کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
4679034 | 1634872 | 2009 | 12 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Neodymium isotopes in Archean seawater and implications for the marine Nd cycle in Earth's early oceans
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موضوعات مرتبط
مهندسی و علوم پایه
علوم زمین و سیارات
علوم زمین و سیاره ای (عمومی)
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چکیده انگلیسی
Published neodymium (Nd) isotopic data for Archean iron-formations (IF) suggest that, overall, seawater throughout the Archean typically displayed 143Nd/144Nd close to bulk Earth values, with ÐNd(t) between â 1.5 and + 2.5. Neodymium isotopic ratios in seawater during deposition of the ~ 3.8 Isua (Greenland) IF likely displayed positive ÐNd(3.8 Ga) of + 2.5, as suggested by IF-G, an Isua reference IF that is considered the best archive for Early Archean seawater. Seawater 143Nd/144Nd ratios dominated by radiogenic Nd (positive ÐNd(t)) seem to have persisted for much of the Archean, as IF from the Pietersburg greenstone belt, South Africa, suggest seawater ÐNd(2.95 Ga) â¥Â + 1. However, similarly aged (~ 2.9 Ga) IFs from South Africa indicate that significant variations in seawater 143Nd/144Nd occurred, and clearly show influences from isotopically distinct crustal sources. These variations are apparently related to depositional environment, with cratonic margin, shallow-water IFs possessing a continental ÐNd(t) of â 3, while IFs associated with sub-aqueous mafic volcanics display more radiogenic, positive ÐNd(t) values. Such variation in seawater 143Nd/144Nd is not possible in an isotopically well-mixed ocean, and similar to today, it appears that marine Nd cycling in the Archean produced water masses with distinct Nd isotopic ratios. Since the presence of banded iron-formations requires a reducing Archean ocean capable of transporting Fe, metal-oxide precipitation and scavenging processes near deep sea hydrothermal vent systems would not have scavenged mantle Nd, i.e., Nd sourced from alteration of oceanic crust. We propose that bulk anoxic seawater prior to 2.7 Ga possessed relatively constant positive ÐNd(t) of + 1 to + 2, whereas local shallow-water masses associated with exposed evolved crust could possess distinctly different, lower ÐNd(t).
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Earth and Planetary Science Letters - Volume 283, Issues 1â4, 15 June 2009, Pages 144-155
Journal: Earth and Planetary Science Letters - Volume 283, Issues 1â4, 15 June 2009, Pages 144-155
نویسندگان
Brian W. Alexander, Michael Bau, Per Andersson,