Hemispheric Geochemical Dichotomy of the Mantle Is a Legacy of Austral Supercontinent Assembly and Onset of Deep Continental Crust Subduction
Hemispheric Geochemical Dichotomy of the Mantle Is a Legacy of Austral Supercontinent Assembly and Onset of Deep Continental Crust Subduction
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DOI：
10.1029/2022av000664
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发表时间：
2022-12-01
期刊：
影响因子：
8.4
通讯作者：
Macdonald, F. A.
中科院分区：
文献类型：
--
作者：
Jackson, M. G.;Macdonald, F. A.
具有极端富集的地幔放射性同位素特征（包括表明俯冲大陆地壳的低Nd - 143/Nd - 144比值）的大洋热点与对南半球地幔进行采样的地幔柱通道有关。然而，导致俯冲大陆地壳在澳大利亚地幔中聚集的机制尚不清楚。我们表明，沉积物的俯冲、俯冲侵蚀物质以及下大陆地壳的拆沉都无法产生这种空间上连贯的澳大利亚地球化学区域。然而，在新元古代晚期广泛出现大陆超高压变质岩带期间，与冈瓦纳 - 泛大陆拼合相关的大陆碰撞主要发生在南半球，这标志着上大陆地壳深俯冲的开始。我们提出，在大约650 - 300 Ma澳大利亚超大陆拼合期间，古代裂解 - 被动边缘上大陆地壳的深俯冲导致更多的上大陆地壳进入南半球地幔。北半球地幔柱源中不存在类似的富集地幔区域，原因有二。首先，在300 Ma之后（当大陆漂移到北半球时）俯冲的大陆地壳没有足够的时间通过对北半球地幔进行采样的地幔柱返回地表。其次，在650 Ma首次出现已知的大陆超高压岩石之前，上大陆地壳的深俯冲并不常见，这限制了它在地球历史早期俯冲到北半球（和南半球）地幔中。我们的模型意味着澳大利亚富集地幔区域是近期形成的，解释了澳大利亚和北半球地幔柱源之间的地球化学二分性，并且可能解释了为什么澳大利亚热点数量是北半球热点数量的两倍。
Oceanic hotspots with extreme enriched mantle radiogenic isotopic signatures-including low Nd-143/Nd-144 indicative of subducted continental crust-are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions-associated with the assembly of Gondwana-Pangea-were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra-high-pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted-passive margins during ca. 650-300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma-when the continents drifted into the northern hemisphere-has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra-high-pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots.