Search

Your search keyword '"Sial, A.N."' showing total 188 results
Start Over Author "Sial, A.N."
188 results on '"Sial, A.N."'

32. Chapter 3 The São Francisco Palaeocontinent

Author

Sial, A.N., Dardenne, M.A., Misi, A., Pedreira, A.J., Gaucher, C., Ferreira, V.P., Silva Filho, M.A., Uhlein, A., Pedrosa-Soares, A.C., Santos, R.V., Egydio-Silva, M., Babinski, M., Alvarenga, C.J.S., Fairchild, T.R., and Pimentel, M.M.

Published
2009
Full Text
View/download PDF

35. 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon

Author

Valley, J.W., Lackey, J.S., Cavosie, A.J., Clechenko, C.C., Spicuzza, M.J., Basei, M.A.S., Bindeman, I.N., Ferreira, V.P., Sial, A.N., King, E.M., Peck, W.H., Sinha, A.K., and Wei, C.S.

Subjects
Rocks, Igneous -- Chemical properties, Rocks, Igneous -- Structure, Zircon -- Chemical properties, Zircon -- Structure, Earth -- Crust, Earth -- Research, Earth -- Analysis, Earth sciences
Abstract

Analysis of [[delta].sup.18]O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time. This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation of supracrustal materials into melts, which commonly have [[delta].sup.18]O values higher than in primitive mantle magmas. This study summarizes data for [[delta].sup.18]O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly evolved magmatic [[delta].sup.18]O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity of values throughout the Archean, and comparison to the composition of the 'modern' mantle indicate that [[delta].sup.18]O of primitive mantle melts have remained constant ([+ or -] 0.2[per thousand]) for the past 4.4 billion years. The range and variability of [[delta].sup.18]O in all Archean zircon samples is subdued ([[delta].sup.18]O(Zrc)=5-7.5[per thousand]) ranging from values in high temperature equilibrium with the mantle (5.3 [+ or -] 0.3[per thousand]) to slightly higher, more evolved compositions (6.5-7.5[per thousand]) including samples from: the Jack Hills (4.4-3.3 Ga), the Beartooth Mountains (4.0-2.9 Ga), Barberton (3.5-2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have been analyzed with magmatic [[delta].sup.18]O above 7.5[per thousand]. The mildly evolved, higher Archean values (6.5-7.5[per thousand]) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons. During the Proterozoic, the range of [[delta].sup.18]O(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After ~1.5 Ga, high [[delta].sup.18]O zircons (8 to > 10[per thousand]) became common in many Proterozoic and Phanerozoic terranes reflecting [[delta].sup.18]O(whole rock) values from 9 to over 12[per thousand]. The appearance of high [[delta].sup.18]O magmas on Earth reflects non-uniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived material into magmas within the crust.

Published
2005

36. Close spatial relationship between plutons and shear zones

Author

Weinberg, R.F., Sial, A.N., and Mariano, G.

Subjects
Geology -- Research, Shear zones, Earth sciences
Abstract

This paper explores the relationship between granitic plutons and transcurrent shear zones using examples from the Borborema province, northeastern Brazil. Syntectonic phitons of this province generally have en cornue shapes and were emplaced preferentially within or at the margins of wide, regional-scale transcurrent shear zones, particularly where shear zones intersect major lithological boundaries. It is argued that the plutons' shapes and their closeness to shear zones are indicative of a causal relationship between granite plutons and shear zones. With the support of numerical models, it is further argued that pluton emplacement in transcurrent terranes is controlled by low-meanpressure magma traps at the shoulders of shear zones, resulting from strain incompatibility caused by regional variations in rheological properties. Keywords: granites, emplacement, shear zones, rheology, transcurrent faults, Borborema.

Published
2004

37. Chemostratigraphy of carbonates from the minas supergroup, quadrilatero ferrifero (iron quadrangle), Brazil: a stratigraphic record of early proterozoic atmospheric, biogeochemical and climatic change

Author

Bekker, A.; Kaufman, A. J., Sial, A.N.; Romano, A. W., Karhu, J. A.; Pimentel, M. M, Ferreira, V. P., and Noce, C. M.

Subjects
Climatic changes -- Research, Carbonates -- Research, Earth sciences
Abstract

Absence of evidence in Brazil of the three early Paleoproterozoic glacial events was found in North America. This was due to the enhanced weathering due to the rifting of the subcontinent and the position of the low to middle latitudes was also contributed to climatic changes.

Published
2003

38. Oxygen isotope compositions and magmatic epidote from two contrasting metaluminous granitoids, NE Brazil

Author

Ferreira, V.P., Valley, J.W., Sial, A.N., and Spicuzza, M.J.

Subjects
Brazil -- Natural history, Isotope geology -- Research, Earth sciences
Abstract

Oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids (Emas and Sao Rafael plutons) from northeastern Brazil. The I-type Emas pluton has high [[delta].sup.18]O (WR) values (11.5-11.8[per thousand]), whereas the Sao Rafael pluton has low [[delta].sup.18]O (WR) values (7.5-8.1[per thousand]), but Sr and Nd are characteristics of S-type granitoids. Measured mineral-mineral fractionations suggest continuous sub-solidus inter-mineral isotope exchange among all minerals except zircon. There is a large and consistent quartz-epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote. Oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations, as [[delta].sup.18]O (epidote)

Published
2003

48. Reply to comments by Sanjay K. Mukhopadhyay, Sucharita Pal, J. P. Shrivastava on the paper by Sial et al. (2016) Mercury enrichments and Hg isotopes in Cretaceous–Paleogene boundary successions: Links to volcanism and palaeoenvironmental impacts. Cretaceous Research 66, 60–81

Author

Sial, A.N., Chen, Jiubin, Lacerda, L.D., Frei, R., Tewari, V.C., Pandit, M.K., Gaucher, C., Ferreira, V.P., Cirilli, S., Peralta, S., Korte, C., Barbosa, J.A., and Pereira, N.S.

Published
2017
Full Text
View/download PDF

Catalog

Books, media, physical & digital resources
See catalog results