compositional trends of diamond microinclusions are consistent with those of diamond-forming media ranging continuously between a highly carbonatitic endmember and a highly silicic endmember. The microinclusions exhibit general enrichment of most incompatible elements, which is probably indicative of their crystallization during partial melting of mantle peridotites and eclogites. Our results also suggest that the diamond analyzed in this study may have formed as a result of interaction between carbonate-silicate melts and peridotitic wall-rocks at the base of a thick lithospheric mantle at depths below 300 km. The trace element distributions in the studied diamond microinclusions show a general similarity to those previously found in the parental kimberlites and carbonatites. These data suggest that diamonds may have crystallized either directly from a kimberlitic/carbonatitic melt or from a proto-kimberlitic fluid/melt, which was derived from a source also common to kimberlites. This is supported by differences in the major element compositions of diamond-forming fluids/melts and kimberlites.4 page(s