Magnesium isotopic evidence for carbon transfer and sequestration in the forearc mantle wedge
Description
The removal and sequestration of subducted carbon in the forearc mantle have major implications for Earth’s carbon budget, yet the processes responsible and the ultimate fate of this carbon remain poorly constrained because direct evidence from natural samples is scarce. Mantle-hosted felsic intrusions emplaced into forearc mantle peridotites provide a rare opportunity to trace slab-derived mass transfer and volatile recycling within the shallow mantle. Here we report the first high-precision Mg isotope data for mantle-hosted felsic intrusions from the Myitkyina Ophiolite, Myanmar, to evaluate their origin and the role of transferring carbonate-bearing slab fluids into the forearc mantle. The intrusions exhibit uniformly light Mg isotope compositions, with δ26Mg values ranging from −1.15 ± 0.03‰ to −0.30 ± 0.03‰ (average = −0.69‰, n = 8), significantly lower than normal mantle values (−0.25 ± 0.07‰). These low δ26Mg values are not attributable to low-temperature alteration, magmatic differentiation, or thermal-chemical diffusion, and are therefore interpreted to reflect source signatures. Coupled variations in δ26Mg, Ba/Th, Pb/Ce, and Sr-Nd isotopes indicate that the source was modified by slab-derived fluids carrying both fluid-mobile elements and isotopically light Mg. Trace-element modeling suggests that these mantle-hosted felsic intrusions were derived from partial melting of hydrated gabbroic source rocks at forearc depths, while mixing models show that their coupled Mg-Sr-Nd isotopic variations can be explained by source metasomatism caused by carbonate-bearing slab fluids generated through approximately 10 to 60% carbonate dissolution. These results provide direct natural evidence that carbonate dissolution in slab-derived aqueous fluids can transfer carbon into the forearc mantle and generate isotopically distinct metasomatized source rocks. Mantle-hosted felsic intrusions in ophiolites, therefore, represent a promising archive for tracking carbon transfer and sequestration in the forearc mantle wedge.
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Institutions
- Laoshan LaboratoryShandong, Qingdao