Pohlner, Johannes E.El Korh, AfifeChiaradia, MassimoKlemd, Reiner...
11页
查看更多>>摘要：The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Munchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/Sigma Fe = 0.06 to 0.30). Their delta Fe-56 values (+0.00 to +0.17 parts per thousand; mean + 0.08 +/- 0.01 parts per thousand) mostly resemble those of MORB (+0.07 to +0.17 parts per thousand). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier delta Fe-56 values (+0.09 to +0.22 parts per thousand) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10 parts per thousand) are typical for pelitic sediments. It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that delta Fe-56 values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Munchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic delta Fe-56 values throughout the subduction-exhumation cycle, whereas delta Fe-56 values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.
查看更多>>摘要：The effect of sulfate (SO42- ) on the precipitation of As(V) and Fe(III) and the stability of the generated solid phase as well as its underlying mechanisms, which strongly influences the transportation and fate of As in acidic waters (such as acid mine drainage), is still not well known. This work systematically investigated the precipitation, decomposition, and settling properties of the ferric arsenate phase in the presence of different amount of SO42- at pH 1.2 and 1.8. The results showed that on the one side, the presence of SO42- delayed the removal of As and Fe from the aqueous phase and enhanced the solubility of solid ferric arsenate. The geochemical modelling results showed that the coexisting SO42- induced the formation of aqueous SO42- -Fe(III) complexes, inducing the decomposition of produced ferric arsenate solid phase. On the other side, the addition of SO42- promoted the growth rate of generated ferric arsenate and accelerated the settling of solids. The particle size distribution and transmission electron micorscopy analyses elucidated that the presence of SO42- significantly enlarged the ferric arsenate aggregate size. The infrared spectroscopy and S K-edge X-ray absorption near edge structure results proved that SO42- is incorporated into the structure of ferric arsenate by forming ferric arsenate sulfate solid solution, with each SO42- tetrahedron coordinating with approximately two FeO6 octahedra. The S K-edge extended X-ray absorption fine structrure result gave an averaged S-Fe interatomic distance of 3.21 ??. These results demonstrated that the presence of SO42- enlarged the aggregate size of ferric arsenate dominantly via the S-O-Fe bridging, and then accelerated the settling of SO4-ferric arsenate. The contrasting role of SO42- in influencing As obtained in this study is significant for understanding the mobilization and fate of As and the formation mechanisms of ferric arsenate minerals in acidic systems.
查看更多>>摘要：In Maoming oilshales, the presence of races B and L of Botryococcus braunii (B. braunii) was confirmed by the detection of botryococcanes and C-40 lycopanes and their derivatives. However, there were no specific biomarkers for race A of B. braunii. The contribution of race A of B. braunii has been implied by the distribution of nonspecific n-alkanes with an odd carbon-number preference and a predominance of n-C-27. However, confirmation of its contribution to oilshales has been lacking until now. We found that n-C-27 is a dominant n-alkane in most Maoming samples which exhibits a positive delta C-13 excursion (approximately 2-3 parts per thousand) compared to other homologues. In addition, the extremely heavy delta C-13 values of botryococcanes (-6.0 parts per thousand to -8.5 parts per thousand) are rationalized to reflect bicarbonate assimilation in race B of B. braunii. The positive excursion of the delta C-13 values of n-C-27 extracted from the same samples could be attributed to bicarbonate assimilation by race A in a similar manner. Bicarbonate assimilation by race A would also explain the isotopic enrichment of n-C-14 similar to n-C-33 alkanes in the range of -20.1 parts per thousand to -26.1 parts per thousand. They are close to the values of some higher plant sources (C-4 plant) and largely different than that of the reported algal n-alkanes (<-30.6%.). The bicarbonate consumption mechanism utilized by both races A and B of B. braunii in Maoming oilshales was triggered by the sharp decrease in pCO(2) during the Late Eocene period, recorded globally at the Eocene-Oligocene transition. The presence of race A of B. braunii in most of Maoming sediments indicates that it was wide spread and populated in Maoming Basin over a long period of time, serving as the main biological source material of oilshales on a large scale.
查看更多>>摘要：Zircon is a ubiquitous accessory phase that carries important isotopic and geochemical information. Experimental work indicates that zircon solubility in silicate melts increases exponentially with decreasing silica; accordingly, crystallizing mafic magmas would only precipitate zircon from the last drops of residual melts, likely of granophyric composition. However, this view is inconsistent with the abundance of syn-magmatic zircons in many mafic rocks, in which zircon often occupies textural positions compatible with early crystallization. Given that factors other than temperature and magma composition negligibly affect the solubility of zircon, its precipitation from mafic magmas must involve the formation of small zircon-saturated transient zones. Here we explored that possibility using 2D finite elements to model the crystallization of MORB melts confined in pores. We found that zircon-saturated volumes may form locally at the growing mineral-melt interfaces if the growth rate of a low KdZr mineral (<0.2) is much faster than the diffusion rate of the rejected Zr4+ away into the melt, thus leading to the precipitation of zircon in low-Zr mafic magmas. Local saturation close to interfaces growing in confined pores also explains the wide range of crystallization temperatures displayed by zircons of mafic rocks, the variety of their inclusions and textures, and in the particular case of the Atlantic seafloor, why syn-magmatic zircons are more common in troctolites and olivine gabbros than in the more evolved clinopyroxene or amphibole-rich gabbros. This mechanism can also account for the formation of other accessory phases composed of elements scarcely partitioned in major minerals.
Sjostrom, J. K.Bindler, R.Cortizas, A. MartinezBjorck, S....
14页
查看更多>>摘要：Atmospheric mineral dust not only interacts with the climate system by scattering incoming solar radiation and affecting atmospheric photochemistry, but also contributes critical nutrients to marine and terrestrial ecosystems. In a high-resolution analysis of paleodust deposition, peat development and soil dust sources, we assess the interplay between dust deposition and bog development of the Davidsmosse bog in south-western Sweden. Analyses of the 5400-year record (458 cm) included radiocarbon dating, bulk density, ash content, chemical and mineralogical composition and carbon stable isotopes, subsequently explored using principal component analysis. Fourteen dust events (DEs) were recorded (cal BP) in the peat sequence: 3580???3490; 3280; 3140; 3010???2840; 2740; 2610; 2480; 2340; 2240???2130; 1690; 1240; 960, 890???760, and 620???360. The majority of the DEs were coupled to increases in peat accumulation rates and increased nutrient content (N, P and K) suggesting that the DEs contributed with nutrients to the bog ecosystem, promoting increased accumulation. We also analyzed the chemical and mineral composition of potential mineral source deposits (separated into 6 grain-size fractions) from sites within a 4 km radius as well as aeolian dunes closer to the coast (25 km). The composition deposited on the present-day bog surface indicates that the bulk of the contemporary minerals have a local origin (<1.5 km), but the DEs may be of a more distant origin. The results also indicate that quartz and plagioclase feldspar content consistently increase with increasing grain-size, both in the source samples as well as in the peat sequence, and that the Si/Al ratio can be used to infer grain size changes in the peat. Two longer phases saw numerous DEs, between 2800 and 2130 cal BP and a stepwise increase from 960 towards 360 cal BP. The episodic character of the events, together with the inferred coarse grain size, suggest that the particles were deposited by (winter) storms. Future studies should include grain size analysis as well as a more in-depth comparison with regional paleo dust and storm records to increase knowledge on both transport processes (creep, saltation, suspension) and the climate processes driving late Holocene dust and storm events in Scandinavia.
NSTL
Elsevier