Maximiliano Martins

Morphology and defects of diamond grains in carbonado: clues to carbonado genesis

The morphology and defects in diamond grains comprising Brazilian carbonado have been investigated using X-ray diffraction, photo- and X-ray luminescence, electron paramagnetic resonance and Raman spectroscopy. Paramagnetic and non-paramagnetic defects in the diamond structure indicate that many of the studied carbonado samples were annealed under mantle conditions, although for a relatively short period of time. Diamond grains show various growth morphological forms with low degrees of dissolution. Individual diamond grains are characterised by a number of important features, such as reentrant apices and incomplete growth layers on faces. We suggest that micron-sized diamond single crystals of predominantly octahedral and cubooctahedral shape grew under conditions of decreasing carbon supersaturation. The temperature decrease serves as a plausible driving force for crystallisation. During the second stage of carbonado formation, mass crystallisation of diamond has occurred. The necessary C supersaturation was likely caused by crystallisation of other minerals, leading to decrease in the volume and/or structure of the parent solution.

Chapter 49 The Neoproterozoic Macaúbas Group, Araçuaí orogen, SE Brazil

Abstract The Neoproterozoic Macaúbas Group records accumulation in the precursor basin of the Araçuaí orogen, located on the eastern margin of the São Francisco craton (SE Brazil). The Macaúbas basin evolved from a late Tonian continental rift to a passive margin that lasted at least until c. 660 Ma. It was orogenically inverted during the late Neoproterozoic Brasiliano event. The Macaúbas Group includes the pre-glacial Matão, Duas Barras and Rio Peixe Bravo formations, the glaciogenic Serra do Catuni, Nova Aurora and Lower Chapada Acauã formations, and the post-glacial Upper Chapada Acauã and Ribeirão da Folha formations. In the central sector of the Araçuaí orogen, the oldest glaciogenic unit of the group, the Serra do Catuni Formation, overlies the sandstone-conglomerate package of the Duas Barras Formation. The Serra do Catuni Formation consists of massive diamictite with minor sandstone and rare pelite, deposited mostly in a proximal glaciomarine environment. This unit passes upward and eastward into the Lower Chapada Acauã Formation, a thick succession of stratified diamictite, graded sandstone, pelite, transitional basalt and rare carbonate. This distal glaciomarine unit is covered by the diamictite-free Upper Chapada Acauã Formation, which passes eastward into the Ribeirão da Folha Formation, which includes fine-grained turbidite, pelite and ocean-floor rocks. In the northern sector of the Araçuaí orogen, the sandstone-pelite succession of the pre-glacial Rio Peixe Bravo Formation is covered by the Nova Aurora Formation, the glaciomarine unit rich in diamictite and Fe-rich diamictite, with minor graded sandstone and rare pelite. The Nova Aurora Formation is covered by the sandstone-pelite package of the Upper Chapada Acauã Formation. The pre-glacial and glaciogenic successions record the continental rift to transitional stages of the Macaúbas basin. The post-glacial succession represents proximal and distal passive margin to ocean floor environments. The Serra do Catuni Formation seems to be a proximal glaciomarine equivalent of the Jequitaí glacio-terrestrial deposits located on the São Francisco craton.

Fluid phases in carbonado and their genetic significance

Using the method of pyrolysis—gas chromatography, the composition of fluid inclusions in Brazilian carbonado and associating monocrystalline diamonds were investigated for the first time. Temperature trends were obtained for the composition of the released gas phase; based on this data, it was concluded that the diamonds include two generations of inclusions: those decrepitating at temperatures below 500°C and above 500°C. It was found that the fluid released from carbonado at high temperatures is anomalously enriched in carbon monoxide, which is strongly different from the characteristics of both the products of intracrustal mineral, rock, and ore formation and the results of crust—mantle interaction.