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1.
A system for precisely determining normalized by comparing unknown to standard in a solid-source quadruple-collector mass spectrometer is outlined. This is made possible by a mathematical approximation in the data-reduction process. 相似文献
2.
L. D. Ashwal R. A. Armstrong R. J. Roberts M. D. Schmitz F. Corfu C. J. Hetherington K. Burke M. Gerber 《Contributions to Mineralogy and Petrology》2007,153(4):389-403
Nepheline-bearing gneisses from the 75 km2 Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence
of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain
U = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements
except Hf (HfO2 = 0.53–0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores,
indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism.
U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 ± 4 Ma (MSWD = 1.7), which we interpret as the time of
magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 ±
7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization
of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon
megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf array to represent a mixing line
defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization;
hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic
volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma
during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other Pan-African metamorphic ages that record
collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record
of intra-continental rifting and of continental collision in southern Malawi. 相似文献
3.
Barium silicate minerals such as celsian, ganterite, armenite, as well as Ba-bearing and Ba-free white mica from the Berisal Complex, Simplon Nappe, Swiss Alps, were dated by 39Ar-40Ar. Ages of Ba-free micas are ca. 17 Ma, while Ba and parentless 40Ar are correlated in Ba silicates, suggesting common inheritance from the Paleozoic orthogneissic protolith. The release pattern of reactor-produced 39Ar (or 37Ar) from hydrated and anhydrous minerals is very similar, with apparent activation energies of ca. 180 kJ/mole and a conspicuous kink around 900 °C. White micas release Ar at higher temperature than the literature determination of their dehydroxylation. In addition to Ar, we studied the degassing of monoisotopic 131Xe produced from Ba during neutron irradiation. Xe is degassed at higher temperature than Ar, and again all analyzed silicates have the same apparent activation energy of ca. 300 kJ/mole. The decoupling of Ar and Xe rules out delamination as the dominant degassing mechanism in mica and implies that recoiled rare gas atoms mostly reside inside the T-O-T layers of the mica structure. The near-identical apparent activation energies in such diverse silicates as tecto-, phyllo- and cyclosilicate requires instead that the in-vacuo gas release kinetics are the same in all three silicates. As the only structural element common to these three silicate families are silica tetrahedra, it is possible that their well-known rotation plays a decisive role for in-vacuo Ar degassing; additional high-temperature in situ structural investigations on feldspars and micas would be needed to help constrain the mechanisms of laboratory Ar release. 相似文献
4.
The value of studying artificially-produced radionuclides in the marine environment is discussed in the wider context of monitoring sediments for other metal pollutants. Differences in the dispersion of selected radionuclides discharged from the same known source are used to illustrate the sort of changes in phase or speciation which must occur with other metal pollutants, but which cannot be demonstrated directly because of the multiplicity of sources. Various chemical and physical procedures are discussed which can be applied to a study of heavy metal associations with marine sediments, as are questions such as the existence, or otherwise, of mechanisms for the slow release of adsorbed material from the buried sediment back into the water column. Suggestions are made as to how radionuclide studies may be helpful in answering questions such as these, which are considered to be vital for the meaningful interpretation of sediment analyses. 相似文献
5.
E. A. Laycock K. Spence D. P. Jefferson S. Hetherington B. Martin C. Wood 《Environmental Geology》2008,56(3-4):521-528
This research aimed to specify an optimum replacement stone for Truro Cathedral. A variety of petrographically and visually similar material to the original Bath stone was initially selected. The stones were subjected to three different durability tests; Sodium sulphate crystallisation and large scale testing with both accelerated and climatic freeze-thaw cyclic loading. The most suitable stone was determined as the one with the best performance characteristics overall. 相似文献
6.
Freshwater Protozoa belonging to the genus Loxodes accumulate barium sulphate as discrete, intracellular, spherical granules. These organisms may be extremely abundant () in biologically productive lakes. In two such lakes a sharp peak in the concentration of particulate barium was observed in the region of the oxycline. In both cases this coincided with a peak in the abundance of Loxodes but not with an elevated concentration of particulate iron, also suggested to concentrate Ba by physical-chemical mechanisms. A relatively constant proportion of the total paniculate Ba over a concentration range of three orders of magnitude could be accounted for by accumulation in Loxodes. Certain Protozoa clearly play an important role in the biogeochemical cycle of barium in freshwaters. 相似文献
7.
