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Amorphous silicates, abundant in primitive carbonaceous chondrites, are among the most primitive materials from the early Solar System. They show evidence of some aqueous alteration in the meteorite parent bodies, but it is not clear how this highly reactive material changed at an early stage after contact with water. Herein, we report in-situ experiments on the aqueous alteration of amorphous silicate nanoparticles (typically 70 nm in diameter); we used two different compositions that are similar to forsterite (MgO/SiO2 = 2.02) and enstatite (MgO/SiO2 = 1.15) in the simple MgO–SiO2 system to understand basic reaction principles at the onset of the aqueous alteration. The experiments were performed in pure water at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM), and pH measurements. The in-situ TEM images of the nanoparticles—in particular those with the forsterite composition—gradually became difficult to recognize in water. The pH value of the water also increased with time, suggesting that preferential Mg2+ dissolution occurred from the amorphous silicates right after mixing with water. The in-situ XRD patterns showed that magnesium silicate hydrate (M-S-H), which is a poorly crystalline phase like a phyllosilicate, newly appeared. The M-S-H seems to have been formed via a dissolution–precipitation process. Its formation rate from amorphous silicates was considerably higher than from crystalline silicates, because amorphous silicates are highly metastable and have high solubility in water. M-S-H formation from the forsterite composition, which has a highly unstable amorphous structure, is ten times faster than from the enstatite composition. The M-S-Hs show string-like or tiny fragmental textures in the final dried products that are very similar to those observed in the matrices of some primitive carbonaceous chondrites. M-S-H would have been the initial product formed in the aqueous alteration of amorphous silicates in the meteorites; thus, it is an important tracer of early aqueous activity at low temperatures in the early Solar System. By comparing the in-situ observations with those obtained after drying the experimental samples, we found two types of M-S-Hs: epigenetic M-S-Hs—which have a slightly Si-rich composition—formed during drying, and syngenetic M-S-Hs formed by in-situ alteration. Carbonaceous chondrites may also contain these two types of hydrous silicates, and this should be investigated to understand the conditions for aqueous alteration in the early Solar System in more detail. The present study clearly showed the importance of Mg/Si ratio in the precursor materials, although the actual chondrites are in more complicated multi-component system. Future experiments based on the present results can extend the investigation to the system containing Fe, S, and other components as in carbonaceous chondrites.

Base-metal sulfides (BMSs) are minerals that host platinum-group elements (PGE) in mantle peridotites and significantly control the bulk PGE content. They have been investigated in detail down to the sub-micrometer scale to elucidate PGE behavior in the Earth's interior. Base-metal sulfides are supposedly subjected to supergene and seawater weathering, leading to the redistribution of PGEs at low temperatures. Careful and thorough measurements of BMSs are thus required to elucidate PGE behavior in the Earth's interior. In the present study, a sub-micrometer-sized PGE-bearing sulfide inclusion in a clinopyroxene crystal in a harzburgite xenolith from Tahiti (Society Islands, French Polynesia) was investigated in detail (down to the sub-micrometer scale) using transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDS). The sulfide inclusion is of carbonatitic metasomatic origin, as it is enveloped by carbonaceous glass, and forms a planar inclusion array with other PGE-bearing sulfide inclusions. The following sulfide phases were identified using TEM-EDS: Fe- and Ni-rich monosulfide solid solutions (MSSs), Fe- and Ni-rich pentlandite, sugakiite, heazlewoodite, chalcopyrite, and Cu-Ir-Pt-Rh-thiospinel (cuproiridsite–malanite–cuprorhodsite). We established the formation process of the metasomatic PGE-bearing sulfide inclusion by considering morphological and mineral characteristics in addition to the chemical composition. A primary MSS first crystallized from metasomatic sulfide melt at ca. 1000 °C, followed by the crystallization of an intermediate solid solution (ISS) below 900 °C. A high-form (high-temperature origin) Fe-rich pentlandite simultaneously crystallized with the primary MSS below ca. 850 °C and recrystallized into a low-form (low-temperature origin) Fe-rich pentlandite below ca. 600 °C. The primary MSS decomposed to Fe- and Ni-rich MSSs, low-form Ni-rich pentlandite, sugakiite, and heazlewoodite. The ISS decomposed to chalcopyrite below ca. 600 °C. Meanwhile, a Cu-Ir-Pt-Rh-thiospinel crystallized directly from the evolved Cu-rich sulfide melt below ca. 760 °C. Thus, Ir, Pt, and Rh preferentially partitioned into the melt phase during the crystallization process of the metasomatic sulfide melt. Metasomatic sulfide melts could be a significant medium for the transport and condensation of Pt together with Ir and Rh during the fractionation process in the Earth's interior. We hypothesize that the compositional variability of PGEs in carbonatites is due to the separation of sulfide melt leading to the loss of PGEs in the carbonatitic melts.

