New observations on high‐pressure phases in a shock melt vein in the Villalbeto de la Peña meteorite: Insights into the shock behavior of diopside     

Marina Martinez  Adrian J. Brearley  Josep M. Trigo‐Rodríguez  Jordi Llorca 《Meteoritics & planetary science》2019,54(11):2845-2863

The petrology and mineralogy of shock melt veins in the L6 ordinary chondrite host of Villalbeto de la Peña, a highly shocked, L chondrite polymict breccia, have been investigated in detail using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and electron probe microanalysis. Entrained olivine, enstatite, diopside, and plagioclase are transformed into ringwoodite, low‐Ca majorite, high‐Ca majorite, and an assemblage of jadeite‐lingunite, respectively, in several shock melt veins and pockets. We have focused on the shock behavior of diopside in a particularly large shock melt vein (10 mm long and up to 4 mm wide) in order to provide additional insights into its high‐pressure polymorphic phase transformation mechanisms. We report the first evidence of diopside undergoing shock‐induced melting, and the occurrence of natural Ca‐majorite formed by solid‐state transformation from diopside. Magnesiowüstite has also been found as veins injected into diopside in the form of nanocrystalline grains that crystallized from a melt and also occurs interstitially between majorite‐pyrope grains in the melt‐vein matrix. In addition, we have observed compositional zoning in majorite‐pyrope grains in the matrix of the shock‐melt vein, which has not been described previously in any shocked meteorite. Collectively, all these different lines of evidence are suggestive of a major shock event with high cooling rates. The minimum peak shock conditions are difficult to constrain, because of the uncertainties in applying experimentally determined high‐pressure phase equilibria to complex natural systems. However, our results suggest that conditions between 16 and 28 GPa and 2000–2200 °C were reached.  相似文献   

Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002     

Lysa J. Chizmadia  Adrian J. Brearley 《Geochimica et cosmochimica acta》2008,72(2):602-625

Transmission electron microscopy (TEM) was used to study the microtextural and mineralogical characteristics of fine-grained rims in the unbrecciated CM2 chondrites, Y-791198 and ALHA81002, in an effort to provide constraints on the origins of the rims themselves. Our TEM observations show that the rims in Y-791198 are composed of two distinct types of region, sulfide-poor and sulfide-rich, that are intermixed in a complex manner at the micron to submicron level. The sulfide-poor regions are largely composed of amorphous silicate material or nanocrystalline serpentine, but rare fibrous and coarse-grained serpentine grains have also been identified. No fine-grained cronstedtite or tochilinite were observed, although coarse-grained lumps of tochilinite are present in the rims. In contrast, the sulfide-rich regions are characterized by the presence of a myriad, nanometer-sized Fe, Ni sulfide grains (pentlandite with some Ni-rich pyrrhotite) embedded within an amorphous silicate similar in composition to that of the sulfide-poor regions. The sulfide-rich regions also contain rare phases such as olivine, and Fe, Ni metal grains with grain sizes that are always >100 nm in size. Z-contrast scanning transmission electron microscopy (STEM) reveals that the fine-grained rims consist of a mosaic of irregularly-shaped sulfide-poor and sulfide-rich-regions with sizes of about 0.2-0.5 μm, that have been compacted together during parent body lithification. Despite aqueous alteration, the distinct mineralogical characteristics of these different regions are preserved on a fine-scale and probably represent primitive heterogeneity in the dust from which these rims formed.Serpentine is much better developed and more widespread in the fine-grained rims of ALHA81002 than Y-791198. Complex mats of serpentine fibers are commonly found and cronstedtite and tochilinite are plentiful. Anhydrous minerals such as olivine are rare and have usually been replaced by serpentine. Like Y-791198, all the fine-grained rims studied in ALHA81002 show the same mineral assemblages and textural characteristics throughout and between rims. The homogeneity of the mineralogy, textural relationships and degree of hydration in the rims of these two chondrites is more consistent with parent-body alteration than with pre-accretionary alteration.  相似文献   

Riverine dominance of a nearshore marine demersal food web: evidence from stable isotope and C/N ratio analysis     

AM de Lecea  AJ Smit  ST Fennessy 《African Journal of Marine Science》2016,38(1):S181-S192

