Constraints on the 232Th/238U ratio (κ) of the continental crust
Constraints on the 232Th/238U ratio (κ) of the continental crust
复制标题
大陆地壳 232Th/238U 比值 (κ) 的约束
DOI：
10.1029/2002gc000497
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发表时间：
2003
期刊：
影响因子：
3.7
通讯作者：
Donald L. Turcotte
中科院分区：
文献类型：
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作者：
Debajyoti Paul;William M. White;Donald L. Turcotte
本文研究了热流、Th-U系统学和铅同位素比值对陆壳232Th/238U比值(κC)的制约。大洋中脊玄武岩采集的亏损上地幔的232Th/238U比值(κUM)很好地约束在2.5±0.1。下地幔的~(232)Th/~(238)U比值(κ~(Lm))可受如下约束。大洋岛玄武岩的平均κ为3.78。如果OIB来自地幔热柱，并且如果热柱代表下地幔，则κLm≤3.78。或者，如果下地幔是“原始的”，则κLm类似于κ的球粒陨石的232Th/238U比值S=4±0.2。物质平衡计算假定κUM=2.5，κLm≤4具有产热限制，表明κC在5-6范围内。对κC实质上大于4的进一步支持来自陆壳的铅同位素组成。我们估计下陆壳的208Ph/204Pb值为37.36~37.84,208Ph/206Pb值为1.046~1.059，相当于232Th/238U比值(κC(Ph))为4.25−4.30。一个四储层(上、下陆壳、亏损上地幔和原始下地幔)的铅同位素演化正向输运模型再现了亏损地幔中κUM和κUM(Pb值)的差异，表明要达到κC(Pb值)为4.2 5，地壳中目前的232Th/238U比值(κC)必须为≈5。因此，几条证据集中在κC值约为5或略高。
This study examines constraints on the 232Th/238U ratio of the continental crust (κC) imposed by heat flow, Th‐U systematics and Pb isotope ratios. The 232Th/238U ratio of the depleted upper mantle (κUM) as sampled by mid‐ocean ridge basalt is well‐constrained to be 2.5 ± 0.1. The 232Th/238U ratio of the lower mantle (κLM) can be constrained as follows. Oceanic island basalts (OIB) have average κ of 3.78. If OIB are derived from mantle plumes and if plumes are representative of the lower mantle, then κLM ≤ 3.78. Alternatively, if the lower mantle is “primitive”, then κLM is similar to the chondritic 232Th/238U ratio of κS = 4 ± 0.2. Mass balance calculations assuming κUM = 2.5, and κLM ≤ 4 with heat production constraints suggest that κC is in the range 5–6. Further support for κC substantially greater than 4 comes from the Pb isotopic composition of the continental crust. We estimate the 208Pb/204Pb of the lower continental crust to be 37.36 to 37.84 and the 208Pb*/206Pb* ratio of the total continental crust to be 1.046 to 1.059, which corresponds to a time‐integrated 232Th/238U ratio (κC(Pb)) of 4.25 − 4.30. A four‐reservoir (upper and lower continental crust, depleted upper mantle, and primitive lower mantle) forward transport model of Pb isotope evolution that reproduces the difference in κUM and κUM(Pb) values in the depleted mantle suggests that to achieve a κC(Pb) of 4.25, the present 232Th/238U ratio in the crust (κC) must be ≈5. Thus several lines of evidence converge on a value κC of around 5 or slightly higher.