Phyllosilicate Matrix Research Articles

Coevolution of phyllosilicate, carbon, sulfide, and apatite in Ryugu's parent body

AbstractWe analyzed an asteroid Ryugu sample returned to Earth by JAXA's Hayabusa2 mission using nanoIR, SEM, and TEM microscopy. We identified multiple distinct carbon reservoirs within the phyllosilicate matrix and demonstrate infrared spectral affinities for some of the carbon to insoluble organic matter (IOM). TEM studies of Ryugu samples have allowed us to better understand the interrelationship between the crystallographic orientations of phyllosilicates and the secondary minerals such as carbonate, sulfide, and apatite. Transport of elements provides a unifying theme for understanding these interrelationships.

AFM-IR nanospectroscopy of nanoglobule-like particles in Ryugu samples returned by the Hayabusa2 mission

Context. The JAXA Hayabusa2 mission returned well-preserved samples collected from the carbonaceous asteroid Ryugu, providing unique non-terrestrially weathered samples from a known parent body. Aims. This work aims to provide a better understanding of the formation and evolution of primitive asteroidal matter by studying the fine scale association of organic matter and minerals in Ryugu samples. We characterized the samples by IR nanospectroscopy using infrared photothermal nanospectroscopy (AFM-IR) technique. This technique overcomes the diffraction limit (of several microns) of conventional infrared microspectroscopy (µ-FTIR). The samples were mapped in the mid-IR range at a lateral spatial resolution about a hundred times better than with µ-FTIR. This provided us with unique in situ access to the distribution of the different infrared signatures of organic components at the sub-micron scale present in the Ryugu whole-rock samples as well as to the characterization of the compositional variability of Ryugu in the insoluble organic matter (IOM) chemically extracted from the Ryugu samples. Methods. The AFM-IR maps of whole-rock particles and IOM residues from Ryugu samples were recorded with a lateral resolution of tens of nanometers. Spectra were recorded in the 1900–900 cm−1 spectral range by AFM-IR (Icon-IR) for all samples, and additional spectra were recorded from 2700 to 4000 cm−1 for one IOM sample by an optical photothermal IR (O-PTIR) technique using a mIRage® IR microscope. Results. Organic matter is present in two forms in the whole-rock samples: as a diffuse phase intermixed with the phyllosilicate matrix and as individual organic nanoparticles. We identify the Ryugu organic nanoparticles as nanoglobule-like inclusions texturally resembling nanoglobules present in primitive meteorites. Using AFM-IR, we record for the first time the infrared spectra of Ryugu organic nanoparticles that clearly show enhanced carbonyl (C=O) and CH contributions with respect to the diffuse organic matter in Ryugu whole-rock and IOM residue.

Three-dimensional textures of Ryugu samples and their implications for the evolution of aqueous alteration in the Ryugu parent body

Samples collected from the surface/subsurface of C-type asteroid 162173 Ryugu by the Hayabusa2 mission were nondestructively analyzed in three dimensions (3D). Seventy-three small particles (approximately 10–180 µm in size) were observed using X-ray nanotomography, with an effective spatial resolution of approximately 200 nm. Detailed descriptions of these samples in terms of mineralogy, petrology, and variations among particles were reported. The 57 most common particles consisted of a phyllosilicate matrix containing mineral grains, mainly magnetite, pyrrhotite, dolomite and apatite. The remaining particles were mostly monomineralic particles (pyrrhotite, dolomite, breunnerite, apatite, and Mg-Na phosphate) with two unique particles (calcite in a Al2Si2O5(OH)4 matrix, and CaCO3, phyllosilicate, and tochilinite-chronstedtite inclusions in a carbonaceous material matrix). The results confirmed that the samples correspond to Ivuna-type carbonaceous chondrites (CI chondrites) or related materials. Many small inclusions of voids and carbonaceous materials were detected in pyrrhotite, dolomite, breunnerite, and apatite. However, no fluid inclusions were observed, except for those in pyrrhotite that have already been reported. Magnetite exhibited a wide variety of morphologies, from irregular shapes (spherulites, framboids, plaquettes, and whiskers) to euhedral shapes (equants, rods, and cubes), along with transitional shapes. In contrast, the other minerals exhibit predominantly euhedral shapes (pyrrhotite: pseudo-hexagonal plates, dolomite: flattened rhombohedrons, breunnerite: largely flattened rhombohedrons, and apatite: hexagonal prisms) or aggregates of faceted crystals, except for Mg-Na phosphate. The matrices were heterogeneous with variable phyllosilicate particle sizes, Mg/Fe ratios, density (1.7 ± 0.2 g/cm3), nanoporosities (36 ± 9 %), and abundances of nanograins of Fe(-Ni) sulfides. The macroporosity of the particles was estimated as 12 ± 4 %.The observed textural relationships among the minerals suggest a precipitation sequence of: magnetite (spherulite → plaquette/framboid → rod/equant) → pyrrhotite (pentlandite → pyrrhotite) → apatite → dolomite → breunnerite → coarse phyllosilicates. Fe-bearing olivine (or low-Ca pyroxene) might have precipitated later than dolomite, indicating a high Mg activity in the aqueous solution. This precipitation sequence corresponds to a transition from irregular crystal forms (as seen in some magnetite) to regular forms of euhedral crystals (observed in some magnetite and other minerals). Based on the precipitation sequence and mineral morphologies, together with previously reported observations, a model for aqueous alteration in the Ryugu parent body was proposed as follows: CO2-H2O ice, amorphous silicates (GEMS-like material), and some minerals (mostly metal, sulfides, and anhydrous silicates) accumulated to form the parent body of Ryugu. Amorphous silicates and Fe-Ni metal quickly dissolved into the melted ice to form a highly supersaturated aqueous solution. Poorly-crystalized phyllosilicate and spherulitic magnetite precipitated first, followed by plaquette/framboidal magnetites with decreasing degree of supersaturation due to precipitation. Pseudo-hexagonal pyrrhotite plates were formed by dissolution and reprecipitation under relatively low supersaturation. Subsequently, apatite, dolomite, and breunnerite precipitated in this order in response to decreasing supersaturation.

