Earth and Planetary Science Letters最新文献_第3页

Lower mantle water distribution from ab initio proton diffusivity in bridgmanite 从桥粒石中的质子扩散性得出下地幔水的分布情况

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-16 DOI: 10.1016/j.epsl.2024.119095

Chris E. Mohn , Razvan Caracas , Clinton P. Conrad

Proton self diffusion coefficients for bridgmanite at lower mantle conditions are calculated from ab initio molecular dynamics simulations. We find that the proton self diffusion coefficient,

D^{self}

is nearly constant ∼ 10⁻⁸ m² s⁻¹ along the lower mantle geotherm but increases by nearly one order of magnitude from ∼10⁻¹⁰ m² s⁻¹ to ∼ 10⁻⁹ m² s⁻¹ along a cold slab geotherm to about 1800 km depth. These rates imply that the proton diffusion length scale is less than 10 km in lower mantle peridotite in the 150-200 million years timescale for slab material to sink through the lower mantle. Cold wet slabs probably lose less than one percent of their total water content to the ambient mantle on their journey through the lower mantle, indicating that recycled water is far from homogeneously distributed since slab delivery is highly heterogeneous. We estimate that 0.1 to 0.3 ocean masses (<100 ppm wt%) of recycled water may be currently stored in slab remnant materials within the lower mantle. This water is likely not entrained by plumes but is instead captured by background mantle flow before returning to the mid-ocean ridges. By contrast, deep-rooted mantle plumes may entrain materials containing primordial-like water from the lowermost mantle or the core, and preserve these anomalies in fairly small-scale heterogeneities. Over the age of the Earth, the proton diffusion length scale is a few tens of km, which places constraints on the size of possible primordial water reservoirs isolated from convective mixing, and indicates little flux of water across the core-mantle boundary.

通过ab initio分子动力学模拟，计算了桥芒石在下地幔条件下的质子自扩散系数。我们发现，质子自扩散系数 Dself 沿下地幔地温几乎恒定为 ∼ 10-8 m2 s-1，但沿冷板块地温至约 1800 千米深度则增加了近一个数量级，从 ∼10-10 m2 s-1 增加到 ∼ 10-9 m2 s-1。这些速率意味着，在板块物质通过下地幔下沉的1.5亿-2亿年时间尺度内，质子在下地幔橄榄岩中的扩散长度尺度小于10千米。冷湿板坯在穿过下地幔的过程中，可能损失不到其总含水量的百分之一到环境地幔中，这表明由于板坯的输送是高度异质的，所以回收的水远远不是均匀分布的。我们估计，目前可能有 0.1 到 0.3 个大洋质量（100 ppm wt%）的循环水储存在下地幔内的板块残余物质中。这些水很可能没有被羽流夹带，而是在返回大洋中脊之前被背景地幔流捕获。相比之下，根深蒂固的地幔羽流可能会夹带来自最底层地幔或地核的含有类似原始水的物质，并将这些异常保留在相当小尺度的异质中。在地球年龄段，质子扩散的长度尺度为几十公里，这就限制了可能与对流混合隔绝的原始水储层的大小，并表明穿越地核-地幔边界的水通量很小。

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引用次数: 0

Mechanisms and timing of carbonaceous chondrite delivery to the Earth 碳质软玉运抵地球的机制和时间

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-15 DOI: 10.1016/j.epsl.2024.119112

Francis Nimmo , Thorsten Kleine , Alessandro Morbidelli , David Nesvorny

The nucleosynthetic isotope signatures of meteorites and the bulk silicate Earth (BSE) indicate that Earth consists of a mixture of “carbonaceous” (CC) and “non-carbonaceous” (NC) materials. We show that the fraction of CC material recorded in the isotopic composition of the BSE varies for different elements, and depends on the element's tendency to partition into metal and its volatility. The observed behavior indicates that the majority of material accreted to the Earth was NC-dominated, but that CC-dominated material enriched in moderately volatile elements by a factor of ∼10 was delivered during the last ∼2–10% of Earth's accretion. The late delivery of CC material to Earth contrasts with dynamical evidence for the early implantation of CC objects into the inner solar system during the growth and migration of the giant planets. This, together with the NC-dominated nature of both Earth's late veneer and bulk Mars, suggests that material scattered inwards had the bulk of its mass concentrated in a few, large CC embryos rather than in smaller planetesimals. We propose that Earth accreted a few of these CC embryos while Mars did not, and that at least one of the CC embryos impacted Earth relatively late (when accretion was 90–98% complete). This scenario is consistent with the subsequent Moon-forming impact of a large NC body, as long as this impact did not re-homogenize the entire Earth's mantle.

