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

[摘要] 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 δ18O values of magmas with δ18O-diverse zircons indicate that magma generation happens by remelting of variably hydrothermally altered, and thus diverse in δ18O, protoliths from which the host magma batch, minute or voluminous, inherited low-δ18O 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 δ18O 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 long-lived 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 fluxes. Next, we use thermomechanical finite element modeling of rhyolite genesis and to explain (1) the formation of magma batches in stress fields by dike capture or deflection 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 fluid phase (heated meteoric water); and (3) mixing time scales and their efficacies on extended horizontal scales, as expressed by marker method particle tracking. The envisioned 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 made of dispersed magma batches. Although not the focus of this work, dispersed magma batches may be stable in the long term, but their coalescence creates ephemeral, short-lived eruptable magma bodies that erupt nearly completely.

[学科分类] 地质学