Intact Pacific oceanic crust captured as mafic xenoliths in a petit-spot volcano

IF 2.6 3区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Marine Geology Pub Date : 2025-02-10 DOI:10.1016/j.margeo.2025.107497

Kazuto Mikuni , Naoto Hirano , Shiki Machida , Norikatsu Akizawa , Shigekazu Yoneda , Akihiro Tamura , Tomoyuki Mizukami , Yasuhiro Kato , Tomoaki Morishita

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Abstract

Researchers have gained insights into the lithology and geochemistry of oceanic crust through the investigation of mid-ocean ridge basalts (MORBs), rocks exposed along the spreading ridges (e.g., East Pacific Rise), ophiolites, and samples retrieval through seafloor drilling. However, obtaining samples of old and deep oceanic crust, particularly formed at fast spreading ridges, remains challenging in ocean basins. Much of what we know about the matured oceanic crust comes from studies of ophiolite sequences, whose origins are often uncertain.

This study investigates the whole-rock and mineral chemistry, Sr–Nd–Pb isotopic ratios, and petrography of basaltic, doleritic, and gabbroic xenoliths in a petit-spot volcano on the northwestern Pacific Plate in order to characterize the deeper lithologies of in situ old oceanic crust which have not been confirmed. The major-element compositions of mafic xenoliths were non alkaline and generally align with the global MORBs. Their entirely N-MORB-like trace-element-patterns and the radiogenic isotopic ratios, aligning with the Pacific MORB and drilled uppermost basaltic crusts in the vicinity of the study area, indicate that these mafic xenoliths are fragments of oceanic crust formed at the fast-spreading Izanagi–Pacific Ridge at 130–140 Ma unrelated to petit-spot volcanism. The apparent correlation between grain size and chemical composition may reflect the lithological variation within oceanic crust owing to crystal fractionation and replenishment at the MOR. The presence of “granoblastic” dolerite supports that the contact metamorphism of lower sheeted dikes due to melt lenses is a ubiquitous phenomenon beneath the fast-spreading ridge axis. While quantitative extraction of the original depth information for these mafic xenoliths is challenging, these results substantiate the hypothesis on the Penrose-type lithostratigraphy of the oceanic crust formed at fast-spreading ridge and its relationship with chemical composition, established through studies in limited areas, even in old oceanic crust that had never been examined before.

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完整的太平洋地壳在一个小点火山中被捕获为基性捕虏体

研究人员通过对洋中脊玄武岩（morb）、沿扩展脊（如东太平洋隆起）暴露的岩石、蛇绿岩以及通过海底钻探获取样品的调查，对海洋地壳的岩性和地球化学有了深入的了解。然而，在海洋盆地中获取古老的深海地壳样本，特别是在快速扩张的山脊上形成的地壳样本，仍然具有挑战性。我们对成熟海洋地壳的了解大多来自对蛇绿岩序列的研究，而蛇绿岩序列的起源往往不确定。本文研究了西北太平洋板块一个小点火山的玄武岩、碎屑岩和辉长岩包体的全岩和矿物化学、Sr-Nd-Pb同位素比值和岩石学特征，以表征尚未确定的原位古海洋地壳的深部岩性。基性捕虏体的主要元素组成是非碱性的，与全球morb基本一致。它们的微量元素模式和放射性成因同位素比值与研究区附近的太平洋MORB和钻取的最上层玄武岩地壳一致，表明这些基性捕虏体是在130 ~ 140 Ma快速扩张的伊沙那吉-太平洋洋脊形成的海洋地壳碎片，与小点火山作用无关。晶粒尺寸与化学成分之间的明显相关性可能反映了海洋地壳内部由于晶态分馏和多晶态补给而发生的岩性变化。“花岗质”白云岩的存在表明，在快速扩张的脊轴之下，由于熔融透镜作用而形成的下片状岩脉的接触变质作用是普遍存在的。虽然定量提取这些基性捕虏体的原始深度信息具有挑战性，但这些结果证实了在快速扩张脊形成的海洋地壳的penrose型岩石地层学及其与化学成分的关系的假设，这些假设是通过在有限区域的研究建立的，甚至是在以前从未研究过的古老海洋地壳中。

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来源期刊

Marine Geology 地学-地球科学综合

CiteScore

6.10

自引率

6.90%

发文量

175

审稿时长

21.9 weeks

期刊介绍： Marine Geology is the premier international journal on marine geological processes in the broadest sense. We seek papers that are comprehensive, interdisciplinary and synthetic that will be lasting contributions to the field. Although most papers are based on regional studies, they must demonstrate new findings of international significance. We accept papers on subjects as diverse as seafloor hydrothermal systems, beach dynamics, early diagenesis, microbiological studies in sediments, palaeoclimate studies and geophysical studies of the seabed. We encourage papers that address emerging new fields, for example the influence of anthropogenic processes on coastal/marine geology and coastal/marine geoarchaeology. We insist that the papers are concerned with the marine realm and that they deal with geology: with rocks, sediments, and physical and chemical processes affecting them. Papers should address scientific hypotheses: highly descriptive data compilations or papers that deal only with marine management and risk assessment should be submitted to other journals. Papers on laboratory or modelling studies must demonstrate direct relevance to marine processes or deposits. The primary criteria for acceptance of papers is that the science is of high quality, novel, significant, and of broad international interest.