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Sci Rep. 2019 Aug 29;9(1):12586. doi: 10.1038/s41598-019-48778-x.

Diamond-inclusion system recording old deep lithosphere conditions at Udachnaya (Siberia).

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Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, I-35131, Padova, Italy.
Dipartimento di Scienze della Terra e dell'Ambiente, Università degli Studi di Pavia, Via Ferrata 1, I-27100, Pavia, Italy.
Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, I-35131, Padova, Italy.
Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185, Roma, Italy.
Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, Via Weiss 8, I-34127, Trieste, Italy.
Institute of Geology and Mineralogy, Russian Academy of Sciences Siberian Branch, Novosibirsk, 630090, Russia.
Department of Geology and Geophysics, Novosibirsk State University, Pirogova 2, 630090, Novosibirsk, Russia.
Dipartimento di Ingegneria Industriale, Università degli Studi di Padova, Via Marzolo 9, I-35131, Padova, Italy.
School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK.


Diamonds and their inclusions are unique fragments of deep Earth, which provide rare samples from inaccessible portions of our planet. Inclusion-free diamonds cannot provide information on depth of formation, which could be crucial to understand how the carbon cycle operated in the past. Inclusions in diamonds, which remain uncorrupted over geological times, may instead provide direct records of deep Earth's evolution. Here, we applied elastic geothermobarometry to a diamond-magnesiochromite (mchr) host-inclusion pair from the Udachnaya kimberlite (Siberia, Russia), one of the most important sources of natural diamonds. By combining X-ray diffraction and Fourier-transform infrared spectroscopy data with a new elastic model, we obtained entrapment conditions, Ptrap = 6.5(2) GPa and Ttrap = 1125(32)-1140(33) °C, for the mchr inclusion. These conditions fall on a ca. 35 mW/m2 geotherm and are colder than the great majority of mantle xenoliths from similar depth in the same kimberlite. Our results indicate that cold cratonic conditions persisted for billions of years to at least 200 km in the local lithosphere. The composition of the mchr also indicates that at this depth the lithosphere was, at least locally, ultra-depleted at the time of diamond formation, as opposed to the melt-metasomatized, enriched composition of most xenoliths.

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