The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of...

Olivine with diamond-imposed morphology included in diamonds. Syngenesis or protogenesis?

A new suite of 173 clinopyroxene grains from heavy-mineral concentrates of the diamondiferous Novinka kimberlite (Upper Muna field, Yakutia) has been analyzed for major and minor elements with an electron microprobe to perform a thermobarometric study and model the thermal structure of the Archean Upper Muna lithospheric mantle. Scrupulous evaluation of propagation of analytical uncertainties on pressure estimates revealed that (1) the single-clinopyroxene geobarometer can be very sensitive to analytical uncertainties for particular clinopyroxene compositions, and that (2) most clinopyroxenes from Novinka have compositions that are sensitive to analytical uncertainties, notwithstanding their apparent compositional suitability for single-clinopyroxene thermobarometry based on previously proposed application limits. A test on various mantle clinopyroxenes containing different proportions of the sensitive elements Cr, Na, and Al allowed us to identify clinopyroxene compositions that produce unacceptably high propagated errors and to define appropriate analytical conditions (i.e., higher beam currents and longer counting times for specific elements) that allow precise P-T estimates to be obtained for sensitive compositions. Based on the results of our analytical test, and taking into account the intrinsic limitations of the single-clinopyroxene thermobarometer, we have designed a new protocol for optimum thermobarometry, which uses partly revised compositional filters. The new protocol permits precise computation of the conductive paleogeotherm at Novinka with the single-clinopyroxene thermobarometer of Nimis and Taylor (2000). Thermal modeling of the resulting P-T estimates indicates a ~34 mW/m 2 surface heat flow, a thermal lithosphere thickness of ~225 km, and an over 100 km thick “diamond window” beneath Novinka in the middle Paleozoic (344–361 Ma). We estimate that appropriate analytical conditions may extend the applicability of single-clinopyroxene thermobarometry to over 90% of clinopyroxene-bearing garnet peridotites and pyroxenites and to ~70% of chromian-diopside inclusions in diamonds. In all cases, application to clinopyroxenes with Cr/(Cr+Al) mol P > 4.5 GPa.

/pdf/error-sources-in-single-clinopyroxene-thermobarometry-and-a-d105miv6j6.pdf

Error sources in single-clinopyroxene thermobarometry and a mantle geotherm for the Novinka kimberlite, Yakutia

This paper considers the lithospheric structure and evolution of the wider Barents–Kara Sea region based on the compilation and integration of geophysical and geological data. Regional transects ar ...

/pdf/tectonic-implications-of-the-lithospheric-structure-across-lxi8ssul7l.pdf

Tectonic implications of the lithospheric structure across the Barents and Kara shelves

Thermo-chemical structure of the lithospheric mantle underneath the Siberian craton inferred from long-range seismic profiles

Thermal regime of sedimentary basins in the Tarim, Upper Yangtze and North China Cratons, China

Thermal evolution of the continental crust beneath the northeastern Siberian craton was studied based on the interpretation of apatite fission-track ages. The samples selected for AFT dating were collected from depths between 2 and 3 km along a 1000-km-long profile, from the crystalline basement of the Siberian platform. The AFT ages range from 185 to 222 Ma, indicating that in the late Triassic-early Cretaceous, the top of the crystalline basement was cooled below ~100 ° C. Once the apatite cooled below this temperature, it began to accumulate and preserve tracks produced by spontaneous fissioning of 238 U, and the number of tracks pre- served is effective in determining the ages of events using the apatite fission-track method (AFT). The study showed that the apatite from Archean rocks was largely formed at 1.8-1.9 Ga as a result of a Pale- oproterozoic metamorphic overprinting during the terrane collision and the subsequent accretion of the Siberian craton. The last thermal event, the self-heating of the collision prism, was terminated by cooling at ~1.3 Ga. At that time, the Rb-Sr isotopic system became closed and the upper crust passed the ~300°e isograd. The calcu- lation results showed that on further cooling, the ~100°e isograd was passed at 1143 Ma. This age estimate could be obtained using AFT dating if the above event had been the last one in the thermal history of the Sibe- rian craton. The obtained track ages indicate the existence of a repeated, significantly younger, heating of the crystalline crust due to some local reason. The present-day temperature at the sampling depth is 29°e based on the geothermal gradient curve, which was derived from the surface heat flow rate of 25 mW/m 2 and the crustal heat production value calculated from the concentrations of radioactive elements in the crust. In the near geological past, the maximum heat flow reaching 45 mW/m 2 was determined from the mineral equilibria in the mantle xenoliths from the 245-135 Ma kimber- lites. Based on the respective geotherm, the crustal temperature in the study area did not exceed 53°e . It is obvi- ous that an enhanced heat flux during kimberlite magmatism would not cause additional heating of the crust in the Mesozoic above ~ 100°e . The emplacement of flood basalts at 250 Ma was undoubtedly most important but underestimated the thermal event in the Mesozoic history of the Siberian platform. Numerous dolerite dykes were intruded at a great dis- tance, far beyond the limits of the volcanic facies of the Putorana plateau, extending onto the study area. They were probably sourced from sills, which were emplaced along the base of the crust during basaltic underplating. The calculation results suggest that the reason of the crust's heating in the Mesozoic (222-185 Ma) might have been the emplacement of a large sill (or sills) into the base of the crust at 250 Ma. This heating of the top of the crystalline basement of the Siberian platform in the Mesozoic above ~100°e suggests that the respective heating event may have occurred at the base of the sedimentary cover and, thus may help in obtaining the age dates of the processes of the transformation of organic matter and oil generation.

/pdf/thermal-evolution-of-the-northeastern-siberian-platform-in-1ac39bheof.pdf

Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core

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Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core