Early global mantle chemical and isotope heterogeneity revealed by the komatiite-basalt record: The Western Australia connection
Early global mantle chemical and isotope heterogeneity revealed by the komatiite-basalt record: The Western Australia connection
复制标题
科马提岩-玄武岩记录揭示了早期全球地幔化学和同位素异质性：与西澳大利亚的联系
DOI：
10.1016/j.gca.2021.11.030
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发表时间：
2022
影响因子：
5
通讯作者：
Locmelis, M.
中科院分区：
文献类型：
--
作者：
Puchtel, I.S.;Nicklas, R.W.;Slagle, J.;Horan, M.;Walker, R.J.;Nisbet, E.G.;Locmelis, M.
虽然地幔化学成分的非均质性现在已经很好地确定了，但这种非均质性的起源和持续时间仍在争论中。为了进一步研究地球早期的非均质性，我们对西澳大利亚Pilbara克拉通的~ 3.53 Ga Coonterunah、~ 3.34 Ga Kelly、~ 3.18 Ga Ruth Well和Regal体系的科马长岩和玄武岩进行了一组Sm-Nd、Lu-Hf、Re-Os和Hf-W同位素以及亲石和亲铁元素丰度数据。Sm-Nd、Lu-Hf和Re-Os同位素数据的等时线与各自的科马马岩-玄武岩熔岩的公认就位年龄一致。地幔源区的长期演化规律为：147sm /144Nd = 0.200 ~ 0.214, 176lu /177Hf = 0.0355 ~ 0.0395，涵盖了太古宙和元古代马母岩—玄武岩体系的Sm/Nd和Lu/Hf时间积分范围。与其他早太古宙科马岩浆岩和玄武岩不同，皮尔巴拉熔岩的143nd - 176hf耦合同位素系统没有提供早期岩浆海洋作用参与其地幔源演化的证据。不同程度的地幔部分熔融和熔体萃取可以解释太古代早期地幔域中Sm/Nd和Lu/Hf时间积分比值的大变化。相对于变化较大的Nd-Hf分系统，γ - 187os初始值在+0.9 ~−0.4的狭窄范围内变化，表明皮尔巴拉地幔源演化为球粒质时间积分Re/Os。早期低度地幔熔融事件将Sm从Nd中分离出来，将Lu从Hf中分离出来，但对Re/Os比值的影响不大，这一模型调和了全球观测到的不相容亲石微量元素消耗与近球球状Re/Os之间的明显差异。这反过来又意味着玄武岩地壳的早期形成和长期隔离，高度富含不相容的亲石微量元素。计算出的科马地岩地幔源的总HSE丰度范围从Coonterunah的~ 30%到Regal系统的~ 70%，与现代BSE的估计相比，表明从3.53 Ga到3.18 Ga， HSE丰度增加了2.4倍。所有四种科马铁矿-玄武岩系统均表现出+11.4 ~ +7.7 ppm的正182w异常。在Pilbara komatii -玄武岩源中，182w /184W组成与计算的HSE丰度呈负相关，与大分异星子的粒状晚期增生最为一致。对科马地岩系统的182w - hse组合数据进行回归，可以估计出晚期增生前BSE的W同位素组成为+17±7。这一估计与月球+25±5的估计相似，并进一步支持了地月系统中最初共同的W同位素组成的概念。通过对全球科马地岩地幔源现有HSE丰度数据的回归，估计地幔内晚期吸积物质完全均匀化的时间为2.5±0.2 Ga。计算表明，晚期吸积星子在地幔中的平均存活时间为1.9±0.2 Ga，这限制了冥古宙和太古宙HSE的平均地幔搅拌速率。
Although the heterogeneous nature of the chemical composition of Earth's mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the ∼3.53 Ga Coonterunah, ∼3.34 Ga Kelly, and ∼3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term147Sm/144Nd = 0.200 to 0.214 and176Lu/177Hf = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled143Nd-176Hf isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains.In contrast to the highly variable Nd-Hf systematics, the initial γ187Os values vary within a narrow range from +0.9 to −0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements.The calculated total HSE abundances in the komatiite mantle sources range from ∼30% in the Coonterunah to ∼70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4× increase in HSE abundances from 3.53 to 3.18 Ga.All four komatiite-basalt systems exhibit positive182W anomalies ranging between +11.4 and +7.7 ppm. The182W/184W compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined182W-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 ± 7. This estimate is similar to that of the Moon of +25 ± 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system.Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 ± 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth's mantle of 1.9 ± 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean.