Very fast silicic magma genesis in caldera and rift environments based on isotope zoning in zircons, experiments, and thermal modeling (original) (raw)

Abstract

Large-volume sub-liquidus silicic rocks are erupted in caldera environments with short repose time. Modern in situ isotopic methods have recently permitted analysis of isotopic and trace elemental abundances on micron to smaller scale and demonstrate strong crystalline heterogeneity. We review recent discoveries of isotopically (O, U-Pb) zircons in large volume ignimbrites (Snake River Plain, Kamchatka, and Iceland). We report results from a long-duration isotope exchange experiment with natural zircon and rutile that was held for 4 months at 850°C and 0.3 kbars in a silica-rich solution doped with 18O, 2H, 7Li and 10B. The length-scales of in-diffusion were examined by depth profiling using time-of-flight (TOF) and Cameca 1270 high sensitivity dynamic SIMS. Rutile and zircon developed ~2 µm and ≤0.13µm Fickian profiles, respectively, suggesting that rutile diffusion coefficients were at ~400 times greater than zircon's, and both are consistent with the wet diffusion coefficient...

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