Abstract
The Aso-4 caldera-forming event (86.4 ± 1.1 ka, VEI-8) is the second largest volcanic eruption Earth experienced in the past 100 ka. The ignimbrite sheets produced during this event are some of the first ever described compositionally zoned pyroclastic flow deposits exhibiting clear compositional, mineralogical and thermal gradients with stratigraphic position. Large quantities of the deposits are composed of crystal-poor, highly evolved juvenile pumices, while late-erupted pyroclastic flows are in many cases dominated by crystal-rich and less silicic scoria. These petrological gradients in the Aso-4 deposits have been linked to extensive magma mixing of two compositionally distinct magmas in a complex upper crustal reservoir. However, new studies on several other zoned ignimbrites suggest that magma mixing alone is not sufficient to fully explain such strong compositional gradients in the deposits. These gradients are expected to be dominantly caused by the recharge-induced reactivation of extracted melt caps and their complementary cumulate in the upper crust. Here, we investigate bulk rock and matrix glass data with detailed analyses of mineral chemistry in order to re-evaluate the Aso-4 deposits in light of these latest developments. Reverse chemical zoning in phenocrysts, Sr enrichment in euhedral rims of plagioclase and the presence of mafic minerals (clinopyroxene, olivine) indicate recharge of hot, mafic magmas shortly prior to eruption, inducing a mixing signature. However, the marked enrichment in some elements in bulk-rock analyses and the presence of highly evolved minerals (some in the form of glomerocrysts) in the late-erupted, crystal-rich units, provide clear evidence for crystal accumulation in these scoria. Mass balance modeling of P2O5, Sr and SiO2 supports the extraction of melt-rich lenses within an upper crustal mush zone, leaving a partly cumulative evolved crystal residue. We therefore propose an origin of the compositionally zoned Aso-4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt caps within its associated cumulate mush. This complex reservoir was reactivated by mafic recharge shortly prior to eruption, imparting an additional mixing signature to the deposits.
Highlights
Large-scale caldera-forming eruptions are typical examples of rare, but high-impact events, with major consequences on a local to global scale (Self, 2006; Self, 2015; Newhall et al, 2018; Papale and Marzocchi, 2019; De Maisonneuve and BergalKuvikas, 2020; Geshi, 2020)
We propose an updated model relating the origin of the voluminous Aso-4 ignimbrites to the formation of extracted silicic melt caps within a complementary cumulate mush, reactivated prior to eruption by lessdifferentiated recharge, inducing a faint but significant magma mixing signature
Juvenile clasts were sampled at different stratigraphic heights from, among others, the 4I-1, 4I-3 and 4II-2 units described in Kaneko et al, (2007)
Summary
Large-scale caldera-forming eruptions are typical examples of rare, but high-impact events, with major consequences on a local to global scale (Self, 2006; Self, 2015; Newhall et al, 2018; Papale and Marzocchi, 2019; De Maisonneuve and BergalKuvikas, 2020; Geshi, 2020). Typical volcanic products of large-scale caldera eruptions are “zoned ignimbrites”, characterized by strong compositional, mineralogical and thermal gradients from early to late-erupted material Those ignimbrites are usually produced by the rapid emptying of a complex, zoned magma reservoir in a single geological “instant” (i.e., on the order of days; Lipman, 1967; Wörner and Schmincke, 1984; Bacon and Druitt, 1988; Milner et al, 2003; Bachmann et al, 2014; Bachmann and Huber, 2016; Forni et al, 2016) and are interpreted to reflect reservoir conditions just prior to eruption. Reevaluation of bulk rock, groundmass glass and mineral chemistry were performed in order to test the previous hypotheses in light of the latest findings on the generation of zoned ignimbrites
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