陨石和大块硅酸盐地球（BSE）的核合成同位素特征表明，地球由 "碳质"（CC）和 "非碳质"（NC）物质混合组成。我们的研究表明，BSE 的同位素组成中记录的 CC 物质的比例因不同元素而异，并取决于该元素分化为金属的倾向及其挥发性。观察到的行为表明，地球吸积的大部分物质以NC为主，但以CC为主的物质在地球吸积的最后∼2-10%期间富含中度挥发性元素，富集系数为∼10。CC物质很晚才被输送到地球，这与巨行星生长和迁移过程中CC物体很早就被植入内太阳系的动力学证据形成了鲜明对比。这一点，再加上地球晚期的贴面和火星主体都以NC为主的性质，表明向内散布的物质的大部分质量都集中在少数大型CC胚胎中，而不是较小的行星碎片中。我们提出，地球吸积了其中几个 CC 胚胎，而火星没有，至少有一个 CC 胚胎在相对较晚的时候（吸积完成 90-98% 时）撞击了地球。只要这种撞击没有使整个地球的地幔重新同质化，那么这种情况就与随后一个大型NC天体撞击月球形成的情况是一致的。

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引用次数: 0

Weak paleointensities from 1.6 Ga Greenland dykes: Further evidence for a billion-year period of paleomagnetic dipole low during the Paleoproterozoic 来自 1.6 Ga 格陵兰堤坝的弱古密度：古近代十亿年古地磁偶极低值期的进一步证据

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-10 DOI: 10.1016/j.epsl.2024.119110

Simon J. Lloyd , Andrew J. Biggin , Henry Halls , Steve Denyszyn

The Paleoproterozoic era is the longest in Earth's history, with significant changes hypothesised to have occurred in the deep Earth's physical and chemical conditions at this time. It has been suggested that the paleomagnetic field became weaker at this time (∼2.4 Ga) and remained weak for the next billion years. Paleomagnetism is intrinsically linked to, and is able to inform on, ancient deep Earth processes; a weak dipole strength sustained over this time period may have implications for both core and mantle evolution.

We test this hypothesis here in a two-fold approach: (1) A paleointensity study on the widespread ca. 1.6 Ga diabase/dolerite Melville Bugt dyke swarm. The swarm extends along the west coast of Greenland for more than 1000 km and intruded over ∼13 million years, capturing polarity reversals of Earth's magnetic field. (2) A detailed statistical analysis on the long-term trend in average dipole moment from an improved paleointensity dataset (PINT.org) that has recently undergone a major update.

Five of the Greenland dykes produce paleointensity results ranging from 1.4 μT to 5.1 μT (virtual dipole moment range 0.3–1.2 × 10²² Am²) during the mid-point of this extended period of ‘dipole low’. Our statistical study robustly confirms that this one-billion-year period was indeed associated with an anomalously weak dipole moment (2.7 × 10²² Am²) relative to 500-million-year intervals before and after, which were almost twice as strong. Sampling of more geographically diverse rocks from this time is needed to yield a clear picture of the long-term time evolution of the dipole moment.

古近纪是地球历史上最长的时期，据推测，地球深部的物理和化学条件在这一时期发生了重大变化。有人认为，古地磁场在这一时期（2.4 Ga ∼ 2.4 Ga）变得较弱，并在接下来的十亿年中一直保持较弱的状态。古地磁与古老的地球深部过程有着内在联系，并能够为其提供信息；在这一时期持续较弱的偶极子强度可能会对地核和地幔的演化产生影响。我们在这里从两个方面对这一假设进行了验证：（1）对广泛分布的约 1.6 Ga 的辉长岩/辉绿岩 Melville Bugt 堤群进行古强度研究。该岩群沿格陵兰岛西海岸延伸 1000 多公里，侵入时间超过 1300 万年，捕捉到了地球磁场的极性反转。(2) 对最近经过重大更新的改进型古强度数据集（PINT.org）中平均偶极矩的长期趋势进行了详细的统计分析。在这一延长的 "偶极低 "时期的中点，格陵兰岛的五个堤坝产生了从 1.4 μT 到 5.1 μT（虚拟偶极矩范围为 0.3-1.2 × 1022 Am2）的古强度结果。我们的统计研究有力地证实，相对于之前和之后的 5 亿年，这 10 亿年的偶极矩（2.7 × 1022 Am2）确实异常微弱，而之前和之后的偶极矩几乎是前者的两倍。要清楚地了解偶极矩的长期时间演化，还需要对这一时期更多不同地理位置的岩石进行取样。

