Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts
Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts
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化学不平衡、岩石圈厚度和洋岛玄武岩的来源
DOI：
10.1093/petrology/egz012
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发表时间：
2019
影响因子：
3.9
通讯作者：
Afonso, Juan C
中科院分区：
文献类型：
--
作者：
Grose, Christopher J;Afonso, Juan C
我们检查了 OIB（海洋岛玄武岩）中的 REE（稀土元素）和同位素（Sr-Hf-Nd-Pb）特征作为岩石圈厚度的函数，并表明数据可以分为薄板（<12Ma）和厚板（>12Ma）子集。与地球物理约束的热板块模型的比较表明，分界年龄（~12Ma）对应于约50km的岩石圈厚度。厚板 OIB 显示出不相容的元素和同位素富集，而薄板熔岩则显示出类似 MORB 或轻微富集的值。我们认为，厚板 OIB 中的富集特征源自干固相线以下深度的低度熔化，而 MORB 和薄板 OIB 中的贫化特征则表明较高程度的熔化。我们使用均质肥沃橄榄岩的熔化模型测试了稀土元素系统学的定量解释。使用主要上地幔矿物的实验分配系数，我们的平衡熔融模型无法解释这些数据。然而，对于相同的均质岩性，使用新的晶粒尺度不平衡熔融模型可以解释数据。不平衡模型能够通过减少分配为低度熔化的不相容元素的数量来解释数据。为了探索不平衡现象的新细节水平，我们采用蒙特卡洛波茨模型来表征微观结构的织构演化，该微观结构经历粗化和相变过程，同时微量元素在减压地幔中固相和熔体之间扩散分配。我们进一步采用逆向方法来研究解释 OIB 数据的模型所需的热化学性质。数据和理论都表明，尽管 E-MORB 提供了最佳拟合，但在扩张山脊上喷发的 OIB 包含接近 MORB 值的特征。这表明 MORB 和 OIB 是由成分上无法区分的来源产生的，尽管同位素数据表明来源是异质的。此外，还发现热机械岩石圈-软流圈边界 (TLAB) 的温度、OIB 源的温度 (Tp, oib) 以及熔化过程中的平衡程度（即晶粒尺寸）的后验分布。TLAB 被限制为 1200–1300°C，Tp、oibis 被限制为 <1400°C。然而，我们认为 Tp、oibas 对所有 OIB 的描述的约束是临时的，因为它是来自全局数据集的统计推断。可能存在特殊的岛屿或岛屿群，例如经典的“热点”（夏威夷、留尼汪岛等），并且这些岛屿可能源自热源。另一方面，根据相同的统计论据，它们的起源可能是异常水合或富集的。 OIB 源中的平均晶粒尺寸约为 1–5mm，尽管这也是临时的，因为强烈依赖于分配系数、上升速率和熔化函数的知识。我们还进行了反演，其中允许分配系数与其实验值不同。在这些反演 TLAB 和 Tp 中，oib 没有变化，但当分配系数与其实验值相差很大时，可以发现接近平衡的实现。我们还研究了受我们的逆模型约束的 OIB 源中的整体成分。对结晶效应的修正为之前提出的地幔成分提供了模糊的确认，其中贫化的地幔提供了最差的拟合。我们的模型中没有包含同位素，但我们简要评估了岩石圈厚度对同位素的影响。尽管 REE 数据不需要……
We examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness and show that the data can be divided into thin- (<12 Ma) and thick-plate (>12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates that the demarcation age (∼12 Ma) corresponds to a lithospheric thickness of about 50 km. Thick-plate OIB show incompatible element and isotopic enrichments, whereas thin-plate lavas show MORB-like or slightly enriched values. We argue that enriched signatures in thick-plate OIB originate from low-degree melting at depths below the dry solidus, while depleted signatures in MORB and thin-plate OIB are indicative of higher-degree melting. We tested quantitative explanations of REE systematics using melting models for homogeneous fertile peridotite. Using experimental partition coefficients for major upper mantle minerals, our equilibrium melting models are not able to explain the data. However, using a new grain-scale disequilibrium melting model for the same homogeneous lithology the data can be explained. Disequilibrium models are able to explain the data by reducing the amount of incompatible element partitioning into low degree melts. To explore new levels of detail in disequilibrium phenomena, we employ the Monte-Carlo Potts model to characterize the textural evolution of a microstructure undergoing coarsening and phase transformation processes simultaneous with the diffusive partitioning of trace elements among solid phases and melt in decompressing mantle. We further employ inverse methods to study the thermochemical properties required for models to explain the OIB data. Both data and theory show that OIB erupted on spreading ridges contain signatures close to MORB values, although E-MORB provides the best fit. This indicates that MORB and OIB are produced by compositionally indistinguishable sources, although the isotopic data indicate that the source is heterogeneous. Also,a posterioridistributions are found for the temperature of the thermomechanical lithosphere-asthenosphere boundary (TLAB), the temperature in the source of OIB (Tp, oib) and the extent of equilibrium during melting (i.e. grain size).TLABhas been constrained to 1200–1300°C andTp, oibis constrained to be <1400°C. However, we consider the constraints onTp, oibas a description of all OIB to be provisional, because it is a statistical inference from the global dataset. Exceptional islands or island groups may exist, such as the classical ‘hotspots’ (Hawaii, Reunion, etc) and these islands may originate from hot sources. On the other hand, by the same statistical arguments their origins may be anomalously hydrated or enriched instead. Mean grain size in the source of OIB is about 1–5 mm, although this is also provisional due to a strong dependence on knowledge of partition coefficients, ascent rate and the melting function. We also perform an inversion in which partition coefficients were allowed to vary from their experimental values. In these inversionsTLABandTp, oibare unchanged, but realizations close to equilibrium can be found when partition coefficients differ substantially from their experimental values. We also investigated bulk compositions in the source of OIB constrained by our inverse models. Corrections for crystallization effects provided ambiguous confirmations of previously proposed mantle compositions, with depleted mantle providing the poorest fits. We did not include isotopes in our models, but we briefly evaluate the lithospheric thickness effect on isotopes. Although REE data do not require a …
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DOI：
--
发表时间：
1992
期刊：
影响因子：
--
作者：
Zhenwei Qin
通讯作者：
Zhenwei Qin
影响因子：
5.2
作者：
J. Afonso;S. Zlotnik;Manel Fernández
通讯作者：
J. Afonso;S. Zlotnik;Manel Fernández
影响因子：
5.3
作者：
DAVIS, EE;LISTER, CRB
通讯作者：
LISTER, CRB
影响因子：
5
作者：
Mark E. Schneider;D. Eggler
通讯作者：
Mark E. Schneider;D. Eggler
扩散控制化学反应的熔体-固体流动
DOI：
--
发表时间：
1992
期刊：
影响因子：
--
作者：
H. Iwamori
通讯作者：
H. Iwamori