INSIGHTS FROM VOLCANIC ASH TOWARD UNDERSTANDING THE VOLCANIC-PLUTONIC CONNECTION: SHORT AND LONG TIMESCALE GEOCHEMICAL PROCESSES PRESERVED IN CRETACEOUS CORDILLERAN VOLCANISM

This study examines the geochemical relationships between volcanic ash and its intrusive counterparts in continental arc settings. Are erupted products biased to certain compositions? Are there compositional differences between large and small eruptions? To what extent do these relationships change with the temporal evolution of the system? To examine these questions, we conduct a case study of Cretaceous Western Interior Seaway ash.

We examine a 7 million year sediment core containing 390 bentonitized ash layers interbedded with carbonates that preserves a detailed record of Cordilleran volcanism. Plutonic compositions and additional ash data from the Cretaceous Western Interior Seaway are compiled from the literature for a statistically robust examination of these questions. We assess spatial and temporal geochemical trends on timescales of both single eruptions and of the system.

We employ micro-XRF to assess spatial geochemical variability within bentonites and their associated carbonates. Individual ash beds exhibit different trends between chemistry and depth in the ash layer. By examining the boundaries of the ash and carbonates, we extract information about the relative timing and nature of short-timescale processes of deposition and alteration.

One challenge of studying bentonites is that they are heavily altered, obscuring original protolith composition. However, the elements Ti, Zr, and Nb remain immobile in chemical weathering processes, allowing us to backtrack from bentonites to original eruption chemistry. We assess the relationship between immobile element ratios and major oxide concentrations in reported data from the inferred plutonic counterparts. Such relationships can be determined for the major oxides SiO₂, MgO, CaO, TiO₂, FeO, Al₂O₃, and K₂O. We measure immobile element ratios in bentonites using LA-ICP-MS, allowing reconstruction of the pre-alteration protolith composition.

Important uses of such reconstructions include tracking the temporal evolution of a system, assessing the potential role of ash in nutrient flux, relating eruption composition with size, and comparing erupted and unerupted compositions to provide evidence to understand elusive yet critical relationships between plutonism and volcanism.