Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada
Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada
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DOI：
10.1093/petrology/42.6.1095
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发表时间：
2001-06
影响因子：
3.9
通讯作者：
S. S. Schmidberger-S.;D. Francis
中科院分区：
文献类型：
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作者：
S. S. Schmidberger-S.;D. Francis
太古宙地幔根部的橄榄岩通常含有丰富的微量元素，尽管它们是难熔的，许多太古代克拉通下面的地幔具有元素成分。为了将反常高地震速度的微量元素预算限制在加拿大克拉通下方 350-400 km 岩石圈深处，微量元素和稀有元素（Jordan，1988；Grand，1994）确定了一系列冷难熔根的地球元素（REE）丰度的存在，这些根在可熔的主要紫榴石橄榄岩和石榴辉石岩中耗尽。尼科斯金伯利岩与肥沃地幔的比较（Boyd & Mertzman，加拿大北极萨默塞特岛上的管道，其组成石榴石 1987 年；McDonough，1990 年）。这些深部残留橄榄岩和单斜辉石，以及宿主金伯利岩。这些难熔地幔根可能有助于太古宙捕虏体的稳定性，其可熔主量元素贫乏，但由于其密度较低和不相容的微量元素而在大陆岩石圈中富集，例如与周围相比粘度较高的大离子亲石元素（LILE）、Th、U和轻稀土元素（LREE）。大量软流圈地幔（Boyd & McCallister，1976；基于单斜辉石 dan 模态丰度的 Jorbalance 计算，1979；Pollack，1986）。地幔捕虏体（石榴石）和金伯利岩和碱性玄武岩各自的 REE 含量产量差异是我们计算和分析的 Nikos 整体窗口的 REE 含量之间唯一的差异。它们提供的岩石轻稀土元素含量达 70-99%，这表明了化学成分的重要证据，以及上地幔深度超过 200 公里的少量间质金伯利岩液态化的存在的重要证据。研究（0·4‐2 wt%）解释了过量的轻稀土元素丰度。这些地幔捕虏体使我们能够表征结果，表明橄榄岩在被其宿主金伯利岩污染之前，实际上已经耗尽或平坦的丰度和主要、次要和痕量轻稀土模式的分布。橄榄岩中的轻稀土元素及其成分与单斜辉石中的 Sr 富集以及低 Zr 和 Sr 丰度矿物之间的关系。大多数克拉通下橄榄岩样本中的低温橄榄岩（800-1100°C）中的石榴石与高温橄榄岩（1200-1400°C）相比经历了复杂的熔体萃取历史，这表明其化学成分发生变化并导致浅层岩石圈在地球化学上不同于加拿大克拉通北缘下方残留地幔中可熔主要元素的深层岩石圈损耗。萨默塞特，如 Fe、Al 和 Ca（例如 Nixon，1987；Herzberg，1993；地幔根似乎以深度分带为特征，Boyd 等人，1997）。与此相反，它们的消耗可能要追溯到其在太古宙稳定的时期。不相容的主要元素，然而，许多橄榄岩
Peridotites that sample Archean mantle roots are frequently in- INTRODUCTION compatible trace element enriched despite their refractory major The mantle underlying many Archean cratons has anelement compositions. To constrain the trace element budget of the omalously high seismic velocities to depths of 350–400 km lithosphere beneath the Canadian craton, trace element and rare ( Jordan, 1988; Grand, 1994), indicating the presence of earth element (REE) abundances were determined for a suite of cold refractory roots, depleted in the fusible major elegarnet peridotites and garnet pyroxenites from the Nikos kimberlite ments compared with fertile mantle (Boyd & Mertzman, pipe on Somerset Island, Canadian Arctic, their constituent garnet 1987; McDonough, 1990). These deep residual peridotite and clinopyroxene, and the host kimberlite. These refractory mantle roots probably contribute to the stability of Archean xenoliths are depleted in fusible major elements, but enriched in continental lithosphere because of their lower density and incompatible trace elements, such as large ion lithophile elements higher viscosity compared with that of the surrounding (LILE), Th, U and light rare earth elements (LREE). Mass asthenospheric mantle (Boyd & McCallister, 1976; Jorbalance calculations based on modal abundances of clinopyroxene dan, 1979; Pollack, 1986). Mantle xenoliths that are and garnet and their respective REE contents yield discrepancies hosted by kimberlites and alkaline basalts are our only between calculated and analyzed REE contents for the Nikos bulk window into the subcontinental lithosphere. They provide rocks that amount to LREE deficiencies of 70‐99%, suggesting essential evidence on the chemical composition and evoluthe presence of small amounts of interstitial kimberlite liquid tion of the upper mantle to depths of >200 km. Studies (0·4‐2 wt %) to account for the excess LREE abundances. These of these mantle xenoliths enable us to characterize the results indicate that the peridotites had in fact depleted or flat abundance and distribution of major, minor and trace LREE patterns before contamination by their host kimberlite. LREE elements in peridotites and between their constituent and Sr enrichment in clinopyroxene and low Zr and Sr abundances minerals. Most subcratonic peridotite samples have in garnet in low-temperature peridotites (800‐1100°C) compared undergone a complex history of melt extraction that has with high-temperature peridotites (1200‐1400°C) suggest that the changed their chemical composition and resulted in shallow lithosphere is geochemically distinct from the deep lithosphere depletion of the residual mantle in fusible major elements beneath the northern margin of the Canadian craton. The Somerset such as Fe, Al and Ca (e.g. Nixon, 1987; Herzberg, 1993; mantle root appears to be characterized by a depth zonation that Boyd et al., 1997). In contrast to their depletion in may date from the time of its stabilization in the Archean. incompatible major elements, however, many peridotite