Rhyolites—Hard to produce, but easy to recycle and sequester: Integrating microgeochemical observations and numerical models

Abstract

Recent discoveries of isotopically diverse minerals, i.e., zircons, quartz, and feldspars, in large-volume ignimbrites and smaller lavas from the Snake River Plain (SRP; Idaho, USA), Iceland, Kamchatka Peninsula, and other environments suggest that this phenomenon characterizes many silicic units studied by in situ methods. This observation leads to the need for new models of silicic magma petrogenesis that involve double or triple recycling of zircon-saturated rocks. Initial partial melts are produced in small quantities in which zircons and other minerals undergo solution reprecipitation and inherit isotopic signatures of the immediate environment of the host magma batch. Next, isotopically diverse polythermal magma batches with inherited crystals merge together into larger volume magma bodies, where they mix and then erupt. Concave-up and polymodal crystal size distributions of zircons and quartz observed in large-volume ignimbrites may be explained by two or three episodes of solution and reprecipitation. Hafnium isotope diversity in zircons demonstrates variable mixing of crustal melts and mantle-derived silicic differentiates. The low δ 18 O values of magmas with δ 18 O-diverse zircons indicate that magma generation happens by remelting of variably hydrothermally altered, and thus diverse in δ 18 O, protoliths from which the host magma batch, minute or voluminous, inherited low-δ 18 O values. This also indicates that the processes that generate zircon diversity happen at shallow depths of a few kilometers, where meteoric water can circulate at large water/rock ratios to imprint low δ 18 O values on the protolith. We further review newly emerging isotopic evidence of diverse zircons and their appearance at the end of the magmatic evolution of many longlived large-volume silicic centers in the SRP and elsewhere, evidence indicating that the genesis of rhyolites by recycling their sometimes hydrothermally altered subsolidus predecessors may be a common evolutionary trend for many rhyolites worldwide, especially in hotspot and rift environments with high magma and heat fl uxes. Next, we use thermomechanical fi nite element modeling of rhyolite genesis and to explain (1) the formation of magma batches in stress fi elds by dike capture or defl ection as a function of underpressurization and overpressurization, respectively; (2) the merging of neighboring magma batches together via four related mechanisms: melting through the screen rock and melt zone expansion, brittle failure of a separating screen of rocks, buoyant merging of magmas, and explosive merging by an overpressurized interstitial fl uid phase (heated meteoric water); and (3) mixing time scales and their effi cacies on extended horizontal scales, as expressed by marker method particle tracking. The en visioned advective thermomechanical mechanisms of magma segregation in the upper crust may characterize periods of increased basaltic output from the mantle, leading to increased silicic melt production, but may also serve as analogues for magma chambers ...

Estimates of Volume and Magma Input in Crustal Magmatic Systems from Zircon Geochronology: The Effect of Modeling Assumptions and System Variables

“…For zircons retrieved in volcanic products, if evidences exist for re-heating above the zircon saturation temperature and rejuvenation (e.g., Bachmann and Bergantz, 2003) in the period preceding an eruption, our method will need to be applied with caution. Resorption or lack of zircon crystallization in the period preceding the eruption would produce a depletion of the younger portions of the zircon populations, which, in turn, would result in a decrease of the mode, median, and standard deviation recalculated following our approach (Bindeman and Simakin, 2014). Figure 6 shows how a decrease of the value of these parameters leads to an underestimation especially in the final volume of the magmatic reservoir.…”

Melt evolution and residence in extending crust: Thermal modeling of the crust and crustal magmas

“…Crustal melting is more favored in regions where dry melts ascend from the mantle particularly when intruding hydrous lithologies (e.g., Bindeman and Simakin, 2014). It is also significantly controlled by tectonic extension (Fig.…”

Magma reservoir dynamics at Toba caldera, Indonesia, recorded by oxygen isotope zoning in quartz

“…Recent work on magma petrogenesis of the Yellowstone hot spot track, for example, has revealed oxygen isotope diversity in zircon that includes both high- and low-δ 18 O values relative to ‘normal’ rhyolites9. For instance, the higher δ 18 O values in Yellowstone zircon are thought to represent assimilation of unaltered crust by ascending magmas, whereas the lower end of the δ 18 O zircon spectrum has been attributed to recycling of hydrothermally-altered (low-δ 18 O) rhyolitic material into the Yellowstone magma system.…”

“…Unlike zircon, however, quartz can grow to relatively large crystal sizes (e.g., up to 2 mm across in this study), which permits spatially detailed isotopic profiling with SIMS. This level of detail is more challenging to obtain in zircon as crystal size is typically ≤200 μm in most volcanic and metamorphic rocks, which limits intra-crystal analyses to usually a single core and perhaps one or two rim analyses (e.g., refs 2 and 9). Quartz may therefore be especially useful in certain high-silica magmatic settings, such as the Toba caldera in Indonesia, whose erupted products host plentiful quartz phenocrysts.…”