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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >A multi-isotopic and trace element investigation of the Cretaceous-Tertiary boundary layer at Stevns Klint, Denmark-inferences for the origin and nature of siderophile and lithophile element geochemical anomalies
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A multi-isotopic and trace element investigation of the Cretaceous-Tertiary boundary layer at Stevns Klint, Denmark-inferences for the origin and nature of siderophile and lithophile element geochemical anomalies

机译：丹麦Stevns Klint的白垩纪-第三纪边界层的多同位素和微量元素研究-推论了嗜铁和嗜石元素地球化学异常的起源和性质

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摘要

Os, Sr, Nd and Pb isotope data were collected from a profile across the Cretaceous-Tertiary (K-T) boundary layer at Stevns Klint, Denmark. ε_(Nd) [T = 65 Ma] values from within the boundary layer (Fish Clay) are lower by ～1ε unit than those of the underlying Maastrichtian limestone and the overlying Danian chalk sequences. Systematic profile-upward changes of Pb, Sr and Os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the Fish Clay. While Os, Nd and Pb isotopes indicate the admixing of less radiogenic components to the Fish Clay, Sr isotopes show elevated radiogenic values in the boundary layer, relative to the carbonate sequences beneath and above it. The sudden change in lithophile (e.g., Sr, Pb and Nd) isotope compositions at the base of the Fish Clay and profile-upward trends of ~(87)Sr/~(86)Sr and ~(206)Pb/~(204)Pb ratios towards those of the overlying Danian chalk are interpreted to reflect recovery from enhanced, acid rain-induced continental (local?) weathering input to the seawater. However, a continental crustal source is invalid for the siderophile element Os. In the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements (PGEs) and certain moderately siderophile elements (Cr, Ni, Co, V) in K-T boundary sediments, including Stevns Klint [Shukolyukov and Lugmair, Science 282 (1998) 927-929], and supported by the finding of projectile debris [Bauluz et al., Earth Planet. Sci. Lett. 182 (2000) 127-136] and the occurrence of abundant Ni-rich spinel at many K-T sites [Robin et al., Nature 363 (1993) 616-617; Kyte, Nature 396 (1998) 237-239], we favor to explain the sudden drop of ~(187)Os/~(188)Os ratios from 0.210 to 0.160 at the K-T boundary to derive from global fall-out of extraterrestrial matter. The present ~(186)Os/~(188)Os ratio of 0.119836 ± 0.000004 measured in the basal layer of the Fish Clay is within the uncertainty a chondritic value. WE therefore exclude the possibility of a major contribution of PGEs to the sediment from iron meteorites. Chondrite-normalized (Ru/Ir)_N ratios of ～0.95 ± 0.14 and (Os/Ir)_N ratio of ～0.93 ± 0.14 in the Fish Clay cannot distinguish between abundance ratios of different types of chondrites, and strongly sub-chondritic (Pt/Ir)_N ratios of ～0.62 ± 0.09 (2σ) suggest differential PGE remobilization through the sedimentary column (and consequently the alteration of inter-element ratios). PGEs and the moderately siderophile elements Cr, Ni, V, and Co form an elemental association with systematically upward-decreasing concentrations in the Fish Clay. Low Co/Ni ratios of ～0.12 in the Fish Clay relative to values of ～0.35 in the over- and underlying carbonate sequences support mixing of meteorite-derived (Co/Ni ～0.05) and terrestrial upper mantle/crustal (Co/Ni > ～0.3) sources. While lithophile element isotope data indicate an increased continental crustal input to the Fish Clay at the K-T transition, the uncertainty with respect to possible post-depositional alteration of abundance patterns of siderophile and moderately siderophile elements-though not affecting the chondritic isotopic composition of Os-does not allow confirmation of indications from chromium isotopes for a carbonaceous (CV-type) meteorite as the preferred K-T impactor type by Shukolyukov and Lugmair [Science 282 (1998) 927-929].
机译：Os，Sr，Nd和Pb同位素数据是从横跨丹麦Stevns Klint的白垩纪-第三系（K-T）边界层的剖面中收集的。边界层（鱼黏土）内的ε_（Nd）[T = 65 Ma]值比下伏的马斯特里赫特灰岩和上覆的大白垩纪序列低约1ε单位。自从鱼泥沉积以来，子产物的原位生长不能解释边界层中铅，锶和s同位素的组成和浓度的系统性向上分布变化。尽管Os，Nd和Pb同位素表明鱼骨中含有较少的放射源成分，但Sr同位素相对于其下方和上方的碳酸盐序列，在边界层中显示出较高的放射源值。鱼粘土底部的亲石物质（如Sr，Pb和Nd）同位素组成的突然变化以及〜（87）Sr /〜（86）Sr和〜（206）Pb /〜（204）的剖面上升趋势铅对上覆白垩系白垩的比率被解释为反映了从酸雨引起的大陆性（局部）风化增加后的海水中恢复的量。但是，大陆铁源对于铁粒亲铁元素Os无效。根据铬同位素的证据表明，KT边界沉积物中的铂族元素（PGE）和某些中等嗜铁性元素（Cr，Ni，Co，V）是宇宙起源的，包括Stevns Klint [Shukolyukov和Lugmair，Science 282（ （1998）927-929]，并得到了射弹碎片的发现[Bauluz等，地球行星。科学来吧182（2000）127-136]和在许多K-T位点出现富Ni的尖晶石[Robin等人，Nature 363（1993）616-617;和N.M.等，J.Biol.Chem。182（2000）127-136]。 Kyte，Nature 396（1998）237-239]，我们倾向于解释〜（187）Os /〜（188）Os在KT边界突然从0.210下降到0.160的比率，这是由于地球外物质的整体衰减而产生的。在鱼黏土基底层中测得的当前〜（186）Os /〜（188）Os比值为0.119836±0.000004，在不确定度范围内为软骨素值。因此，我们排除了PGE对铁陨石沉积物的重大贡献的可能性。鱼泥中的球粒归一化（Ru / Ir）_N比为〜0.95±0.14和（Os / Ir）_N比为〜0.93±0.14无法区分不同类型的球粒体的丰度比和强次软骨体（Pt / Ir）_N比值约为0.62±0.09（2σ），表明PGE在沉积柱中的迁移不同（因此元素间比值发生了变化）。 PGEs和适度嗜铁亲铁元素Cr，Ni，V和Co形成了元素键，并且鱼黏土中的浓度逐渐升高。鱼泥中的Co / Ni比值低至〜0.12，而上，下碳酸盐序列中的Co / Ni比值低至〜0.35，这支持了陨石衍生的（Co / Ni〜0.05）和陆地上地幔/地壳（Co / Ni> 〜0.3）来源。虽然嗜石元素元素同位素数据表明，在KT过渡期增加了向鱼泥的大陆地壳输入，但沉积后嗜铁亲铁元素和中度嗜铁亲和元素的丰度模式可能发生变化的不确定性，尽管不影响Os-的软骨状同位素组成不允许通过Shukolyukov和Lugmair的铬同位素确认碳酸盐（CV型）陨石作为首选的KT撞击器类型[科学282（1998）927-929]。

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