MAGNESIUM ISOTOPIC EVIDENCE FOR ANCIENT SUBDUCTED OCEANIC CRUST IN LOMU-LIKE POTASSIUM-RICH VOLCANIC ROCKS

Continent-derived sediments which were recycled into the deep mantle through subduction event have been considered as a crucial component in the source region of potassium-rich volcanic rocks globally. However, little attention has been paid to the fate of the subducted oceanic crust. Whether the recycled oceanic crust also contributes to the genesis of potassium-rich magmas or not, and how it influences the source signatures remain unclear. Here, we present Mg isotope data for a set of well-characterized samples from Northeast China, including ultrapotassic lavas, potassic lavas, transitional lavas, and sodic lavas, with the goal of examining the role of subducted oceanic crust in the potassium-rich magmatism that has a LOMU-like source signature. These Cenozoic lavas overall are lighter in Mg isotopic compositions than the normal mantle (δ²⁶Mg = -0.25 ± 0.07, [1]), and display considerable Mg isotopic variations, with δ²⁶Mg ranging from -0.61 to -0.23. The covariation of δ²⁶Mg with TiO₂ in these rocks suggests their light Mg isotopic compositions were derived from recycled oceanic crust in the form of carbonated eclogite in the source region. The strong correlation between δ²⁶Mg and (Gd/Yb)_N as well as Sr-Pb isotopes also indicates a multicomponent and multistage origin of these rocks. Our studies here find, for the first time, the presence of recycled oceanic crust in the source region of LOMU-like potassium-rich magmas, indicating that Mg isotopes can be used as a tracer of carbonated eclogites in the magma source. These novel findings also provide a better understanding of the surficial carbon recycle at depth via subduction, and its return to surface through the volcanism.

[1] Teng et al. (2010) GCA 74, 4150-4166.