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Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon 总被引：162，自引：0，他引：162
Protolith zircon in high‐grade metagranitoids from Queensland, Australia, partially recrystallized during granulite‐grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed‐system solid‐state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re‐setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub‐grain rotation or local dissolution/re‐precipitation, but probably by grain‐boundary migration and defect diffusion. The formation of metamorphic zircon by solid‐state recrystallization is probably common to high‐grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon‐derived U–Pb ages and subsequent tectonic models. 相似文献
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Models of corundum origin from alkali basaltic terrains: a reappraisal 总被引：10，自引：0，他引：10
F. Lin Sutherland Paul W. O. Hoskin C. Mark Fanning Robert R. Coenraads 《Contributions to Mineralogy and Petrology》1998,133(4):356-372
Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned “magmatic” suite
(BGY suite) and subsidiary vari-coloured “metamorphic” suites. The BGY corundums have distinctive trace element contents (up
to 0.04 wt% Ga2O3 and low Cr/Ga and Ti/Ga ratios <1). Different melt origins for BGY corundums are considered here from their inclusion and
intergrowth mineralogy, petrologic associations and tectonic setting. Analysed primary inclusion minerals (over 100 inclusions)
cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magnetite, gahnospinel, rutile-ilmenite solid
solution, calcic plagioclase, Ni-rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This widens a previous
inclusion survey; New England, East Australia corundums contain the most diverse inclusion suite known from basalt fields
(20 phases). Zircon inclusion, intergrowth and megacryst rare earth element data show similar patterns, except for Eu which
shows variable depletion. Temperature estimates from magnetite exsolution, feldspar compositions and fluid inclusion homogenization
suggest that some corundums crystallized between 685–900 °C. Overlap of inclusion Nb, Ta oxide compositions with new comparative
data from niobium-yttrium-fluorine enriched granitic pegmatites favour a silicate melt origin for the corundums. The feasibility
of crystallizing corundum from low-volume initial melting of amphibole-bearing mantle assemblages was tested using the MELTS
program on amphibole-pyroxenite xenolith chemistry from basalts. Corundum appears in the calculations at 720–880 °C and 0.7–1.1 GPa
with residual feldspathic assemblages that match mineral compositions found in corundums and their related xenoliths. A model
that generates melts from amphibole-bearing lithospheric mantle during magmatic plume activity is proposed for BGY corundum
formation.
Received: 3 January 1997 / Accepted: 8 July 1998 相似文献
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Anders Scherstén Hans Årebäck David Cornell Paul Hoskin Anton Åberg Richard Armstrong 《Contributions to Mineralogy and Petrology》2000,139(1):115-125
Zircons from anatectic melts of the country rocks of three Proterozoic mafic–ultramafic intrusions from the Sveconorwegian Province in SW Sweden were microanalyzed for U–Th–Pb and rare earth elements. Melting and interaction of the wall rocks with the intrusions gave rise to new magmas that crystallized zircon as new grains and overgrowths on xenocrysts. The ages of the intrusions can be determined by dating this newly crystallized zircon. The method is applied to three intrusions that present different degrees of complexity, related to age differences between intrusion and country rocks, and the effects of post-intrusive metamorphism. By careful study of cathodoluminescent images and selection of ion probe spots in zircon grains, we show that this approach is a powerful tool for obtaining accurate and precise ages. In the contact melts around the 916?±?11?Ma Hakefjorden Complex, Pb-loss occurred in some U-rich parts of xenocrystic zircon due to the heat from the intrusion. In back-veins of the 1624?±?6?Ma Olstorp intrusion we succeeded in geochemically distinguishing new magmatic from xenocrystic zircon despite small age differences. At Borås the mafic intrusion mixed with country rock granite to form a tonalite in which new zircon grew at 1674?±?8?Ma. Reworking of zircon occurred during 930+33/–34?Ma upper amphibolite facies Sveconorwegian metamorphism. Pb-loss was the result of re-equilibration with metamorphic fluids. REE-profiles show consistent differences between xenocrystic, magmatic, and metamorphic zircon in all cases. They typically differ in Lu/LaN, Ce/Ce*, and Eu/Eu*, and igneous zircon with marked positive Ce/Ce* and negative Eu/Eu* lost its anomalies during metamorphism. 相似文献
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Organic-free sediment was ignited at several temperatures. Significant weight loss (approximately 20%) occurred at temperatures above and below those commonly used for organic determinations. This weight loss should be taken into consideration when organic content of sediment is determined by ignition. 相似文献
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Living barnacles eaten by sheepshead fishes results in the production of broken barnacle plate sediment. The yearly rate of production of broken barnacle plates is 4.9 kg m–2 and varies seasonally, with the largest mean flux in summer (22.5 g m–2 day–1) and the least in winter (1.9 g m–2 day–1). The mean grain size mode of broken barnacle plates is positively correlated with the flux of broken barnacle plates. Experiments with exclusion and inclusion cages support the postulation that increases in flux and size of broken barnacle plates are caused by the feeding activity of larger sheepshead. 相似文献
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Paul W.O. Hoskin 《Geostandards and Geoanalytical Research》1999,23(1):69-76
The fluorine concentration in NIST SRM 610 was determined by SIMS to be 295 16 μg g−1 (5.40% relative standard deviation). Accuracy of the measurement was determined from a calibration involving the synthesis of glass calibration samples doped with varying concentrations of fluorine and characterized by electron microprobe analysis and SIMS. The calibration was accurate to about 5%. Multiple analyses of the calibration samples and SRM 610 in three different analytical sessions combine to produce a low relative standard deviation of the mean (0.23% RSD) in the mean fluorine value for SRM 610. Analytical uncertainty in the fluorine value was 5.40% (RSD), originating from a combination of calibration and ion counting uncertainties as determined from multiple analyses. Evaluation of the SIMS technique using the new fluorine value in SRM 610 shows that this element can be determined with a precision and accuracy superior to that of EPMA. Measurements of fluorine in igneous and hydrothermal zircon suggest that F-ligands may have been responsible for Zr transportation in hydrothermal fluids also responsible for W-Au mineralisation. Other applications for low-level fluorine determinations may include melt inclusions and nominally anhydrous mineral phases, particularly mantle phases. 相似文献
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Biodeposition rates were studied for a fouling community with a biomass of 6–10 kg per m2 dry wt including shells in which the barnacle Balanus eburneus was a dominant species. The fouling community filtered Indian River lagoon water containing 2–15 mg per 1 mud-size particles and deposited them as sand-size fecal pellets. Measurements of the fecal pellet flux by sediment traps indicated seasonal variations between 16.7 and 74.8 g per m2 per day. A significant correlation was found between fecal pellet flux and temperature (r=0.90; p<0.001). The average flux of fecal pellet deposition was four times greater than the average flux of suspended particle settling without biological influence. Suspended sediment concentration did not significantly affect the rate of biodeposition. Annual biodeposition was 18 kg per m2. 相似文献
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