Scales of preservation and root causes of mantle heterogeneities in the Iapetan Ocean convecting upper mantle

土卫洋对流上地幔的保存规模及地幔异质性的根源

• 批准号: NE/J00457X/1
• 负责人: Brian O'Driscoll
• 金额: $ 7.54万
• 依托单位: Keele University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ00457X%2F1
• 关键词: Scales; preservation; root; causes; mantle

Although the terrestrial mantle comprises ~80 vol.% of our planet, its compositional architecture is not well understood despite the importance such knowledge holds for constraining Earth's thermal and chemical evolution over ~4.5 billion years of geological time. Our lack of detailed insight into the mantle stems in part from the fact that it is rarely exposed at our planets surface, making direct observation and study difficult. It is clear from recent study, however, that the mantle cannot be assumed to be compositionally homogenous or static over geological time. Peridotites from the ocean basins (abyssal peridotites) and from ophiolites preserve evidence for a convecting upper mantle that is chemically and isotopically heterogeneous at regional (100's km) and small (cm-to-m) scales. Complex formation and alteration upper mantle histories involving processes of melt-depletion, refertilisation (whereby originally refractory residues such as harzburgites become lherzolites again via melt addition) and melt-rock reaction have been held responsible, but the causes, timing and distribution of such processes are poorly resolved.Ophiolites, which represent partially-to-wholly preserved slivers of obducted oceanic mantle, are particularly valuable resources for assessing the timing, causes and extent of mantle heterogeneity, as they allow field-based observation to be coupled with geochemical investigation on otherwise inaccessible mantle material. Furthermore, ophiolites preserve a range of oceanic mantle lithologies (e.g., harzburgites, lherzolite and dunite) and such variation allows detailed assessment of the distribution and relative timing of events acting upon the mantle that is preserved. A distinctive attribute of some ophiolites, which contrasts with abyssal peridotites, is the presence of podiform chromitite seams, typically in the region of the petrological Moho, which are often associated with Platinum-group element mineralization. The timing and genesis of ophiolite podiform chromitites is controversial, but it has been suggested that they represent zones of focused melt channeling in supra-subduction zone settings.The Shetland (UK) and Leka (Norway) supra-subduction zone ophiolites comprise oceanic lithosphere separated at ~620 Ma on either side of a mid-ocean ridge and subsequently obducted over continental crust ~130 Ma later, each on opposite sides of the northern Iapetus Ocean. A pilot study already carried out on the Shetland ophiolite by the PI and Project Partner reveals that it preserves evidence for a complex sequence of melt depletion, percolation and refertilisation events that occurred over the lifetime of the Iapetus mantle. The critical observation made from the pilot dataset is that later mantle events only partially overprint the compositional heterogeneities developed from earlier mantle processes and that the relatively high degrees of partial melting associated with the supra-subduction zone are very effective at generating such heterogeneity. This important observation will be tested in the proposed research by 1) extending the Shetland study to greater levels of detail; 2) inclusion of a comparative study of carefully selected samples from the well-preserved Leka ophiolite; 3) drawing comparisons with existing geochemical and isotopic datasets from ophiolites that formed in other (e.g., mid-ocean ridge) tectonic settings. In order to achieve this, the powerful combination of the Re-Os isotopic system and highly-siderophile element (Os, Ir, Ru, Rh, Pt, Pd, Re, Au) abundance measurements will be utilised to discriminate between the processes responsible for generating mantle heterogeneities such as melt depletion, refertilisation and melt-rock reaction. Thus, profound insight will be gained into the chemical evolution of a piece of oceanic mantle and the development of compositional heterogeneity therein, from outcrop to oceanic plate scales, over much of the lifetime of the Iapetus Ocean.

