Hydrocarbon generation reactions occur within organic-rich shales as a response to thermal maturation. Here, we report observations on samples from the organic-rich Mississippian Barnett shale gas ...system (Fort Worth Basin, Texas, USA) at varying stages of thermal maturation. A multiscale characterization was achieved using a combination of compositional organic geochemistry and spectromicroscopy techniques, including synchrotron-based scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM). We document the chemical evolution of the macromolecular structure of Barnett Shale kerogen with increasing maturity, from an immature kerogen containing a significant aliphatic component and an important concentration of oxygen and sulfur-containing functional groups to an overmature kerogen dominated by poorly condensed aromatic structures. In addition to the presence of bitumen in samples of oil window maturity, very likely genetically derived from thermally degraded kerogen, the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity, likely resulting from the formation of gaseous hydrocarbons by secondary cracking of bitumen compounds.
► Detection of geochemical heterogeneities at the nanoscale within gas shale samples. ► Chemical and structural evolution of type II kerogen with increasing maturity. ► In-situ insights into the fate of bitumen as a response to maturation. ► Identification of nanoporous pyrobitumen within samples of gas window maturity.
Experimental studies on binary, ternary and quaternary Cu–Fe–Ni–S systems are fundamental for the investigation of magmatic sulfide deposits, the main source of Ni, Co and platinum group elements ...(PGE). Previous experimental studies successfully formulated our general understanding of the evolution of magmatic sulfide systems but, yet, in many cases could not explain some of the key geological, mineralogical and geochemical features of sulfide ore deposits. The challenges are imposed by not well-defined solidus of the Cu-rich sulfide melts, yet poorly constrained phase stability at subliquidus conditions, and poorly resolved subsolidus evolution of magmatic sulfide phases. In this study we aim at better understanding how base metal sulfides crystallize from the evolving sulfide liquid during cooling from superliquidus to room temperatures. We report on controlled cooling (15 °C/day) experiments from 1100 to 25 °C in evacuated silica tubes of a single composition of the quaternary Cu–Ni–Fe–S system similar to the Merensky Reef sulfide ore. Run products were sampled at various temperatures along the cooling path and examined on the microscopic scale by back scattered electron imaging and on the nanometer scale by transmission electron microscopy. The compositions of coexisting phases were analysed using electron microprobe. We show that the sulfide melt (SM) coexists with monosulfide solid solution (MSS) above 950 °C and persists to 700 ± 25 °C before it crystallizes to intermediate solid solution (ISS). The transition to subsolidus state is clearly traced by abrupt change in the Cu/(Fe + Ni) distribution between MSS and Cu-rich phase (either SM or ISS) and in the composition of SM or ISS. The compositional jump at ca. 700 °C is also accompanied by the inverse change in the proportions of coexisting phases indicating significant subsolidus reactions and evolution with decreasing temperature. Pentlandite commences crystallization around MSS grains between 550 and 450 °C and coarsens to a granular-type pentlandite at 450 °C by diffusion of nano pentlandite exsolutions from MSS. Pentlandite exsolves and grows as “flames” and “brushes” in both MSS and ISS at lower temperatures (< 250 °C). Mass balance calculations suggest that 32% of pyrrhotite and 7% of pentlandite in magmatic deposits like the Merensky Reef exsolve from ISS. Results imply that magmatic sulfide systems evolve to lower temperatures than previously thought, leading to significant ore metal fractionation and redistribution. Base metal sulfide phases re-equilibrate extremely fast during cooling, ensuring that primary phase compositions and textures are inevitably and completely eradicated during cooling histories even as short as a few days. The sulfide mineral assemblage, texture and modal abundance of magmatic sulfide phases could be used as a proxy for the reconstruction of the parent sulfide liquid evolution in the deposit. The newly established mechanism and timing of base metal sulfides crystallization provides possible explanation of PGE distribution in base metal sulfides.
