Lithospheric Origin for a Diamond from the Rio Sorriso area, Mato Grosso State, Brazil

A Angellotti¹Giulia Marras¹M Morana²S Chariton³V Stopponi⁴L Medeghini¹C Romano⁵Alessandra Correale⁶Luca Bindi²F V Kaminsky⁷Vincenzo Stagno^1*

• ¹Sapienza University of Rome, Rome, Italy
• ²University of Florence, Florence, Tuscany, Italy
• ³Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois, United States
• ⁴CNR - Istituto Officina dei Materiali, Basovizza, Italy
• ⁵Roma Tre University, Rome, Lazio, Italy
• ⁶INGV, Palermo, Sicily, Italy
• ⁷V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry (RAS), Moscow, Moscow Oblast, Russia

Diamonds are unique samples that enlarge our vision of physical and chemical reactions occurring in the deep, inaccessible Earth's mantle. By investigating the minerals and fluids entrapped within them, it is possible to constrain key conditions relevant to diamond formation, such as pressure, temperature and oxygen fugacity. In this study, we investigated a diamond from the Rio Sorriso area, Juína (Brazil), a site known for the high abundance of discovered sublithospheric diamonds. The studied diamond contains both colorless and greenish optically visible inclusions of Na-Cr-bearing diopside, high-Mg olivine, and enstatite. Thermobarometric estimates of the polished and entrapped inclusions suggest that the diamond likely formed at pressures between 4 and 5 GPa and temperatures of 1050-1150 °C. Major and trace elements data from one polished clinopyroxene provide evidence of interaction between the local peridotite and an oxidized Na-rich carbonated melt, a generally proposed growth medium from which diamonds might have crystallized. Our study, thus, demonstrates that diamonds from underneath the Amazonian craton did not originate solely at lower mantle depths but also within a metasomatized lithospheric mantle.