Analysis of Cation Composition in Dolomites on the Intact Particles Sampled from Asteroid Ryugu.

Characterization of the elemental distribution of samples with rough surfaces has been strongly desired for the analysis of various natural and artificial materials. Particularly for pristine and rare analytes with micrometer sizes embedded on specimen surfaces, non-invasive and matrix effect-free analysis is required without surface polishing treatment. To satisfy these requirements, we proposed a new method employing the sequential combination of two imaging modalities, i.e., microenergy-dispersive X-ray fluorescence (micro-XRF) and Raman micro-spectroscopy. The applicability of the developed method is tested by the quantitative analysis of cation composition in micrometer-sized carbonate grains on the surfaces of intact particles sampled directly from the asteroid Ryugu. The first step of micro-XRF imaging enabled a quick search for the sparsely scattered and micrometer-sized carbonates by the codistributions of Ca2+ and Mn2+ on the Mg2+- and Fe2+-rich phyllosilicate matrix. The following step of Raman micro-spectroscopy probed the carbonate grains and analyzed their cation composition (Ca2+, Mg2+, and Fe2+ + Mn2+) in a matrix effect-free manner via the systematic Raman shifts of the lattice modes. The carbonates were basically assigned to ferroan dolomite bearing a considerable amount of Fe2+ + Mn2+ at around 10 atom %. These results are in good accordance with the assignments reported by scanning electron microscopy-energy-dispersive X-ray spectroscopy, where the thin-sectioned and surface-polished Ryugu particles were applicable. The proposed method requires neither sectioning nor surface polishing; hence, it can be applied to the remote sensing apparatus on spacecrafts and planetary rovers. Furthermore, the non-invasive and matrix effect-free characterization will provide a reliable analytical tool for quantitative analysis of the elemental distribution on the samples with surface roughness and chemical heterogeneity at a micrometer scale, such as art paintings, traditional crafts with decorated shapes, as well as sands and rocks with complex morphologies in nature.

Phyllosilicates with embedded Fe‐based nanophases in Ryugu and Orgueil

AbstractSamples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L3 edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe3+) than in Ryugu (<50% Fe3+). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

The mineralogy and alteration history of the Yamato-type (CY) carbonaceous chondrites

The CY chondrites are a group of thermally metamorphosed carbonaceous chondrites. Although they share similarities with the CM and CI chondrites, their primary properties argue for a distinct classification. Previous studies have highlighted their isotopically heavy bulk compositions (δ17O = 10 ‰, δ18O = 21 ‰, Δ17O = 0 ‰) and exceptionally high sulphide abundances (10–30 vol%). In this work we explore their petrography and alteration history. The CYs accreted low abundances of chondrules (15–20 area%) with average apparent diameters slightly larger (∼320–340 µm) than the CM chondrites. In contrast to the CMs, the CYs record an early episode of brecciation prior to the main window of aqueous alteration. Subsequent fluid activity produced a range of alteration extents with both CY2 and CY1 chondrites documented. Phyllosilicate minerals in the CYs were a mix of serpentine and saponite (including occurrences of Na-saponite) with minor quantities of chlorite (within chondrules). An initial generation of Fe-sulphides formed by sulfidation of metal, and by precipitation from S-rich fluids. Three generations of carbonates are recognized, an early generation that infilled voids left by brecciation and co-precipitated with sulphide, a later generation that co-precipitated with magnetite and a final Fe-Mg-bearing generation which formed large (>100 µm) clasts. Only the first-generation carbonates are found in the CY2s, while the CY1s preserve all three generations. Phosphates occur as Ca-apatite or rarely as Mg-bearing apatite and have hydroxylapatite compositions, indicating low halogen activities in the alteration fluids. Refractory oxides (ilmenite and Cr-spinel) occur as precipitates adhering to the margins of phyllosilicates. They formed late in the alteration sequence and attest to oxidizing conditions. During the late-stages of aqueous alteration Fe-sulphides were replaced by magnetite. Thermal metamorphism (Stage II-IV: ∼300–750 °C) overprinted aqueous alteration leading to dehydration and recrystallization of the phyllosilicate matrix and the decomposition of some carbonate phases. Most Fe-sulphide grains survived heating without decomposition as initial partial decomposition from pyrrhotite to troilite under closed system conditions led to elevated ƒS2 gas and resulted in a stabilizing effect. Retrograde reactions between trapped S2 gas and metal/magnetite formed a final generation of Fe-sulphides. The survival of Fe-sulphides and their stochiometric troilite compositions are evidence for near-closed system heating. Analysis of organic matter by Raman spectroscopy supports an interpretation of short-duration heating (on the scale of minutes to days), at peak temperatures between 750 and 900 °C. Thus, an impact event was the most likely cause of metamorphic heating.

On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective.

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ18O, Δ17O, and ε54Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10's of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation.

Genesis and 40Ar/39Ar dating of K-Mn oxides from the Stavelot Massif (Ardenne, Belgium): Insights into Oligocene to Pliocene weathering periods in Western Europe

The occurrence of Ordovician Fe-Mn-rich sediments (Ottré Formation, Salm Group) in the Stavelot inlier (NE Ardenne, Rhenish Massif, Belgium) has drawn the attention of geoscientists for more than one century, as they contain natural “coticule” whetstones and numerous weathered Mn deposits. Two locations expose the Fe-Mn-bearing rocks: the Chevron area to the north is located at an altitude of 230–380 m, and the Salmchâteau at 480–570 m. The Mn-bearing sediments have undergone greenschist facies metamorphism in relation to the Caledonian and Variscan orogenesis, and more recently weathering processes, transforming the early mineral assemblages (mainly rhodochrosite and/or kutnohorite, spessartine, muscovite, chloritoïd, andalusite, hematite) into secondary Mn oxides,—hydroxides and—oxyhydroxides (cryptomelane, nsutite, lithiophorite, manganite, pyrolusite), clays (mainly kaolinite) and Fe oxides (goethite). In this paper, we investigate the weathering processes of these Mn-rich rocks by i) new mineralogical, petrographical and geochemical data, and ii) new 40Ar/39Ar geochronology of K-Mn oxides.Petrographical and geochemical data indicate that rhodochrosite (and/or kutnohorite) is the main source of Mn, from which Mn oxides have precipitated. Spessartine and other Mn-silicates have poorly contributed to the Mn mobilization. Pure Mn oxides, namely manganite, nsutite, and pyrolusite, successively replace rhodochrosite under increasing O2 conditions due to the weathering fluids, while the formation of cryptomelane and then lithiophorite in veins is associated to the recombination of K+, Li+ and Al3+ released from the phyllosilicate matrix of the host slate, probably in relation with sligh pH changes. This multistage character of weathering processes is well identified in the Salmchâteau area, where weathering conditions are stronger than in the Chevron area because of the elevation. This also positions the Mn levels close to the roll front within the lower saprolite and the fissured horizon, respectivelly. Such differences in the weathering grade of these areas determined a weathering-type deposit in the Salmchâteau area and a carbonate-hosted sedimentary deposit in the Chevron area. Weathering deposits are accompanied by an enrichment in Mn and associated transition metals (As, Co, Cu, Ni). Dating of pure cryptomelane samples corroborate the multistage character of the ore and yield at least four weathering stages: Chattian (26.33 ± 0.17 Ma), Aquitanian (22.2 ± 0.6 and 20.0 ± 0.2 Ma), Serravalian-Tortonian (11.79 ± 0.10 to 10.12 ± 0.18 Ma) and Pliocene or younger (5.30 ± 0.76 and 4.11 ± 0.57 Ma). A relatively continuous weathering of the Massif since the late Oligocene, or older weathering phases, cannot be excluded as the upper part of the saprolite (and laterite) have been removed. As climatic conditions evolve from tropical to temperate by the Oligocene, the uplift of the Ardenne is probably the main factor triggering the development of weathering mantle, together with the increasing seasonality and high precipitation. These ages with others obtained in supergene ores of Western European massifs define a long-lasting weathering wave since the late Oligocene until the uppermost Neogene. The absence of older periods in the 40Ar/39Ar record of these deposits indicates that the upper part of the weathering mantle was recently stripped and limit their economic interest.

