Elucidating parameters governing elemental transfer during seafloor alteration: Application of machine-learning based Protolith Reconstruction Model (PRM) to seafloor altered basalt, the South and Northwest Pacific regions.

Seafloor alteration controls the element transfer from the Earth's surface to towards the deep Earth. Element transfer in altered basalt is usually recognized as chemical differences between altered and non-altered samples located closely (i.e., within the same drilling site; Zhang & Smith-Duque, 2014). However, non-altered samples or "protolith" are often unavailable due to complete alteration. Consequently, our knowledges on element transfer variations during seafloor alteration and their governing parameters in a global perspective are still limited.In this study, we revealed the regional trend of element transfer from seafloor altered basalt samples by reconstructing their protolith composition via machine learning models (Protolith Reconstruction Models for metabasalt (PRM); Matsuno et al. 2022 Sci. Rep.). We compiled the compositional data (sixteen elements; Rb, Ba, U, K, La, Ce, Pb, Sr, Nd, Y, Yb, Lu, Zr, Th, Ti, Nb) of 237 altered basalts from seven drilling sites in South Pacific (i.e., S10-50°, W100-180°) and Northwest Pacific (i.e., N10-50°, W100-180°) regions. These regions show contrasting ages and sedimentation rates (Northwest: 130-170 Myr, 15 mm/yr; South: 13.5-103.7 Myr, 0 .01-0.1 mm/yr). We applied PRM to obtain the protolith compositions and then evaluated the element transfer during alteration. For both region samples, there are up to 100-fold enrichment in Rb, Ba, U, K, and Pb, while there are no significant enrichment in La, Ce, Sr, Nd, Y, Yb, and Lu. Enrichments of Rb and K are strongly correlated (R~0.85), and related to the amount of secondary minerals such as Saponite (R~0.5). The characteristics of Ba, U, and Pb enrichment are different between the Northwest and South Pacific regions: in South Pacific, these elements are enriched in intensively altered samples with reddish color, associated with Rb and K enrichment. In the Northwest Pacific, the Ba, U, and Pb enrichments occurred independently each other. These regional characteristics imply that the sediment layer thickness may control the extent of circulation and/or sources of hydrothermal fluid.