Cr-spinel records metasomatism not petrogenesis of mantle rocks

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.

Fig. 1

X-ray elemental map of the studied Cr-Spinel. a Backscattered electron (BSE) image of a Cr-spinel grain with a homogenous core surrounded by a small magnetite rim. Al (b) and Cr (c) X-ray map for the same grain show reverse zoning for Al and Cr. Asymmetrical and heterogeneous distribution of Al-Cr within the core (d) and gradual increase of Al-Cr from core to rim (b, c, e, f) of three different Cr-spinel grains. White arrows point to Al halos around inclusions within the Cr-spinel grains (d) and high Al content in small grains with tightly curved rims (b–e). The results of the Atom probe tomography (APT) specimens for the core (M8 and M10) and rim (M5 and M6) are shown in Fig. 5. Mag = magnetite, Srp = serpentine, IC = inner core and R = rim

Fig. 2

Plots of the studied Cr-Spinel chemical composition for different zones within the grains. a Cr# (Cr/Cr + Al) vs. Mg# (Mg/Mg + Fe⁺²) plot of all the raw datasets for Cr-spinel grains in the studied rocks. Such data are usually not published in previous literature. The data span between recommended fields used in previous literature for abyssal peridotites and fore arc (FA)-peridotites^,. b Representative/average Cr-spinel data for each sample. Such sample-average data are usually used in previous mantle petrology studies. This plot shows that our samples had a fore arc to abyssal peridotite origin. c Al₂O₃ vs Cr₂O₃ and d Cr# vs Mg# plots of the studied Cr-spinel grains with low Al₂O₃ and high Cr₂O₃ and Cr# in their cores, and high Al₂O₃ and Low Cr₂O₃ and Cr# in their rims (reverse zoning). Partial melting trend and the brown line between abyssal peridotites (Cr# < 0.60) and FA-peridotites (Cr# > 0.60) are from Dick and Bullen. e All datasets have trends parallel to both the melting trend and our newly defined metasomatism trend. f Datasets of different zones (from core to rim) from four grains from a single sample (sample # A4D) that show large Al-Cr heterogeneity. The data span across the whole range between abyssal peridotites and FA- peridotites. g Plots of the studied Cr-spinel grains compared with the compositions of both modified spinel/rims and non-modified spinel/cores of published mantle peridotite xenoliths^,,,

Fig. 3

Trace elements concentration of different zones in the studied Cr-Spinel. a–d Covariation between Al₂O₃ (wt %) content and fluid-mobile elements (FME: Li, Rb, Sr, and Cs). All the elements show positive a correlation with Al content and an increase from core to rim. e FME and transition elements normalized to primitive mantle compare with the average content of subduction inputs including altered oceanic crust (AOC), global subducted sediments (GLOSS II) and marine sediments, melt inclusions in Cr-spinel from Avacha peridotite xenoliths, and spinel in refractory/depleted peridotites. The Cr-spinel show high enrichment in FME, attributed to slab-derived fluid/melt interaction with host peridotites

Fig. 4

EBSD microstructural data from two Cr-spinel grains from sample A4D. Images comprise greyscale image of EBSD pattern quality overlain by misorientation maps measured relative to the orientation of spinel lattice at the position shown by the white cross. For grain a total misorientation is 10°, for b the total misorientation is 15°. The change in misorientation in each grain corresponds to presence of late fractures seen in the pattern quality image. Total misorientation within individual fracture-bound regions of the grain is <1°, indicating that there is no plastic deformation within the grains

Fig. 5

Atom probe tomography results. Atom maps of Mg, Cr, Al, and Fe are presented for needle-shaped specimens from the core (M8 and M10) and the rim (M5 and M6). The specimens were extracted from sample A4D and their location is indicated on Fig. 1b. The composition in atomic % is indicated for each specimen (See Supplementary Data 2). The composition of the core is enriched in Cr and depleted in Al compared to the rim

Fig. 6

Spinel Cr# and Mg# for the fore arc peridotites. a Cr-spinel database from arc-peridotites including fore arc settings (composed of mantle wedge xenoliths (e.g., the Kamchatka arc)^, and dredged samples form present-day oceanic arcs (e.g., the Izu-Bonin-Mariana arc)^,) and back arc- peridotites (i.e., Mariana Trough)^,. b Cr-spinel database from abyssal/ Mid-ocean ridge (MOR) peridotites. See Supplementary Data 3 for the arc and MOR-Cr-spinel datasets. All the arc and MOR-peridotites dataset plot within the Cr-spinel form mantle wedge peridotites. c All arc peridotite Cr-spinel datasets (without filtering) plotted against MOR-peridotite one, showing their overlapping fields. d Filtered dataset for both arc- and MOR-peridotite are still completely overlapping. For filter details, see the methods. The shaded area in (c, d) marks the overlapping range of Cr# (0.15–0.65) between the arc-peridotite and MOR-peridotite fields