The Thukela Bank, KwaZulu-Natal, supports a diverse ecosystem and South Africa’s only prawn fishery. Oceanographic studies suggest riverine input is not important for the biology of this system, whereas biological studies suggest the contrary, with prawn catches increasing with increased fluvial run-off. The aim of this study was to determine (i) the importance of riverine and marine organic matter for the Thukela Bank food web; and (ii) whether there are seasonal changes in the Thukela River stable isotope values, and, if so, whether these are reflected in the isotope values of demersal organisms. Estuarine organic matter, sediments and demersal organisms were collected from several sites across the bank in the wet and dry seasons of 2008, 2009 and 2010. Marine particulate organic matter was also collected in 2010 and analysed for δ13C and δ15N, as well as C/N ratios. There were strong seasonal changes in isotopic values of organic matter and fauna, especially faunal δ13C. There was an apparent time-lag in organisms assimilating riverine organic matter isotopic values, with the isotopic signature of demersal organisms reflecting that of riverine organic matter from the previous season, which is likely the result of tissue turnover time. In 2010, Thukela Bank sediment organic matter was of riverine origin and this maintained the demersal food web. We conclude that Thukela River organic matter is an important input to the food web of the Thukela Bank, indicating that any future damming of the catchment area could have serious consequences for this ecosystem.  相似文献   

Microstructural evidence for a disequilibrium condensation origin for hibonite‐spinel inclusions in the ALHA77307 CO3.0 chondrite          下载免费PDF全文

Jangmi Han  Adrian J. Brearley  Lindsay P. Keller 《Meteoritics & planetary science》2015,50(12):2121-2136

Two hibonite‐spinel inclusions (CAIs 03 and 08) in the ALHA77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique combined with transmission electron microscopy. These hibonite‐spinel inclusions are irregularly shaped and porous objects and consist of randomly oriented hibonite laths enclosed by aggregates of spinel with fine‐grained perovskite inclusions finally surrounded by a partial rim of diopside. Melilite is an extremely rare phase in this type of CAI and occurs only in one inclusion (CAI 03) as interstitial grains between hibonite laths and on the exterior of the inclusion. The overall petrologic and mineralogical observations suggest that the hibonite‐spinel inclusions represent high‐temperature condensates from a cooling nebular gas. The textural relationships indicate that hibonite is the first phase to condense, followed by perovskite, spinel, and diopside. Texturally, melilite condensation appears to have occurred after spinel, suggesting that the condensation conditions were far from equilibrium. The crystallographic orientation relationships between hibonite and spinel provide evidence of epitaxial nucleation and growth of spinel on hibonite surfaces, which may have lowered the activation energy for spinel nucleation compared with that of melilite and consequently inhibited melilite condensation. Hibonite contains abundant stacking defects along the (001) plane consisting of different ratios of the spinel and Ca‐containing blocks within the ideal hexagonal hibonite structure. This modification of the stacking sequence is likely the result of accommodation of excess Al in the gas into hibonite due to incomplete condensation of corundum from a cooling gas under disequilibrium conditions. We therefore conclude that these two hibonite‐spinel inclusions in ALHA77307 formed by high‐temperature condensation under disequilibrium conditions.  相似文献   

Zoned chondrules in Semarkona: Evidence for high‐ and low‐temperature processing     

Jeffrey N. Grossman  Conel M. O'D. Alexanders  Jianhua Wang  Adrian J. Brearley 《Meteoritics & planetary science》2002,37(1):49-73

Abstract— At least 15% of the low‐FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins. We have studied zoned chondrules using electron microprobe methods (x‐ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved. Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water). Calcium is depleted in areas of volatile enrichment. Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another; Mn follows Na in some chondrules but not in others; Fe and Mg are unzoned. D/H ratios increase in the water‐rich areas of zoned chondrules. Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside. Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification. Systematic variations in zoning patterns among pyroxene‐ and olivine‐rich chondrules may indicate that fractionation of low‐ and high‐Ca pyroxene played some role in Ti, Cr, Mn, Si, Al, and some Ca zoning. But direct condensation of elements into hot chondrules, secondary melting of late condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub‐parallel zoning profiles of many elements, the presence of H₂O, or elemental abundance patterns. Zoning of moderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid. This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material. Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, low‐FeO chondrules appear to have fully altered mesostasis.  相似文献   

