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首页> 外文期刊>Geochemistry, geophysics, geosystems >The coupled W-182-Nd-142 record of early terrestrial mantle differentiation
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The coupled W-182-Nd-142 record of early terrestrial mantle differentiation

机译：W-182-Nd-142耦合记录的早期地幔分化

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摘要

New Sm-Nd, Lu-Hf, Hf-W, and Re-Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) and W abundances, are reported for the 3.55 Ga Schapenburg komatiites, South Africa. The Schapenburg komatiites define a Re-Os isochron with an age of 355087 Ma and initial Os-187=+3.70.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 295% of those in the present-day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long-lived Sm-147 and Lu-176 nuclides (initial Nd-143=+2.40.1, Hf-176=+5.7 +/- 0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in Nd-142 and W-182 (W-182=-8.4 +/- 4.5, Nd-142=-4.9 +/- 2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in Nd-142 and W-182. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first approximate to 30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal-liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after Hf-182 had gone extinct, to account for the observed initial excesses in Nd-143 and Hf-176. The survival of early-formed W-182 and Nd-142 anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks.
机译：据报道，新的Sm-Nd，Lu-Hf，Hf-W和Re-Os同位素数据与高度亲铁元素（HSE，包括Re，Os，Ir，Ru，Pt和Pd）和W丰度相结合南非的3.55 Ga沙芬堡科马提派人。沙芬堡科马蒂岩矿定义了一个Re-Os等时线，其年龄为355087 Ma，初始Os-187 = + 3.70.2（2SD）。据估计，沙芬堡科马特岩系地幔源中的绝对HSE丰度仅为当今大块硅酸盐地球（BSE）的295％。科马铁矿源自富含长寿命Sm-147和Lu-176核素（初始Nd-143 = + 2.40.1，Hf-176 = + 5.7 +/- 0.3、2SD）的衰变产物的地幔。相比之下，相对于现代地幔，科摩铁矿在Nd-142和W-182中被耗尽（W-182 = -8.4 +/- 4.5，Nd-142 = -4.9 +/- 2.8，2SD）。这些结果构成了Nd-142和W-182耦合耗竭的陆地岩石中的首次观测。这种同位素的消耗需要从最早约30 Ma的太阳系历史内形成的地幔区域中衍生出科马铁矿，并由于早期岩浆海洋中的晶体-液体分离而在地球化学上富含高度不相容的微量元素。在Hf-182灭绝后，该地幔区域还必须经历随后的熔体耗尽，以解释Nd-143和Hf-176中观察到的初始过量。早先形成的W-182和Nd-142异常在地幔中的存在，直到至少3.55 Ga为止，这表明早期行星分化的产物在Hadean的后期行星增生和对流地幔混合中都得以生存。此外，这项工作使得可变的后期增生本身不可能解释太古代岩石中所有观察到的W同位素变化。

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