共查询到20条相似文献，搜索用时 515 毫秒
1.
2.
Recent exploratory studies have suggested the potential of magnetic susceptibility (MS) as a rapid and low‐cost sourcing technique for lithic archaeological materials. Most commercially available susceptibility instruments, however, do not have the sensitivity to characterize weakly susceptible cherts and silicified woods. Comparative results from nine chert, two silicified wood, a porcellanite, and four obsidian sources using a highly sensitive, calibrated, and magnetically‐shielded instrument allow exploration of MS variability within and between geological sources. Color, texture, grain size, and large inclusions of cherts and obsidians are not straightforward determinants of MS. Weathering rinds (patinas) and cortex from a variety of cherts yield reduced MS values, as does a recrystalized cobble of Rio Maior flint, indicating that magnetic mineral removal during chemical weathering and diagenesis is more prevalent than staining or other mineral penetration of cobbles. In situations of multiple overlapping MS ranges, analysis is limited to discriminating cherts with high or low MS range distributions, for example in an atypicality index. Some silicified woods, obsidians, and porcellanites possess a much greater range of intersource variability, and thus sourcing analysis is more likely to be broadly successful using the susceptibility of these materials. Unexpected results from an archaeological test using a historic lithic assemblage from Azinheira, Portugal, indicate that assemblage MS may be influenced by practices of raw material selection. Understanding variability in lithic MS has relevance for the design of most source geochemical sampling, as well as the behavioral interpretations that result from such investigations. © 2002 Wiley Periodicals, Inc. 相似文献
3.
Ellery Frahm 《Geoarchaeology》2012,27(2):166-174
As new analytical techniques are brought to sourcing studies and researchers compile data into multi‐laboratory databases, systematic evaluation is essential. The importance of precision and accuracy is clear, but Shackley (2005) also calls for “archaeological accuracy.” Hughes (1998) offered a framework to consider precision and accuracy alongside the concepts of reliability and validity. These four concepts can serve as a foundation to evaluate archaeological sourcing data and procedures, but adoption of Hughes’ framework has been nearly nonexistent. Unfortunately, Hughes’ formulations of reliability and validity are somewhat at odds with their conventional definitions, hindering his framework. Furthermore, the concept of precision has become outdated in analytical circles, and superfluous terms (e.g., replicability) have emerged in the archaeological literature. Here I consider the basis of Hughes’ framework and how its four components, when applied consistently by the sourcing community, are best applied to evaluate analytical data and techniques for sourcing. 相似文献
4.
Weathering of chert artifacts from three Lapita sites (ca. 3000–2000 B.P.) in the Southeast Solomons is examined. It is shown that the rate of weathering is strongly influenced by soil pH, mineralogy, and prehistoric burning of the material. The concentrations of some major elements (Na, Cl, K, Al) are shown to have been altered during the weathering process. Implications of this work for the sourcing and dating of cherts are discussed. 相似文献
5.
Otumba and Sierra de Pachuca obsidian deposits in Central Mexico have been important sources of raw material since pre‐Hispanic times. Numerous archaeological investigations have suggested that the economical and political expansion of major Mesoamerican societies were linked to the control of obsidian sources and distribution of quarried material. Sierra de Pachuca contains several obsidian flows and numerous quarries throughout the region that were preferentially exploited by different cultures. The Otumba Volcanic Complex has four important obsidian domes, but three of them have not been studied in detail. A geochemical characterization of subsources from the Sierra de Pachuca and Otumba Volcanic Complex is an important step toward future sourcing of obsidian artifacts that would help provide insight into spheres of influence and trade by past cultures in Central Mexico. Having this purpose in mind, inductively coupled plasma mass spectrometry (ICP‐MS) was used to analyze obsidian samples collected from five separated locations at Sierra de Pachuca and four at Otumba, followed by statistical analysis (density‐based spatial clustering of applications with noise, DBSCAN). We were able to distinguish three chemically distinctive subsources in Sierra de Pachuca and three in Otumba. This study illustrates the importance of accurate characterization of obsidian raw material when attempting to define subsource usage. 相似文献
6.
JIANGUO WANG DAIZHAO CHEN DAN WANG DETIAN YAN XIQIANG ZHOU QINGCHEN WANG 《Sedimentology》2012,59(3):809-829
The Ediacaran–Cambrian transition was one of the most critical intervals in Earth history. During this interval, widespread chert was precipitated, commonly as a stratal wedge in carbonates, along the southern margin of the Yangtze Platform, South China. The chert wedge passes into a full chert succession further basinward to the south‐east. Four lithotypes of chert are identified across the marginal zone in western Hunan: mounded, vein, brecciated and bedded chert. The mounded chert is characterized by irregular to digitiform internal fabrics, generally with abundant original vesicles and/or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals. The host chert (or matrix) of these mounds is dominated by amorphous cryptocrystalline silica, commonly disseminated with pyrite. The vein chert, with minor quartz locally, generally cross‐cuts the overlying dolostone and chert horizons and terminates under the mounded and/or bedded chert bodies. The brecciated chert commonly occurs as splayed ‘intrusions’ or funnel‐shaped wedges and cross‐cuts the topmost dolostones. The bedded chert, the most common type, generally is thin to medium‐bedded and laminated locally; it is composed of amorphous silica with minor amounts of black lumps. Microthermometry of fluid inclusions from vein and void‐lining minerals (mainly quartz and barite) revealed homogenization temperatures from 120 to 180°C for the trapped primary fluids. Compositionally, these chert deposits generally are pure, with SiO2 > 92 wt%, and only minor Fe2O3 and Al2O3 contents, most of which show positive Europium anomalies in rare earth element patterns, especially for the mounded chert. All these data suggest that the marginal zone chert deposits resulted from a low‐temperature, silica‐rich hydrothermal system, in which the mounded chert was precipitated around the releasing vents, i.e. as silica chimneys. The vein and splayed brecciated chert, however, was formed along the syndepositional fault/fracture conduits that linked downward, while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor. These petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation; rapid tectono‐depositional and oceanic changes during the Ediacaran–Cambrian transition in South China. 相似文献
7.
This article extends the earlier work (Haavisto et al. 2008. Optical spectrum based measurement of flotation slurry contents. Int. J. Miner. Process. 88 (3–4), 80–88), where visible and near-infrared (VNIR) reflectance spectroscopy was used together with an X-ray fluorescence (XRF) analyzer to improve the assaying of zinc flotation concentrate. Especially the sampling interval of the assay could be drastically reduced by the presented approach. In this study, a multichannel VNIR spectrum analyzer is utilized to measure the spectrum of the seven most important slurry lines in copper and zinc flotation circuits. Recursive data-based modeling is applied to the VNIR spectrum data and XRF assays to calculate and adaptively maintain the calibration model. The accuracy of the VNIR assays is evaluated in all the lines, and the benefits of the obtained high frequency assays in detecting oscillations and sudden grade changes are demonstrated. 相似文献
8.
We introduce the use of Nd isotopes as a high‐precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin. © 2000 John Wiley & Sons, Inc. 相似文献
9.
Origin and variability of a terminal Proterozoic primary silica precipitate,Athel Silicilyte,South Oman Salt Basin,Sultanate of Oman 
下载免费PDF全文
下载免费PDF全文 The Precambrian–Cambrian Athel Silicilyte is a 400 m thick, salt‐encased siliceous succession in the South Oman Salt Basin. It is a self‐sourcing hydrocarbon reservoir and comprises up to 95% microcrystalline quartz and exhibits wavy discontinuous lamination, comprising thin, alternating organic‐rich and silica‐rich layers. Textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water, within the water column at the sulphidic/oxic interface. The unique occurrence of the Athel Silicilyte in the terminal Proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly. Basin anoxia, coupled with the growth of microbial mats, low pH and high silica pore water saturations, created optimal chemical conditions for silica precipitation. Volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the Athel Basin. At the basin margins, and within the late highstand systems tract, volumes of microcrystalline quartz decreased as the volume of detrital sediment increased. Mass‐balance calculations indicate that silica‐enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column. In conclusion, precipitation of the Athel Silicilyte was driven by the coincidence of basin restriction, limited clastic input, availability of organic matter and water column anoxia. The observation that there are few documented examples of chert deposits younger than ca 700 Ma, prior to the Cambrian explosion, suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica‐secreting organisms, direct precipitation from sea water was restricted. This is tentatively related to the gradual increase in alkalinity of sea water through the Palaeo‐Proterozoic and Meso‐Proterozoic, such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input. 相似文献
10.
