Pressure-dependent compatibility of iron in garnet: Insights into the origin of ferropicritic melt

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• 作者：JunBo Zhang ; YongSheng Liu ; ^{yshliu@hotmail.com} ; WenLi Ling ; Shan Gao
• 关键词：Iron ; Low-velocity anomaly ; Recycled lower continental crust ; Eclogitic garnet ; Ferropicrite
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2017
• 出版时间：15 January 2017
• 年：2017
• 卷：197
• 期：Complete
• 页码：356-377
• 全文大小：1917 K
• 卷排序：197

文摘

Iron-rich silicate melts in the Earth’s deep mantle have been seismologically and geochemically inferred in recent years. The origin of local enrichments in iron and low-velocity seismic anomalies that have been detected in dense mantle domains are critical to understanding the mantle’s evolution, which has been canonically explained by long-term chemical reactions between the Earth’s silicate mantle and its liquid iron outer core. However, the Pleistocene alkaline ferropicrites (∼0.73 Ma) from Wudi, North China, show chemical and Sr–Nd–Os isotopic features that suggest derivation from the preferential melting of silica-deficient eclogite, a lithology of delaminated mafic lower continental crust that had stagnated at mid-upper mantle depths during the Mesozoic decratonization of the North China block. These rocks are characterized by substantial enrichment in iron (14.9–15.2 wt% Fe2O3), relative depletion in silica (40–41 wt% SiO2) and decoupled Y and heavy rare earth element (HREE) compositions. These ferropicrites have particularly higher Y/Yb ratios than the other Cenozoic basalts from North China. The pressure-dependent compatibility of Fe, Y and Yb in eclogitic garnet can adequately explain the Fe-enrichment and Y-HREE decoupling of the Wudi ferropicrites and indicates that the eclogites were melted at pressures of 5–8 GPa, as also constrained by previous high-P–T experiments. This melting depth ties together a seismically imaged high-velocity anomaly that extends from 150 km to 350 km in depth under the study area, which has been commonly interpreted as evidence for the stagnation of the missing, delaminated continental lithosphere. Our findings provide an alternative mechanism to produce an extremely iron-rich mantle reservoir in addition to core–mantle interaction. Iron-rich silicate melts that form by this process are likely to be denser than the ambient mantle peridotite (and therefore drive flow downward) and may play a more significant role in the deep-mantle geophysical and geochemical diversities than previously considered.