虽然地球地幔约占80体积%，尽管这些知识对于限制地球在45亿年地质时间内的热演化和化学演化具有重要意义，但它的组成结构还没有得到很好的理解。我们对地幔缺乏详细的了解，部分原因是它很少暴露在我们的行星表面，这使得直接观察和研究变得困难。然而，从最近的研究中可以清楚地看出，不能假定地幔在地质时期内是成分均匀或静止的。来自海洋盆地（深海橄榄岩）和蛇绿岩的橄榄岩保存对流上地幔的证据，在区域（100公里）和小（厘米到米）尺度上的化学和同位素的异质性。复杂的形成和蚀变上地幔的历史，包括熔融-贫化，再受精过程（其中最初的难熔残余物如方辉橄榄岩通过熔体添加再次变成二辉橄榄岩）和熔体-岩石反应的作用，但这些过程的原因、时间和分布尚不清楚。蛇绿岩代表了部分或全部保存的仰冲洋幔的薄片，对于评估地幔不均匀性的时间、原因和程度来说，这些数据是特别宝贵的资源，因为它们使实地观察与对否则无法获得的地幔物质的地球化学调查相结合。此外，蛇绿岩保存了一系列海洋地幔岩性（例如，方辉橄榄岩、二辉橄榄岩和纯橄榄岩），这种变化使得能够详细评估作用于保存的地幔的事件的分布和相对时间。与深海橄榄岩形成对比的一些蛇绿岩的一个独特属性是豆荚状铬铁矿接缝的存在，通常位于岩石学莫霍面区域，通常与铂族元素矿化有关。蛇绿岩豆荚状铬铁矿的形成时间和成因存在争议，但有人认为它们代表了超俯冲带环境中集中熔融通道的区域。设得兰群岛（英国）和莱卡（挪威）超俯冲带蛇绿岩构成了大洋岩石圈，在大洋中脊两侧分离约620 Ma，随后在约130 Ma后仰冲到大陆地壳上。分别位于土卫八北方大洋的两侧PI和项目合作伙伴已经对设得兰群岛蛇绿岩进行了一项试点研究，研究结果表明，它保留了在Iapetus地幔的生命周期中发生的熔体耗尽、渗透和再受精事件的复杂序列的证据。从试点数据集的关键观察是，后来的地幔事件只是部分地覆盖了从早期地幔过程中发展的成分不均匀性，相对较高程度的部分熔融与超俯冲带是非常有效的产生这种不均匀性。这一重要的观察结果将在拟议的研究中进行测试，方法是：1）将设得兰群岛的研究扩展到更详细的层次; 2）包括对保存完好的莱卡蛇绿岩中精心挑选的样本进行比较研究; 3）与其他蛇绿岩中形成的现有地球化学和同位素数据集进行比较（例如，洋中脊）构造环境。为了实现这一目标，Re-Os同位素系统和高度亲铁元素（Os，Ir，Ru，Rh，Pt，Pd，Re，Au）丰度测量的强大组合将被用来区分负责产生地幔非均质性的过程，如熔体贫化，再受精和熔体-岩石反应。因此，将获得深刻的洞察到一块海洋地幔的化学演化和成分的不均匀性的发展，从露头到海洋板块尺度，在大部分的Iapetus海洋的生命周期。

项目成果

The effects of subduction-zone melting on the oceanic mantle: insights from ophiolite peridotites

俯冲带熔融对大洋地幔的影响：来自蛇绿岩橄榄岩的见解

• 发表时间: 2014
• 作者: Brian O'Driscoll

Oceanic and Continental Lithospheric Mantle in the 1.95-Ga Jormua Ophiolite Complex, Finland: Implications for Mantle and Crustal Evolution

芬兰 1.95 Ga Jormua 蛇绿岩杂岩中的海洋和大陆岩石圈地幔：对地幔和地壳演化的影响

• DOI: 10.1093/petrology/egad080
• 发表时间: 2023
• 期刊: Journal of Petrology
• 影响因子: 3.9
• 作者: Finlayson, Valerie A;Haller, Mitchell;Day, James M;Ginley, Stephen;O'Driscoll, Brian;Kontinen, Asko;Hanski, Eero;Walker, Richard J
• 通讯作者: Walker, Richard J

Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides

苏格兰喀里多尼德群岛南设得兰群岛将地幔橄榄岩鉴定为可能的 Iapetan 蛇绿岩条

• DOI: 10.1144/jgs2016-074
• 发表时间: 2016
• 期刊: Journal of the Geological Society
• 影响因子: 2.7
• 作者: Day J

Scales of preservation and root causes of heterogeneities in the convecting upper mantle of the Iapetan Ocean

Iapetan洋对流上地幔的保存规模和异质性的根本原因

• 发表时间: 2012
• 作者: Brian O'Driscoll

Lithological, Chemical and Chronological Constraints on Melt Extraction from the Mantle Section of the ?492 Ma Shetland Ophiolite Complex, Scotland

苏格兰 492 Ma 设得兰蛇绿岩杂岩地幔部分熔体提取的岩性、化学和年代学限制

• 发表时间: 2015
• 作者: B O'Driscoll