THE “CLINOPYROXENE” PALEOGEOTHERM BENEATH THE OBNAZHENNAYA KIMBERLITE PIPE AND THIСKNESS OF LITHOSPHERE UNDER THE KUOYKA FIELD (SIBERIAN CRATON, YAKUTIA)

The mantle paleogeotherm under the Obnazhennaya kimberlite pipe (Kuoika field, Siberian craton) was reconstructed using the chemical composition of clinopyroxene xenocrystals and the FITPLOT program. The lithosphere thickness 187–193 km and surface heat flow 41–42 mW/m2 were measured for the Obnazhennaya pipe at the time of kimberlite magmatism in the Mesozoic. The lithosphere thickness was found to be much smaller than that in the central part of the Siberian craton (210–230 km), where Paleozoic diamond-bearing kimberlite pipes-deposits are located. It is however comparable to the highly diamond-bearing Kimberley field in the Kaapvaal craton (South Africa). The absence of diamonds in the pipes of the Kuoika field, but poor diamondiferous Dyanga pipe, might be associated with the more intense metasomatic alteration of the rocks within the lithospheric mantle of this region in the Mesozoic time, as compared to the central part of the Siberian craton in the Middle Paleozoic time

«КЛИНОПИРОКСЕНОВАЯ» ПАЛЕОГЕОТЕРМА ПОД КИМБЕРЛИТОВОЙ ТРУБКОЙ ОБНАЖЕННОЙ: МОЩНОСТЬ ЛИТОСФЕРЫ ПОД КУОЙКСКИМ ПОЛЕМ (СИБИРСКИЙ КРАТОН, ЯКУТИЯ)

The mantle paleogeotherm under the Obnazhennaya kimberlite pipe (Kuoika field, Siberian craton) was reconstructed using the chemical composition of clinopyroxene xenocrystals and the FITPLOT program. The lithosphere thickness 187–193 km and surface heat flow 41–42 mW/m2 were measured for the Obnazhennaya pipe at the time of kimberlite magmatism in the Mesozoic. The lithosphere thickness was found to be much smaller than that in the central part of the Siberian craton (210–230 km), where Paleozoic diamond-bearing kimberlite pipes-deposits are located. It is however comparable to the highly diamond-bearing Kimberley field in the Kaapvaal craton (South Africa). The absence of diamonds in the pipes of the Kuoika field, but poor diamondiferous Dyanga pipe, might be associated with the more intense metasomatic alteration of the rocks within the lithospheric mantle of this region in the Mesozoic time, as compared to the central part of the Siberian craton in the Middle Paleozoic time.На основе химического состава ксенокристаллов клинопироксена и с использованием программного пакета FITPLOT была реконструирована мантийная палеогеотерма под кимберлитовой трубкой Обнаженной (Куойкское поле, Сибирский кратон). Значения мощности литосферы и поверхностного теплового потока для трубки Обнаженной на момент кимберлитового магматизма (мезозой) составили 187–193 км и 41–42 мВт/м2. Полученное значение мощности литосферы значительно меньше, чем в центральной части Сибирского кратона, где располагаются алмазоносные кимберлитовые трубки-месторождения среднепалеозойского возраста (210–230 км), однако оно сопоставимо с таковым в районе высокоалмазоносного поля Кимберли (Каапвальский кратон, Южная Африка). Отсутствие алмазов в трубках Куойкского поля, за исключением убогоалмазоносной трубки Дьянга, может быть связано с интенсивным метасоматическим преобразованием пород литосферной мантии данного региона в мезозое по сравнению с центральной частью Сибирского кратона в среднем палеозое

In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass–ceramics–Influence of ZrO2 and TiO2 nucleating agents

International audienceThe evolutionofNi2þ environmenthasbeensystematicallyinvestigatedusingopticaland in situ X-rayabsorption spectroscopy(XAS)todeterminetheinfluence ofnucleatingagents(TiO2 and/or ZrO2) duringthe formationofspinelinmagnesiumaluminosilicateglass–ceramics.Theresultswerecomplementedby in situ X-raydiffractiondata.AccordingtoXASandopticalspectroscopy,thenatureofnucleatingagents doesnotmodifysignificantly theNienvironmentininitialglasses.However,ithasarelativelystrong influence intheobservedcrystallizationsequence.Ni2þ ions donotentertheZr-containingcrystalline phaseofZrO2 or ZrTiO4 but aNi2þ coordinationchangefromthe fivefold coordinatedsites,with asmallamountoftetrahedralsitesinparentglasses,to [6]Ni2þ and [4]Ni2þ sites inspinel(inglassesnucleated byZrO2 and/or TiO2) orin β-quartzsolidsolutions(inglassesnucleatedbyZrO2) hasbeenfound