Synchrotron powder X-ray diffraction (XRD) experiments and transmission electron microscopy (TEM) observations of heat-treated sillimanite at various pressures were conducted to clarify the detailed phase relation between sillimanite and mullite. Under TEM, heat-treated sillimanite frequently showed anti-phase boundary (APB)-like textures with a displacement vector of ½[001]sil. Additional scanning TEM energy-dispersive X-ray spectroscopy analysis of regions with APB-like texture showed that they were clearly enriched in Al and accompanied by very fine, Si-rich glass inclusions, which indicates that the APB-like textures are composed of fine mullite. Moreover, synchrotron XRD patterns of these samples clearly showed double peaks of newly formed mullite and remnant sillimanite, indicating that the compositional transformation from sillimanite to mullite and glass is discontinuous. We separately determined the cell parameters of the sillimanite and mullite from the XRD pattern and found that the b axial length of the sillimanite increased with the treatment temperature, reflecting disordering of tetrahedral Al and Si in the sillimanite. In contrast, the positions of the deconvoluted mullite peaks indicated that the a axial length of mullite decreased as experimental pressure increased, owing to enrichment of the Si component. By projecting the cell parameters onto the a–b axial plane, the detailed changes in the crystallographic state of the sillimanite and mullite could be easily and comprehensively identified. On the basis of our results, we propose a new P-T diagram for the Al2SiO5 system that shows the transformation boundary between sillimanite and mullite + SiO2-rich melt and the contour of the Al/Si order parameter of sillimanite.

The microtexture of graphite exposed on the polished surface was studied using confocal laser scanning microscopy, laser Raman spectroscopy, and focused ion beam–transmission electron microscopy (FIB–TEM) to elucidate the effect on surface condition and crystallinity of graphite by polishing process. The polished surface of the graphite was divided into a flat part with no irregularities and a grooved band with a width of

Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale ( i.e. scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded. Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8 wt% Na 2 O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides.

High-resolution synchrotron powder X-ray diffraction (XRD) experiments were conducted to clarify the transformation of sillimanite to mullite (mullitization) and determine the mullitization temperature (Tc). We were able to distinguish sillimanite and mullite in the XRD patterns, despite their very similar crystallographic parameters, and to detect the appearance of small mullite peaks among sillimanite peaks. Analysis of the Johnson–Mehl–Avrami (JMA) equation for mullitization ratio (ζ) revealed that at temperatures T ≥1240°C the mullitization had the same kinetics. The activation energy E at T ≥1240°C obtained from the Arrhenius plot was 679.8 kJ mol−1. In analysis using a time–temperature–transformation diagram for mullitization, a mullitization curve of ζ = 1% can be described as $$\hbox{ln } t=({{1}\over{n}}) \hbox{ ln }\hbox{ln} \left( {{1}\over{1-\zeta}} \right)- \hbox{ln }{{k}_{0}}+A/[RT{{\left( T-{{T}_{c}} \right)}^{2}}]+{{E}_{A}}/RT$$ where t is time, n is a reaction-mechanism-dependent parameter determined as 0.324 by JMA-analysis, k0 is the frequency factor, EA is the activation energy for atomic diffusion, and $A/{\left( {{T_c} - T} \right)^2}$. represents the activation energy for nucleation. The results of fitting the data to this equation were Tc = 1199°C, A = 3.9 × 106 kJ mol−1 K−2, EA = 605 kJ mol−1, and k0 = 3.65 × 1015. We conclude that the boundary between sillimanite and mullite + SiO2 in the phase diagram is ~1200°C. This article is protected by copyright. All rights reserved.

The stabilization of coesite is a diagnostic indicator of ultrahigh-pressure metamorphism and in many cases it implies that a rock has been subducted to a minimum depth of 80 km. Coesite typically occurs as rare relicts in rigid host minerals, but most commonly transforms into α-quartz pseudomorphs during exhumation. The abundance of coesite-bearing rocks in orogens worldwide is a contentious issue in the petrological community, despite evidence from numerical modeling that suggests that coesite formation should be a common geological process during ultrahigh-pressure metamorphism. This knowledge gap must be addressed to improve the understanding of the geological aspects of subduction-zone geodynamics. Here we report that minuscule coesites (