The behavior of magnesium isotopes in low-grade metamorphosed mudrocks

摘要: Magnesium isotopic compositions of mudrocks metamorphosed at sub-greenschist facies from three lower Paleozoic basins (northern Lake District, southern and Southern Uplands) in the British Caledonides were measured order to understand behavior Mg isotopes during diagenesis low-grade metamorphism. Carbonate-free northern District have heavy d 26 values varying � 0.17 +0.25. By contrast, carbonate-bearing Uplands vary more widely, with ranging 0.74 0.08. Acid leaching experiments on latter show that leachates higher Ca/Al Ca/K ratios than residues due dissolution leachable carbonates. The (� 1.54 0.21) are always corresponding (d = 0.39 +0.09), consistent isotopically light A rough, negative correlation between Mg/Al for residual silicate fraction suggests their controlled by relative proportion illite/muscovite chlorite. Global clastic sediments display highly variable negatively correlated CaO/Al2O3 CaO/TiO2, implying carbonates introduce sediments, although end member itself has a wide range Mg, depending its mineralogy. mudrocks, as well carbonate fractions, do not systemically metamorphism proceeds metamorphism, suggesting limited isotope fractionation low-temperature metamorphic dehydration (<300 C). general decrease (by mass) contributed increasing grade or decomposition expelled isotopes. Thus, fractions faithfully reflect provenance signatures. Our study shows can be used sedimentary diagenesis, may prove useful tracer recycled into mantle given heterogeneous