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引用次数: 0

Magnetotelluric image of the Patagonian slab window: Constraints on upper mantle physical properties and sources of intraplate magmatism 巴塔哥尼亚板块窗口的磁位图象：对上地幔物理特性和板内岩浆活动来源的制约

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-09 DOI: 10.1016/j.epsl.2024.119105

Jaime Araya Vargas , Jorge Sanhueza , Daniel Díaz , María José Segovia , Francisco Pastén-Araya , Katarzyna Ślęzak

The Patagonian slab window (PSW) is a region of the Southamerican subduction zone where the absence of subducted slabs is interpreted, due to the subduction of the Chile Mid-Ocean Ridge at the Chile Triple Junction. Here we report the results of a long-period magnetotelluric (MT) study conducted in two 300 km-long trench-parallel transects crossing the northern boundary of the PSW in the proximal backarc. We modeled the MT data using 3-D inversion, obtaining an electrical resistivity model of the continental crust and upper mantle up to a depth of ∼150 km. Our model shows a heterogeneous resistivity structure in the uppermost mantle, dominated by resistivities >300 Ωm below the array of sites even within the PSW, and some low-resistivity zones (LRZs, <10 Ωm) mainly at the edge of the array. Using petrophysical models, we estimated the mantle temperature, water content, melt fraction, and viscosity based on obtained resistivity values and a preexistent model of P-wave velocity (Vp) at 50 km and 100 km depth. These estimates suggest that the uppermost mantle within the PSW region is heterogeneous and dominated by high-viscosity blocks, compatible with the continental mantle lithosphere or even subducted slabs. Based on relatively hot and low-viscosity zones estimated in the periphery of LRZs, we interpret the presence of asthenospheric mantle in areas where LRZs coincide with relatively low Vp. According to this interpretation, asthenospheric upwelling in the study area at depths ≤150 km would be localized rather than ubiquitous over the interpreted extent for the PSW. Such localized asthenosphere upwelling processes in the past could explain the scattered distribution of Neogene basaltic lavas in the southern Patagonia backarc. The continental crust exhibits LRZs in the upper and lower crust. Remarkably, ensembles of LRZs at different crustal depths within the presumable area of the PSW were found below the General Carrera Lake, and towards the North Patagonian Icefield, likely indicating the presence of hidden intraplate magmatic and/or hydrothermal systems.

巴塔哥尼亚板块窗口（Patagonian slab window，PSW）是南美洲俯冲带的一个区域，据解释，由于智利三交界处的智利大洋中脊的俯冲，该区域没有俯冲板块。在此，我们报告了在两条长达 300 公里的海沟平行横断面上进行的长周期磁测（MT）研究的结果，这两条横断面穿越了近端弧背的 PSW 北部边界。我们利用三维反演对磁法数据进行了建模，获得了深度达 150 千米的大陆地壳和上地幔电阻率模型。我们的模型显示了最上层地幔的异质电阻率结构，主要是电阻率>300 Ωm，甚至在PSW内的站点阵列下方，以及一些低电阻率区（LRZs，<10 Ωm），主要位于阵列边缘。利用岩石物理模型，我们根据获得的电阻率值和已有的 50 千米和 100 千米深度的 P 波速度（Vp）模型，估算了地幔温度、含水量、熔融率和粘度。这些估算结果表明，PSW 区域内的最上层地幔是异质的，主要是高粘度块体，与大陆地幔岩石圈甚至俯冲板块相吻合。根据在LRZ外围估计的相对热的低粘度区，我们解释了在LRZ与相对低Vp相吻合的地区存在着星体层地幔。根据这一解释，研究区域内深度≤150千米的星体层上升流将是局部的，而不是在所解释的PSW范围内普遍存在的。过去这种局部的星体层上升流过程可以解释新近纪玄武岩熔岩在巴塔哥尼亚背弧南部的分散分布。大陆地壳的上部和下部均表现出低辐射区。值得注意的是，在General Carrera湖以下和北巴塔哥尼亚冰原方向，在推测的PSW区域内的不同地壳深度发现了成组的LRZ，这很可能表明板内存在隐藏的岩浆和/或热液系统。

