Shui-Jiong Wang

Tracing carbonate–silicate interaction during subduction using magnesium and oxygen isotopes

Subduction of carbonates and carbonated eclogites into the mantle plays an important role in transporting carbon into deep Earth. However, to what degree isotopic exchanges occur between carbonate and silicate during subduction remains unclear. Here we report Mg and O isotopic compositions for ultrahigh pressure metamorphic marbles and enclosed carbonated eclogites from China. These marbles include both calcite- and dolomite-rich examples and display similar O but distinct Mg isotopic signatures to their protoliths. Their δ(26)Mg values vary from -2.508 to -0.531‰, and negatively correlate with MgO/CaO ratios, unforeseen in sedimentary carbonates. Carbonated eclogites have extremely heavy δ(18)O (up to +21.1‰) and light δ(26)Mg values (down to -1.928‰ in garnet and -0.980‰ in pyroxene) compared with their protoliths. These unique Mg-O isotopic characteristics reflect differential isotopic exchange between eclogites and carbonates during subduction, making coupled Mg and O isotopic studies potential tools for tracing deep carbon recycling.

Magnesium isotope evidence for a recycled origin of cratonic eclogites

The Mg isotopic compositions of garnet and clinopyroxene mineral separates and wholenrocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from thenKoidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa,nprovide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxenenin both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mgnisotope partitioning, with δ 26 Mg varying from –2.15‰ to –0.46‰ in garnets and from –0.49‰nto +0.35‰ in clinopyroxenes. Bulk δ 26 Mg values (–1.38‰ to +0.05‰), constructed from garnetnand clinopyroxene data, are similar to results from rock powders (–1.60‰ to +0.17‰),nsuggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositionsnof the xenoliths. The δ 26 Mg values of low-MgO eclogites (–0.80‰ to +0.05‰) exceednthe range of mantle peridotite xenoliths (–0.25‰ ± 0.04‰), consistent with the eclogite9snderivation from recycled altered oceanic crust. Similarly variable δ 26 Mg values in high-MgOneclogites (–0.95‰ to –0.13‰), together with their high MgO and low FeO contents, suggestnthat high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO neclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogitesnpreserve a record of Mg isotopic compositions produced by low-pressure, surficial isotopenfractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneitynof the mantle.