Variations in Late Quaternary coastal deformation along the Pacific margin of Canada reflect not only differences in glacial history, but also local non-glacial tectonics. A forebulge developed to the north in the Queen Charlotte Basin (QCB) from 13.2 until after 9.7 ka BP resulting in exposure of the currently submerged shelf; whereas similar exposure was not evident to the south in the Georgia Basin. A semi-quantitative effective elastic lithosphere thickness of between 10 and 20 km is estimated for the QCB. We propose that this warm, relatively thin lithosphere combined with tectonic movement influenced crustal displacement, particularly the decoupling of the North American plate along the Queen Charlotte (QC) Fault, which may have allowed the QCB to behave like a hinge in response to glacial unloading. This would explain large magnitude crustal tilting (2.1 m km−1) observed adjacent to the QC Fault. The 50 m of depression observed in the early- and mid-Holocene may imply that the western edge of the North America Plate ‘flexed’ down below present elevation, producing a second wave of response to ice loading and unloading. 相似文献
8.
The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; 87Sr/86Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated 87Sr/86Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution. 相似文献
9.
Greg Brown Jonathan Rhodes Daniel Lunney Ross Goldingay Kelly Fielding Nicole Garofano Scott Hetherington Marama Hopkins Jo Green Skye McNamara Angie Brace Lorraine Vass Linda Swankie Clive McAlpine 《Transactions in GIS》2019,23(6):1184-1203
Geographic citizen science has much potential to assist in wildlife research and conservation, but the quality of observation data is a key concern. We examined the effects of sampling design on the quality of spatial data collected for a koala citizen science project in Australia. Data were collected from three samples—volunteers (n = 454), an Internet panel (n = 103), and landowners (n = 35)—to assess spatial data quality, a dimension of citizen science projects rarely considered. The locational accuracy of koala observations among the samples was similar when benchmarked against authoritative data (i.e., an expert‐derived koala distribution model), but there were differences in the quantity of data generated. Fewer koala location data were generated per participant by the Internet panel sample than the volunteer or landowner samples. Spatial preferences for land uses affecting koala conservation were also mapped, with landowners more likely to map locations for residential and tourism development and volunteers less likely. These spatial preferences have the potential to influence the social acceptability of future koala conservation proposals. With careful sampling design, both citizen observations and land use preferences can be included within the same project to augment scientific assessments and identify conservation opportunities and constraints. 相似文献
10.
Callum J. Hetherington Mattias Lundmark Stefan Graeser Reto Gieré 《International Journal of Earth Sciences》2008,97(1):51-69
The petrographic and geochemical features of two zoisite–celsian gneiss outcrops from the Berisal Complex, characterised by
a syn-kinematic mineral assemblage that contains celsian and barium white micas and a maximum whole-rock BaO content of 8.36 wt%,
are described. The outcrops are enclosed in a larger body of garnet-bearing two-mica augen-gneiss, which has intrusive contacts
with the surrounding garnet–biotite–muscovite paragneiss, and also contains small outcrops of two-mica clinozoisite gneiss.
The zoisite–celsian gneisses are strongly enriched in all alkaline-earth elements, are depleted in the alkali elements, and
have high Zr/Hf and Nb/Ta ratios compared with the surrounding gneisses. The zoisite–celsian, two-mica clinozoisite, and garnet-bearing
two-mica augen-gneisses have Al/CNK molar ratios >1, and the zoisite–celsian gneisses are also enriched in Zr, Y, and Nb.
Chondrite-normalised patterns for the rare earth elements (REE) show light REE enrichment, with a negative Eu anomaly. On
the basis of field and geochemical data it is argued that the barium anomalies in the Berisal complex are the result of igneous
fractionation of barium into anorthosite-dominated cognate inclusions within a larger volume of calc-alkaline peraluminous
melt. On the basis of U–Pb zircon ages, a conservative estimate for the age of magma crystallisation is placed at 460 ± 10 Ma,
and thus was related to significant late Ordovician granitoid magmatism in Gondwana-derived microcontinents during collision
between Avalonia-Cadomia and Gondwana. 相似文献