ABSTRACT Stable natural graphene occurs in graphite- and phyllosilicate-bearing low-grade metamorphic rocks. The present work simulates the process of geological formation of graphene using a piston-cylinder apparatus, promoting diffusion between talc and graphite at 700ºC and 900 MPa for a period of 24 h. The experimental products were analyzed by optical and scanning electron microscopy for description of the microstructures formed between the mineral precursors. The talc-graphite diffusion zone was also analyzed by Raman spectroscopy. Results indicate that graphite becomes progressively more deformed near the talc diffusion zone and eventually undergoes cleavage. Graphene becomes stable on the talc substrate in the center of diffusion zone. Therefore, the search for deposits of natural graphene and other nanomaterials in geological context is promising.

Abstract We present pressure calibration results for piston-cylinder –” and 1” high temperature furnace assemblies in the 200-600 MPa range aiming to expand its applicability to simulate upper crustal conditions. The furnace assemblies were made up by crushable MgO, graphite heater, pyrex glass spacer, and NaCl sleeve. Twenty-one experiments were carried out over a range of pressures (200-600 MPa) and temperatures (870-945°C) to determine the liquidus curve of NaCl. The obtained curve slope fits that given by Siewert et al. (1998), but offsets by ~ -55 MPa, implying the need of a constant P upward correction in the 300-600 MPa and ca. 300-400 MPa pressure ranges for the 3/4” and 1” assemblies, respectively. However, if the NaCl melting points (918-920°C) obtained by Bohlen (1984) at 500 MPa are used as reference, no pressure correction is needed. Larger offsets (to -120 MPa) were obtained under pressure conditions around and lower than 300 MPa with the 1” assembly. Our results confirm the applicability of Bristol-type end-loaded piston-cylinders coupled with appropriate furnace assemblies in assessing relatively low-pressure conditions (down to ~ 200 MPa, ~ 7 km depth) found in the Earth’s upper crust, where important magmatic, hydrothermal and metamorphic processes occur.

ABSTRACT: Syn-orogenic Neoproterozoic granitic magmatism (ca. 630-600 Ma) in southern Brasília Orogen includes large batholiths of compositionally expanded high-K calc-alkaline granites (lato sensu) and small occurrences constituted by anatetic leuco-granites associated to the regional metamorphism. We present petrographic, mineralogical and geochemical data for a representative sample of the Bragança Paulista-type granite and estimates for magmatic crystallization conditions. The sample corresponds to a porphyritic hornblende-biotite monzogranite (CI ≈ 20) comprising perthitic microcline megacrysts within a medium-grained matrix with plagioclase (An 30 Ab 70 ), quartz, microcline and mafic minerals. The main mafic minerals are biotite (0.51 ≤ mg# ≤ 0.57) and Ca-amphibole (potassic-magnesio-hastingsite, 0.47 ≤ mg# ≤ 0.52), with similar volumes; accessory phases (ca. 2 vol.%) are Fe-Ti oxides (ferrian ilmenite, titanohematite, hematite and magnetite), allanite, apatite, zircon, titanite and sulphides. Our results indicate crystallization at 510 ± 60 MPa and temperatures from ca. 970ºC (close-to-liquidus) to 755 ± 45ºC (close-to-solidus), under oxidizing conditions, compatible with 0 ≤ ΔNNO ≤ +1. Estimated water and halogen fugacity ratios are ƒ H2O /ƒ HF ≥ 10 5.8 , 10 3.5 ≤ ƒ H2O /ƒ HCl ≤ 10 3.8 and ƒ HF /ƒ HCl ≤ 10 -2.2 . The obtained pressure, integrated with available regional geobarometric data, suggests a relative uplift rate about 0.2-0.3 (km/Ma) from the main regional metamorphism to the post-orogenic magmatism in the area.