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引用次数: 0

Shallow crustal rupture in a major MW 7.5 earthquake above a deep crustal seismic swarm along the Noto Peninsula in western Japan 日本西部能登半岛深层地壳地震群上方发生的 MW 7.5 级大地震中的浅层地壳破裂

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-08 DOI: 10.1016/j.epsl.2024.119107

Chengli Liu , Yefei Bai , Thorne Lay , Ping He , Yangmao Wen , Xiaoran Wei , Neng Xiong , Xiong Xiong

A damaging M_W 7.5 earthquake struck the western coast of Japan along the Noto Peninsula on January 1, 2024. The initiation of large shallow earthquakes along the Noto Peninsula, particularly above deeper long-duration patchy seismic swarms, presents an unusual seismic phenomenon that warrants in-depth investigation of their interactions. The 2024 earthquake nucleated with an initial low average rupture velocity of 0.5–1.0 km s^-1 near the up-dip end of a long-lasting seismic swarm that commenced in November 2020. Analysis of dense seismic, geodetic, and tsunami observations provides good resolution of large shallow slip in the crust below the peninsula and extending offshore to the northeast, revealing a heterogeneous slip distribution characterized by bilateral two-stage rupture expansion during the faulting. Up to 8 m of slip occurred in several patches along ∼150 km of the southeastward-dipping thrust fault, which extends to near the seafloor along the northwest side of the peninsula. Up to 5 m of uplift occurred along the peninsula's northwestern coast. Up-dip fluid migration appears to have weakened the shallow fault prior to failure and influenced the initial slow rupture expansion, highlighting the need to monitor the evolution of worldwide swarms.

2024 年 1 月 1 日，能登半岛沿日本西海岸发生了一次破坏性 MW 7.5 地震。能登半岛沿岸浅层大地震的发生，尤其是在较深的长持续时间的成片地震群之上，是一种不寻常的地震现象，需要对其相互作用进行深入研究。2024 年地震在 2020 年 11 月开始的持续时间较长的地震群的上倾端附近以 0.5-1.0 km s-1 的初始低平均破裂速度成核。通过对密集的地震、大地测量和海啸观测数据进行分析，可以很好地解决半岛下方地壳向东北方向延伸的大面积浅层滑移问题，揭示了断层期间以双边两级断裂扩展为特征的异质滑移分布。沿半岛西北侧延伸至海底附近的向东南倾斜的推力断层的 150 千米范围内发生了多处高达 8 米的滑移。半岛西北沿岸出现了高达 5 米的隆起。在断裂之前，上倾流体迁移似乎削弱了浅层断层，并影响了最初缓慢的断裂扩展，这凸显了监测世界性地震群演变的必要性。

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引用次数: 0

Thermal evolution of the lunar magma ocean 月球岩浆海洋的热演化

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-08 DOI: 10.1016/j.epsl.2024.119109

Line Colin , Chloé Michaut , Stéphane Labrosse , Bernard Bourdon

The energy of the giant impact was large enough to generate an initially fully molten Moon. During the solidification of this lunar magma ocean (LMO), an anorthosite crust formed by flotation of light anorthite crystals. Lunar anorthosites show crystallization ages as young as 4.360 Gyr, suggesting a long-lived LMO or a rather young Moon. Existing models for LMO solidification are for a specific phase diagram based on one compositional model. However, the LMO solidification timescale depends on the lunar bulk composition and on the appearance of anorthite in the crystallization sequence.

Here, we propose a physically robust 1D model for LMO evolution based on a simple anorthite/olivine-pyroxene eutectic phase diagram. Cumulates first settle at the ocean base for about a thousand years. This first stage results in an unstable thermal profile for the cumulates that can lead to their overturn. In the second stage, simultaneous crystallization of anorthite and cumulates leads to the formation of a buoyant lid that considerably slows down LMO cooling.

We explore the impact of an initially hydrated composition, which reduces the stability of plagioclase, of the eutectic position and of the crust thermal conductivity. We show that cumulates overturn may reduce or extend the LMO solidification time depending on its duration. The total LMO solidification timescale ranges between 45 and 250 Myr. Given the most reliable age of 4.360 Gyr for FAN sample 60025, which derives from more than 99% of crystallization, we estimate an age of 4400 to 4560 Myr for the Moon.