The Bushveld Complex in South Africa hosts the world’s largest resources of platinum-group elements (PGEs), which are mainly mined from three ore bodies, namely the Merensky Reef, the UG-2 ...chromitite, and the Platreef. In these ores, the PGEs are bimodally distributed, occurring both as discrete platinum-group minerals (PGMs) and hosted by sulfides. The presence of PGEs in sulfides has been demonstrated by electron probe microanalysis, laser ablation induced coupled plasma mass spectrometry, secondary ion mass spectrometry, and particle-induced X-ray emission. However, evidence is lacking on the mineralogical siting of the PGEs, e.g., whether they occur in solid solution, as nano-inclusions, and/or micro-inclusions. Therefore, in the present study, a combination of focused ion beam and transmission electron microscopy was used which allows to obtain crystal structural relationships between the host mineral and incorporated trace elements and revealing the physicochemical state of the PGE in sulfides. The present study confirms the existence of micrometer-sized discrete PGMs in the ores. Further, the PGEs occur in a number of forms, namely (1) as discrete nano-inclusions of PGMs, (2) as patchily distributed solid solution, (3) ordered within the pentlandite crystal structure, substituting for Ni and/or Fe (superlattice), and (4) as homogenous solid solution. Nanometer-sized PGMs (nPGMs) show no orientation relationship with the host sulfide mineral. Consequently, they are discrete phases, which were trapped within pentlandite during sulfide growth. Heterogeneous and patchy distributions of Rh and Ir within the pentlandite lattice suggest that Rh and Ir were already present within the sulfide liquid. The absence of possible reaction partners (e.g., Bi, As, and Sn) necessary for the formation of discrete PGMs forced Rh and Ir to remain in the crystal lattice of pentlandite and down-temperature exsolution caused patchy distribution patterns of Rh and Ir. High concentrations of Rh and Ir in pentlandite initiate ordering of the randomly distributed PGE in form of nanometer-sized lamellae resulting in the formation of a superlattice. Palladium is homogenously distributed within the pentlandite lattice, even at high Pd concentrations, and in addition also occurs as nPGMs.
Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element ...exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.
Diamonds originating from the transition zone or lower mantle were previously identified based on the chemistry of their silicate or oxide mineral inclusions. Here we present data for such a ...super-deep origin based on the internal pressure of nitrogen in sub-micrometer inclusions in diamonds from Juina, Brazil. Infrared spectroscopy of four diamonds, rich in such inclusions revealed high concentrations of fully aggregated nitrogen (average of 900 ppm, all in B centers) and almost no platelets. Raman spectroscopy indicated the presence of solid, cubic δ-N2 at 10.9±0.2 GPa (corresponding to a density of 1900 kg/m3). Transmission electron microscopy of two diamonds found two generations of octahedral inclusions: microinclusions (average size: 150 nm, average concentration: 100 ppm) and nanoinclusions (20–30 nm, 350 ppm). EELS detected nitrogen and a diffraction pattern of one nanoinclusion yielded a tetragonal phase, which resembles γ-N2 with a density of 1400 kg/m3 (internal pressure = 2.7 GPa). We also observed up-warping of small areas (∼150 nm in size) on the polished surface of one diamond. The ∼2 nm rise can be explained by a shallow subsurface microinclusion, pressurized internally to more than 10 GPa.
Using available equations of state for nitrogen and diamond, we calculated the pressures and temperatures of mechanical equilibrium of the inclusions and their diamond host at the mantle geotherm. The inclusions originated at the deepest part of the transition zone at pressures of ∼22 GPa (630 km) and temperatures of ∼1640 °C. We suggest that both generations are the result of exsolution of nitrogen from B centers and that growth took a few million years in a subducting mantle current. The microinclusions nucleated first, followed by the nanoinclusions. Shortly after the exsolution events, the diamonds were trapped in a plume or an ascending melt and were transported to the base of the lithosphere and later to the surface.
•A new type of inclusions in “super-deep” diamonds.•Raman spectroscopy indicates solid molecular δ-N2 under 10.9±0.2 GPa.•This is the highest internal pressure ever recorded in a natural inclusion.•The inclusions originated at ∼22 GPa, 1640 °C at the base of the transition zone.•This is the geological story behind the formation of “voidites”.
•We report optical absorption spectra of pyrolite at high P-T conditions.•We show that light absorption in pyrolite is enhanced with P and T.•We constrain the radiative thermal conductivity of the ...lower mantle.•Radiative heat transfer is blocked at core-mantle boundary conditions.
The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ∼3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ∼0.8 W/m/K at 1000 km to ∼0.35 W/m/K at the CMB, the latter being ∼30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ∼8.5 TW, on the lower end of previous QCMB estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.
