Gamal El Dien Hamed, Arai Shoji, Doucet Luc-Serge, Li Zheng-Xiang, Kil Youngwoo, Fougerouse Denis, Reddy Steven M, Saxey David W, Hamdy Mohamed
Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, GPO Box U1987, Perth, WA, 6845, Australia.
Geology Department, Faculty of Science, Tanta University, 31527, Tanta, Egypt.
Nat Commun. 2019 Nov 8;10(1):5103. doi: 10.1038/s41467-019-13117-1.
Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel's core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes.
地幔熔体为研究与全球板块构造相关的过程提供了一个窗口。镁铁质-超镁铁质岩石中铬尖晶石（Cr-尖晶石）的成分已被广泛用于追踪大地构造环境、地幔熔融程度和大洋中脊扩张速率。其假设是Cr-尖晶石的核心成分（Cr# = Cr/(Cr + Al)）是均匀的，对形成后的改造不敏感，因此是一个可靠的岩石成因指标。然而，我们证明，在岛弧下和大洋中脊下地幔中，流体/熔体-岩石相互作用会改变Cr-尖晶石的成分。交代作用会在Cr-尖晶石中产生铝-铬的不均匀性，降低Cr/Al比值，从而改变Cr#，使Cr#无法作为大地构造和地幔熔融的有效指标。我们的分析还表明，Cr-尖晶石是流体活动元素的潜在汇聚地，特别是在俯冲带环境中。因此，Cr-尖晶石中Cr#的不均匀性可作为交代过程的优良示踪剂。
