The behavior of magnesium isotopes in low-grade metamorphosed mudrocks

Abstract

Magnesium isotopic compositions of mudrocks metamorphosed at sub-greenschist facies from three lower Paleozoic basins (northern Lake District, southern Lake District, and Southern Uplands) in the British Caledonides were measured in order to understand the behavior of Mg isotopes during diagenesis and low-grade metamorphism. Carbonate-free mudrocks from the northern Lake District have heavy δ26Mg values varying from −0.17 to +0.25. By contrast, Mg isotopic compositions of carbonate-bearing mudrocks from the southern Lake District and Southern Uplands vary more widely, with δ26Mg ranging from −0.74 to −0.08. Acid leaching experiments on the latter show that the leachates have higher Ca/Al and Ca/K ratios than the residues due to the dissolution of leachable carbonates. The δ26Mg values of leachates (−1.54 to −0.21) are always lower than the corresponding residues (δ26Mg=−0.39 to +0.09), consistent with isotopically light Mg in carbonates. A rough, negative correlation between δ26Mg and Mg/Al for the residual silicate fraction of mudrocks suggests that their Mg isotopic compositions are controlled by the relative proportion of illite/muscovite and chlorite. Global clastic sediments display highly variable Mg isotopic compositions that are negatively correlated with CaO/Al2O3 and CaO/TiO2, implying that carbonates introduce light Mg isotopes to sediments, although the silicate end member itself has a wide range of δ26Mg, depending on its mineralogy. Magnesium isotopic compositions of mudrocks, as well as their silicate and carbonate fractions, do not vary systemically as metamorphism proceeds from diagenesis to low-grade metamorphism, suggesting limited Mg isotope fractionation during low-temperature metamorphic dehydration (<300°C). The general decrease of Mg fraction (by mass) contributed by carbonate with increasing metamorphic grade suggests that dissolution or decomposition of carbonates during metamorphism expelled light Mg isotopes. Thus, the Mg isotopic compositions of the silicate fractions in clastic sediments more faithfully reflect their provenance signatures. Our study shows that Mg isotopes can be used to study sedimentary diagenesis, and Mg isotopes may prove a useful tracer of sediments recycled into the mantle given their heterogeneous δ26Mg values.