巨大撞击的能量足以产生一个最初完全熔融的月球。在这个月球岩浆洋（LMO）的凝固过程中，轻质正长岩晶体浮选形成了正长岩地壳。月球正长岩的结晶年龄可年轻到 4.360 Gyr，这表明月球岩浆海洋或月球相当年轻。现有的 LMO 凝固模型是基于一种成分模型的特定相图。然而，LMO 的凝固时标取决于月球块体成分和结晶序列中正长岩的出现。在此，我们基于简单的正长岩/橄榄石-辉石共晶相图，提出了一个物理上稳健的 LMO 演化一维模型。积聚物首先在海洋底部沉积约一千年。第一阶段的结果是积聚体的热曲线不稳定，可能导致其倾覆。在第二阶段，正长岩和积云的同时结晶导致浮力盖的形成，从而大大减缓了 LMO 的冷却速度。我们探讨了初始水合成分的影响（它降低了斜长石的稳定性）、共晶位置和地壳导热性的影响。我们的研究表明，积聚物的倾覆可能会缩短或延长 LMO 的凝固时间，这取决于其持续时间。LMO 的总凝固时间范围在 45 到 250 Myr 之间。鉴于 FAN 样品 60025 最可靠的年龄为 4.360 Gyr（来自 99% 以上的结晶），我们估计月球的年龄为 4400 至 4560 Myr。

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引用次数: 0

Coupling magma ascent models with volatile diffusion chronometry 将岩浆上升模型与挥发性扩散计时法结合起来

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-08 DOI: 10.1016/j.epsl.2024.119099

O. Bernard, F. Costa

The pre- and syn-eruptive magma decompression rate is recognized as a key parameter modulating eruption dynamics, with explosive eruptions being generally associated with much larger decompression rates than effusive ones. Magma decompression rates cannot be directly measured and thus are typically inferred from petrological, geochemical, numerical modelling, and seismic data. Most studies use petrological information of volatile element diffusive equilibration in glass and crystals to infer a single value for the magma ascent rate for a given eruption, even though numerical volcano conduit simulations show that changes of velocity are expected during magma ascent. Here we integrate magma ascent conduit models with diffusion chronometry of volatiles in melt embayments and phenocrysts to obtain a more comprehensive understanding of magma ascent rates. We find that incorporating a more realistic boundary condition that depends on the magma ascent path with variable velocities gives time estimates that can be up to a factor of 7 longer than from the standard assumption of constant magma ascent rate. Therefore, previous magma ascent rates from diffusion chronometry of volatiles in crystals and melts with a fixed boundary condition may be significantly overestimated. Overall, we show that coupling of magma ascent models with diffusion chronometry can provide more robust inferences of magma ascent and thus improve the understanding of the role of this parameter into the explosive and effusive eruption controls.

爆发前和同步爆发的岩浆减压率被认为是调节喷发动力学的一个关键参数，爆炸性喷发的减压率通常比喷出性喷发大得多。岩浆减压率无法直接测量，因此通常通过岩石学、地球化学、数值模拟和地震数据来推断。大多数研究使用玻璃和晶体中挥发性元素扩散平衡的岩石学信息来推断特定喷发的岩浆上升速率的单一值，尽管火山导管数值模拟显示岩浆上升过程中速度会发生变化。在这里，我们将岩浆上升导管模型与熔体内湾和表晶中的挥发物扩散计时法结合起来，以获得对岩浆上升速率更全面的了解。我们发现，根据岩浆上升路径的不同速度，结合更现实的边界条件，得出的时间估计值比岩浆上升速率恒定的标准假设最多可延长 7 倍。因此，以前根据晶体和熔体中挥发物的扩散时间测定法得出的岩浆上升速率在边界条件固定的情况下可能会被大大高估。总之，我们的研究表明，将岩浆上升模型与扩散计时法结合起来，可以提供更可靠的岩浆上升推断，从而提高人们对这一参数在爆炸和喷发控制中的作用的认识。

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引用次数: 0

Mantle plume – mid ocean ridge interactions revealed from hotspot melt production rate along the Walvis Ridge-Rio Grande Rise 从沃尔维斯海脊-里奥格兰德海隆沿线的热点熔体生成率揭示地幔羽流-中洋脊的相互作用

