Effects of secondary alteration on the composition of free and IOM-derived monocarboxylic acids in carbonaceous chondrites     

José C. Aponte  Yi Wang  Adrian J. Brearley  Yongsong Huang 《Geochimica et cosmochimica acta》2011,75(9):2309-2323

Monocarboxylic acids (MCAs) are important astrobiologically because they are often the most abundant soluble compounds in carbonaceous chondrites (CCs) and are potential synthetic end products for many biologically important compounds. However, there has been no systematic study on the effect of parent body alteration on molecular and isotopic variability of MCAs. Since MCAs in meteorites are dominated by low molecular weight (C1-C8), highly volatile compounds, their distributions are likely to be particularly sensitive to secondary alteration processes. In contrast, the aliphatic side chains of insoluble organic matter (IOM) in CCs, whose composition has been shown to be closely related to the MCAs, may be far more resistant to secondary alteration. In the present study, we determined the distributions and isotopic ratios of free and IOM-derived MCAs in six carbonaceous chondrites with a range of classifications: Murchison (CM2), EET 87770 (CR2), ALH 83034 (CM1), ALH 83033 (CM2), MET 00430 (CV3) and WIS 91600 (C2). We compare mineralogical and petrological characteristics to the MCAs distributions to better define the processes leading to the synthesis and alteration of meteoritic MCAs. Our results show that aqueous and especially thermal alteration in the parent bodies led to major loss of free MCAs and depletion of straight relative to branched chain compounds. However, the MCAs derived from aliphatic side chains of IOM are well preserved despite of secondary alterations. The molecular and isotopic similarities of IOM-derived MCAs in different chondrite samples indicate very similar synthetic histories for organic matter in different meteorites.  相似文献   

Mineral chemistry of igneous melanite garnets from analcite-bearing volcanic rocks,Alberta, Canada     

Donald B. Dingwell  Mark Brearley 《Contributions to Mineralogy and Petrology》1985,90(1):29-35

The mineral chemistry of melanite garnets from the Crowsnest volcanic rocks of SW Alberta, Canada, has been investigated by using electron microprobe scans, quantitative analyses and multivariate statistical analysis. The garnets occur with aegirine-augite, sanidine, analcite and rare plagioclase as phenocrysts in trachyte and phonolite flows, agglomerates and tuffs. Wavelength dispersive microprobe scans reveal complex zonation patterns, both normal and oscillatory. The results of fifty quantitative analyses were subjected to R-mode factor analysis to delineate the chemical exchanges producing the zonation. The chemical zonation of the garnets may be attributed to four independent binary exchanges; Al-Fe³⁺, Si-Ti, Ca-Mn and Mg-Fe²⁺. The stoichiometry of these garnets, based on microprobe and wet chemical Fe analyses, combined with the strongly antithetic behavior of Si and Ti lead us to infer that the Ti in these garnets is dominantly tetravalent. It is clear from this study that quantitative modelling of the processes of crystal growth and zonation of melanite garnets in alkaline, undersaturated igneous rocks should be aimed at simulating the four chemical exchanges listed above. Present address: Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton St. N.W., Washington, DC 20008  相似文献   

Microchondrules in two unequilibrated ordinary chondrites: Evidence for formation by splattering from chondrules during stochastic collisions in the solar nebula          下载免费PDF全文

E. Dobrică  A. J. Brearley 《Meteoritics & planetary science》2016,51(5):884-905

The diversity of silicate, glassy spherules analogous to chondrules, called microchondrules, and the implications for their presence in unequilibrated ordinary chondrites (UOCs) were investigated using different electron microscope techniques. Our observations show that the abundance of microchondrules in UOCs is much larger than the values proposed by previous studies. We identified two different types of microchondrules, porous and nonporous, embedded within fine‐grained matrices and type I chondrule rims. The porous microchondrules are characterized by distinctive textures and chemical compositions that have not been recognized previously. Additionally, we show detailed textures and chemical compositions of protuberances of silicate materials, connected to the chondrules and ending with microchondrules. We suggest that microchondrules and protuberances formed from materials splattered from the chondrules during stochastic collisions when they were still either completely or partially molten. The occurrence and distinct morphologies of microchondrules and protuberances suggest that rather than just a passive flash melting of chondrules, an additional event perturbed the molten chondrules before they underwent cooling. The bulk chemical compositions suggest that (1) nonporous microchondrules and protuberances were formed by splattering of materials that are compositionally similar to the bulk silicate composition of type I chondrules, and (2) the porous microchondrules could represent the splattered melt products of a less evolved, fine‐grained dust composition. The preservation of protuberances and microchondrules in the rims suggests that the cooling and accretion rates were exceptionally fast and that they represent the last objects that were formed before the accretion of the parent bodies of OCs.  相似文献   

