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.

尽管陆地地幔占我们星球的约80卷，但尽管这种知识对于限制地质时间超过45亿年的地质时间的限制，但它的组成结构尚未得到充分的理解。我们缺乏对地幔茎的详细见解，部分原因是它很少在我们的行星表面暴露，从而使直接观察和研究变得困难。但是，从最近的研究可以明显看出，在地质时期，不能认为地幔在构图上是同质的或静态的。来自海盆（深渊橄榄岩）和蛇绿岩的橄榄岩为对流的上地幔提供了证据，该地幔在区域（100 km）和小（cm-to-m）尺度上是化学和同位素异质性的。复杂的形成和变化的上地幔历史涉及涉及融化过程的过程，置换过程（在这些过程中（诸如Harzburgites之类的耐火残基再次通过融化而添加）和融化摇滚反应负责，但是这些过程的原因和分布较差。评估地幔异质性的时机，原因和程度的特别有价值的资源，因为它们允许基于现场的观察以及对其他难以接近的地幔材料的地球化学研究。此外，Ophiolites保留了一系列海洋地幔岩性（例如Harzburgites，Lherzolite和Dunite），并且这种变化允许详细评估作用于保留的地幔的事件的分布和相对时机。某些蛇绿岩与深渊橄榄岩对比的独特属性是存在Podiormal铬铁矿接缝，通常在岩石学Moho的区域，通常与铂群元素矿化有关。蛇绿岩杆状染色体的时间和生成是有争议的，但有人建议它们代表着在上缩合区设置中集中融化通道的区域。该地区（英国）和挪威（挪威）Supra-Supra-Sucduction ofhiolites在〜620 mA上分离了OB层次，并在〜620 mA上分离出来。大陆壳〜130 mA后，每座北部的北部海洋的相对侧面。 PI和项目合作伙伴已经在设得兰群岛蛇纤维上进行的一项试点研究表明，它保留了在IAPETUS MANTLE一生中发生的复杂熔体耗竭，渗透和介绍事件的复杂序列的证据。从试点数据集做出的批判性观察是，后来的地幔事件仅部分叠加了从早期地幔过程开发的组成异质性，并且与上造物相关的部分熔化相对较高的程度非常有效地产生这种异质性。 1）将设得兰群岛的研究扩展到更高的细节； 2）包括保存完好的Leka Ophiolite的经过精心选择的样品的比较研究； 3）与来自其他（例如中洋脊）构造设置中形成的蛇绿岩的现有地球化学和同位素数据集进行了比较。为了实现这一目标，将利用Re-OS同位素系统和高度支持的元素（OS，IR，RU，RH，PT，PD，RE，AU）的强大组合来区分负责构成甲壳质异质性的过程，例如产生融合耗竭性，转介，推荐，推荐，介绍和融化反应。因此，在iApetus海洋的大部分时间内，将获得深刻的见识，从露头到海洋板块的化学演变，以及从露头到海洋板块鳞片的形成异质性的发展。

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

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

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

Multi-stage fluid infiltration and metasomatism in supra-subduction zone mantle: evidence from halogens and noble gases in the Leka Ophiolite Complex, Norway

• DOI: 10.1016/j.gca.2021.04.028
• 发表时间: 2021-07-07
• 期刊: GEOCHIMICA ET COSMOCHIMICA ACTA
• 影响因子: 5
• 作者: Carter, Elliot J.;O'Driscoll, Brian;Clay, Patricia L.
• 通讯作者: Clay, Patricia L.