In addition to a series of finds of diamond in mafic volcanic and ultramafic massive rocks in Kamchatka, Russia, a carbonado-like diamond aggregate was identified in recent lavas of the active Avacha ...volcano. This aggregate differs from ‘classic carbonado’ by its location within an active volcanic arc, well-formed diamond crystallites, and cementing by Si-containing aggregates rather than sintering. The carbonado-like aggregate contains inclusions of Mn–Ni–Si–Fe alloys, native β-Mn, tungsten and boron carbides, which are uncommon for both carbonado and monocrystalline diamonds. Mn–Ni–Si–Fe alloys, trigonal W2C and trigonal B4C are new mineral species that were not previously found in the natural environment. The formation of the carbonado-like diamond aggregate started with formation at ~850–1000°C of tungsten and boron carbides, Mn–Ni–Si–Fe alloys and native β-Mn, which were used as seeds for the subsequent crystallization of micro-sized diamond aggregate. In the final stage, the diamond aggregate was cemented by amorphous silica, tridymite, β-SiC, and native silicon. The carbonado-like aggregate was most likely formed at near-atmospheric pressure conditions via the CVD mechanism during the course or shortly after one of the volcanic eruption pulses of the Avacha volcano. Volcanic gases played a great role in the formation of the carbonado-like aggregate.
•Carbonado-like diamonds identified in lavas of the active Avacha volcano.•They contain inclusions of Mn–Ni–Si–Fe alloys, native β-Mn, WC, and BC.•The assemblage was formed via the mechanism of CVD.
A series of uncommon micro- and nano-inclusions has been identified in diamonds from the Rio Soriso placer deposit in Mato Grosso State, Brazil. The micro-inclusions are variable in size, from 1 to ...300 µm. Usually, they are polymineralic, being formed predominantly by intergrowths of carbonates, silicates and other minerals. Carbonates are represented mostly by dolomite and occasionally by calcite. Silicates found are coesite, wollastonite-II, cuspidine and monticellite. Sulphides and ilmenite form micro-inclusions as well. Nano-inclusions are different from micro-inclusions not only in size (not exceeding 200 nm); they are usually included in micro-inclusions. Among nano-inclusions, halides (NaCl, KCl, CaCl
2 and PbCl
2), anhydrite, spinel, phlogopite, PbO
2, TiO
2 with an α-PbO
2 structure, native Fe, and other phases are identified. All these minerals are of the eclogitic association: they are either associated with coesite or are included in a diamond with a light, ‘organic’ carbon isotopic composition (with
δ
13C from −
14 to −
25‰ PDB). This confirms our earlier conclusion that diamonds from the Juina area may have formed as a result of subduction of the crustal material to depths of at least the lower transition zone or even the lower mantle. The pressure estimates for the investigated diamonds vary in a range from 3 to 10 GPa. An interesting assemblage of calcite + cuspidine + wollastonite + monticellite + fluid identified in one of the studied diamonds is considered as a product of a reaction of wollastonite + fluid forming cuspidine + monticellite. The presence of numerous pores, cavities and bubbles in mineral inclusions, and identification of an association of volatile-containing mineral inclusions, such as halides (NaCl, KCl, CaCl
2, and PbCl
2), fluorine-containing silicate cuspidine, and phlogopite emphasize the important role of volatiles, particularly chlorine and fluorine in the formation of the diamonds.
A series of polycrystalline diamond grains were found within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. A grain from the Aynyn River area is studied in detail with TEM. Diamond ...crystallites, 2-40 µm in size are twinned and have high dislocation density. They are cemented with tilleyite Ca5(Si2O7)(CO3)2, SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Among SiC grains, three polymorphs were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si=0.505-1.925. Native silicon is an open-framework allotrope of silicon S24, which has been observed, to date, as a synthetic phase only; this is a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-C-O material, similar in composition to tilleyite; amorphous carbon in contact with diamond, which includes particles of crystalline graphite; and amorphous SiO2. No regularity in the distribution of the amorphous material was observed. In the studied aggregate, diamond crystallites and moissanite are intensively twinned, which is characteristic for these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are related not to 'classic' diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called 'kamchatite'. Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond.
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