Bimetallic Synergy Effects of Phyllosilicate‐Derived NiCu@SiO2 Catalysts for 1,4‐Butynediol Direct Hydrogenation to 1,4‐Butanediol

AbstractHydrogenation of 1,4‐butynediol (BYD) to 1,4‐butanediol (BDO) is a two‐step process, with an initial hydrogenation of BYD to 1,4‐butenediol (BED) and the subsequent hydrogenation of BED to BDO. However, the BYD hydrogenation also involves many side reactions originated from the isomerization of BED. In order to inhibit the isomerization pathways, phyllosilicate‐derived bimetallic NiCu@SiO2 catalysts have been developed for efficient C≡C/C=C hydrogenation in this work. Due to the formation of phyllosilicate matrix and highly dispersed metal nanoparticles, NiCu@SiO2 showed total BYD conversion with extremely high BDO selectivity compared to a conventional impregnated Ni/SiO2 catalyst. A remarkable result of NiCu@SiO2 catalysts is that a new type of bimetallic catalytic sites responsible for the high hydrogenation activity can be differentiated from the Ni phyllosilicate matrix by the induction of Cu species, and these neighboring bimetallic sites with the help of weak acid phyllosilicate interface, can realize to stabilize the activated BED species (allyl alcohol form) adsorbed on the cooperative active sites, thus to avoid its isomerization to aldehyde form and unexpected C=O hydrogenolysis. Consequently, it enhanced the selectivity to the diol products BDO significantly. Owing to the benign improvement of three center synergy effect, 9Ni1Cu@SiO2 possesses the optimum BYD direct hydrogenation ability with a rarely reported high selectivity of 90.5–94.5 % at 50 °C and 1 MPa.

A divergent heritage for complex organics in Isheyevo lithic clasts

Primitive meteorites are samples of asteroidal bodies that contain a high proportion of chemically complex organic matter (COM) including prebiotic molecules such as amino acids, which are thought to have been delivered to Earth via impacts during the early history of the Solar System. Thus, understanding the origin of COM, including their formation pathway(s) and environment(s), is critical to elucidate the origin of life on Earth as well as assessing the potential habitability of exoplanetary systems. The Isheyevo CH/CBb carbonaceous chondrite contains chondritic lithic clasts with variable enrichments in 15N believed to be of outer Solar System origin. Using transmission electron microscopy (TEM-EELS) and in situ isotope analyses (SIMS and NanoSIMS), we report on the structure of the organic matter as well as the bulk H and N isotope composition of Isheyevo lithic clasts. These data are complemented by electron microprobe analyses of the clast mineral chemistry and bulk Mg and Cr isotopes obtained by inductively coupled plasma and thermal ionization mass spectrometry, respectively (MC-ICPMS and TIMS). Weakly hydrated (A) clasts largely consist of Mg-rich anhydrous silicates with local hydrated veins composed of phyllosilicates, magnetite and globular and diffuse organic matter. Extensively hydrated clasts (H) are thoroughly hydrated and contain Fe-sulfides, sometimes clustered with organic matter, as well as magnetite and carbonates embedded in a phyllosilicate matrix. The A-clasts are characterized by a more 15N-rich bulk nitrogen isotope composition (δ15N=200–650‰) relative to H-clasts (δ15N=50–180‰) and contain extremely 15N-rich domains with δ15N<5000‰. The D/H ratios of the clasts are correlated with the degree of clast hydration and define two distinct populations, which we interpret as reflecting mixing between D-poor fluid(s) and distinct organic endmember components that are variably D-rich. High-resolution N isotope data of 15N-rich domains show that the lithic clast diffuse organic matter is typically more 15N-rich than globular organic matter. The correlated δ15N values and C/N ratios of nanoglobules require the existence of multiple organic components, in agreement with the H isotope data. The combined H and N isotope data suggest that the organic precursors of the lithic clasts are defined by an extremely 15N-poor (similar to solar) and D-rich component for H-clasts, and a moderately 15N-rich and D-rich component for A-clasts. In contrast, the composition of the putative fluids is inferred to include D-poor but moderately to extremely 15N-rich H- and N-bearing components. The variable 15N enrichments in H- and A-clasts are associated with structural differences in the N bonding environments of their diffuse organic matter, which are dominated by amine groups in H-clasts and nitrile functional groups in A-clasts. We suggest that the isotopically divergent organic precursors in Isheyevo clasts may be similar to organic moieties in carbonaceous chondrites (CI, CM, CR) and thermally recalcitrant organic compounds in ordinary chondrites, respectively. The altering fluids, which are inferred to cause the 15N enrichments observed in the clasts, may be the result of accretion of variable abundances of NH3 and HCN ices. Finally, using bulk Mg and Cr isotope composition of clasts, we speculate on the accretion regions of the various primitive chondrites and components and the origin of the Solar System’s N and H isotope variability.

Fe(II)- and sulfide-facilitated reduction of 99Tc(VII)O4− in microbially reduced hyporheic zone sediments

Redox-reactive, biogeochemical phases generated by reductive microbial activity in hyporheic zone sediments from a dynamic groundwater–river interaction zone were evaluated for their ability to reduce soluble pertechnetate [99Tc(VII)O4−] to less soluble Tc(IV). The sediments were bioreduced by indigenous microorganisms that were stimulated by organic substrate addition in synthetic groundwater with or without sulfate. In most treatments, 20μmolL−1 initial aqueous Tc(VII) was reduced to near or below detection (3.82×10−9molL−1) over periods of days to months in suspensions of variable solids concentrations. Native sediments containing significant lithogenic Fe(II) in various phases were, in contrast, unreactive with Tc(VII). The reduction rates in the bioreduced sediments increased with increases in sediment mass, in proportion to weak acid-extractable Fe(II) and sediment-associated sulfide (AVS). The rate of Tc(VII) reduction was first order with respect to both aqueous Tc(VII) concentration and sediment mass, but correlations between specific reductant concentrations and reaction rate were not found. X-ray microprobe measurements revealed a strong correlation between Tc hot spots and Fe-containing mineral particles in the sediment. However, only a portion of Fe-containing particles were Tc-hosts. The Tc-hot spots displayed a chemical signature (by EDXRF) similar to pyroxene. The application of autoradiography and electron microprobe allowed further isolation of Tc-containing particles that were invariably found to be ca 100μm aggregates of primary mineral material embedded within a fine-grained phyllosilicate matrix. EXAFS spectroscopy revealed that the Tc(IV) within these were a combination of a Tc(IV)O2-like phase and Tc(IV)–Fe surface clusters, with a significant fraction of a TcSx-like phase in sediments incubated with SO42−. AVS was implicated as a more selective reductant at low solids concentration even though its concentration was below that required for stoichiometric reduction of Tc(VII). These results demonstrate that composite mineral aggregates may be redox reaction centers in coarse-textured hyporheic zone sediments regardless of the dominant anoxic biogeochemical processes.

An integrated AMS, structural, palaeo- and rock-magnetic study of Eocene marine marls from the Jaca-Pamplona basin (Pyrenees, N Spain); new insights into the timing of magnetic fabric acquisition in weakly deformed mudrocks

Abstract In this paper, we present a combined magnetic anisotropy (AMS), structural, palaeo- and rock-magnetic study of Eocene marine mudrocks from the western sector of the Jaca-Pamplona Basin (southwestern Pyrenees, north Spain). Comparison of structural, AMS and palaeomagnetic data reveals a subtle, but evident tectonic overprint affecting the phyllosilicate matrix of the mudrocks and the remanence carriers. The particular structural setting of the studied rocks has allowed us to combine palaeomagnetic and structural data with the AMS results in order to establish a relative chronology between sedimentation, blocking of magnetic fabrics, acquisition of magnetic remanence and deformation. Our data suggest that the blocking of the magnetic fabrics and the lock-in of the remanence occurred simultaneously during the early stages of gentle warping that affected the Jaca-Pamplona basin throughout the Mid-Late Eocene. According to the origin of the remanence carriers and the synsedimentary nature of the Mid-Late Eocene warping, the blocking of the magnetic fabrics and the lock-in of the remanence can be bracketed to a very short time span, of a few (&lt;15?) ka, after sediment deposition. Our findings confirm previous interpretations claiming a very early origin of the magnetic fabric blocking in mudrocks undergoing weak deformation. They therefore validate the use of magnetic fabrics as palaeostress indicators and suggest that AMS data might also provide a useful tool for detecting preferred paths for the migration of geofluids during the early stages of deformation.

