Role of recycled oceanic crust in the potassium and argon budget of the Earth: Toward a resolution of the “missing argon” problem
Role of recycled oceanic crust in the potassium and argon budget of the Earth: Toward a resolution of the “missing argon” problem
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回收的洋壳在地球钾和氩预算中的作用：解决“氩缺失”问题
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发表时间：
2004
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通讯作者：
J. C. Lassiter
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作者：
J. C. Lassiter
根据大洋中脊玄武岩和大陆地壳中大约 12,000-13,000 的 K/U 比率，并假设地球的球粒状 U 含量，硅酸盐地球的 K 含量通常估计为大约 250 ppm。鉴于此 K 含量，地球历史上 40K 衰变产生的 40Ar 中只有约 50% 目前存在于大气中。由于上地幔已彻底脱气且几乎不含 40Ar，因此“缺失”的 Ar 长期以来一直被视为下地幔富含气体（因此对流隔离）的证据。 “Ar 缺失”问题的另一种解决方案是硅酸盐地球的 K 含量远低于 250 ppm。之前的估计未能考虑到再循环洋壳的成分，该洋壳可能占地幔质量的 6-9%，并含有地球 U 的 40%。由于 K 在俯冲产生的含水流体中比 U 更易溶解，因此再循环、脱水的洋壳的特点是 K/U 比率极低。这在榴辉岩 (~1000) 和 HIMU 型玄武岩 (~4000) 的低 K/U 比中得到了证明，这些玄武岩被认为来自含有大量再生地壳的来源。海岛玄武岩由含有不同数量和类型的回收材料的地幔生成，具有高度可变的 K/U 比，但各个岛链的平均 K/U 比与 206Pb/204Pb 呈负相关，这表明一些海岛玄武岩中的低 K/U 是一个长期存在的源特征。在 K 和 U 质量平衡计算中包含回收的地壳储层，可估计块状硅酸盐地球 K/U 约为 7000–9000，或 K 含量约为 150–195 ppm。因此，许多被认为储存在地幔深处的“失踪”40Ar 可能并不存在，地球上约 60-90% 的 40Ar 存在于大气或大陆地壳中。剩余的 40Ar 可以容纳在对流地幔中，而无需隔离深的、未脱气的储层。
The K content of the Silicate Earth is commonly estimated at ∼250 ppm on the basis of K/U ratios of ∼12,000–13,000 in mid‐ocean ridge basalts and continental crust, and assuming a chondritic U content for the Earth. Given this K content, only ∼50% of the 40Ar generated over Earth history from the decay of 40K currently resides in the atmosphere. Because the upper mantle is thoroughly degassed and contains little 40Ar, the “missing” Ar has long been taken as evidence for a gas‐rich (and therefore convectively isolated) lower mantle. An alternative solution of the “missing Ar” problem is that the K content of the Silicate Earth is substantially lower than 250 ppm. Previous estimates have failed to take into account the composition of recycled oceanic crust, which could comprise up to 6–9% of the mantle by mass and contain up to 40% of the Earth's U. Because K is more soluble than U in subduction‐generated hydrous fluids, recycled, dehydrated oceanic crust is characterized by extremely low K/U ratios. This is evidenced in the low K/U ratios of eclogites (∼1000), and in HIMU‐type basalts (∼4000), which are thought to derive from sources containing significant quantities of recycled crust. Ocean island basalts, which are generated from mantle containing varying amounts and types of recycled material, possess highly variable K/U ratios, but average K/U ratios from individual island chains are negatively correlated with 206Pb/204Pb, indicating that the low K/U in some ocean‐island basalts is a long‐lived source feature. Inclusion of a recycled crust reservoir in K and U mass balance calculations results in an estimate of the Bulk Silicate Earth K/U of ∼7000–9000, or a K content of ∼150–195 ppm. As a result, much of the “missing” 40Ar thought to be stored in the deep mantle may not exist, with ∼60–90% of the Earth's 40Ar residing in the atmosphere or continental crust. The remaining 40Ar can be accommodated in the convecting mantle without isolation of a deep, undegassed reservoir.