The population genetic structure of the waratah anemone (Actinia tenebrosa) around New Zealand     

AJ Veale  SD Lavery 《新西兰海洋与淡水研究杂志》2013,47(4):523-536

The genetic structure of populations is often shaped by the reproductive system and larval properties of the species. The waratah anemone (Actinia tenebrosa) reproduces through both asexual clones, which have very short-distance dispersal, and sexual larvae, which are believed to disperse much greater distances. The impact of this mixed strategy on the New Zealand population structure of Actinia tenebrosa was investigated using microsatellite markers. The analysis incorporated 24 sampling locations from around New Zealand and one Australian location, using four microsatellite markers, n = 420. We observed low connectivity and high genetic differentiation between all locations sampled, with a distinct pattern of isolation by distance. The most distinct grouping of locations sampled was the north-east of the North Island from Cape Reinga to East Cape, which was identified by SAMOVA and STRUCTURE analyses as being moderately diverged from the remainder of the country. A tentative correlation was observed between genetic clustering and biogeographic regionalisation, where the distribution of a number of genetic clusters matched previously defined biogeographic regions. Within each location sampled, large numbers of clones were present and a latitudinal cline was observed in the relative contribution of asexually and sexually generated recruits, with an increase in asexual recruits on the South Island's east coast.  相似文献   

Mineralogy and oxygen isotope systematics of magnetite grains and a magnetite‐dolomite assemblage in hydrated fine‐grained Antarctic micrometeorites     

Elena Dobric&#x;  Ryan C. Ogliore  Ccile Engrand  Kazuhide Nagashima  Adrian J. Brearley 《Meteoritics & planetary science》2019,54(9):1973-1989

We report the mineralogy and texture of magnetite grains, a magnetite‐dolomite assemblage, and the adjacent mineral phases in five hydrated fine‐grained Antarctic micrometeorites (H‐FgMMs). Additionally, we measured the oxygen isotopic composition of magnetite grains and a magnetite‐dolomite assemblage in these samples. Our mineralogical study shows that the secondary phases identified in H‐FgMMs have similar textures and chemical compositions to those described previously in other primitive solar system materials, such as carbonaceous chondrites. However, the oxygen isotopic compositions of magnetite in H‐FgMMs span a range of ?¹⁷O values from +1.3‰ to +4.2‰, which is intermediate between magnetites measured in carbonaceous and ordinary chondrites (CCs and OCs). The δ¹⁸O values of magnetites in one H‐FgMM have a ~27‰ mass‐dependent spread in a single 100 × 200 μm particle, indicating that there was a localized control of the fluid composition, probably due to a low water‐to‐rock mass ratio. The ?¹⁷O values of magnetite indicate that H‐FgMMs sampled a different aqueous fluid than ordinary and carbonaceous chondrites, implying that the source of H‐FgMMs is probably distinct from the asteroidal source of CCs and OCs. Additionally, we analyzed the oxygen isotopic composition of a magnetite‐dolomite assemblage in one of the H‐FgMMs (sample 03‐36‐46) to investigate the temperature at which these minerals coprecipitated. We have used the oxygen isotope fractionation between the coexisting magnetite and dolomite to infer a precipitation temperature between 160 and 280 °C for this sample. This alteration temperature is ~100–200 °C warmer than that determined from a calcite‐magnetite assemblage from the CR2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibrated OCs, CIs, and CMs. This suggests that the sample 03‐36‐46 could come from a parent body that was large enough to attain temperatures as high as the OCs, CIs, and CMs, which implies an asteroidal origin for this particular H‐FgMM.  相似文献   

Hydrothermal evolution of the morphology,molecular composition,and distribution of organic matter in CR (Renazzo-type) chondrites     

Hitesh G. Changela  Corentin Le Guillou  Sylvain Bernard  Adrian J. Brearley 《Meteoritics & planetary science》2018,53(5):1006-1029

