Description: Approximately 700 diamond crystals were identified in volcanic (mainly pyroclastic) rocks of the Tolbachik volcano, Kamchatka, Russia. They were studied with the use of SIMS, scanning and transmission electron microscopy, and utilization of electron energy loss spectroscopy and electron diffraction. Diamonds have cube-octahedral shape and extremely homogeneous internal structure. Two groups of impurity elements are distinguished by their distribution within the diamond. First group, N and H, the most common structural impurities in diamond, are distributed homogeneously. All other elements observed (Cl, F, O, S, Si, Al, Ca, and K) form local concentrations, implying the existence of inclusions, causing high concentrations of these elements. Most elements have concentrations 3–4 orders of magnitude less than chondritic values. Besides N and H, Si, F, Cl, and Na are relatively enriched because they are concentrated in micro- and nanoinclusions in diamond. Mineral inclusions in the studied diamonds are 70–450 nm in size, round- or oval-shaped. They are represented by two mineral groups: Mn-Ni alloys and silicides, with a wide range of concentrations for each group. Alloys vary in stoichiometry from MnNi to Mn2Ni, with a minor admixture of Si from 0 to 5.20–5.60 at%. Silicides, usually coexisting with alloys, vary in composition from (Mn,Ni)4Si to (Mn,Ni)5Si2 and Mn5Si2, and further to MnSi, forming pure Mn-silicides. Mineral inclusions have nanometer-sized bubbles that contain a fluid or a gas phase (F and O). Carbon isotopic compositions in diamonds vary from –21 to –29‰ δ13CVPDB (avg. = –25.4). Nitrogen isotopic compositions in diamond from Tolbachik volcano are from –2.32 to –2.58‰ δ15NAir. Geological, geochemical, and mineralogical data confirm the natural origin of studied Tolbachik diamonds from volcanic gases during the explosive stage of the eruption.

Description: We demonstrate for the first time the presence of iron carbides in placer diamonds from the northeastern region of the Siberian craton. It was found that the inclusions are polycrystalline aggregates, and iron carbides filling the fissures in the diamonds, thus providing clear evidence that the iron melts were captured first. Iron carbides were identified in diamonds containing mineral inclusions of eclogitic (Kfs, sulfide) and peridotitc (olivine) paragenesis. Iron carbides with minor amounts of admixed nickel were detected in a diamond sample containing an olivine inclusion (0.3 wt% Ni), indicating that the iron melt was not in equilibrium with the mantle peridotite.The low nickel contents of the iron carbides provide the best evidence that the subducted crust is a likely source of the iron melt. Diamonds containing carbide inclusions are characterised by a relatively low nitrogen aggregation state (5–35%), which is not consistent with the high temperature of the transition zone. Therefore, we have reason to assume that the studied diamonds are from the lower regions of the lithosphere. Considering all factors, the model for the interaction of the ascending asthenospheric mantle with the subducting slab seems to be more realistic.

Description: We have studied volcanogenic diamonds in the context of a discussion of their genesis, including some assumption on their artificial origin. The carbon isotope composition of diamonds collected from the eruption products of Tolbachik Volcano (δ13CVPDB from -22 to -29 ‰) is within the range of the δ13CVPDB values of natural diamonds, including those from kimberlites. The 15NAir values of the Tolbachik diamonds, measured for the first time (-2.58 and -2.32 ‰), correspond to δ15NAir of volcanic gases and differ from that of atmospheric nitrogen (δ15NAir = 0 ‰), which may be expected in synthetic diamonds. In the studied volcanogenic diamonds, as in synthetic ones, the nitrogen impurity is unaggregated. However, such an unaggregated form of nitrogen is specific to many natural diamonds (e.g., variety II diamonds, according to Orlov’s classification). Impurity elements (Cl, F, O, S, Si, Al, Ca, and Na) are locally concentrated in volcanogenic diamonds; they are a constituent of micro- and nanoinclusions in them. The high contents of F and Cl in the studied diamonds are correlated with the composition of volcanic gases; there is no reason to expect a similar correlation in synthetic diamonds. Moreover, the studied cube-octahedral Tolbachik diamonds have a number of accessory forms, some of which are not observed in synthetic diamonds. Their surfaces are frequently covered with films composed of Mg-Fe and Ca-Mg silicates, aluminosilicates, sulfates, metal alloys, and native Al. Mineral inclusions in the studied diamonds are Mn-Ni-Si alloys and silicides varying in composition from (Mn,Ni)4Si to (Mn,Ni)5Si2, Mn5Si2, and pure Mn silicide MnSi. Summing up the obtained data, we conclude that volcanogenic diamonds form in a strongly reducing environment, in which silicides and native metals and their alloys are stable. The predominant cube-octahedral morphology of these diamonds and the unaggregated nitrogen impurity point to their short-term residence under high-temperature conditions. This makes them similar, to some extent, to synthetic diamonds. There are, however, clear differences as well. Volcanogenic diamonds are similar in compositional peculiarities, including isotope compositions, to natural diamonds that form under most unfavorable conditions, such as cuboids, balases, carbonado, and some diamonds of the eclogite paragenesis. They also resemble diamonds found in situ in harzburgite and chromitite of ophiolites. This suggests a specific mechanism of formation of both volcanogenic and ophiolitic diamonds in the oceanic lithosphere.

