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Phytoliths contain occluded organic compounds called phytC. Recently, phytC content, nature, origin, paleoenvironmental meaning and impact in the global C cycle have been the subject of increasing debate. Inconsistencies were fed by the scarcity of in situ characterizations of phytC in phytoliths. Here we reconstructed at high spatial resolution the 3-D structure of harvested grass short cell (GSC) phytoliths using 3-D X-ray microscopy. While this technique has been widely used for 3-D reconstruction of biological systems it has never been applied in high-resolution mode to silica particles. Simultaneously, we investigated the location of phytC using nanoscale secondary ion mass spectrometry (NanoSIMS). Our data evidenced that the silica structure contains micrometric internal cavities. These internal cavities were sometimes observed isolated from the outside. Their opening may be an original feature or may result from a beginning of dissolution of silica during the chemical extraction procedure, mimicking the progressive dissolution process that can happen in natural environments. The phytC that may originally occupy the cavities is thus susceptible to rapid oxidation. It was not detected by the NanoSIMS technique. However, another pool of phytC, continuously distributed in and protected by the silica structure, was observed. Its N/C ratio (0.27) is in agreement with the presence of amino acids. These findings constitute a basis to further characterize the origin, occlusion process, nature and accessibility of phytC, as a prerequisite for assessing its significance in the global C cycle.

During the past two decades, several studies of fluid inclusions hosted in some opaque ore minerals using near-infrared microscopy have been performed. Results indicated that this method can be applied to several sulfidic ores and metal oxides depending on their electronic band structures and infrared-active vibration modes. Infrared transmittance of individual ore minerals can be best characterized using Fourier transform infrared spectroscopy. Infrared microscopic observations are limited to the near-infrared region to about 2.3 μm depending on the IR sensitivity of the IR camera. The trace element content in ore minerals can be another limiting factor for optical observations in near-infrared light. Still, IR transmittance gradually decreases upon heating caused by shifting of IR absorption edges for higher wavelengths. Possibilities and limitations of studying fluid inclusions hosted in opaque minerals by near-infrared light microthermometry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are discussed.

Infrared (IR) studies of fluid inclusions in opaque minerals provide direct insights into the ore-forming fluids. However, the challenge posed by the “warming effect” of IR light during microthermometry remains unresolved. Here we address this problem and show that the deviation in temperature of phase changes within fluid inclusions caused by IR light is more common than what was thought before. Our results reveal that transparent to translucent minerals (e.g., quartz, sphalerite) also absorb IR energy. Since IR absorption is influenced by the host mineral, the fluid inclusions hosted by different minerals exhibit different degrees of deviation in temperature during microthermometry. The Fourier transform infrared (FTIR) spectra do not display a consistent correlation between the band gap energy of a mineral and its absorption of IR energy. Minerals with low band gap energy, such as pyrite, absorb limited IR energy, resulting in small deviations of fluid inclusion data. In contrast, this deviation could be significant for fluid inclusions hosted in minerals with a relatively high band gap energy, such as iron-rich sphalerite and wolframite. Substitution of iron increases the absorption of IR energy in these minerals. The substitution of trace elements may also play a role. Our quantitative analyses confirm that using the lowest possible IR light intensity with the smallest diaphragm minimizes the “warming effect” of IR light. We also propose an improved cycling method as a better option where high IR light intensity is required.

The Waterberg platinum deposit is an extraordinary example of a vein-type hydrothermal quartz-hematite-PGE (platinum-group element) mineralization. This study concentrates on the geochemical character of the ores and the platinum-group mineral (PGM) assemblage by application of reflected-light and scanning electron microscopy followed by electron probe microanalysis. The PGM-bearing quartz veins show multiple banding indicating numerous pulses of fluid infiltration. Mineralization was introduced contemporaneously with the earliest generation of vein quartz and hematite. High oxygen and low sulfur fugacities of the mineralizing fluids are indicated by hematite as the predominant opaque mineral and the lack of sulfides. The 'Waterberg type' mineralization is characterized by unique metal proportions, namely Pt>Pd>Au, interpreted as a fingerprint to the cradle of the metals, namely rocks and ores of the Bushveld Complex, or reflecting metal fractionation during ascent of an oxidized, evolving fluid. The PGM assemblage signifies three main depositional and alteration events. (1) Deposition of native Pt and Pt-Pd alloys (>90% of the PGM assemblage) and Pd-Sb-As compounds (Pt-rich isomertieite and mertieite II) from hydrothermal fluids. (2) Hydrothermal alteration of Pt by Cu-rich fluids and formation of Pt-Cu alloys and hongshiite [PtCu]. (3) Weathering/oxidation of the ores producing Pd/Pt-oxides/hydroxides. Platinum-group element transport was probably by chloride complexes in moderately acidic and strongly oxidizing fluids of relatively low salinity, and depositional temperatures were in the range 400-200°C. Alternatively, quartz and ore textures may hint to noble metal transport in a colloidal form and deposition as gels. The source of the PGE is probably in platiniferous rocks or ores of the Bushveld Complex which were leached by hydrothermal solutions. If so, further Waterberg-type deposits may be present, and a prime target area would be along the corridor of the Thabazimbi-Murchison-Lineament where geothermal springs are presently still active.

Thunderbayite (IMA2020-042), ideally TlAg3Au3Sb7S6, is a new mineral from the Hemlo gold deposit, Marathon, Ontario, Canada. It occurs as very rare anhedral rims up to 70 µm across in contact with aurostibite and associated spatially with stibarsen, biagioniite and native gold in a calcite matrix. Thunderbayite is opaque with a metallic lustre and shows a black streak. In reflected light, thunderbayite is weakly bireflectant and faintly pleochroic from grey-blue to slightly greenish grey-blue. Under crossed polars, it is weakly anisotropic with bluish to light-blue rotation tints. Internal reflections are absent. Reflectance percentages for the four Commission on Ore Mineralogy wavelengths (Rmin, Rmax) are: 37.9, 38.4 (471.1 nm); 35.3, 36.0 (548.3 nm); 33.9, 34.4 (586.6 nm); and 32.0, 32.5 (652.3 nm), respectively. A mean of five electron-microprobe analyses gave Ag 14.91(16), Au 27.40(22), Tl 9.37(9), Sb 39.80(34) and S 8.61(7), for a total of 100.09 wt.%, corresponding, on the basis of a total of 20 atoms, to Tl1.00Ag3.01Au3.03Sb7.12S5.84. Thunderbayite is triclinic, space group P1, with a=8.0882(5), b=7.8492(5), c=20.078(1) Å, α=92.518(5), β=93.739(5), γ=90.028(6)°, V=1270.73(9) Å3 and Z=2. The five strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are: 4.04 (100) (200); 3.92 (80) (020); 2.815 (50) (220/2̄20); 2.566 (45) (1̄17); and 2.727 (40) (01̄7). The crystal structure [R1=0.0220 for 5521 reflections with I>2σ(I)] can be considered as a strongly deformed pyrite-type structure with several metal-metal bonds. Thunderbayite shows close similarities with criddleite, TlAg2Au3Sb10S10, from an optical, chemical and structural point of view. The new mineral is named for the Thunder Bay district, Ontario, in which the Hemlo gold deposit is located.

The present study reports the occurrence of orthopyroxene megacrysts from the Chilka Lake anorthosite massif, Eastern Ghats, India. An insight into the mineral chemistry of different phases, coupled with detailed field and petrographic evidences from this study, shed light on a long debate on the origin of orthopyroxene megacrysts in anorthosite massifs. The megacrysts contain exsolved lamellae of plagioclase and opaque oxides (ilmenite, rutile) oriented along orthopyroxene cleavage planes. The trace element distribution patterns of the megacryst and matrix plagioclase are mirror reflections of each other and mutually complementary. The calculated compositions of melts in equilibrium with these two phases show comparable patterns for LREE (light rare earth elements, La–Sm), but differ markedly in terms of HREE (heavy rare earth elements, Eu–Lu), suggesting that the megacrysts and matrix plagioclases did not crystallize simultaneously. We infer that the orthopyroxene megacrysts have a longer crystallization history, initially as a low-Ca non-quad member of the pyroxene group at pressure ≥ 10 kbar, incorporating some amount of Ca, Al and Ti in their structure. Subsequently, they have been carried by a plagioclase crystal mush to mid-crustal levels at pressure ~ 4–6 kbar following a near-isothermal decompression that may be linked to the emplacement of the anorthosite massif, giving rise to the exsolution lamellae of plagioclase and opaque oxides.