Experimental Study of the Compaction of Phyllosilicate-Bearing Sand at Elevated Temperature and with Controlled Pore Water Pressure

Relative to the time-dependent compaction behavior of clean quartz sands, little experimental work has been carried out on phyllosilicate-rich siliciclastic rocks (mudstones). Compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix, coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water. Time dependence of compaction also arises in nature owing to the generation of excess pore pressure, which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation. A suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite + muscovite, hydrostatically compacted at a range of effective pressures between 10 MPa and 210 MPa, with a constant pore water pressure of 70 MPa and at temperatures ranging between 300 and 450°C. A small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate + quartz mixture. Volume strains were measured by (a) direct measurement of sample length and (b) measurement of amount of pore water expelled at constant pore pressure, and interpreted with the aid of quantitative petrography on compacted samples. Control experiments were performed on (i) dry, clean sand, (ii) dry sand + phyllosilicate, and (iii) wet, clean sand. Dry, clean sand showed almost no compaction. Wet, clean sand showed a degree of compaction similar to dry phyllosilicate + sand, but wet phyllosilicate + sand showed dramatically more compaction. Compaction rate φ for the wet, mixed phyllosilicate/sand samples is described by ![Formula][1] in which σ is effective stress (MPa), φ is porosity (%), R is the gas constant, and T is temperature (K). Petrographic study showed interpenetration of grain contacts, formation of quartz overgrowths, a decrease in the amount of quartz embedded in the phyllosilicate matrix, and an increase in mean grain size of quartz with progressive compaction. These effects are attributed to solution and redeposition of quartz. The empirical compaction law was integrated over a natural burial history. The porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate. There is reasonable qualitative agreement when compared with porosity/depth relations reported in the literature. [1]: /embed/graphic-1.gif

The strata-bound tin deposit Nam Salu, Kelapa Kampit, Indonesia

The Nam Salu horizon at Kelapa Kampit on Belitung Island, Indonesia, is the richest stratabound tin mineralization in Southeast Asia. It belongs to a sequence of Carboniferous-Permian sediments and volcanics which have been intruded by granitoids of Triassic age. The Nam Salu horizon is a steeply dipping, 35-m-thick layer of stilpnomelane-chlorite-(biotite) phyllite which is exposed over a length of 3 km. The horizon carries high-grade cassiterite ore (1-2% Sn) 100 m along strike up to a height of 100 m. This orebody, which is being mined in the Nam Salu open pit, is located symmetrically on both sides of the Khoi-Wa fault which intersects the horizon at an angle of about 45 degrees .The horizon exhibits semicontinuous banding in the 0.2- to 15-mm range which is characteristically defined by its varying iron mineral content (magnetite, pyrrhotite, pyrite, ilmenite, and siderite), averaging 8 vol percent. Eutaxitic textures are preserved in one-third of the rock indicating its derivation from tuff.In the open pit, the horizon contains disseminations of abundant fine-grained (2-200-mu m) cassiterite which is randomly distributed in the felted phyllosilicate matrix. The banded rock is intersected by veinlets with an average thickness of 0.1 mm which postdate the disseminated cassiterite mineralization and constitute approximately 1 percent of the total rock volume. The early-phase veinlets are filled with pyrite, phyllosilicates, siderite, pyrrhotite, quartz, sphalerite, chalcopyrite, magnetite, arsenopyrite, sphene, and cassiterite (in order of decreasing frequency). Late-stage veinlets filled with galena, sphalerite, carbonates, fluorite, and bismuth minerals are subordinate.The Nam Salu open-pit orebody is characterized by high concentrations of Fe (avg 39.95% Fe 2 O 3 total Fe) which vary inversely with SiO 2 , Al 2 O 3 , TiO 2 , and Zr concentrations. The contents of CaO and NaO 2 are very low (avg 2 O (3.98%), Rb (1,270 ppm), and Cs (325 ppm) are high. The chemical composition corresponds to a mixture of iron-rich sediment (iron-formation) and basalt (texturally tuff) which has been altered by metasomatic processes. The hydrothermal fluids which interacted with the host rock produced a strong enrichment in some alkali metals by biotitization together with the introduction of Sn, Be, Bi, F, Nb, Pb, and W as well as a depletion in Na 2 O, CaO, and Sr. The hanging-wall and footwall rocks, which are composed of metasandstone, metasiltstone, and chert, have been much less reactive to hydrothermal fluids than the Nam Salu horizon and are distinguished by their low Sn and Rb concentrations.The alkali (Sn) metasomatism cannot be linked in time to the precipitation of the iron-rich sediment or the basaltic volcanism which deposited the water-laid tuff, since seawater had no or insignificant access to the operating hydrothermal system. This is evident from the high Rb and Cs concentrations as well as from the high ratios Rb/K (0.04) and Cs/K (0.01) in the Nam Salu horizon. The reaction with heated seawater would produce a depletion of Rb and Cs relative to K. Seawater has very low Rb/K and Cs/K ratios because Rb and Cs are preferentially removed from seawater through incorporation into marine sediments. Only residual fluids separating from a granite magma are known to produce rocks which have Rb and Cs concentrations as high as the Nam Salu horizon. The most likely source of the Sn-bearing fluids is granitic magmatism, though its nearest manifestations are the biotite granite outcrops 20 km to the east and 30 km to the west of Kelapa Kampit, respectively.

