Geochemical variability in distal and proximal glass from the Youngest Toba Tuff eruption

Abstract

The Youngest Toba Tuff (YTT, erupted at ca. 74 ka) is a distinctive and widespread tephra marker across South and Southeast Asia. The climatic, human and environmental consequences of the YTT eruption are widely debated. Although a considerable body of geochemical data is available for this unit, there has not been a systematic study of the variability of the ash geochemistry. Intrinsic (magmatic) and extrinsic (post-depositional) chemical variations bring fundamental information regarding the petrogenesis of the magma, the distribution of the tephra and the interaction between the ash and the receiving environment. Considering the importance of the geochemistry of the YTT for stratigraphic correlations and eruptive models, it is central to the YTT debate to quantify and interpret such variations. Here, we collate all published geochemical data on the YTT glass, including analyses from 67 sites described in the literature and three new samples. Two principal sources of chemical variation are investigated: (i) compositional zonation of the magma reservoir and (ii) post-depositional alteration. Post-depositional leaching is responsible for up to ca. 11 % differences in Na2O/K2O and ca. 1 % differences in SiO2/Al2O3 ratios in YTT glass from marine sites. Continental tephras are 2 % higher in Na2O/K2O and 3 % higher in SiO2/Al2O3 with respect to the marine tephra. We interpret such post-depositional glass alteration as related to seawater-induced alkali migration in marine environments. Crystal fractionation and consequential magmatic differentiation, which produced order-of-magnitude variations in trace element concentrations reported in the literature, also produced major element differences in the YTT glass. FeO/Al2O3 ratios vary by about 50 %, which is analytically significant. These variations represent magmatic fractionation involving Fe-bearing phases. We also compared major element concentrations in YTT and Oldest Toba Tuff (OTT) ash samples, to identify potential compositional differences that could constrain the stratigraphic identity of the Morgaon ash (western India); no differences between the OTT and YTT samples were observed.