The early eruptive phase of the 2010 eruption at the Fimmvorethuhals Pass, east of Eyjafjallajokull volcano, produced poorly evolved basalts with mildly alkaline affinity, and benmoreitic tephra were emitted during the second explosive phase from the summit vent of the volcano. In this study, textural features and chemical zoning preserved in olivine crystals of the early erupted basalts have been used to define the timescales of differentiation processes and magma ascent before the eruption. These lavas contain a mineral assemblage constituted by olivine (Fo70-88) and plagioclase (An57-83) in similar proportions with scarce clinopyroxene and opaque oxides. Olivine occurs as euhedral or embayed crystals characterized by different core compositions and zoning patterns. Three main olivine populations have been found, namely crystals with: (1) wide Fo88 cores with normal zoning toward narrow rims (P1); (2) ∼Fo81 cores with either no zoning or slight reverse zoning patterns toward the rims (P2); (3) ∼Fo77 cores with reverse zoning at the rims (P3). The olivine reverse zoning indicates that these poorly evolved magmas experienced mixing processes in addition to limited fractional crystallization at different levels of the plumbing system. Timescales of transfer dynamics before the eruption have been estimated through Fe-Mg diffusion modeling on these olivine populations. The olivine-melt equilibration through diffusion was triggered by interaction of magmas differing in their evolutionary degree. P1 and P2 crystals recorded a first event of interaction in a ∼22 km deep reservoir that took place about one month before the emission of the analyzed products. Only part of P2 crystals records reverse zoning due to interaction with more basic magma bearing P1 crystals (which consequently develop normal zoning), suggesting fast timescales of magma mixing that prevented the complete homogenization. A second mixing event, which is evident in the P3 olivines, occurred at shallower levels (5-6 km of depth) ∼15 days before the emplacement and can be considered the triggering mechanism leading to the eruption at the Fimmvorethuhals Pass. Integration of our timescales with seismic data relative to the hypocenter migration indicate rates of magma ascent throughout the deep plumbing system of ∼0.01 m/s. This study provides evidence that magmas emitted at Eyjafjallajokull volcano, and more in general at similar other volcanic systems in ocean ridge settings, can undergo complex processes during their storage and transport in the crust, chiefly due to the presence of a multilevel plumbing system.

This study presents mineralogical characterization of opaque assemblages from I- and S-type granites from the Araçuaí orogen, southeastern Brazil that belong respectively, to the pre- and syn-collisional stages of the orogeny. Although these granites are geochemically well-characterized, with a robust geochemical, isotopic and geochronological database, their opaque minerals have not yet been investigated, and they provide important information about the oxygen fugacity and temperature conditions of their magmas. I-type granites (G1 Supersuite) consist of biotite hornblende granites and their opaque assemblage is ilmenite + pyrite + pyrrhotite ± magnetite ± Fe-Ti oxides ± chalcopyrite. S-type rocks (G2 Supersuite) are biotite muscovite sillimanite granites with ilmenite + graphite + pyrrhotite + pyrite as opaques. Our results combined with literature data show that ranges for oxygen fugacity (fO2) are: I-type granitoids containing magnetite and free of pyrite and phyrrhotite likely crystallized under fO2 between 10−15 bars and 10–8.5 bars, whereas magnetite free rocks containing pyrite and pyrrhotite should have crystallized with fO2 higher than 10−18 bars and lower than 10−15 bars. Regarding S-type granites, they must have crystallized under fO2 lower than 10−18 bars.