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-07 DOI: 10.1016/j.epsl.2024.119102

Yongliang Bai , Kunpeng Cheng , Dongdong Dong , Xinjian Song , Sanzhong Li , Shiguo Wu , Zhenjie Wang

Mantle plume and mid-ocean ridge (MOR) interactions have a profound impact on geological processes, such as large igneous province eruptions, mid-ocean ridge jumps and plume flow towards the MOR. The Walvis Ridge and Rio Grande Rise provide an excellent opportunity to study these interactions, as they have a record of during- and post-interaction stages. We have estimated the hotspot melt production rate along the Walvis Ridge - Rio Grande Rise. Our findings are as follows: (1) The hotspot melt production rate during plume-MOR interaction is seven times higher than that after the interaction, primarily because a thinner lithospheric coverage over the plume allows for greater decompression melting and facilitates the ascent of melt through the lithospheric mantle to accrete to the crust. (2) The waning of the Tristan-Gough plume is indicated by a decreasing melt production rate and an increase in inter-volcano spacing. (3) The linear ridges on the western side of the Guyot Province likely result from mantle plume flow beneath the lithosphere from the hotspot to the Mid-Atlantic Ridge. (4) Fracture zones enhance hotspot melt production rates.

地幔羽流与大洋中脊（MOR）的相互作用对地质过程有着深远的影响，如大型火成岩群的喷发、大洋中脊的跃升和流向大洋中脊的羽流。沃尔维斯海脊和格兰德河海隆是研究这些相互作用的绝佳机会，因为它们有相互作用期间和之后阶段的记录。我们估算了沃尔维斯海脊-格兰德河隆起沿线的热点熔体生成率。我们的研究结果如下(1) 羽流-MOR相互作用期间的热点熔体生成率是相互作用后的七倍，这主要是因为羽流上方的岩石圈覆盖层较薄，减压熔化作用较大，有利于熔体穿过岩石圈地幔上升到地壳。(2) 特里斯坦-戈夫火山羽流的减弱表现为熔体生成率下降和火山间距增大。(3) 盖奥特省西侧的线状海脊可能是岩石圈下的地幔羽流从热点流向大西洋中脊的结果。(4) 断裂带提高了热点熔体的生成率。

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引用次数: 0

How brittle detachments form and evolve through space and time 脆性脱落是如何形成并在空间和时间中演变的

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-07 DOI: 10.1016/j.epsl.2024.119108

C. Zuccari , F. Mazzarini , E. Tavarnelli , G. Viola , L. Aldega , V. Moretto , R. Xie , G. Musumeci

We report the first absolute deformation ages for the Mykonos Detachment that juxtaposes, in the context of the current Aegean rifting, Miocene siliciclastic deposits in the hanging wall against metabasites and synkinematic granites in the footwall. We identified and characterised 16 brittle structural facies (BSFs) within the detachment fault architecture through fieldwork, optical microscopy, and X-ray diffraction. BSFs dating by K-Ar of synkinematic mixed layer illite-smectite shows that the preserved BSFs formed during repeated slip events, thus constraining protracted faulting between 13.5 and 6.5 Ma. Dating, structural and mineralogical characterisation allowed for the time-constrained evaluation of the activation/de-activation of the involved deformation mechanisms and of the processes, including (i) fault zone nucleation, (ii) deformation partitioning, (iii) cataclasis, gouge formation and (iv) the final deformation localisation, that govern the evolution of brittle detachments, shaping the local current upper crustal structure. Our results provide new insights into the understanding of wide active and fossil rift systems.

我们首次报告了米克诺斯支脉的绝对变形年龄，在当前爱琴海断裂的背景下，该支脉将悬壁的中新世硅质沉积物与底壁的偏闪长岩和同生花岗岩并列在一起。我们通过实地考察、光学显微镜和 X 射线衍射，确定并描述了剥离断层结构中的 16 个脆性结构面（BSF）。通过同步混合层伊利石-闪长岩的 K-Ar 测定，BSFs 的年代显示，保存下来的 BSFs 是在反复滑动过程中形成的，从而确定了 13.5 至 6.5 Ma 之间的漫长断层。通过年代测定、结构和矿物学特征描述，可以对所涉及的变形机制的激活/去激活过程以及包括(i) 断层区成核、(ii) 变形分区、(iii) 角砾岩、沟槽形成和(iv) 最终变形定位在内的各种过程进行有时间限制的评估，这些过程控制着脆性脱离的演化，塑造了当地当前的上地壳结构。我们的研究结果为了解广阔的活动断裂和化石断裂系统提供了新的视角。

