Amorphous Groundmass Research Articles

From rock to magma and back again: The evolution of temperature and deformation mechanism in conduit margin zones

Explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing. The zone at the margin of a volcanic conduit—thought to play an important role in the outgassing of magma and therefore pore pressure changes and explosivity—is the boundary through which heat is exchanged from the hot magma to the colder country rock. Using a simple heat transfer model, we first show that the isotherm for the glass transition temperature (whereat the glass within the groundmass transitions from a glass to an undercooled liquid) moves into the country rock when the magma within the conduit can stay hot, or into the conduit when the magma is quasi-stagnant and cools (on the centimetric scale over days to months). We then explore the influence of a migrating viscous boundary on compactive deformation micromechanisms in the conduit margin zone using high-pressure (effective pressure of 40 MPa), high-temperature (up to 800 °C) triaxial deformation experiments on porous andesite. Our experiments show that the micromechanism facilitating compaction in andesite is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass Tg (i.e., rock). In this regime, porosity is only reduced within the bands of crushed pores; the porosity outside the bands remains unchanged. Further, the strength of andesite is a positive function of temperature below the threshold Tg due to thermal expansion driven microcrack closure. The micromechanism driving compaction above Tg (i.e., magma) is the distributed viscous flow of the melt phase. In this regime, porosity loss is distributed and is accommodated by the widespread flattening and closure of pores. We find that viscous flow is much more efficient at reducing porosity than cataclastic pore collapse, and that it requires stresses much lower than those required to form bands of crushed pores. Our study therefore highlights that temperature excursions can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the material within the conduit margin zone, with possible implications for pore pressure augmentation and explosive behaviour.

Organic petrology and geochemistry of mudrocks from the lacustrine Lucaogou Formation, Santanghu Basin, northwest China: Application to lake basin evolution

Exploration for tight oil in the frontier Santanghu Basin of northwest China has resulted in recent commercial discoveries sourced from the lacustrine Upper Permian Lucaogou Formation, already considered a “world class source rock” in the Junggar Basin to the west. Here we apply an integrated analytical program to carbonate-dominated mudrocks from the Lucaogou Formation in Santanghu Basin to document the nature of organic matter (OM) in the context of an evolving lake system. The organic-rich samples (TOC 2.8–11.4wt%; n=10) were widely spaced from an ~200m cored section, interpreted from textural and mineralogical evidence to document transition from a lower under-filled to an overlying balanced-filled lake. Organic matter is dominated by moderate to strongly fluorescent amorphous material with Type I geochemical signature (HI values 510–755; n=10) occurring in a continuum from lamellar stringers, 10–20μm thick, some ≥1mm in length (possible microbial mat; preserved only in lower under-filled section) to finely-disseminated amorphous groundmass intimately intermixed with mineral matrix. Biomarkers for methanotrophs and photosynthetic cyanobacteria indicate a complex microbial consortium. A unicellular prasinophyte green alga(?), similar to Tasmanites in marine rocks, is present as discrete flattened discs 50–100μm in diameter. Type III OM including vitrinite (some fluorescent) and inertinite also is abundant. Solid bitumen, indicating local kerogen conversion, fills voids and occurs throughout the cored section. Vitrinite reflectance values are 0.47–0.58%, consistent with strong OM fluorescence but may be “suppressed”. Other proxies, e.g., biomarker parameters, indicate the Lucaogou Formation is in the early oil window at this location. On average, slightly more amorphous OM and telalginite are present in the lower section, consistent with a shallow, stratified, saline environment with low sediment dilution. More inertinite is present in the upper section, indicating greater terrestrial influx and consistent with higher quartz and plagioclase content (dominantly authigenic chalcedony and albite). Laminated mudstones in the upper section indicate anoxia prevented bioturbation from benthic grazing, also indicating stratified water column conditions. A decrease upsection in authigenic dolomite with reciprocal increase of ankerite/siderite is consistent with decreasing salinity, as is an overall decrease in gammacerane index values. These observations suggest evolution from a shallow, stratified evaporative (saline) setting to a deeper, stratified freshwater basin with higher water input during Lucaogou deposition. The evolution from an under-filled to balance-filled lake in Santanghu Basin is similar to Lucaogou deposition in Junggar Basin, suggesting similar tectonic and climatic controls. Paleoclimate interpretations from other researchers in this area suggested an evolution from semi-arid to humid conditions during the Roadian; we interpret that the evolution from an under-filled to balanced-filled lake seen in our data is in response to climate change, and may represent increased groundwater delivery to the Santanghu Basin.