Since lenses of chert are common within the volcano-sedimentary succession hosting the massive sulphide deposits of the Iberian Pyrite Belt (Spain and Portugal), we examined numerous chert occurrences, both petrographically and geochemically, to test their possible value for massive sulphide exploration. The chert is found at two main lithostratigraphic levels (upper and lower) that are also interpreted as massive-sulphide bearing. In both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments; we have not been able to observe the relationships between massive sulphides and chert, but some of the large orebodies of the Province (Lousal, La Zarza, Tharsis, Planes-San Antonio body of Rio Tinto, Neves) are described as being locally capped by chert facies. Four main types are recognized among the chert and associated facies: (1) red hematitic chert?±?magnetite; (2) radiolarian and/or sedimentary-textured (conglomeratic) chert with hematite and/or Mn oxides; (3) pale sulphidic chert; (4) rhodonite and/or Mn carbonate?±?magnetite facies. In the Spanish part of the Province the radiolarian chert is confined to the upper level; the distribution of the other types appears to be haphazard. The hydrothermal origin of the South Iberian chert is shown by its high Fe-Mn and low Co-Ni-Cu contents. The presence of small positive Ce anomalies indicates a shallow marine environment (shelf or epicontinental sea), which is consistent with the volcanological and sedimentological data. The chert was emplaced below the sea floor through chemical precipitation and/or through alteration and replacement of the country rock, residual traces of which are ghost phenocrysts and high Al, Ti and rare earth contents. Macro- and microscopic relationships indicate that the oxide facies (hematite?±?magnetite) formed first, probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies; a phase of Mn?carbonate and silicate + quartz?±?chlorite + sulphides appears to be even later. It was not possible, through discrimination, to isolate a chert that could be considered as representing a lateral marker of massive sulphides; moreover, both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides. Such is also indicated by the lead isotopic signature of the chert, which is appreciably more radiogenic than that of the massive sulphides; the lead enrichment in the sulphidic chert facies indicates the participation of a different source (sediments, sea water) from that of the massive sulphides. The hypothesis of an independent hydrothermal “chert” event can thus be envisaged, wherein the chert reflects submarine low-temperature hydrothermal activity that is most apparent during a “break” within the volcano-sedimentary succession and which may locally have competed with the high-temperature hydrothermal activity giving rise to the massive sulphides. The interest of the chert thus rests in its palaeodynamic significance, as a marker of periods of volcanic quiescence, and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides. 相似文献
11.
RICHARD J. BEHL 《Sedimentology》2011,58(2):325-351
Chert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata. 相似文献
12.
MU Maosong YANG Debin QUAN Yikang HAO Leran YANG Haotian WANG Anqi XU Wenliang 《《地质学报》英文版》2019,93(Z2):41-42
The Qilian orogen along the NE edge of the Tibet‐Qinghai Plateau records the evolution of Proto‐Tethyan Ocean that closed through subduction along the southern margin of the North China block during the Early Paleozoic. The South Qilian belt is the southern unit of this orogen and dominated by Cambrian‐Ordovician volcano‐sedimentary rocks and Neoproteozoic Hualong complex that contains similar rock assemblages of the Central Qilian block. Our recent geological mapping and petrologic results demonstrate that volcano‐sedimentary rocks show typical rock assembles of a Cambrian‐early Ordovician arc‐trench system in Lajishan Mts. along the northern margin of the Hualong Complex. Island arc rocks including basalt, andesite, dacite, rhyolite, and breccia is in fault contact with ophiolite complex consisting of mantle peridotite, serpentinite, gabbro, dolerite, plagiogranite, and basalt. Accretionary complexes are tectonically separated from the ophiolite‐arc rocks, with various rock assemblages spatially. They consist of pillow basalt, basalt breccia, tuff, chert, and limestone blocks with a seamount origin within the scaly shale in Dingmaoshan and Donggoumeikuang areas, and basalt, chert, and sandstone blocks within muddy shale matrix and mélange at Lajishankou area. Abundant radiolarians occur in red chert, and trilobite, brachiopod, and coral fossils occur within Dingmaoshan limestone blocks. Although partial basalt or chert blocks are highly disrupted, duplex, thrust fault, rootless intrafolial fold, tight fold, and penetrative foliation are well‐developed at Donggoumeikuang area. Spatially, accretionary complexes lie structurally beneath ophiolite complex and above the turbidites of the Central Qilian block. Ophiolite and accretionary complexes are also overlapped by late Ordovician molasse deposits sourced from Cambrian arc‐trench system and the Central Qilian block. These observations demonstrate that a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt formed during the early Paleozoic southward subduction of the South Qilian Ocean collided with the Central Qilian block prior to the late Ordovician. 相似文献
13.