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引用次数: 0

Magmatic N2 degassing dominates nitrogen loss during sediment subduction and granitoid genesis 沉积物俯冲和花岗岩成因过程中，岩浆 N2 脱气主导了氮的损失

IF 4.8 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Earth and Planetary Science Letters

Pub Date : 2024-11-06 DOI: 10.1016/j.epsl.2024.119094

Yunzhe Chen , Peng Wu , Tiago Angelo , Christopher Spencer , Long Li

Subducting sediments contain much more abundant nitrogen (N) than the other slab components. The fate of sedimentary N along subduction directly determines the quantity of surficial N that can be released back to the atmosphere/crust or further delivered to the deep mantle, which has a strong impact on the long-term evolution of the N budgets and isotopic signatures of Earth's major reservoirs. Previous studies on the fate of subducted sedimentary N have been intensively focused on assessing the extent of metamorphic N devolatilization from metasedimentary rocks varying from ultrahigh-pressure eclogites facies (subducted to as deep as 90 km) in cold subduction zones to medium-pressure amphibolite facies in extremely hot subduction zones. However, the fate of sedimentary N after these metamorphic stages, particularly during sediment melting and subsequent magmatic crystallization, has not been examined so far. Here, we present the N data of the Cretaceous peraluminous granitoids in the Oman-United Arab Emirates area, which were formed by melting of subducted pelagic sediments followed by intrusion and crystallization in the lithospheric mantle before the entire lithospheric section was obducted to form the Oman-United Arab Emirates ophiolites. The 27 studied samples show surprisingly low N contents (4.0 - 13.7 ppm; mean: 6.9 ± 2.3 ppm; 1σ) and a narrow δ¹⁵N range (mostly in the range of –1.4‰ to +2.2‰; mean: +0.4 ± 1.5‰; 1σ). Data modeling indicates that the N in these granitoids contains little mantle contribution and was predominantly inherited from their source rocks. Compared with potential source rocks, these granitoids display orders of magnitude lower N contents but comparable δ¹⁵N values, indicating that the significant N loss during the genesis of these granitoids was associated with little N isotope fractionation. This suggests that magmatic N₂ degassing (with minor N isotope fractionation) rather than metamorphic N devolatilization (with much larger N isotope fractionation) is the dominant mechanism driving the N loss from subducted sediments.

俯冲沉积物中含有比其他板块成分丰富得多的氮。沉积氮在俯冲过程中的去向直接决定了表层氮能够释放回大气/地壳或进一步输送到地幔深处的数量，这对地球主要储层的氮预算和同位素特征的长期演化有很大影响。以往对俯冲沉积 N 的归宿的研究主要集中在评估变质 N 从变质岩中脱落的程度，这些变质岩从寒冷俯冲带的超高压夕卡岩面（俯冲深度达 90 千米）到极热俯冲带的中压闪长岩面都有。然而，迄今为止还没有研究过沉积 N 在这些变质阶段之后的去向，特别是在沉积物熔融和随后的岩浆结晶过程中的去向。在此，我们介绍了阿曼-阿联酋地区白垩纪围岩花岗岩的氮数据，这些花岗岩是在整个岩石圈断面被俯冲形成阿曼-阿联酋蛇绿岩之前，由俯冲沉积物熔融后在岩石圈地幔中侵入和结晶形成的。所研究的 27 个样本显示出令人惊讶的低 N 含量（4.0 - 13.7 ppm；平均值：6.9 ± 2.3 ppm；1σ）和狭窄的 δ15N 范围（大部分在-1.4‰至 +2.2‰之间；平均值：+0.4 ± 1.5‰；1σ）。数据建模表明，这些花岗岩中的氮含量很少来自地幔，而主要来自其源岩。与潜在的源岩相比，这些花岗岩中的氮含量低了几个数量级，但δ15N值却相当，这表明这些花岗岩在成因过程中的大量氮损失与很少的氮同位素分馏有关。这表明，岩浆N2脱气（N同位素分馏小）而非变质N脱溶（N同位素分馏大得多）是驱动N从俯冲沉积物中流失的主要机制。

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