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Título

SPECTROSCOPIC CHARACTERIZATION OF LITHOSPHERIC DIAMONDS FROM GUIRATINGA, MATO GROSSO

Texto do resumo

The State of Mato Grosso is globally acknowledged for its sublithospheric diamonds from the Juína area. Despite of its renown, other occurrences in this cratonic region remain understudied. As an example of other diamond districts, we highlight the Rio Garças Diamond District (DDRG) in SE Mato Grosso. Despite of its small area, it hosts several occurrences of diamonds in alluvial and paleoalluvial deposits near Guiratinga. In the DDRG, the geological substrate of alluvial deposits consists of sedimentary rocks from the Aquidauana Formation, dating back to the Permo-Carboniferous. The diamonds are suggested to be originated from Cretaceous kimberlite bodies intruded in the Paraná Basin, such as those in the Poxoréu Diamond District. In this study, we present the spectroscopic characterization of 65 alluvial diamonds from DDRG. Our goal is to provide further insights into the mantle compositions and to enhance the understanding of the geological processes in the lithospheric mantle beneath the Amazonian Craton. The described diamonds are typically colorless or light-yellow in collor, occasionally with a brownish hue, and are characterized by dominant dodecahedral, octahedral, or trioctahedral shapes. Some diamonds exhibit distinctive textures, such as grooves and resorption features. Mineral inclusions within these diamonds were analyzed by Raman microspectroscopy to identify mineral paragenesis, while Fourier transform infrared spectroscopy (FTIR) was applied to classify the diamonds based on their nitrogen aggregation states. The main phases identified are, in order of abundance, olivine (doublet at 823 and 856 cm-1), enstatite (doublet at 657 and 673, and other peaks at 332, 387, 657, 673, 947 and 1000 cm-1), ferrosilite (doublet at 667 and 771, with additional peaks at 311 and 451 cm-1), and hematite (226, 295, 413, 495, 615 and 662 cm-1). Regarding nitrogen aggregation states, these diamonds are predominantly characterized as type IaAB diamonds, marked by the A center (1282 cm-1) and B center (1175 cm-1). However, some type IaA (A centers only) and type IaB (B centers only) diamonds are also present. All of these types exhibit hydrogen impurities (1307 cm-1). The spectroscopic characterization indicates that the Guiratinga diamonds were formed in lithospheric conditions, with peridotitic/harzburgitic paragenesis (olivine, enstatite and ferrosilite). The occurrence of hematite without the presence of any fractures or cracks precludes the classification as epigenetic inclusions. Therefore, the formation of this mineral as a syngenetic phase under lithospheric mantle conditions requires the presence of oxygen and water in the system, potentially derived from fluids associated with a subduction zone. The predominance of intermediate aggregation states indicates average mantle residence temperatures, the minor presence of diamonds with fully aggregated nitrogen (Type IaB), which suggests residence in the mantle for an extended period and/or at abnormally high temperatures, may be an indication of the presence of distinct diamond populations in the DDRG.

Palavras Chave

Lithospheric diamonds; Guiratinga; Raman; FTIR; Amazonian Craton