Mechanisms of the transformations between the α, β and γ polymorphs of Mg2SiO4 at 15 GPa     

A. J. Brearley  D. C. Rubie  E. Ito 《Physics and Chemistry of Minerals》1992,18(6):343-358

Data on the mechanisms of mantle phase transformations have come primarily from studies of analogue systems reacted experimentally at low pressures. In order to study transformation mechanisms in Mg₂SiO₄ at mantle pressures, forsterite () has been reacted in the stability field of -phase, at 15 GPa and temperatures up to 900° C, using a multianvil split-sphere apparatus. Transmission electron microscope studies of samples reacted for times ranging from 0.25–5.0 h show that forsterite transforms to -phase by an incoherent nucleation and growth mechanism involving nucleation on olivine grain boundaries. This mechanism and the resultant microstructures are very similar to those observed at much lower pressures in analogue systems (Mg₂GeO₄ and Ni₂SiO₄) as the result of the olivine to spinel () transformation. Metastable spinel () also forms from Mg₂SiO₄ olivine at 15 GPa, in addition to -phase, by the incoherent nucleation and growth mechanism. With time, the spinel progressively transforms to the stable -phase. After 1 h, spinels exhibit a highly striated microstructure along {110} and electron diffraction patterns show streaking parallel to [110] which indicates a high degree of structural disorder. High resolution imaging shows that the streaking results from thin lamellae of -phase intergrown with the spinel. The two phases have the orientation relationship [001]//[001] and [010]//[110] so that the quasi cubic-close-packed oxygen sublattices are continuous between both phases. These microstructures are similar to those observed in shocked meteorites and show that spinel transforms to -phase by a martensitic (shear) mechanism. There is also evidence that the mechanism changes to one involving diffusion-controlled growth at conditions close to equilibrium.  相似文献   

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars     

Marina Martínez  Charles K. Shearer  Adrian J. Brearley 《Meteoritics & planetary science》2023,58(9):1229-1245

Secondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB-TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron-scale vein consisting of fluorapatite and an SiO₂-rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F-bearing chlorapatite and the SiO₂-rich phase filled the center of the vein. The presence of nanoporosity and nanometer-scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F-, Si-rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre-terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low-shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late-stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.  相似文献   

Microstructural evidence for complex formation histories of amoeboid olivine aggregates from the ALHA77307 CO3.0 chondrite          下载免费PDF全文

Jangmi Han  Adrian J. Brearley 《Meteoritics & planetary science》2015,50(5):904-925

The microstructures and compositions of olivine and refractory components in six amoeboid olivine aggregates (AOAs) in the Allan Hills A77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique with transmission electron microscopy. In the AOAs, refractory components (perovskite, melilite, spinel, anorthite, and Al‐Ti‐bearing diopside) provide evidence of a high degree of textural and compositional heterogeneity, suggesting that these phases have formed by disequilibrium gas–solid condensation at high temperatures under highly dynamic conditions. We infer different possible reactions of early‐condensed solid minerals (perovskite and spinel) with a nebular gas, forming diopside with wide ranges of Al and Ti contents and/or anorthite. The progressive, incomplete consumption of spinel in these reactions may have resulted in the Cr enrichment in the remaining, unreacted spinel in the AOAs. In contrast to the refractory components, olivines in the AOAs have equilibrated textures with 120° triple junctions, indicating that the AOAs were subjected to high‐temperature annealing after agglomeration of olivine and refractory components. Because the AOAs consist of fine‐grained olivine grains with numerous pores, the annealing is constrained by experimental data to have occurred for a short duration of the order of a few hours to tens of hours depending on the annealing temperature. In comparison, the effects of annealing on the refractory components are minimal, probably due to pinning of grain boundaries in the multiphase assemblages that inhibited grain growth.  相似文献   

Global Climate Change — the Latest Assessment: Does Global Warming Warrant a Health Warning?     