Isolated olivines in the Yamato 82042 CM2 chondrite: The tracing of major condensation events in the solar nebula

Abstract— Yamato 82042 is an unusual CM2 chondrite consisting mainly of phyllosilicates, a few olivines and carbonates, very minor sulphides and trace metal. Olivine occurs: (1) as isolated grains dispersed in the phyllosilicate matrix, (2) as constituents of mineral aggregates or accretionary fragments associated with abundant phyllosilicates and minor sulphides, and (3) as objects which resemble barred olivine chondrules also associated with phyllosilicates. Olivine, from all occurrences, ranges in composition from 0.26 to 22.6 weight % FeO, but generally contains less than 1.25 wt.% FeO. Minor element contents, particularly Ca, Al, and Cr, are relatively high and are generally correlated, as reported for olivines in other carbonaceous chondrites. However, we report here uncorrected trends for the same minor elements which occur in distinct areas (volumes) within the same olivines. These compositional trends may be due to condensation of olivine from a vapor of non‐solar composition and partial mobilization of Ca during later annealing. If this is the case, the data may be used to trace changes in the Ca/Al ratio of the parent medium during the formation of these olivines, provided that it is possible to distinguish the effects of any post‐formation annealing which could have redistributed the minor elements. Some isolated olivines show distinctive minor element zoning which severely limits the possibility of any post‐formation redistribution of these elements. Accordingly, these isolated olivines indeed retain evidence of early condensation processes in the solar nebula, though non‐classic conditions are implied for their formation.

The effect of pre-existing microcavities on mechanical rock performance in sedimentary and crystalline rocks

Diamond and percussion drill performances, and mechanical rock properties in 11 sedimentary and crystalline rock types are reported. Pre-existing or natural microcavities in sandstones (high porosity materials) and crystalline rocks (low porosity materials) each have a significant and different effect on drilling rates and strength properties in their respective rock types. A gradual change in sedimentary porosity produces a similar gradual change in drilling rates and strength properties. A small change in crystalline rock porosity produces a profound change in strength properties and a lesser effect on drill penetration rates. Brazilian disc fracture modes in the sandstones were predominantly inter-granular and through the phyllosilicate matrix. Crystalline rocks exhibited three fracture modes: inter-granular along grain boundaries, intra-granular along cleavage direction and intra-granular chonchoidal fractures. The mechanical performance of sandstones can be directly related to porosity. The mechanical performance of crystalline rocks cannot be directly related to porosity as measured in this paper. It is assumed other mechanisms control fracture processes in these rock types.

Tectonic Significance of Lithicwacke-Polymictic Conglomerate Petrofacies Association Within Upper Cretaceous Torchlight Sandstone, Big Horn Basin, Wyoming: ABSTRACT