A cornucopia of presolar and early solar system materials at the micrometer size range in primitive chondrite matrix

Abstract— We have used a variety of complementary microanalytical techniques to constrain the mineralogy, trace‐element distributions, and oxygen‐isotopic compositions in a 50 × 50 μm area of Acfer 094 matrix. The results reveal the exceptional mineralogical and compositional heterogeneity of this material at the sub‐μm level. We observe μm‐scale and sub‐μm grains with elemental associations suggesting feldspar, metal with widely varying Ni contents, and a Cr‐Fe alloy (in addition to forsterite, pyroxene, sulfide, ferrihydrite, and amorphous groundmass previously described). A new class of μm‐scale CAI (μCAI) is also observed, which show sub‐μm compositional zoning, and a range of oxygen isotopic compositions. Unlike the larger CAIs in Acfer 094, which are uniformly 16O‐enriched, two of the three μCAIs we analyzed are isotopically normal. We also observed a Li‐rich hotspot that detailed analysis by ToF‐SIMS suggests may be a LiCr‐oxide grain. Within the resolution of the NanoSIMS, this grain has isotopically normal Li. Finally, in our 50 × 50 μm area, we positively identified a presolar grain that is the most 18O‐rich silicate found so far in meteorites. The grain may originate from an asymptotic giant branch (AGB) star, or more likely, a supernova. In line with previous TEM studies (Greshake 1997), we find no evidence for clastic material (e.g., fragmental chondrules) in the matrix of Acfer 094: although the matrix is volatile‐depleted, this depletion does not appear to result from dilution of a primordial starting material with (depleted) chondrule fragments. Assuming that matrix experienced the depletion event, our data on the detailed mineralogy of Acfer 094 are currently equivocal in constraining the nature of that event. We observe carrier phases for several elements consistent with conditions approaching equilibrium condensation; however, the presence of an amorphous groundmass is suggestive of more rapid cooling.

An example of oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter

A palynologically monotypic assemblage from the Eocene Jatta Gypsum Formation has (Pakistan) been analyzed microscopically and chemically. Scanning electron microscopy shows that the palynomorphs are solid to spongeous dinoflagellate (Division Dinophyta) remains embedded in an amorphous ground mass. Unlike dinoflagellate resting and temporary cysts, a resistant outer wall is absent and the fossils are presently interpreted to be the internal contents (“dinocasts”) of a motile thecate dinoflagellate. Chemical analysis shows that the microfossils are highly aliphatic and consist of polymerized free lipids, likely to have been predominantly C 18 and C 16 fatty acids with a predominantly C 9 mid-chain functionality. We suggest that these “dinocasts” have been formed post-mortem by oxidative polymerization of these lipids, derived from cellular membranes and storage vesicles. Additionally, lipids from outside may have contributed, during or after polymerization of the cell contents. Our hypothesis is supported by chemical analysis of autoxidized vegetable oil and contrasts with the generally accepted opinion that algal remains are preserved via the so-called selective preservation pathway only. In the present case, the unique morphological preservation of the “dinocasts” enabled linking of the post-mortem polymerized free lipids to their microalgal source. However, upon regular, less ideal, morphological preservation conditions such linkage is impossible since “amorphous” particles would result. We propose, therefore, that oxidative polymerization of lipids may be responsible for much more of the amorphous marine organic matter in sediments than presently acknowledged.

Super-critical carbonation of glass-fibre reinforced cement. Part 2: Microstructural observations

The deterioration of glass-fibre reinforced cement/concrete (grc) arises to a substantial extent from the alkalinity and calcium hydroxide content of the matrix. Carbonation of the matrix significantly lowers both factors, but under normal circumstances the reaction proceeds too slowly to be of practical use in improving durability. If carbonation is effected using super-critical carbon dioxide the reaction can be completed within hours rather than years, rendering it potentially attractive as a treatment for enhancing the durability of grc. The efficacy of such treatment is dependent on the moisture content of the samples prior to treatment expressed in terms of a degree of drying (DOD), as established in Part 1 of this paper. Grc samples thus treated, were examined using petrography, scanning electron microscopy and mercury intrusion porosimetry before and after a period of accelerated ageing. Treatment transforms the matrix into a groundmass of amorphous material intimately mixed with microcrystalline calcium carbonate. Dark inclusions appear to be remnants of unhydrated cement particles. The mechanical property enhancements detailed in Part 1 are attributed to the precipitation of calcium carbonate reducing porosity. The nature of the carbonation reaction was found to be dependent on DOD. In samples indicated by differential thermal analysis to be completely carbonated, some uncarbonated unhydrated cement and uncarbonated amorphous groundmass remained in isolated patches.