Abstract. Numerous bedded manganese deposits sporadically distributed throughout the Tamba district, southwestern Japan are intercalated within chert sequence. It is well known that radiolarian remains are commonly included in both bedded manganese deposits and host cherts. The Gen‐otani mine, one of these deposits, is located at Otani, Keihoku‐Shimonaka, northern Kyoto City. Chemical composition and age of the chert sequence at the mine were examined. Mainly according to SiO2 and MnO contents together with lithology, the chert sequence is divided into three sections; lower massive chert, middle bedded manganese deposit and upper bedded chert sections. Radiolarian faunas consisting of middle Jurassic species such as Eucyrtidiellum unumaense, Dictyomitrella(?) kamoensis, Parvicingula dhimenaensis, Sethocapsa aitai, Sethocapsa kodrai, Transhsuum brevicostatum, Tricolocapsa plicarum, Unuma echinatus and others were extracted from both the middle manganese section and overlying bedded chert of the upper section. This examination reveals that the bedded manganese deposit at the Gen‐otani mine formed until Bajocian to early Bathonian (middle Middle Jurassic) in age. 相似文献
14.
E. Calvin Alexander 《Precambrian Research》1975,2(4):329-344
KAr isochron techniques can provide, in principle, an experimental reconstruction of the time evolution of the atmospheric 40Ar/36Ar ratio if minerals can be found which contain samples of argon from the ancient atmosphere and which have had a simple geologic history. Authigenic sedimentary minerals with low potassium content appear to be the best candidates. An experimental reconstruction of the evolution of the atmospheric 40Ar/36Ar ratio will serve as a test of various models for the chemical and thermal evolution of the Earth.40Ar39Ar studies of five chert samples from the Swaziland sequence and the Bulawayan and Gunflint Formations indicate that lower Precambrian cherts do not contain appreciable samples of the ancient atmospheric argon and have experienced complicated geologic histories. The chert sample from the Kromberg Formation contains excess 40Ar. The other four samples yield age spectra which are complicated but which are interpretable in terms of geologically reasonable ages.The lack of evidence for argon loss in the chert data suggests that some cherts may prove to be datable sedimentary minerals. 相似文献
15.
Abstract. Chemistry and sulfur isotopes are analyzed for a series of rocks in the chert‐dominant sequence around the stratiform manganese ore deposit of the Noda‐Tamagawa mine in the northern Kitakami Terrane, northeast Japan. The sequence is litholog‐ically classified into six units in ascending order: lower bedded chert, lower black shale, massive chert, manganese ore, upper black shale, and upper bedded chert. The rocks around the manganese ore deposit exhibit anomalous enrichment in Ni (max. 337 ppm), Zn (102) and U (30) in the upper part of lower bedded chert, Mo (122), Tl (79) and Pb (33) in the lower black shale, MnO, Cu (786) and Co (62) in the manganese ore, and As (247) and Sb (17) in the upper black shale. The aluminum‐normalized profiles reveal zonal enrichment of redox‐sensitive elements around the manganese bed: Zn‐Ni‐Fe‐Mo‐U(‐Co), Tl‐Pb(‐Mo), Mn‐Fe‐Cu‐V‐Cr‐Co(‐Zn) and As‐Sb in ascending order. The uppermost part of the lower bedded chert and black shale exhibit negative Ce/Ce* values, whereas the massive chert, manganese ore and lower part of the upper bedded chert display positive values. The isotopic δ34S values are 0±6 % in the lower part of the lower bedded chert, ‐19 to ‐42 % in the upper part of the lower bedded chert, ‐36 to ‐42 % in the lower black shale, ‐28 to ‐35 % in the massive chert, manganese ore and upper black shale, and ‐23±5 % in the upper bedded chert. Thus, there is a marked negative shift in δ34S values in the lower bedded chert, and an upward‐increasing trend in δ34S through the manganese ore horizon. The present data provide evidence for a change in the paleoceanographic environmental resulting from inflow of oxic deepwater into the stagnant anoxic ocean floor below the manganese ore horizon. This event is likely to have triggered the precipitation of manganese oxyhydroxides. The redistribution of redox‐sensitive elements through the formation of metalliferous black shale and manganese carbonate ore may have occurred in association with bacterial decomposition of organic matter during early diagenesis of initial manganese oxyhydroxides. 相似文献
16.
