Compositions of basaltic arc lavas track temporal changes in the global Sr cycle

Active arc basalts have higher Sr/Nd ratios than the bulk continental crust. The significant delamination of low-density Sr-bearing plagioclase-rich lower arc crust cumulates is unlikely. Here, we compile geochemical data from 1875 – 0 Ma arc basalts (5.5-6.5 wt.% MgO) and demonstrate that Phanerozoic fossil (6.2 – 52.8; average: (26.5 ± 11.5 (1 σ)) and active (27.8 – 67.9; average: 42.1 ± 9.8 (1 σ)) arc basalts have higher average Sr₆/Nd₆ ratios than those of Proterozoic fossil arcs (6.6 – 45.4; average: 16.9 ± 9.8 (1 σ)). There were increases in the average Sr₆/Nd₆ ratios of arc basalts at 800 – 600 and 150 – 100 Ma. The average Sr/Nd ratios of global subducting sediment (12) and depleted mantle (14) are considerably lower than those of active arc basalts. The Sr₆/Nd₆ ratios of active arc basalts do not correlate with Th₆/La₆, ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr and crustal thickness. Active arc basalts have high Nd₆/Sr₆ and Sr₆/Th₆ and low ⁸⁷Sr/⁸⁶Sr ratios. This indicates the high Sr₆/Nd₆ ratios are not influenced by crustal thickness or siliciclastic sediment subduction but rather slab-derived fluids. Higher Sr contents in seawater due to increased continental weathering associated with the rise of the continents in the Neoproterozoic, and increases in the amount of abiogenic and biogenic carbonate being subducted at 800 and 150 Ma, respectively, led to the high Sr₆/Nd₆ ratios of basalts from Phanerozoic fossil and active arcs. The increase in the Sr contents of seawater led to the generation of more Sr-rich basaltic magmas following the dehydration and/or melting of altered oceanic crust. The subduction of pelagic carbonates after 150 Ma resulted in the generation of the high Sr₆/Nd₆ of basaltic lavas from active arcs. Therefore, the compositions of basaltic arc lavas track temporal changes in the global Sr and C cycles.