The Torchlight Sandstone belonging to the Upper Cretaceous Frontier Formation in the Big Horn basin, Wyoming, shows a distinctive lithicwacke-polymictic conglomerate is composed of granule-cobble-sized clasts of quartzite, chert, andesite, and argillite, and phyllite. The survival of phyllite, argillite, and neovolcanic andesite clasts indicate that the detritus underwent very little subaerial transport before it was deposited along the proximal margin of the foreland basin. A petrologically heterogeneous upland source of high to moderate relief is indicated by the clast size and composition. Hydrodynamic structures, in conjunction with textural attributes, and compositional data indicate that detritus moved southeast from its source terrane and was deposited by a high-energy distributary complex. The lithicwacke petrofacies is dominated by higher chert and quartz content with a subordinate amount of labile components including paleovolcanic clasts and fine-grained matrix. The development of phyllosilicate matrix around quartz and chert grains preserved the primary porosity and permeability of the sandstone. Absence of any noticeable quartz overgrowth apparently contributed to the preservation of good reservoir quality in this petrofacies. Considering its (Torchlight Sandstone) close proximity to the thrust belt and to the locus of andesite volcanism in the northwest and west, it is suggested that the extrabasinal detritus withinmore » the foreland basin can provide significant clues as to the timing of the thrust events and volcanicity in the adjacent region. New perspectives for hydrocarbon exploration and regional correlation may be gained by employing this petrofacies association.« less

Quantitative assessment of rock texture and correlation with drillability and strength properties

A dimensionless quantitative measure of rock texture, describing grain: —shape, orientation, degree of grain interlocking and relative proportions of grains and matrix (packing density) has been developed. Data required for the model are obtained by image analysis of thin sections and concerns percentage areas of grains and matrix, length, breadth, perimeter, orientation and area of each grain in the viewing window. The results of rock strength, diamond and percussion drillability tests in eleven sandstones, marbles and igneous rocks are reported, and correlated with the developed texture coefficient. The texture coefficient returns highly statistically significant correlations with rock strength and drillability data. Sandstones have low texture coefficients and high drillability whereas igneous rocks have high texture coefficients and low drillability. With particular reference to percussive drillability it is suggested that extensional crack propagation in the sandstones in an energy efficient process since fracture paths propagate through the weak phyllosilicate matrix. Extensional crack propagation in the igneous rocks is an energy intensive process since a significant proportion of the available drilling energy is consumed in the formation of intra-granular fracture paths. Observational and correlated data are supportive of the suggestion that the texture coefficient is a measure of the resistance of the microstructure of a rock to crack propagation, whether it be inter-or intra-granular. The texture coefficient can be used as a predictive tool for the assessment of drillability and rock strength properties. The technique offers a useful approach in understanding fracture initiation and growth as controlled by the texture of intact rock samples.

The Niger (I) carbonaceous chondrite and implications for the origin of aggregates and isolated olivine grains in C2 chondrites

The origin of olivine grains in C2 carbonaceous chondrites is a controversial topic: directly condensed material or detrital remnants of preexisting chondrules? This study shows that the Niger C2 meteorite is similar to Murchison but reveals several interesting features in relation to the origin of the olivine. Microprobe analysis of olivine (Si, Fe, Mg, Ca, Mn, Cr), glass and nickel-iron inclusions within the grains, and Fe-S-O phase as well as the relationships between the olivine grains in the aggregates, between the grains and the interstitial phyllosilicate matrix, between the inclusions and their host olivine grains, and the morphology of some aggregates all show that two populations of olivine coexist, probably crystallized from chondrule melts rather than by direct condensation from a solar nebula gas. The characteristics of the nickel-iron inclusions within the olivine suggest a magmatic chondrule-making stage from previously condensed materials.

Are carbonaceous chondrites primitive or processed? A review

Carbonaceous chondrites, generally considered to be the most primitive surviving materials from the early solar system, form a distinctive group in terms of bulk Mg/Si, Ca/Si, and Al/Si ratios. The carbonaceous chondrites can be subdivided into five groups (CI, CM, CR, CO, and CV) based on a number of petrologic and chemical criteria. Petrographic observations indicate that most carbonaceous chondrites have been processed, either by thermal metamorphism in the case of CO and CV chondrites or by low‐temperature aqueous alteration in the case of CI, CM, and CR chondrites. Thermal metamorphism resulted in Fe/Mg exchange between chondrules, olivine and pyroxene grains, and matrix, changes in the compositions of metal grains, and textural integration. Aqueous alteration probably produced hydrated phyllosilicate matrix phases and resulted in alteration of chondrules and replacement and vein filling by secondary carbonates and sulfates. The changes incurred during these processes appear to have been largely isochemical. However, if certain constituents behaved as open‐system components, volatile elements or compounds may have been depleted during metamorphism, and isotopic patterns may have been changed during aqueous alteration. The recognition of two different types of postaccretional processes resulting in petrological modifications necessitates a reinterpretation of the classification system for carbonaceous chondrites.