Evidence from HP/UHP metasediments for recycling of isotopically heterogeneous potassium into the mantle

Potassium isotopes may provide a novel approach for fingerprinting recycled sediments in the mantle due to the significant differences in K abundance and isotopic ratio between subducting sediment and the mantle. However, the behavior of K isotopes in sediments during subduction zone metamorphism is still unknown. Here we investigate K isotopic composition of a set of well-characterized high- to ultrahigh-pressure metasediments from the Schistes Lustres nappe (western Alps), which represents marine sediments subducted down to similar to 90 km depth in a cold subduction zone, and their protoliths from the Lavagna nappe (Apennines, Italy). The metasediments display delta K-41(SRM 3141n) values from -0.76 parts per thousand to -0.48 parts per thousand, which are on average lower than the mantle value (-0.43%o) but similar to those of nonmetamorphic equivalents (-0.79%o to -0.49%)). No systemic variation of delta K-41 with metamorphic grade is observed, suggesting negligible K isotope fractionation in these sediments during prograde metamorphism. This is in accord with the limited loss of K during the entire metamorphic history as evidenced by the constancy of K/Rb and K/Cs ratios between metamorphic and non-metamorphic sediments and the absence of correlations of delta K-41 with K/Rb and K/Cs. The heterogeneous delta K-41 values of metasediments are most likely inherited from their protoliths, which experienced different degrees of chemical weathering depending on their provenances. Our results demonstrate that the variable and light K isotopic signatures in subducting sediments could be preserved to depths of at least 90 km along a cold geotherm gradient, indicating that the introduction of sediments into the mantle could produce K isotope heterogeneity in the source regions of mantle-derived lavas.

Automated summary

The study demonstrates that potassium isotopic signatures in subducting sediments, inherited from their protoliths, can be preserved to depths of at least 90 km along a cold geotherm gradient, indicating the potential for producing potassium isotope heterogeneity in the source regions of mantle-derived lavas.