The morphology, molecular composition, and distribution of organic matter (OM) were investigated in a suite of CR chondrites to better constrain its hydrothermal evolution. Multiple focused ion beam sections were extracted from the matrices of seven CR chondrites. Scanning transmission X-ray microscopy and transmission electron microscopy reveal OM ubiquitously distributed across the CR matrices. OM mainly occurs as either discrete submicron rounded or irregularly shaped vein-like particles. Two spectral populations of organic particles were identified by carbon K-edge X-ray absorption near edge structure (XANES): the most abundant one, similar to insoluble organic matter (IOM) residues, contains aromatic, carbonyl, and carboxylic groups. The second population is more aromatic-rich and lacks a distinctive carbonyl peak. An additional, ubiquitous organic component occurs associated with amorphous silicates and phyllosilicates. Less aromatic but aliphatic- and carboxylic-rich, this diffuse OM is interpreted as the result of the redistribution of organic compounds by aqueous fluids. The most altered CR1 GRO 95577 contains a more mature OM and highly aliphatic- and carboxylic-rich diffuse OM. This evolution, from the CR2s to the CR1, is comparable to that of terrestrial gas shale maturation involving cracking reactions, releasing bitumen-like, aliphatic-, and carboxylic-rich compounds, and aromatic residues. Our observations support the accretion of soluble OM and its later polymerization to IOM, as well as the maturation of IOM and its partial oxidation, releasing mobile compounds. The differences in GRO 95577 are clearly attributable to the hydrothermal episode(s), but the relative role of water and temperature on the evolution of OM remains elusive.  相似文献   

Classification of marine bioregions on the east coast of South Africa     

T-C Livingstone  JM Harris  AT Lombard  AJ Smit  DS Schoeman 《African Journal of Marine Science》2018,40(1):51-65

Marine bioregional planning requires a meaningful classification and spatial delineation of the ocean environment using biological and physical characteristics. The relative inaccessibility of much of the ocean and the paucity of directly measured data spanning entire planning regions mean that surrogate data, such as satellite imagery, are frequently used to develop spatial classifications. However, due to a lack of appropriate biological data, these classifications often rely on abiotic variables, which act as surrogates for biodiversity. The aim of this study was to produce a fine-scale bioregional classification, using multivariate clustering, for the inshore and offshore marine environment off the east coast of South Africa, adjacent to the province of KwaZulu-Natal and out to the boundary of the exclusive economic zone (EEZ), 200 nautical miles offshore. We used remotely sensed data of sea surface temperature, chlorophyll a and turbidity, together with interpolated bathymetry and continental-slope data, as well as additional inshore data on sediments, seabed oxygen and bottom temperature. A multivariate k-means analysis was used to produce a fine-scale marine bioregionalisation, with three bioregions subdivided into 12 biozones. The offshore classification was primarily a pelagic bioregionalisation, whereas the inshore classification (on the continental shelf) was a coupled benthopelagic bioregionalisation, owing to the availability of benthic data for this area. The resulting classification was used as a base layer for a systematic conservation plan developed for the province, and provided the methods for subsequent planning conducted for the entire South African EEZ. Validation of the classification is currently being conducted in marine research programmes that are sampling benthic biota and habitats in a sampling design stratified according to the biozones delineated in this study.  相似文献   

Altered primary iron sulfides in CM2 and CR2 carbonaceous chondrites: Insights into parent body processes     

S. A. Singerling  A. J. Brearley 《Meteoritics & planetary science》2020,55(3):496-523

The presence of primary iron sulfides that appear to be aqueously altered in CM and CR carbonaceous chondrites provides the potential to study the effects and, by extension, the conditions of aqueous alteration. In this work, we have used SEM, TEM, and EPMA techniques to characterize primary sulfides that show evidence of secondary alteration. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite (CMs only), magnetite (CMs and CRs), and phyllosilicates (CMs only) in grains that initially formed by crystallization from immiscible sulfide melts in chondrules (pyrrhotite‐pentlandite intergrowth [PPI] grains). Textural, microstructural, and compositional data from altered sulfides in a suite of CM and CR chondrites have been used to constrain the conditions of alteration of these grains and determine their alteration mechanisms. This work shows that the PPI grains exhibit two styles of alteration—one to form porous pyrrhotite‐pentlandite (3P) grains by dissolution of precursor PPI grain pyrrhotite and subsequent secondary pentlandite precipitation (CMs only), and the other to form the altered PPI grains by pseudomorphic replacement of primary pyrrhotite by magnetite (CMs and CRs) or phyllosilicates (CMs only). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation. Our observations show that primary sulfides are sensitive indicators of aqueous alteration processes in CM and CR chondrites.  相似文献