Temporal variations in the composition of Cretaceous Cordilleran arc volcanism through a high-frequency record of bentonites

Continental arc volcanism generates a wide diversity of magma compositions, but the tempos compositional variation are unclear. Here, we investigate 7-million year record volcanic ash layers in Cretaceous Eagle Ford Group, which was deposited Western Interior Seaway western North America. Low oxygen conditions prevented significant bioturbation, allowing for preservation hundreds thin form bentonites. A drill core with an established high-resolution age model provides opportunity to temporal changes composition on <100 kyr timescales. Because intense alteration during diagenesis, protolith compositions were reconstructed from Ti/Zr. We first show that Ti and Zr had limited mobility diagenesis. then show, based compilation data unaltered rocks six continental arcs, relationship between Ti/Zr SiO 2 is similar across arcs different times geographic locations, confirming robust differentiation index indirect measure . Applying empirical Ti/Zr-SiO measurements 52 bentonites allowed reconstruction Ash vary basalt/basaltic andesite dacite/rhyolite, variations not random. fluctuate periods high silica (>60 wt. % ) low (<60 %) over ∼100 If variability these ashes represents broad snapshots Cordilleran arc, results suggest systems may undergo episodic extent magmatic or nature eruption rapid (<100 kyr) • Altered (bentonite) records history eroded arcs. used as reconstruct shows fluctuations