Formation of Archaean continental lithosphere and its diamonds: the root of the problem
Formation of Archaean continental lithosphere and its diamonds: the root of the problem
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DOI：
10.1144/0016-76492008-003
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发表时间：
2008-08
影响因子：
2.7
通讯作者：
D. Pearson;N. Wittig
中科院分区：
文献类型：
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作者：
D. Pearson;N. Wittig
克拉通岩石圈地幔在确定古代大陆的物理行为及其矿物潜力方面起着不可或缺的作用。大量成分数据表明，来自各种构造环境的现代熔融残留物可以像克拉通橄榄岩一样耗尽Al和Ca。克拉通橄榄岩受二次引入辉石和石榴石的强烈影响，无法可靠地确定其熔融程度和深度。橄榄石组成可能是原始熔融过程最可靠的示踪剂，表明典型的克拉通橄榄岩经历了40%或更多的熔融萃取。由橄榄石组成显示的均匀损耗水平，结合轻度不相容的微量元素证据，表明熔融发生在浅深度，主要发生在尖晶石稳定性场。熔体产生模型表明，浅层（<3 GPa）无水熔融不能产生以大量熔体萃取为主的残留物。相反，指出了水的关键作用，暗示在太古宙俯冲带内形成了克拉通橄榄岩。这种熔融作用发生在新太古代的一些克拉通块体中，最初形成了二元残余物，这些残余物在一些克拉通的捕虏体清单中仍然很明显。正辉石分解过程中产生的硅质熔体的释放和向上运移，通过富硅水熔体结晶的正辉石的再富集，使原始岩石圈发生交代作用，形成了大多数克拉通根典型的富含正辉石的难熔碳酸盐。太古宙俯冲带熔融后，俯冲叠加形成克拉通根。在最终的长期构造稳定之前，在短暂的热扰动和物理扰动期间，重力可能是造成瓦状基性地壳损失的原因。大多数钻石是在热活动或构造活动的脉冲中形成于这些克拉通根的底部，最初是在根的构造过程中形成的，随后与可能与全球地幔动力演化脉冲相关的大规模区域岩石圈事件有关。
Cratonic lithospheric mantle plays an integral role in defining the physical behaviour of ancient continents and their mineral potential. Bulk compositional data show that modern-day melting residues from a variety of tectonic settings can be as depleted in Al and Ca as cratonic peridotites. Cratonic peridotites are strongly affected by secondary introduction of pyroxenes and garnet such that the extent and depth of melting cannot be reliably determined. Olivine compositions are probably the most reliable tracer of the original melting process and indicate that typical cratonic peridotites have experienced 40% or more melt extraction. Homogeneous levels of depletion indicated by olivine compositions, combined with mildly incompatible trace element evidence, indicate that melting took place at shallow depths, dominantly in the spinel stability field. Consideration of melt production models shows that shallow (<3 GPa) anhydrous melting is not capable of producing residues dominated by large degrees of melt extraction. Instead, a critical role for water is indicated, implicating the formation of cratonic peridotites within Archaean subduction zones. This melting occurred in the Neoarchaean in some cratonic blocks, initially forming dunitic residues that are still evident in the xenolith inventory of some cratons. Release and migration up-section of siliceous melt produced during orthopyroxene breakdown metasomatizes the proto-lithospheric via re-enrichment in orthopyroxene crystallizing from this hydrous Si-rich melt, forming the variably orthopyroxene-rich refractory harzburgites typical of most cratonic roots. Melting in Archaean subduction zones is followed by subduction stacking to form the cratonic root. Gravitational forces may then be responsible for the loss of imbricated mafic crust during periods of transient thermal and physical disturbances prior to final long-term tectonic stability. Most diamonds form in the base of these cratonic roots during pulses of thermal or tectonic activity, initially during root construction and subsequently associated with large-scale regional lithospheric events that may be correlated to pulses in global mantle dynamic evolution.