RT Watson  AJ McMichael 《Global Change & Human Health》2001,2(1):64-75

Global climate change is a qualitatively distinct, and very significant, addition to the spectrum of environmental health hazards encountered by humankind. Historically, environmental health concerns have focused on toxicological or microbiological risks to health from local exposures. However, the scale of environmental health hazards is today increasing; indeed, the burgeoning human impact on the environment has begun to alter global biophysical systems (such as the climate system). As a consequence, a range of larger-scale environmental hazards to human population health has emerged. This includes the health risks posed by climate change, stratospheric ozone depletion, loss of biodiversity, stresses on terrestrial and ocean food-producing systems, changes in hydrological systems and the supplies of freshwater, and the global spread of persistent organic pollutants. Appreciation of this scale and type of influence on human health entails an ecological perspective — a perspective that recognises that the foundations of long-term good health in populations reside in the continued stability and functioning of the biosphere's "life-supporting" ecological and physical systems. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

Plastic ingestion by estuarine mullet Mugil cephalus (Mugilidae) in an urban harbour,KwaZulu-Natal,South Africa     

T Naidoo  AJ Smit  D Glassom 《African Journal of Marine Science》2016,38(1):145-149

Coastal urban environments have high plastic pollution levels, and hence interactions between plastic debris and marine life are frequent. We report on plastic ingestion by mullet Mugil cephalus in Durban Harbour, KwaZulu-Natal, South Africa. Of 70 mullet (13.0–19.5?cm total length), 73% had plastic particles in their guts, with a mean of 3.8 particles per fish (SD 4.7). Plastic ingestion showed no relation to digestive tract content or fish length. White and clear plastic fibres were ingested most commonly. This urban population of M. cephalus had a higher incidence of plastic ingestion than has been reported in studies on fish from other coastal areas or the oceanic environment.  相似文献   

Primary iron sulfides in CM and CR carbonaceous chondrites: Insights into nebular processes     

S. A. Singerling  A. J. Brearley 《Meteoritics & planetary science》2018,53(10):2078-2106

We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. ( 2015 ). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide-rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron-sized metal inclusions in some PPI grains may have formed by co-crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.  相似文献   

Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ     

John R. Bargar  Christopher C. Fuller  Adrian J. Brearley  Samuel M. Webb 《Geochimica et cosmochimica acta》2009,73(4):889-786

The microbial catalysis of Mn(II) oxidation is believed to be a dominant source of abundant sorption- and redox-active Mn oxides in marine, freshwater, and subsurface aquatic environments. In spite of their importance, environmental oxides of known biogenic origin have generally not been characterized in detail from a structural perspective. Hyporheic zone Mn oxide grain coatings at Pinal Creek, Arizona, a metals-contaminated stream, have been identified as being dominantly microbial in origin and are well studied from bulk chemistry and contaminant hydrology perspectives. This site thus presents an excellent opportunity to study the structures of terrestrial microbial Mn oxides in detail. XRD and EXAFS measurements performed in this study indicate that the hydrated Pinal Creek Mn oxide grain coatings are layer-type Mn oxides with dominantly hexagonal or pseudo-hexagonal layer symmetry. XRD and TEM measurements suggest the oxides to be nanoparticulate plates with average dimensions on the order of 11 nm thick × 35 nm diameter, but with individual particles exhibiting thickness as small as a single layer and sheets as wide as 500 nm. The hydrated oxides exhibit a 10-Å basal-plane spacing and turbostratic disorder. EXAFS analyses suggest the oxides contain layer Mn(IV) site vacancy defects, and layer Mn(III) is inferred to be present, as deduced from Jahn-Teller distortion of the local structure. The physical geometry and structural details of the coatings suggest formation within microbial biofilms. The biogenic Mn oxides are stable with respect to transformation into thermodynamically more stable phases over a time scale of at least 5 months. The nanoparticulate layered structural motif, also observed in pure culture laboratory studies, appears to be characteristic of biogenic Mn oxides and may explain the common occurrence of this mineral habit in soils and sediments.  相似文献