Description: We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating ∼15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo91)) and a natural pyroxene (Bamble orthopyroxene (En87)) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERTIS experiment onboard the BepiColombo mission. For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, ∼450 nm in olivine, 100-250 nm in pyroxene, as well as in developed “nanostratigraphy” of laser-ablated material, like nanophase iron (npFe). In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites.

Description: Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.

Description: The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminum–silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called “HSNL”) on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition. We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston–cylinder experiments at 500–800 °C and 0.25–1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminum silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth’s crust.

Description: A significant characteristic distinguishing Carlin-type Au deposits from other Au deposits is the abundance of invisible Au in arsenian pyrite. Gold occurs primarily as ionic Au1+ in arsenian pyrite and is unstable during subsequent thermal events. In this study, we used the focused ion beam combined with scanning electron microscope (FIB-SEM) techniques, and a transmission electron microscope (TEM) to examine invisible Au and how it evolved through later geologic events that eventually led to the formation of Au nanoparticles. FIB-SEM techniques were used to prepare site-specific TEM foils from four Carlin-type gold deposits, including Getchell and Cortez Hills, Nevada, USA, and Shuiyindong and Jinfeng, Guizhou Province, China. These samples were analyzed to quantify ore pyrite chemistry and evaluate textures at the nanometer scale. In 17 examined TEM foils, we observed widespread Au-rich domains in high-grade Au arsenian pyrites from the Getchell and Cortez Hills Au deposits and the Jinfeng deposit but only 10 Au-bearing nanoparticles, ~10 to 20 nm in diameter. The Au-rich domains exhibit Au (Sb), (Tl), (Hg), and (Cu) peaks in the energy dispersive X-ray (EDX) spectrum without the presence of recognizable nanoparticles. This confirms that Au is invisible even at a nanometer scale and is most likely present in the crystal structure of arsenian pyrite. Stacking faults and nanometer-sized fluid inclusions were commonly observed in Au-bearing arsenian pyrite from the four deposits, implying rapid crystallization. Moreover, unlike the coarsely crystalline arsenian pyrite from Guizhou Carlin-type Au deposits, arsenian pyrite from Carlin-type deposits in Nevada consists of fine-grained polycrystalline aggregates, further implying rapid crystallization. Additionally, curved dislocations were commonly pinned by solid inclusions, reflecting a former annealing process. Combining nanoscale textures with geologic information previously reported for Carlin-type deposits, invisible ionic Au was initially incorporated into the crystal structure of arsenian pyrite during rapid precipitation. Subsequent post-ore magmatic events in both districts initiated the annealing of the ionic Au-bearing arsenian pyrite, leading to the redistribution of trace elements and formation of Au-bearing nanoparticles in the arsenian pyrite. The presence of predominantly ionically bonded Au in arsenian pyrite confirms that ore fluids were not saturated in Au when Au-bearing arsenian pyrite formed, as previously reported for Carlin-type deposits. Ionic Au that was scavenged from an undersaturated ore fluid and incorporated into the arsenian pyrite crystal structure formed the giant Carlin-type Au deposits.

Description: Submicroscopic to micro-diamonds, silicates, wüstite, platinum group minerals (PGM), sulfides and ultra-high pressure (UHP) mineral inclusions have been recovered from podiform and banded chromitites of the Bulqiza ophiolite in Albania. The alignment of acicular silicate inclusions with crystallographic planes of magnesiochromite suggests an exsolution origin. Positive and negative-crystal faces of cubic silicate inclusions are consistent withs a high-temperature paragenesis. Micro-inclusions and nanoparticles (〈500 nm) of Rusingle bondOs bisulfides (laurite; RuS2), Ossingle bondIr, and Os-Ir-Ru alloys, tolovike, irarsite, Irsingle bondNi sulfides, and millerite (NiS) are also common in the Bulqiza chromitites. Dispersed spherical, nano-inclusions of Irsingle bondPt and Ossingle bondIr may reflect formation of the Irsingle bondPt and Ossingle bondIr particles by sulfide liquid immiscibility. The high Cr# of the chromitites, their evolved whole-rock PGE abundances, and IPGE/PPGE ratios (≥ 10) suggest extensive partial melting, likely in a supra-subduction zone (SSZ) environment. This interpretation is supported by the variable 187Re/188Os (0.0103–0.0271) and 187Os/188Os (0.12600–0.13014) ratios and the low γOs isotope values (mean + 0.01) of the chromitites, reflecting derivation from heterogeneously depleted mantle materials. The presence of remarkably zoned zircons with Usingle bondPb ages (164–3354 Ma) represents the addition of crustal materials. Discrete micro-diamonds, PGM, and sulfide inclusions point to a super-reducing (SuR), ultra-high pressure (UHP) origin. We propose a genetic model that involves incorporation of crustal and UHP phases into the mantle by convection or subduction. The UHP and crustal silicates are consistent with a subduction-related origin, but the presence of super-reduced (SuR) inclusions suggests equilibration under subduction-disparate mantle conditions.

