Abstract
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.
Highlights
Mantle melts provide a window on processes related to global plate tectonics
We conduct a correlative microanalysis workflow from the grain-scale to the nano-scale using X-ray elemental mapping, electron microprobe (EMP), laser-ablation-inductively coupled plasma mass spectrometry (LA-ICPMS), electron backscattered diffraction (EBSD), and atom probe tomography (APT) in order to describe the Cr-spinel reverse zonation in mantle peridotites from the Arabian Nubian Shield
Our new analyses and a careful review of published results show that the chemical composition of Cr-spinel can be and severely modified by cryptic metasomatism through fluid/meltrock interaction involving slab-derived fluids in the sub-arc mantle, or even by small volumes of melts in the sub-oceanic mantle
Summary
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 midocean ridge spreading. We conduct a correlative microanalysis workflow from the grain-scale to the nano-scale using X-ray elemental mapping, electron microprobe (EMP), laser-ablation-inductively coupled plasma mass spectrometry (LA-ICPMS), electron backscattered diffraction (EBSD), and atom probe tomography (APT) in order to describe the Cr-spinel reverse zonation in mantle peridotites from the Arabian Nubian Shield. This type of reverse zoning “AlCr heterogeneity” is characterized by Al enrichment and Cr depletion in the rim compared to the core of the studied crystals. We question the robustness of Cr-spinel during post-formation modification, and the suitability of Cr-spinel as a straightforward reliable indicator for tracing mantle melting conditions and tectonic environments
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