Incomplete mixing of isotopically variable melts in the lower oceanic crust and its implications on mantle heterogeneity: Evidence from oceanic cumulates from 14°45′N, Mid-Atlantic Ridge
Incomplete mixing of isotopically variable melts in the lower oceanic crust and its implications on mantle heterogeneity: Evidence from oceanic cumulates from 14°45′N, Mid-Atlantic Ridge
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DOI：
10.1016/j.chemgeo.2024.121940
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发表时间：
2024-01
期刊：
影响因子：
3.9
通讯作者：
Jixin Wang;Bing Li;Fangyi Zhang;R. Liao;Xiaoyan Gu;Congying Li;Weidong Sun
中科院分区：
文献类型：
--
作者：
Jixin Wang;Bing Li;Fangyi Zhang;R. Liao;Xiaoyan Gu;Congying Li;Weidong Sun
深海橄榄岩和洋中脊玄武岩（MORB）只能提供洋中脊下地幔成分和岩浆过程的部分一瞥。近年来，下地壳堆积物已成为捕捉地幔不均匀性和描述MORB生成系统详细过程的潜在候选者。然而，下地壳堆积物在多大程度上能够代表地幔源区的变化，目前还不确定。本文对中大西洋海岭（MAR）下地壳堆积岩进行了详细的地球化学和同位素研究，包括矿物的主量元素和微量元素组成以及斜长石的Sr同位素分析。微量元素组成的熔体平衡与单斜辉石从下地壳堆表现出强烈的相似性，附近的MORB。例外的是，平衡熔体的核心单斜辉石oikocrysts记录的过渡MORB组合物，不同的更丰富的MORB组合物在轮辋。这种现象，连同元素的矿物从核心到轮辋的变化，表明熔体-岩石相互作用的历史，在此期间，更丰富的熔体运输通过矿物，以前结晶从相对更贫化熔体。原位87 Sr/86 Sr比值从0.70249到0.70306不等，跨越了全球MORB Sr同位素值的范围。因此，堆积矿物的微量元素组成和87 Sr/86 Sr比值都证实了亏损和富集熔体的共存。我们的研究结果进一步表明，从异质地幔成分的熔体进行不完全混合的下洋壳，甚至在晶体糊状物，从而保持可变的不均匀性没有看到MORB。通过综合分析地球化学，同位素数据和地球物理信息，从现有的文献中，我们确定了最有可能的地幔成分在研究区的亏损橄榄岩脉与回收的洋壳（辉石岩）。
Abyssal peridotites and mid-ocean ridge basalts (MORB) offer only a partial glimpse into the mantle's composition and the magmatic process beneath the mid-ocean ridges. Recently, lower crustal cumulates have emerged as potential candidates for capturing a broader range of mantle heterogeneity and depicting a detailed process in MORB generating system. However, it remains uncertain to what extent the lower crustal cumulates can represent mantle source variability. In this study, we present detailed in-situ geochemical and isotopic data, including major and trace element compositions of minerals and in-situ Sr isotopic analysis on plagioclase from lower crustal cumulates collected at 14°45′N on the Mid-Atlantic Ridge (MAR). Trace element compositions of melt in equilibrium with clinopyroxenes from the lower crustal cumulates exhibit strong similarities to nearby MORBs. Exception is that the equilibrium melt of the core of a clinopyroxene oikocryst records a transitional MORB composition, different from the more enriched MORB composition at rims. This phenomenon, together with element changing in minerals from cores to rims, suggests a history of melt-rock interactions, during which more enriched melts transport through minerals that had previously crystallized from relatively more depleted melts. The in-situ87Sr/86Sr ratios, which vary from 0.70249 to 0.70306, span the range of the global MORB Sr isotope values. Thus, both the trace element compositions and87Sr/86Sr ratios of cumulate minerals confirm the coexistence of depleted and enriched melts. Our findings furtherly suggest that the melts from heterogenous mantle components underwent incomplete mixing in the lower oceanic crust, even in the crystal mush, thus preserving variable heterogeneities not seen in MORB. Through integrated analyses of geochemistry, isotopic data, and geophysical information from existing literature, we identify the most likely mantle components in the study area as depleted peridotites veined with recycled oceanic crust (pyroxenites).