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 (postdepositional) 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 Na 2 O/K 2 O and ca. 1 % differences in SiO 2 /Al 2 O 3 ratios in YTT glass from marine sites. Continental tephras are 2 % higher in Na 2 O/K 2 O and 3 % higher in SiO 2 /Al 2 O 3 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/Al 2 O 3 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.

Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption, northern Sumatra

“…Reworking of distal deposits in fluvial systems (e.g. Malaysia, India) will average the regional deposition (Gatti, ; Gatti and Oppenheimer, ) and justifies grouping local deposits. For the various deposit types GPP are summarised in Table , with GPP for all samples given in Supplementary Table 2.…”

“…Assigning distal Toba tephra to a particular eruption using glass shard major element compositional data has proved difficult because OTT, MTT and YTT have almost identical compositions (Westgate et al , ; Smith et al , ; Gatti et al , ). Nonetheless, some have suggested that major element discrimination is possible; for example, by using FeO (Westaway et al , ), but this has subsequently been discounted (Gatti et al , ; Pearce et al , ; Westgate and Pearce, ). Biotite major element composition provides a means to distinguish these eruptions (Smith et al , ), and biotite is often found in distal deposits, where small flakes travel easily within the eruption cloud.…”

“…From the major element analyses of YTT glass alone, it is not possible to recognise distinct populations, although some differences are evident in reported analyses associated with post‐depositional environmental variation, e.g. sea‐water‐induced leaching (Gatti et al , ). However, those glass shards analysed for both major and trace elements display a systematic change in major element composition when assigned to populations (see Fig.…”

“…This is particularly true of the YTT across peninsular India, although there has been much debate about the dating of this deposit and its characterisation/identification in archaeological settings, as well as its possible impact on climate and human evolution (Rose and Chesner, 1987;Acharyya and Basu, 1993;Shane et al, 1995;Ambrose, 1998;Pattan et al, 1999Pattan et al, , 2001Pattan et al, , 2010Rampino and Ambrose, 2000;Oppenheimer, 2002Oppenheimer, , 2011Chen et al, 2004;Lee et al, 2004;Gatti et al, 2011;Westaway et al, 2011;Gatti, 2012;Gatti and Oppenheimer, 2012;Petraglia et al, 2012;Williams, 2012;Lane et al, 2013;Roberts et al, 2013;Costa et al, 2014;Neudorf et al, 2014). Assigning distal Toba tephra to a particular eruption using glass shard major element compositional data has proved difficult because OTT, MTT and YTT have almost identical compositions (Westgate et al, 1998;Smith et al, 2011;Gatti et al, 2014). Nonetheless, some have suggested that major element discrimination is possible; for example, by using FeO (Westaway et al, 2011), but this has subsequently been discounted Pearce et al, 2014b;Westgate and Pearce, 2017).…”

Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

“…Although whole-rock geochemistry of volcanic tuffs can provide insight into the petrogenetic history of a magmatic system, tuffs can be subject to postdepositional weathering, reworking and/or detrital mixing, and alteration that can change the whole-rock geochemistry (Gatti et al, 2014). Zircon trace element chemistry has been widely applied to characterize magma chamber processes because zircon chemistry reflects the equilibrium melt composition from which it crystallized (e.g., Barth and Wooden, 2010;Claiborne et al, 2010).…”