Serpentinite Geochemistry Documents the Earliest Dehydration and Decarbonation of the Subducting Slab Beneath the Mariana Forearc
Document Type
Article
Publication Date
2024
Keywords
Earliest Slab Dehydration, Decarbonation, Forearc Serpentinites, Blueschist-facies Metamorphism, Seamount Subductio
Digital Object Identifier (DOI)
https://doi.org/10.1016/j.epsl.2024.118748
Abstract
The geochemistry of Mariana forearc serpentinites sheds light on how the subducting slab starts to devolatilize. International Ocean Discovery Program Expedition 366 targeted three serpentinite seamounts: Yinazao, Fantangisña and Asùt Tesoru, with corresponding slab depths of 13, 14 and 18 km, respectively. The fluid-mobile elements (FMEs) B, Rb, Cs, and Li become more enriched in the eruptive serpentinites with increasing slab depth. Yinazao serpentinites show strongly elevated Sr and Ba contents, which complements depletions in these elements seen in mafic clasts entrained in the serpentinites. Correlations between the FMEs and 87Sr/86Sr of the serpentinites reflect interactions among slab fluid, mantle and the subducting slab. With the enrichments of Sr-Ba in the Yinazao and B-Rb-Cs-Li in the Asùt Tesoru serpentinites, their 87Sr/86Sr converge toward values similar as those of the mafic clasts from values between sediments and the mantle. These observations indicate that sediment is the source of the earliest slab-derived fluids, which experienced extensive interaction first with mantle wedge peridotites to produce high pH fluids, and later with subducted seamount basalts. The decomposition of plagioclase in subducting basalts leads to precipitation of CaCO3 from the high pH serpentinizing fluids and elevated Ca-Sr-Ba contents under low carbonate alkalinity conditions, as seen in the Yinazao serpentinites. The breakdown of clay minerals after lawsonite formation and the lizardite-antigorite transition release geochemically distinct fluids, with high B-Rb-Cs-Li and low Sr. This episode of fluid release results in the dissolution of CaCO3 precipitated in the shallow subduction channel, and the generation of rodingite assemblages and epidote. Finally, these fluids evolve to have high carbonate alkalinity and low Ca-Sr-Ba contents, as seen in Asùt Tesoru serpentinites as well as those in South Chamorro and Conical Seamounts. This study reveals early plagioclase breakdown via high pH fluids in the subduction channel is important both for the generation of hydrous minerals and the precipitation of CaCO3 in the subducting slab, subduction channel, and shallow mantle wedge.
Was this content written or created while at USF?
Yes
Citation / Publisher Attribution
Earth and Planetary Science Letters, v. 637, art. 118748
Scholar Commons Citation
Zhao, Rui-Peng; Li, Hong-Yan; Ryan, Jeffrey G.; Johnston, Raymond M.; Jensen, Owen; and Xu, Yigang, "Serpentinite Geochemistry Documents the Earliest Dehydration and Decarbonation of the Subducting Slab Beneath the Mariana Forearc" (2024). School of Geosciences Faculty and Staff Publications. 2401.
https://digitalcommons.usf.edu/geo_facpub/2401
