Silicon isotopic compositions of altered Oceanic crust samples from IODP U1365 and U1368: Effect of low-temperature seawater alteration

Decoupled Zircon Si–O Isotopes Tracing the Supracrustal Silicification and Komatiitic‐Derived Fluids in the Source of TTGs

“…First, pelitic sediments have relatively low δ 30 Si but high δ 18 O values, indicating desilicification by chemical weathering (Trail et al., 2018). Second, the δ 30 Si values of altered rocks that underwent hydration without silicification (e.g., serpentinization) are dominated by igneous minerals, whereas δ 18 O values vary with the alteration temperature (Abraham et al., 2011; Savage et al., 2013; Yu et al., 2018, 2023). Third, the seawater‐derived authigenic silicification (e.g., chert) and seafloor silicification (e.g., silicified basalt) at low temperatures result in the enrichment of both heavy Si and heavy O isotopes (Abraham et al., 2011; Trail et al., 2018).…”

“…This means the low‐δ 18 O fluid extracted from the basaltic komatiite and komatiite is a significant material in the melting source for the TTGs with decoupled Si–O isotope compositions. The Si isotope composition of overlying basaltic rocks might not be substantially affected by fluid derived from komatiitic rocks, as Si isotopes were resistant to metamorphic and metasomatic processes (André et al., 2006; Yu et al., 2018, 2023). Moreover, the Si isotope fractionation factor between rock and fluid is very low at temperatures of 680–800°C, and basaltic komatiite and komatiite have similar δ 30 Si values (−0.31‰ to −0.24‰) to those of basalt (ca.…”

“…A similar process to that inferred for the profile of Barberton greenstone belts occurs in the modern oceanic crust. Although modern seawater has higher δ 30 Si values than fresh MORB (middle ocean ridge basalt) and bulk silicate Earth, most oceanic basalts and serpentinized rocks have Si isotope compositions similar to the mantle value regardless of their high or low δ 18 O values (Savage et al., 2011; Yu et al., 2018), with the exception being the lighter Si isotopes of small portions of oceanic crust that have undergone strong low‐temperature alteration (e.g., Yu et al., 2023). Si isotopes are resistant to water–rock interaction because modern seawater is deficient in Si and thus cannot cause resolvable Si isotopic variation at the bulk‐rock scale.…”

“…The covariation of Si and O isotopes during fluid alteration and precipitation processes have been investigated previously (e.g., Chowdhury et al, 2023;Deng et al, 2019;Trail et al, 2018;Yu et al, 2018Yu et al, , 2023, which have proposed three evolution paths of the coupled Si-O isotopic variations. First, pelitic sediments have relatively low δ 30 Si but high δ 18 O values, indicating desilicification by chemical weathering (Trail et al, 2018).…”