Description: Several recent papers have purported to find ultra-reduced minerals—as natural examples—within ophiolitic mantle sections, including SiC moissanite, Fe-Si alloys, various metal carbides, nitrides, and borides. All those phases were interpreted to be mantle derived. The phases are recovered from mineral concentrates and are assigned to the deep mantle because microdiamonds and other ultrahigh-pressure (UHP) minerals are also found. Based on these findings, it is claimed that the mantle rocks of ophiolite complexes are rooted in the transition zone (TZ) or even in the lower mantle, at redox states so reduced that phases like SiC moissanite are stable. We challenge this view. We report high-temperature experiments carried out to define the conditions under which SiC can be stable in Earth’s mantle. Mineral separates from a fertile lherzolite xenolith of the Eifel and chromite from the LG-1 seam of the Bushveld complex were reacted with SiC at 1600 K and 0.7 GPa. At high temperature, a redox gradient is quickly established between the silicate/oxide assemblage and SiC, of ~12 log-bar units in fO2. Reactions taking place in this redox gradient allow us to derive a model composition of an ultra-reduced mantle by extrapolating phase compositions to 8 log units below the iron-wüstite equilibrium (IW-8) where SiC should be stable. At IW-8 silicate and oxide phases would be pure MgO end-members. Mantle lithologies at IW-8 would be Fe° metal saturated, would be significantly enriched in SiO2, and all transition elements with the slightest siderophile affinities would be dissolved in a metal phase. Except for the redox-insensitive MgAl2O4 end-member, spinel would be unstable. Relative to an oxidized mantle at the fayalite-magnetite-quartz (FMQ) buffer, an ultra-reduced mantle would be enriched in enstatite by factor 1.5 since the reduction of the fayalite and ferrosilite components releases SiO2. That mantle composition is unlike any natural mantle lithology ever reported in the literature. Phases as reduced as SiC or Fe-Si alloys are unstable in an FeO-bearing, hot, convecting mantle. Based on our results, we advise against questioning existing models of ophiolite genesis because of accessory diamonds and ultra-reduced phases of doubtful origin.

Description: In the Sri Lankan Highlands denudation and chemical weathering represent the low-end member in global weathering rates. Here we report on the causes for these low rates in corestones from a highly weathered saprolite profile. By using electron microprobe and transmission electron microscopy (TEM) analyses, we investigated weathering reactions, and derived rates of pyroxene and biotite oxidation. High-resolution TEM analyses on primary minerals showed that the initial weathering products are non-crystalline and that these form the precursors of secondary minerals (kaolinite and smectite). The dissolution of primary minerals is characterised by sharp reaction fronts in the absence of chemical gradients, hence, dissolution can best be described by a dissolution – reprecipitation process. The first observable weathering reaction is the oxidation of structural Fe(II) in pyroxene and biotite. This oxidation is restricted to distinct zones within the minerals without the formation of oxidized layers. While the oxidation is not accompanied by chemical changes, the oxidation of structural Fe(II) in biotite may cause lattice distortion. Pyroxene and biotite oxidation rates were calculated at the corestone-scale by using the gradient of bulk rock Fe(II)/Fetotal ratios, assuming that oxidation is coupled to the weathering front advance rate. At the mineral-scale, oxidation rates were calculated by using gradients of in situ Fe(II)/Fe(III) ratios measured with electron energy-loss spectroscopy (EELS) within the minerals assuming a coupled oxidation-cation diffusion process. The mineral-scale oxidation rates are significantly higher, log Ominin situ = −11 molmin m−2 s−1, than corestone-scale oxidation rates, log Omin = −13 to −15 mol m−2 s−1. We explain the difference to result from the fact that corestone-scale rates average the oxidation over the entire mineral surface. Because at the corestone-scale pyroxene oxidation rates are also similar to dissolution rates, we infer that oxidation preconditions Fe(II)- bearing primary silicate minerals to weathering. However, although oxidation is initiating chemical weathering in this setting and is limited by the supply of O2, overall conversion of bedrock to saprolite is driven by the formation of porosity by fracture formation during this reaction, which allows for fluid transport and subsequent plagioclase weathering. We conclude that in compact bedrock nm- to µm-scale fracture- and porosity generation are the processes that drive weathering in tectonically quiescent regions. They are also the processes at depth that couple the weathering zone with processes at its surface. Finally, the formation of secondary surface layers at the nm-scale is essential in controlling the equilibrium in the local weathering environment and hence mineral dissolution. These layers are likely also those that control isotope fractionation in the weathering zone.