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At the core of many Earth-scale processes is the question of what the deep mantle is made of. The only direct samples from such extreme depths are diamonds and their inclusions. It is commonly assumed that these inclusions reflect ambient mantle or are syngenetic with diamond, but these assumptions are rarely tested. We have studied inclusion-host growth relationships in two potentially superdeep diamonds from Juina (Brazil) containing nine inclusions of Fe-rich (XFe 0.33 to =0.64) ferropericlase-magnesiowüstite (FM) by X-ray diffractometry, X-ray tomography, cathodoluminescence, electron backscatter diffraction, and electron microprobe analysis. The inclusions share a common [112] zone axis with their diamonds and have their major crystallographic axes within 3°-8° of those of their hosts. This suggests a specific crystallographic orientation relationship (COR) resulting from interfacial energy minimization, disturbed by minor post-entrapment rotation around [112] due to plastic deformation. The observed COR and the relationships between inclusions and diamond growth zones imply that FM nucleated during the growth history of the diamond. Therefore, these inclusions may not provide direct information on the ambient mantle prior to diamond formation. Consequently, a "non-pyrolitic" composition of the lower mantle is not required to explain the occurrence of Fe-rich FM inclusions in diamonds. By identifying examples of mineral inclusions that reflect the local environment of diamond formation and not ambient mantle, we provide both a cautionary tale and a means to test diamond-inclusion time relationships for proper application of inclusion studies to whole-mantle questions. [ABSTRACT FROM AUTHOR]

The fine structure and thermal state of >200-km-thick cratonic lithosphere remain poorly explored because of insufficient sampling and uncertainties in pressure (P) and temperature (T) estimates. We report exceptionally detailed thermal and compositional profiles of the continental lithospheric mantle (CLM) in the Siberian craton based on petrographic, in situ chemical, and P-T data for 92 new garnet peridotite xenoliths from the Udachnaya kimberlite, as well as literature data. The thermal profile is complex, with samples indicating model conductive geotherms between 40 and 35 mW/m² at ~55-130 km, colder (35 mW/m² geotherm) mantle from 140 to 190 km, and hotter layers at the CLM base (190-230 km) and at ~135 km. The latter, previously unidentified, anomalous midlithospheric horizon has rocks up to 150 °C hotter than the 35 mW/m² geotherm, that are rich in garnet and clinopyroxene, have low Mg#, and have melt-equilibrated rare earth element patterns. We posit that this horizon formed in a depth range where ascending melts stall (e.g., via loss of volatiles and redox change), heat wall-rock harzburgites, and transform them to lherzolites or wehrlites. This may explain some seismic midlithosphere discontinuities (MLDs) in cratons. By contrast, we found no rocks rich in metasomatic volatile-rich amphibole, phlogopite, or carbonate matching the MLD, nor layers composed of peridotites with distinct melt-extraction degrees. The CLM below 190 km contains both coarse and variably deformed rocks heated and reworked (Mg#Ol down to 0.86) by localized lithosphere-asthenosphere interaction. [ABSTRACT FROM AUTHOR]