Six chemical subsource groups were identified in the analysis of 84 obsidian samples collected from subsource locations at Coso volcanic field, California. In prehistoric times, Coso provided obsidian for artifacts found from San Francisco Bay to San Diego to Death Valley to the eastern Mojave Desert. Subsource groups were defined by instrumental neutron activation analysis (INAA) of 29 elements followed by cluster analysis, principal component analysis, and bivariate plotting. The new data are compared to previously published INAA and X‐ray fluorescence data. Characterization of 55 obsidian artifacts from archaeological sites located approximately 100 miles from Coso suggests preferential usage of specific subsources as a function of the directionality of travel. The results are consistent with a bimodal (resident and itinerant) model of procurement. This research illustrates the importance of accurate sourcing of obsidian artifacts when attempting to define subsource usage. © 2004 Wiley Periodicals, Inc. 相似文献
17.
18.
引言 河北易县青白口群底部燧石角砾岩(下面简称“角砾岩”)系指震旦亚界蓟县群铁岭组之上、青白口群下马岭组下部之角砾岩。它在太行山北段、中段都有分布,厚度由二、三米至五、六十米不等,在易县南约十四公里的向阳、北考一带发育很好,厚达六十米,顶底接触关系清楚,是研究“角砾岩”成因的良好地段。 相似文献
19.
Spectacularly developed lower Eocene chert in the Corones platform carbonates of the Spanish Pyrenees is concentrated within a restricted, brackish-water, laminated ostracod-rich facies, which also contains abundant sponge spicules. The chert occurs as nodular, bedded and mottled varieties, and four petrographic types of quartz are developed: microquartz; length-fast (LF) chalcedony; megaquartz; and microspheres. δ18 O values of chert range from 29·6‰ to 30·9‰ (SMOW), which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquartz and host carbonate. The chert is closely associated with desiccation cracks and with interstratal dewatering structures. Load casts are silicified, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are filled by microquartz and quartz cements. Ostracod shells within chert are locally fractured; those in the host carbonate are commonly flattened. Late fractures are filled by LF-chalcedony and megaquartz. There is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock. Silica for the Corones cherts was derived from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution, migration of silica-rich fluids and the consequent precipitation of chert. Quartz cements continued to be precipitated into the burial environment. 相似文献
20.
《Gondwana Research》2014,25(1):284-289
The Pilbara Craton in Western Australia contains the best-preserved and most complete record of Archean rocks in the world. As such, they are some of the most studied rocks in the world; paleontologists, isotopic geochemists, geologists and geobiologists have all investigated these rocks for clues about the early biosphere and atmosphere. Here we show using high-resolution transmission electron microscopy that the carbonaceous material found in the Apex chert, and potentially in other associated units, was formed by multiple processes such as abiotic catalytic synthesis and/or biological synthesis. We use these data as well as the geological history of the craton to demonstrate that when the rocks of the Pilbara Craton experienced a high degree of post-depositional hydrothermal alteration, carbonaceous material could have been remobilized and redeposited. As the carbonaceous material within the Apex chert samples was formed over nearly a billion years, bulk chemistry, even at the micron level, will be unable to unambiguously delineate the presence of life in these ancient rocks, although nanoscale observations may provide a way forward in the search for ancient life. 相似文献