Chemistry of maceral and groundmass density fractions of torbanite and cannel coal

Microscopically, torbanite and cannel coal are composed of coarser macerals set in a fine-grained to amorphous groundmass. It is often assumed that the amorphous groundmass is genetically related to the distinct macerals. The separation of macerals and groundmass from 14 late Paleozoic torbanite, cannel, and humic coals permits the analysis of individual constituents using elemental analysis and flash pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Cluster and principal component analyses of the Py-GC/MS data further reveal the chemical similarities and differences between the various constituents. Pyrolyzates of Botryococcus-related alginites are characterized by an abundance of normal alkadienes, alkenes, and alkanes. Even their alkylbenzenes and alkylnaphthalenes exhibit a relatively higher concentration of isomers with a single, linear alkyl side-chain than do other macerals and groundmass. In contrast, vitrinite pyrolyzates are dominated by phenolic and aromatic compounds. Sporinites are enriched in aliphatic, aromatic, and phenolic structures, especially the short chain aliphatics and alkylbenzenes. They are also characterized by a predominance of 1,2-dimethylbenzene and 1-ethyl-2-methylbenzene. The groundmass is further divided into lamalginitic, bituminitic, and vitrinitic. The chemistry of the brightly-fluorescing lamalginitic groundmass is basically similar to that of alginite, but also resembles other groundmass types in normal hydrocarbon and alkylphenol distributions. The vitrinitic groundmass can be described as an “aliphatic-rich” vitrinite. The pyrolyzate of the bituminitic groundmass is characterized by the predominance of long chain normal hydrocarbons. Their pyrolyzates have a chemical nature intermediate between alginite and vitrinite. The relatively higher contents of hopanoids in their pyrolyzates and elemental nitrogen suggest a bacterial role in the formation of the groundmass. Chemical analysis and subsequent multivariate statistical analysis suggest that the groundmass is likely to be a mixture of bacterially-degraded algal and humic organic matter. The proportions of the two primary components vary from sample to sample, as does the extent of degradation. Bacterially-produced hopanoids are also incorporated.

Classification of torbanite and cannel coal: I. Insights from petrographic analysis of density fractions

Torbanite and cannel coal are considered to be coals because of their low mineral content and overall physical morphology. However, the texture and composition of the organic matter in torbanite and cannel coal are similar to the kerogen occurring in oil shales and lacustrine source rocks. Therefore, understanding the nature and origin of organic components in torbanite and cannel coal is of significance in the study of kerogen and petroleum formation. In this research, a set of torbanites and cannel coals from different locations throughout the world were petrographically characterized and processed using a density gradient centrifugation (DGC) technique. Microscopically, the torbanite and cannel coal are composed of coarser maceral particles set in a fine-grained to amorphous groundmass. The groundmass is a mixture of more than one type of substance and accounts for 10 to 80% (by volume) of the torbanites and cannel coals. Botryococcus-related alginite is the most characteristic component of the torbanite. While sporinite typically is the main phytoclast in the cannel coals, in most cases the groundmass is volumetrically the dominant component, determining the overall character of the sample. This observation calls into question the traditional practice of classifying such coals using the alginite to sporinite ratio. Variations in composition, texture and fluorescence permits the recognition of three different types of groundmass: lamalginitic, bituminitic and vitrinitic. High purity alginite, sporinite, vitrinite and varieties of groundmass were separated using the DGC technique. The distribution of density fractions closely reflects the petrographic composition of the various torbanites and cannel coals. Distinct peaks on the density profiles represent the major organic components and peak magnitudes are functions of the percentage of the components, demonstrating that the density gradient profiles can be used to distinguish the different types of torbanite and cannel coal. The separation data also indicate a gradual shift towards higher density from lamalginitic to bituminitic to vitrinitic groundmass.

The in Situ Formation and Development of Some Kaolinite Macrocrystals

Tonsteins, which are kaolinite-rich rocks with an amorphous groundmass containing much organic material, occur widely in European Coal Measure deposits. The nature of these rocks and their peculiar fabric is considered in detail and evidence is adduced which suggests that they were derived from soils and that the original organic components (micro-organisms and fungi) played a significant part in the formation of the kaolinite macrocrystals.